JP3684750B2 - Synthetic aperture radar device and observation process design method for synthetic aperture radar device - Google Patents

Description

なお、rはアンテナと観測点の距離、Δrはスラントレンジ分解能、φは電波入射角度、Δθはエレベーション方向のビーム照射角である。
式(１)より、従来のSCAN SAR型の合成開口レーダ装置は、アンテナが指向するオフナディア角が切り換えられることによりrとφの値が変化するため、合成開口レーダ装置の収集或いは送信するデータ量が変化する。
【００１３】
また、従来のSCAN SAR型の合成開口レーダ装置のアンテナに平面アレーアンテナを用いた場合、アンテナから照射されるアンテナビームのビーム角度によって、アンテナの有効開口長が変化するため、エレベーション方向のビーム照射角Δθが変化し、合成開口レーダ装置の収集或いは送信するデータ量が変化する。
さらに、従来のSCAN SAR型の合成開口レーダ装置から得られる画像のレンジ分解能を一定に保つために、スラントレンジ分解能ΔrをSin(φ)に比例させて制御すると、サブスウォース幅が変化するため、合成開口レーダ装置の収集或いは送信するデータ量が変動する。
このように、従来のSCAN SAR型の合成開口レーダ装置では、アンテナが指向するオフナディア角によって、合成開口レーダ装置が収集或いは地上へ送信する観測データのデータ量が変動し、プラットフォームに搭載されたデータレコーダの単位時間あたりの記録容量、或いはプラットフォームから地上に送信する観測データの送信容量がデータリンクの回線容量を超過するという問題があった。
【００１４】
本発明はこれらの問題に鑑みなされたものであり、画質が高い観測データが得られるSCAN SAR型の合成開口レーダ装置を得ることを目的とする。
また、本発明は、送受信アンテナの指向するオフナディア角が切り換えられても、複数のオフナディア角各々におけるスウォース幅が一定の値に調節され、プラットフォームに搭載されたデータレコーダの単位時間あたりの記録容量、或いはプラットフォームから地上に送信する観測データの送信容量が安定するSCAN SAR型の合成開口レーダ装置を得ることを目的とする。
【００１５】
【課題を解決するための手段】
この発明にかかる合成開口レーダ装置は、移動部と、移動部に設けられ、指向方向が可変の送受信アンテナと、送受信アンテナを介して、第一及び第二の所定期間にそれぞれ観測対象の第一及び第二の観測領域にアンテナビームを照射してパルス信号を送信し、パルス信号が第一及び第二の観測領域でそれぞれ反射されたパルス信号である第一及び第二のパルスエコーを受信する送受信部と、送受信部が受信した第一及び第二のパルスエコーをもとにそれぞれ第一及び第二の観測領域の観測データである第一及び第二の観測データを取得する観測部と、第一及び第二の観測領域それぞれの一部であって、第一及び第二のパルスエコーにおける所望信号の電力と所望信号以外の電力の比が所定値以上である第一及び第二の観測部分領域にそれぞれ対応する第一及び第二の観測データである第一及び第二の観測部分データを抽出して、第一及び第二の観測部分データを合成する合成部と、第一及び第二の所定期間における指向方向を互いに等しくかつ一定とし、第一及び第二の所定期間を第一及び第二の観測部分領域が移動部の進行方向に平行な方向において重複部分を有して隣接するように設定する制御部とを備えたものである。
【００１６】
また、この発明にかかる合成開口レーダ装置は、送受信部は送受信アンテナを介して、第三の所定期間に観測対象の第三の観測領域にアンテナビームを照射してパルス信号を送信し、パルス信号が第三の観測領域で反射されたパルス信号である第三のパルスエコーを受信し、観測部は送受信部が受信した第三のパルスエコーをもとに第三の観測領域の観測データである第三の観測データを取得し、合成部は、第三の観測領域の一部であって、第三のパルスエコーにおける所望信号の電力と所望信号以外の電力の比が所定値以上である第三の観測部分領域に対応する第三の観測データである第三の観測部分データを抽出して、第一乃至第三の観測部分データを合成し、制御部は、第三の観測領域が第一及び第二の観測領域と移動部の進行方向に垂直な方向において隣接するように送受信部及び送受信アンテナを制御するものである。
【００１７】
さらに、この発明にかかる合成開口レーダ装置は、制御部が、第一乃至第三の観測領域それぞれの移動部の進行方向に垂直な方向の長さが互いに等しくなるように送受信部及び送受信アンテナを制御するものである。
【００１８】
また、この発明にかかる合成開口レーダ装置の観測プロセス設計方法は、移動部と、移動部に設けられ、指向方向が可変の送受信アンテナと、送受信アンテナを介して、第一乃至第三の所定期間にそれぞれ観測対象の第一の観測領域、第一の観測領域と移動部の進行方向に平行な方向において重複部分を有して隣接する第二の観測領域、及び第一の観測領域と移動部の進行方向に垂直な方向において隣接する第三の観測領域にアンテナビームを照射してパルス信号を送信し、パルス信号が第一乃至第三の観測領域でそれぞれ反射されたパルス信号である第一乃至第三のパルスエコーを受信する送受信部と、送受信部が受信した第一乃至第三のパルスエコーをもとにそれぞれ第一乃至第三の観測領域の観測データである第一乃至第三の観測データを取得する観測部とを備える合成開口レーダ装置の観測プロセス設計方法において、第一及び第二のパルスエコーにおける所望信号の電力と所望信号以外の電力の比をもとに、第一の観測領域と重畳部分との比である重畳比を算出する第一のステップと、送受信アンテナから照射されるアンテナビームの波長、送受信アンテナの開口部の移動部の進行方向に対して平行な方向の長さである第一のアンテナ開口長、送受信アンテナの開口部の移動部の進行方向に対して垂直な方向の長さである第二のアンテナ開口長、及び送受信アンテナの開口部から第一の観測領域までの距離をもとに、アンテナビームの第一の観測領域における移動部の進行方向に平行な方向の長さである照射幅を算出する第二のステップと、重畳比及び照射幅をもとに、重畳部分の移動部の進行方向に平行な方向の長さである重畳部分幅を算出する第三のステップと、重畳部分幅を移動部が通過する時間、送受信アンテナからパルス信号が送信され、第一の観測領域で反射されて第一のパルスエコーとして送受信アンテナに受信されるまでの時間である遅延時間、第一の観測領域にアンテナビームを照射するときの送受信アンテナの指向方向である第一の指向方向から第三の観測領域にアンテナビームを照射するときの送受信アンテナの指向方向である第三の指向方向に切り換えるために要する時間である切り換え時間、照射幅、及び移動部の移動速度をもとに第一の所定期間の長さを算出する第４のステップを有するものである。
【００１９】
また、この発明にかかる合成開口レーダ装置の観測プロセス設計方法は、送受信アンテナから照射されるアンテナビームの波長、第二のアンテナ開口長、第一の指向方向が鉛直方向と為す角度、第三の指向方向が鉛直方向と為す角度、及び送受信アンテナの指向方向が第一の指向方向であるときの送受信アンテナの開口部から第一の観測領域までの距離をもとに、移動部の進行方向に対して垂直な方向の第一及び第三の観測領域の長さを互いに等しい値に設定する第５のステップを有するものである。
【００２３】
【発明の実施の形態】
発明の実施の形態１．
本発明によるSCAN SAR型の合成開口レーダ装置の一実施形態を図９を用いて説明する。
図９は、実施形態１のSCAN SAR型の合成開口レーダ装置の構成図である。
図９において、６は制御部である。本発明の制御部６は、レンジ方向に隣接する観測領域の画像が密接するように、また、アジマス方向に隣接する観測領域の画像がオーバーラップするように、照射するアンテナビームの幅、アンテナビームを照射する送受信アンテナ２が指向する俯角であるオフナディア角、及び送受信アンテナ２のオフナディア角を切り換えるタイミングを制御する。
また、この実施形態１のSCAN SAR型の合成開口レーダ装置の動作は、図１０に示した従来のSCAN SAR型の合成開口レーダ装置の動作とほぼ同様であり、その説明を省略する。
【００２４】
次に図９に示したSCAN SAR型の合成開口レーダ装置が照射するアンテナビームの照射位置とタイムシーケンスについて、図１を用いて説明する。
図１において、Tciは相関時間を示す。この相関時間Tciとは、任意の位置にあるSCAN SAR型の合成開口レーダ装置の送受信アンテナ２が、所定のオフナディア角で指向する所定の観測帯域であるサブスウォース（以下、第ｉサブスウォースと称する）に、アンテナビームを照射する時間を指し示す。
Tdiは伝播遅延時間を示す。この伝播遅延時間Tdiとは、第ｉサブスウォースに対して照射された送信パルスが、送受信アンテナ２に受信されるまでの時間を指し示す。
Toiは合成開口時間を示す。この合成開口時間Toiとは、第ｉサブスウォースに対して、送受信アンテナ２が指向している時間である。
【００２５】
なお、この合成開口時間Toiは、相関時間Tciと伝播遅延時間Tdiとの和で示される。
Tsiはスキャン切替時間を示す。このスキャン切り換え時間Tsiとは、第ｉサブスウォースから、次なるサブスウォース（以下、第(i+１)サブスウォースと称する）に、送受信アンテナ２のオフナディア角を切り換えるために必要とする時間である。
なお、第(i+１)サブスウォースとは、送受信アンテナ２が指向するオフナディア角を切り換え、この切り換えられたオフナディア角に対応する観測帯域であるサブスウォースを指し示す。また、これらのサブスウォースにアンテナビームを走査させることをスキャンすると言う。
Taiはスキャン周期を示す。このスキャン周期Taiは、第ｉサブスウォースでの観測を開始してから、アンテナビームが指向するオフナディア角が切り換えられN個のサブスウォースで順次観測を行い、送受信アンテナ２が指向すべき全てのサブスウォースでの観測を経た後に、再度第ｉサブスウォースの観測が終了するまでの時間を指し示す。つまり、Ｎ個の観測帯域であるサブスウォースで、例えば、衛星や飛行機といったプラットフォームの位置に基づく各サブスウォースの一部分である観測領域を（N＋１）個、スキャンするために要する時間を指し示す。
【００２６】
Lwiはオーバーラップ長を示す。このオーバーラップ長Lwiとは、同じサブスウォースでアジマス方向にずれた２つの観測領域のオーバーラップするその重畳部分の長さである。
なお、このオーバーラップ長は、所定のＳ／Ａ比を満たす画像を抽出するために画像の端部を切り捨てた場合でも、アジマス方向に隣接する複数の観測領域間で観測もれが生じないように設けられているものである。単純に画像の端を切り捨てた場合には、アジマス方向に隣接する複数の観測領域の間に観測データのない観測領域が生じる恐れがある。この観測漏れを防止するため、制御装置６は、隣接する観測領域を十分にオーバーラップさせて、これらの観測領域にビームスキャンさせる必要がある。
一部オーバーラップする２つの観測領域の観測データを１つに合成する時、この２つの観測領域がオーバーラップするその重畳部分の観測データは、２つの観測データから互いに半分ずつ用いられる。つまり、各観測領域の端部から各々重畳部分の半分の観測データが削除される。また、２つの観測領域の重畳部分の観測データは、各観測領域の観測データを比較選択してもよい。
【００２７】
次に図１に示したSCAN SAR型の合成開口レーダ装置に関するアンテナビームの照射位置とアンテナビームの照射切り換えのタイムシーケンスとの関連づけの中から、特にSCAN SAR型の合成開口レーダ装置が照射するアンテナビームの照射切り換えのタイムシーケンスについて、図２を用いて説明する。
例えば第ｉサブスウォースにおいて、アジマス方向にずれる２つの観測領域が観測漏れのない連続した２つの観測領域として観測されるためには、合成開口レーダ装置の指向するオフナディア角が切り換えられて観測される（N＋１）個の観測領域を合成開口レーダ装置が観測する間に、合成開口レーダ装置が設けられた衛星等のプラットフォームの移動距離が、アンテナビームの照射幅の範囲内に止まっている必要がある。
【００２８】
この条件は式（２）に表され、この式（２）が全てのサブスウォース（i＝１〜N）において成立する必要がある。
【数２】

なお、Ｌは送受信アンテナ２が有する開口部のアジマス方向の開口長であるアンテナ開口長である。
αは送受信アンテナ２が有する開口部のレンジ方向の開口長を調節するウェイトにより決まる係数である。
λはアンテナビームの送信波長である。
Ｖは衛星等のプラットフォームの速度である。
Ｒ_Near,iは第ｉサブスウォースにおける観測領域と送受信アンテナ２が有する開口部との最短距離を指し示すニアレンジ側のスラントレンジである。
【００２９】
また、スキャン周期Taiは式（３）で表される。
【数３】

なお、Tuiは余裕時間であり、アジマス方向で画像をオーバーラップさせるために必要となる。
Tpiはパルス信号長である。
すなわち、制御装置６は、式（２）と余裕時間Tui>０とを満足するように、各パラメータを決定する。
【００３０】
この制御によって、Ｓ／Ａ比が改善される例を次に示す。
なお、Ｓ／Ａを改善するこの制御を行わない場合には、図５に示すように、アジマス方向の位置によって合成開口処理に使用するアンテナビームのゲインが異なるので、特に画像の両端において雑音等価後方散乱係数が劣化する傾向がある。
複数のオフナディア角各々におけるアジマス分解能の最大値Δazは、式（６）で与えられる。
【数４】

なお、Mazはアジマス方向のマルチルック数である。
また、あるサブスウォースにおいてアジマス方向にずれた２つの観測領域の画像のオーバーラップ比γを式（７）で定義する。
【数５】

【００３１】
さらに、SCAN SAR型の合成開口レーダ装置が観測する観測領域におけるアジマス方向のＳ／Ａ比を式（８）で定義する。
【数６】

なお、Min{ }はアジマス角θの最小値を求める関数であり、アジマス角θは式(９)の定義域を有する。
また、式（８）で定義される各観測領域のアジマス方向のＳ／Ａ比は、各観測領域の各アジマス位置において最小のＳ／Ａ比をもって、各観測領域の観測データに基づく画像のＳ／Ａ比と定義している。
また、δは合成開口を行う角度として、式(１０)で与えられる。
【数７】

なお、γは画像のオーバーラップ比(０≦γ＜１)であり、Nbはスキャン数である。
【００３２】
次にSCAN SAR型の合成開口レーダ装置が観測して得られる観測データのアジマス方向の位置であるアジマス位置におけるＳ／Ａ比について図３を用いて説明する。
SCAN SAR型の合成開口レーダ装置のアジマス方向のＳ／Ａ比は、画像の中心から画像の端部へ進むに従い低下することが理論的に分かっている。
なお、図３において、縦軸にはＳ／Ａ比の値が示される。
また、横軸には画像の中心を基準値である０としたアジマス位置が示される。なお、画像のＳ／Ａ比は、画像の中央部を中心として対称な値を示すため、図３中の横軸では画像の中央から画像の一端部までのＳ／Ａ比を示している。この時、画像のアジマス位置は、画像の中央を０、画像の端部を０．５として示している。
【００３３】
図４は、受信信号の周波数帯域の絞り込みを左右するオーバーサンプル比を１．０から１．２５まで変化させた場合の、分解能とＳ／Ａ比との関係である。なお、受信信号とは、送受信アンテナ２より出力されたアンテナビームが、地表面や海面等に反射され、送受信アンテナ２に受信されたエコーのことを指し示す。実施形態１のSCAN SAR型の合成開口レーダ装置は、従来のSCAN SAR型の合成開口レーダ装置に比べて、オーバーラップ比γを上げるに従い、Ｓ／Ａ比が改善されることが実験より分かっている。
なお、図４の左端は、送受信アンテナ２が指向するあるオフナディア角において、観測される例えば２つの観測領域がオーバーラップする部分を有することなく隣接するオーバーラップ比γが０の場合である。これは、従来のSCAN SAR型の合成開口レーダ装置におけるＳ／Ａ比である。
また、左端以外の部分はオーバーラップ比γ＞０の場合であり、この実施形態１のSCAN SAR型の合成開口レーダ装置におけるＳ／Ａ比を示している。なお、この実施形態１のSCAN SAR型の合成開口レーダ装置は、従来のSCAN SAR型の合成開口レーダ装置に比べて、オーバーラップ比γを上げるとアジマス分解能の低下が伴う。
【００３４】
このように、この実施形態１の合成開口レーダ装置は、この合成開口レーダ装置が同じサブスウォースで観測する隣り合う２つの観測領域を一部オーバーラップさせ、２つの観測領域がオーバーラップした重畳部分の観測データは所望の信号電力に対する偽像の信号電力の比であるＳ／Ａ比に基づき選別し、これら２つの観測領域の観測データを１つに合成するため、２つの観測領域の端部で互いに重畳している観測領域の観測データのＳ／Ａ比が高いものが得られる。
【００３５】
また、この実施形態１の合成開口レーダ装置は、この合成開口レーダ装置が有する送受信アンテナ２の指向するオフナディア角を切り換えて、レンジ方向に隣接する複数の観測領域を観測して、これら複数の観測領域の観測データを合成して、１つの観測データを作成するため、アジマス方向に隣り合う２つの観測領域の観測データを高い質で１つの観測データに合成すると共に、レンジ方向に観測領域が拡大された観測データを得ることが出来る。
【００３６】
さらに、この実施形態１の合成開口レーダ装置は、アジマス方向に一部重畳する２つの観測領域の重畳部分の観測データを互いに折半して１つの観測データを合成するという簡素化された合成処理を行うため、比較的質の高い合成された観測データが短時間で得られる。
【００３７】
発明の実施の形態２．
次に本発明の観測プロセスの設計方法について説明する。
この実施形態２では、実施形態１のSCAN SAR型の合成開口レーダ装置で、アジマス方向のＳ／Ａ比が複数のオフナディア角で一定になるように、観測プロセスを設計する。
なお、SCAN SAR型の合成開口レーダ装置は、アジマス方向のＳ／Ａ比を向上させると、アジマス分解能を低下させる。
アジマス方向のＳ／Ａ比を複数のオフナディア角各々で一定にするため、画像のオーバーラップ比γを複数のオフナディア角各々で一定にする。
すなわち、式（１１）を満足させるように画像のオーバーラップ比γｉ、γｊを設定すればよい。なお、γiは第ｉサブスウォースの画像のオーバーラップ比、γｊは第ｊサブスウォースの画像のオーバーラップ比である。
【数８】

【００３８】
この式（１）に基づき、各サブスウォースで共通の画像のオーバーラップ比γが与えられると、観測プロセスを以下のように決定する。
まず、送受信アンテナ２より第ｉサブスウォースに照射されるアンテナビームのビーム照射領域の長さLaiを、式（２）に基づき、式（１２）で決定する。なお、ビーム照射領域とは、観測領域のことを指し示す。また、このビーム照射領域に長さLaiは、観測領域のアジマス方向の長さを指し示す。
【数９】

そこで、各観測領域の観測データに基づく画像のオーバーラップ比γに基づき、隣り合う２つの画像のオーバーラップする重畳部分の長さLwiを導出する式（１３）が得られる。
【数１０】

そして、同一のサブスウォースでアジマス方向にオーバーラップする２枚の画像を合成するための処理時間である余裕時間Tuiは、アジマス方向にオーバーラップする２枚の画像がオーバーラップする重畳部分の長さLwiを衛星等のプラットフォームが通過する時間であり、式（１４）から求められる。
【数１１】

【００３９】
次に、式（３）を満足するようにある観測領域に対してアンテナビームを照射する時間を示す相関時間Tciを決定する。なお、相関時間Tciが決まれば、送受信アンテナ２から照射されたアンテナビームが、地表面や海面に反射されて、この送受信アンテナ２に受信されるまでの時間を示す電波遅延時間Tdiは所定の値に決まる。また、送受アンテナ２が指向するサブスウォースを切り換える為のスキャン切替時間Tsiはハードウェアの性能によって所定の値に決まる。
これで、観測プロセスが決定される。
【００４０】
以上の手順をまとめると次のようになる。
［ステップ（以下、Ａと称する。）１］
式（８）から、所要のＳ／Ａ比を満足する隣り合う画像のオーバーラップ比γを求める。
［Ａ２］
式（１２）から、第ｉサブスォースのビーム照射領域の長さLaiを求める。
［Ａ３］
式（１３）から、隣り合う画像がオーバーラップするその重畳部分の長さLwiを求める。
［Ａ４］
式（１４）から、隣り合う画像をオーバーラップさせ、１つの画像に合成する為の合成処理に要する余裕時間Tuiを求める。
［Ａ５］
式（３）を満足する相関時間Tciを求める。
【００４１】
このように、この実施形態３の合成開口レーダ装置は、この合成開口レーダ装置が有する送受信アンテナ２の指向するオフナディア角を切り換えて、レンジ方向に隣接する複数の観測領域を観測すると共に、これら複数の観測領域のオーバーラップ比を調節するため、レンジ方向に隣接する複数の観測領域の観測データを均質なＳ／Ａ比を有する１つの観測データに合成することができる。
【００４２】
発明の実施の形態３．
次に本発明の他のアンテナビームの観測プロセスの設計方法について説明する。
この実施形態３では、実施形態１のSCAN SAR型の合成開口レーダ装置で、雑音等価後方散乱係数が複数のオフナディア角で一定になるように、観測プロセスを設計する。
実施形態１で述べたSCAN SAR型の合成開口レーダ装置は、アジマス分解能を低下させてアジマス方向のＳ／Ａ比を向上させる。
そこで、雑音等価後方散乱係数を複数のオフナディア角各々で一定にするために、ある観測領域に対する、このある観測領域と隣り合う観測領域とがオーバーラップする重畳部分の比であるオーバーラップ比γを、複数のオフナディア角各々で変える必要がある。
【００４３】
はじめに、SCAN SAR型の合成開口レーダ装置のレーダ方程式を式（１５）に示す。
【数１２】

なお、σ０は雑音等価後方散乱係数、Pnは受信機雑音電力、Ptは送信電力、Mは積分点数、θiは第ｉヒットのオフナディア角、G(θ)はオフナディア角θにおけるアンテナ利得、r(θ)はオフナディア角θにおけるスラントレンジである。
ここで、積分点数が十分に多く、またサブスウォース内のスラントレンジを一定値とみなせば、式（１５）は式（１６）〜式（１９）のように表わされる。
なお、積分点数が十分に多ければ、総和演算を積分で近似することができる。
【数１３】

【００４４】
このように、複数のオフナディア角各々において、雑音等価後方散乱係数が等しくなるように設定するためには、式（１６）に基づく式（２０）が複数のオフナディア角各々で一定なるように画像のオーバーラップ比γを設定すれば良い。
【数１４】

オーバーラップ比γが決定されれば、観測プロセスを決定する手順は実施形態２と同様である。
したがって、複数のオフナディア角各々における各サブスウォースの雑音等価後方散乱係数が等しくなる観測プロセスを決定する手順は、以下に示すステップで構成される。
【００４５】
［ステップ（以下、Ｂと称する。）１］
式（２０）から、各サブスォースにおける隣り合う画像のオーバーラップ比γが決定される。
［Ｂ２］
式（１２）から、第ｉサブスォースのビーム照射領域の長さLaiが求められる。
［Ｂ３］
式（１３）から、隣り合う画像のオーバーラップするその重畳部分の長さLwiが求められる。
［Ｂ４］
式（１４）から、隣り合う画像をオーバーラップさせ、１つの画像に合成する為の合成処理に要する余裕時間Tuiが求められる。
［Ｂ５］
式（３）を満足するように相関時間Tciを求める。
【００４６】
このように、この実施形態３の合成開口レーダ装置は、この合成開口レーダ装置が有する送受信アンテナ２の指向するオフナディア角を切り換えて、レンジ方向に隣接する複数の観測領域を観測すると共に、これら複数の観測領域のオーバーラップ比を調節するため、レンジ方向に隣接する複数の観測領域の観測データを均質な雑音等価後方散乱係数を有する１つの観測データに合成することができる。
【００４７】
発明の実施の形態４．
次に本発明の他のアンテナビームの観測プロセスの設計方法について説明する。
この実施形態４では、実施形態１のSCAN SAR型の合成開口レーダ装置で、アジマス分解能とアジマス方向のＳ／Ａ比とが複数のオフナディア角それぞれで共に一定になるように、観測プロセスを設計する。
実施形態１で述べたSCAN SAR型の合成開口レーダ装置は、アジマス分解能を低下させてアジマス方向のＳ／Ａ比を向上させる。
また、実施形態２に示した観測プロセスを用いれば、全てのサブスウォースにおいて、Ｓ／Ａ比を一定に設定できる。
しかし、アジマス分解能を複数のオフナディア角それぞれに基づく各サブスウォースで一定にするためには、相関時間Tciにオーバーラップ比に関する条件を設定する必要がある。
はじめに、式（３）及び式（６）を変化させ、式（２１）〜式（２２）を得る。
【数１５】

【００４８】
そして、式（２１）〜式（２２）に基づき、式（２３）からアジマス分解能が得られる。
【数１６】

なお、アジマス分解能はレンジ方向に異なる各サブスウォースによって変化し、この式（２３）より得られるアジマス分解能の数値は、そのサブスウォースにおける最大値であり最悪値が示される。
オフナディア角がそれぞれ異なる各サブスウォースで、アジマス分解能を等しく設定するためには、式（２３）を用いて、複数のオフナディア角各々でアジマス分解能を計算し、それら複数の計算結果の中の最大値を共通のアジマス分解能として採用すればよい。
そして、計算結果が最大値とは異なる値であったサブスウォースにおいては、他のサブスウォースに揃えるようアジマス分解能を低下させる。
この場合、アジマス分解能を低下させると、タイムシーケンス中の余裕時間Tuiが増加する。
これらのことより、アジマス分解能とアジマス方向のＳ／Ａ比が複数のオフナディア角各々において共に一定になるような観測プロセスは以下に示すようなステップで構成される。
【００４９】
［ステップ（以下、Ｃと称する。）１］
式（２０）から、各サブスォースにおける隣り合う画像のオーバーラップ比γが決定される。
［Ｃ２］
式（１２）から、第ｉサブスォースのビーム照射領域の長さLaiを求める。
［Ｃ３］a
式（１３）から、隣り合う画像がオーバーラップするその重畳部分の長さLwiを求める。
［Ｃ４］
式（１４）から、隣り合う画像をオーバーラップさせ、１つの画像に合成する為の合成処理に要する余裕時間Tuiを求める。
［Ｃ５］
式（２３）から、各サブスォースで達成できるアジマス分解能を計算する。
［Ｃ６］
各サブスウォースで求められたアジマス分解能の中の最大値を、全てのサブスォースのアジマス分解能として採用する。
［Ｃ７］
式（６）に基づき、相関時間Tciが求められる。
【００５０】
このように、この実施形態３の合成開口レーダ装置は、この合成開口レーダ装置が有する送受信アンテナ２の指向するオフナディア角を切り換えて、レンジ方向に隣接する複数の観測領域を観測すると共に、これら複数の観測領域のオーバーラップ比を調節するため、レンジ方向に隣接する複数の観測領域の観測データを均質なＳ／Ａ比及びアジマス分解能を有する１つの観測データに合成することができる。
【００５１】
発明の実施の形態５．
次に本発明の他のアンテナビームの観測プロセスの設計方法について説明する。
この実施形態５では、実施形態１のSCAN SAR型の合成開口レーダ装置で、アジマス分解能と雑音等価後方散乱係数とが複数のオフナディア角で共に一定になるように、観測プロセスを設計する。
実施形態１で述べたSCAN SAR型の合成開口レーダ装置は、アジマス分解能を低下させて雑音等価後方散乱係数を向上させる。
さらに、実施形態３で示した手順で決定した観測プロセスを用いれば、全てのサブスウォースにおける雑音等価後方散乱係数を一定に設定することができる。
しかしアジマス分解能を複数のオフナディア角それぞれに基づく各サブスウォースで一定にするためには、相関時間Tciにオーバーラップ比に関する条件を設定する必要がある。
【００５２】
実施形態４で示したように、オフナディア角がそれぞれ異なる各サブスウォースでアジマス分解能を等しく設定するためには、式（２３）を用いて複数のオフナディア角各々でアジマス分解能を計算し、得られた複数の計算結果の中の最大値を共通のアジマス分解能として採用すればよい。
なお、計算結果が最大値とは異なる値であったサブスウォースにおいては、他のサブスウォースに揃えるようアジマス分解能を低下させる。
この場合、アジマス分解能を低下させると、タイムシーケンス中の余裕時間Tuiが増加する。
これらのことより、アジマス分解能とアジマス方向のＳ／Ａ比とが、複数のオフナディア角各々において共に一定となるような観測プロセスは、以下に示すようなステップで構成される。
【００５３】
［ステップ（以下、Ｄと称する。）１］
式（２０）から、各サブスォースの画像における隣り合うオーバーラップ比γが決定される。
［Ｄ２］
式（１２）から、第ｉサブスォースのビーム照射領域の長さLaiが求められる。
［Ｄ３］
式（１３）から、隣り合う画像のオーバーラップするその重畳部分の長さLwiが求められる。
［Ｄ４］
式（１４）から、画像をオーバーラップさせ、１つの画像に合成する為の合成処理に要する余裕時間Tuiが求められる。
［Ｄ５］
式（２３）から、各サブスォースで達成できるアジマス分解能が計算される。
［Ｄ６］
各サブスウォースで求められたアジマス分解能の中の最大値を、全てのサブスォースのアジマス分解能として採用する。
［Ｄ７］
式（６）に基づき、相関時間Tciが求められる。
【００５４】
このように、この実施形態３の合成開口レーダ装置は、この合成開口レーダ装置が有する送受信アンテナ２の指向するオフナディア角を切り換えて、レンジ方向に隣接する複数の観測領域を観測すると共に、これら複数の観測領域のオーバーラップ比を調節するため、レンジ方向に隣接する複数の観測領域の観測データを均質な雑音等価後方散乱係数及びアジマス分解能を有する１つの観測データに合成することができる。
【００５５】
発明の実施の形態６．
次に本発明の他の実施形態及び他のアンテナビームの観測プロセスの設計方法について説明する。
この実施形態６は、実施形態１のSCAN SAR型の合成開口レーダ装置の送受信アンテナ２にアレーアンテナを使用し、複数のオフナディア角各々におけるサブスウォースのレンジ方向の距離であるサブスウォース幅が一定となるようにアレーアンテナを制御して、アンテナビームを照射する制御装置及びその制御方法に関するものである。
図６は、この合成開口レーダ装置の制御装置の構成を示す。
図６において、７は移相器、８はウェイティングである。
なお、制御装置６は、ウェイティング８を操作して、送受信アンテナ２の開口部のエレベーション方向の開口径の広狭を制御する。そして、複数のサブスォース各々で、一定のスウォース幅Swに維持するように制御する。
また、図６において、実施形態１と同一又は相当の部分には同一符号を付してその説明を省略し、実施形態１と相違する部分について説明した。
さらに、合成開口レーダ装置は、図示していないが、送受信アンテナ２で受信した受信信号やアンテナビームのエコー等を記録する為の記録部を有する。
また、合成開口レーダ装置は、観測した複数の観測領域の観測データを、若しくは複数の観測データを合成して得られたものを、地表面や海面等にある基地局等に送信する。
【００５６】
この実施形態６に示す合成開口レーダ装置の観測時の概念図を図７に示す。
図７において、２は送受信アンテナである。
また、Hは高度であり、地表面や海面等からプラットフォームまでの距離である。
ρはスラントレンジであり、プラットフォームの進行方向に垂直な方向であるレンジ方向に指向する送受信アンテナ２の開口部から観測する観測領域のある基準点までの距離を指し示す。なお、ある基準点とは、観測している観測領域でのサブスウォース幅における中央位置を示す。
θ０はオフナディア初期角であり、送受信アンテナ２が指向するオフナディア角の初期値である。
θはビーム走査角であり、送受信アンテナ２が指向するオフナディア角からオフナディア初期角を差し引いた値である。
φは電波入射角であり、地表面や海面等において、鉛直方向の逆方向に対して、アンテナビームが照射される方向を指し示す。
この時、送受信アンテナ２から照射されるアンテナビームのエレベーション方向のビーム照射角Δθは式（２４）で求められる。なお、エレベーション方向とは、プラットフォームの進行方向に対して垂直の方向を指し示す。また、サブスウォース幅Swは、式（２５）に基づき求められる。
【数１７】

なお、Dはエレベーション方向のアンテナ有効径、λは波長である。
【００５７】
また、この式（２５）において、スラントレンジρ及び電波入射角φは、SCAN SAR型の合成開口レーダ装置が航空機に搭載された場合と衛星に搭載された場合とで、異なる。
そして、前者の場合、スラントレンジρ及び電波入射角φは、式（２６）〜式（２７）で求められる。
【数１８】

また、後者の場合、スラントレンジρ及び電波入射角φは、式（２８）〜式（２９）で求められる。
【数１９】

式（２５）より、サブスウォース幅Swは、ビームのオフナディア角が大きいほど拡がる。なお、オフナディア角は、オフナディア初期角θ０とビーム走査角θとの和で示される。
そこで、以下に示す手順により、合成開口レーダ装置の制御装置は、送受信アンテナ２のエレベーション方向の有効径Dを制御して、異なる複数の観測領域各々のサブスウォース幅Swを一定にする。
【００５８】
［ステップ（以下、Ｅと称する。）１］
各観測領域におけるサブスウォース幅Swを、式（２４）及び（２５）に基づき、各観測領域に共通のサブスウォース幅Swを設定する。
［Ｅ２］
設定したサブスウォース幅Swを実現するためのビーム照射角Δθを、式（２５）に基づき、複数のサブスウォース各々について求める。
［Ｅ３］
求めたビーム照射角Δθを実現するための送受信アンテナ２のエレベーション方向の有効径Dを、式（２４）に基づき、複数のサブスウォース各々について求める。
【００５９】
このように、実施形態６の合成開口レーダ装置は、レンジ方向に隣接する２つの観測領域各々のレンジ方向の距離を等しく調節して、これらレンジ方向に隣接する２つの観測領域を観測する為、この合成開口レーダ装置の観測部に設けられた記録部への単位時間あたりの記録容量、或いは観測部から例えば地上に送信する際の観測データの送信容量を安定させることができる。
【００６０】
【発明の効果】
以上のように、この発明にかかる合成開口レーダ装置は、移動部と、移動部に設けられ、指向方向が可変の送受信アンテナと、送受信アンテナを介して、第一及び第二の所定期間にそれぞれ観測対象の第一及び第二の観測領域にアンテナビームを照射してパルス信号を送信し、パルス信号が第一及び第二の観測領域でそれぞれ反射されたパルス信号である第一及び第二のパルスエコーを受信する送受信部と、送受信部が受信した第一及び第二のパルスエコーをもとにそれぞれ第一及び第二の観測領域の観測データである第一及び第二の観測データを取得する観測部と、第一及び第二の観測領域それぞれの一部であって、第一及び第二のパルスエコーにおける所望信号の電力と所望信号以外の電力の比が所定値以上である第一及び第二の観測部分領域にそれぞれ対応する第一及び第二の観測データである第一及び第二の観測部分データを抽出して、第一及び第二の観測部分データを合成する合成部と、第一及び第二の所定期間における指向方向を互いに等しくかつ一定とし、第一及び第二の所定期間を第一及び第二の観測部分領域が移動部の進行方向に平行な方向において重複部分を有して隣接するように設定する制御部とを備え、第一及び第二の観測データから、パルスエコーにおける所望信号の電力と所望信号以外の電力の比が所定値以上である第一及び第二の観測部分領域にそれぞれ対応する第一及び第二の観測部分データを抽出して合成する為、これら合成された第一及び第二の観測部分データに基づき得られる第一及び第二の観測部分領域の映像を所望の質で得易くなる。
【００６１】
また、この発明にかかる合成開口レーダ装置は、制御部が、第一乃至第三の観測領域それぞれの移動部の進行方向に垂直な方向の長さが互いに等しくなるように送受信部及び送受信アンテナを制御する為、観測部に設けられた記録部へ記録させる単位時間あたりの記録容量、或いは観測部から例えば地上に送信する観測データの送信容量を安定させることができる。
【００６３】
この発明にかかる合成開口レーダ装置の観測プロセス設計方法は、移動部と、移動部に設けられ、指向方向が可変の送受信アンテナと、送受信アンテナを介して、第一乃至第三の所定期間にそれぞれ観測対象の第一の観測領域、第一の観測領域と移動部の進行方向に平行な方向において重複部分を有して隣接する第二の観測領域、及び第一の観測領域と移動部の進行方向に垂直な方向において隣接する第三の観測領域にアンテナビームを照射してパルス信号を送信し、パルス信号が第一乃至第三の観測領域でそれぞれ反射されたパルス信号である第一乃至第三のパルスエコーを受信する送受信部と、送受信部が受信した第一乃至第三のパルスエコーをもとにそれぞれ第一乃至第三の観測領域の観測データである第一乃至第三の観測データを取得する観測部とを備える合成開口レーダ装置の観測プロセス設計方法において、第一及び第二のパルスエコーにおける所望信号の電力と所望信号以外の電力の比をもとに、第一の観測領域と重畳部分との比である重畳比を算出する第一のステップと、送受信アンテナから照射されるアンテナビームの波長、送受信アンテナの開口部の移動部の進行方向に対して平行な方向の長さである第一のアンテナ開口長、送受信アンテナの開口部の移動部の進行方向に対して垂直な方向の長さである第二のアンテナ開口長、及び送受信アンテナの開口部から第一の観測領域までの距離をもとに、アンテナビームの第一の観測領域における移動部の進行方向に平行な方向の長さである照射幅を算出する第二のステップと、重畳比及び照射幅をもとに、重畳部分の移動部の進行方向に平行な方向の長さである重畳部分幅を算出する第三のステップと、重畳部分幅を移動部が通過する時間、送受信アンテナからパルス信号が送信され、第一の観測領域で反射されて第一のパルスエコーとして送受信アンテナに受信されるまでの時間である遅延時間、第一の観測領域にアンテナビームを照射するときの送受信アンテナの指向方向である第一の指向方向から第三の観測領域にアンテナビームを照射するときの送受信アンテナの指向方向である第三の指向方向に切り換えるために要する時間である切り換え時間、照射幅、及び移動部の移動速度をもとに第一の所定期間の長さを算出する第４のステップと、を有する為、合成された複数の観測部分データそれぞれに基づき得られる複数の観測部分領域の映像を所望の質で得易くなる。
【００６６】
また、この発明にかかる合成開口レーダ装置の観測プロセス設計方法は、送受信アンテナから照射されるアンテナビームの波長、第二のアンテナ開口長、第一の指向方向が鉛直方向と為す角度、第三の指向方向が鉛直方向と為す角度、及び送受信アンテナの指向方向が第一の指向方向であるときの送受信アンテナの開口部から第一の観測領域までの距離をもとに、移動部の進行方向に対して垂直な方向の第一及び第三の観測領域の長さを互いに等しい値に設定する第５のステップを有する為、合成された複数の観測部分データそれぞれに基づき得られる複数の観測部分領域の映像を所望の質で得易くなると共に、観測部に設けられた記録部へ記録させる単位時間あたりの記録容量、或いは観測部から例えば地上に送信する観測データの送信容量を安定させることができる。
【図面の簡単な説明】
【図１】 この発明によるSCAN SAR型の合成開口レーダ装置が、オフナディア角を切り換えると共に観測領域を切り換えるタイムシーケンスが表された概念図である。
【図２】 この発明によるSCAN SAR型の合成開口レーダ装置が、オフナディア角を切り換えるタイムシーケンスのみが表された概念図である。
【図３】 この発明によるSCAN SAR型の合成開口レーダ装置から得られる画像におけるアジマス座標とアジマス方向のＳ／Ａ比との相関が示された実験結果の図である。
【図４】 この発明によるこの実施形態１の合成開口レーダ装置によるＳ／Ａ比改善効果が示された実験結果の図である。
【図５】 この発明によるCAN SAR型の合成開口レーダ装置における、観測点とアンテナビームの観測範囲との関係が示された相関図である。
【図６】 この発明による実施形態６の合成開口レーダ装置の構成の一部が示された構成図である。
【図７】 この発明による実施形態６の合成開口レーダ装置の動作に関する概念図である。
【図８】 従来のCAN SAR型の合成開口レーダ装置における、レンジ方向のＳ／Ａ比を説明するための概念図である。
【図９】 従来のCAN SAR型の合成開口レーダ装置の構成図である。
【図１０】 従来のCAN SAR型の合成開口レーダ装置の動作に関する概念図である。
【図１１】 従来のCAN SAR型の合成開口レーダ装置における、アジマス方向のＳ／Ａ比を説明するための概念図である。
【符号の説明】
１ 送受信機、２ 送受信アンテナ、３ 画像再生装置、４ 画像接続装置、５オフナディア角駆動装置、６ 制御装置、７ 移相器、８ ウェイティング。[0001]
BACKGROUND OF THE INVENTION
The present invention provides observation data relating to a plurality of observation areas that are provided on a moving platform and that are observed by switching the irradiation direction of an antenna beam about the traveling direction of the platform, Synthetic aperture radar apparatus for synthesizing observation data relating to a plurality of observation regions parallel to the traveling direction of the platform observed in the irradiation direction of the antenna beam into one observation data, and an observation process design method for the synthetic aperture radar apparatus It is.
[0002]
[Prior art]
As an example of a synthetic aperture radar device that is provided on a moving platform such as a satellite or an airplane, switches the irradiation direction of the antenna beam output from the antenna around the traveling direction of this platform, and observes the ground surface or the sea surface. There is a SCAN SAR type synthetic aperture radar device devised by Moore et al. Of Univ.of KANSAS.
Synthetic aperture radar equipment is mounted on platforms such as aircraft and satellites, and transmits high-frequency pulse signals to the ground surface or sea surface at a depression angle perpendicular to the platform's orbit, and is reflected from the ground surface or sea surface. It is a radar device that receives a pulse echo as a signal and creates an image of an observation region such as a predetermined ground surface or sea surface.
[0003]
Further, the SCAN SAR type synthetic aperture radar device switches the elevation direction, which is the irradiation direction of the antenna beam emitted from the antenna of the synthetic aperture radar device, at a high speed around the traveling direction of the platform as a plurality. Observe the observation area.
The reason why the method of switching the irradiation direction of the antenna beam at high speed is employed is to avoid the following problems. Rather than switching the irradiation direction of the antenna beam at high speed, if the beam width in the elevation direction of the antenna beam is expanded as shown in FIG. The previously transmitted pulse overlaps with the intended received pulse, causing a problem that a false image is generated in the range direction in the obtained observation data.
[0004]
An example of a SCAN SAR type synthetic aperture radar apparatus provided on a platform such as a satellite or an airplane will be described with reference to FIG.
In FIG. 9, reference numeral 1 denotes a transmitter / receiver, which transmits or receives a high-frequency pulse signal via the transmitting / receiving antenna 2.
Note that a pulse echo that is a pulse signal radiated from the transmission / reception antenna 2 and reflected by the ground surface is amplified and detected by the transmission / reception device 1 via the transmission / reception antenna 2.
An off-nadir angle driving device 5 is connected to the transmission / reception antenna 2 and switches the depression angle or off-nadir angle directed by the transmission / reception antenna 2 that irradiates the antenna beam.
An image reproduction apparatus 3 is connected to the transceiver 1 and creates an image of a predetermined observation area based on the pulse echo received by the transceiver 1.
Note that the observation area where the image is created is a predetermined range for each off-nadir angle directed by the transmission / reception antenna 2.
[0005]
Reference numeral 4 denotes an image connection device, which is connected to the image reproduction device 3 and connects the images of a plurality of observation regions created by the image reproduction device 3 to create and output a radar image that is an image of one observation region. .
Note that the image connection device 4 displays images of a plurality of observation regions having the same off-nadir angle directed by the transmission / reception antenna 2 and different platform positions in one observation region adjacent to the azimuth direction parallel to the traveling direction of the platform. A radar image that is an image of the above is created.
In addition, the image connection device 4 has different off-nadir angles directed to the transmitting / receiving antenna 2 and images of a plurality of observation regions whose platform positions are almost the same, which are adjacent to each other in the range direction perpendicular to the traveling direction of the platform. A radar image that is an image of a region is created.
A control unit 6 is connected to the transceiver 1, the off-nadir angle driving device 5, the image reproducing device 3, and the image connecting device 4 so that images of a plurality of observation areas adjacent in the azimuth direction and the range direction are adjacent to each other. In addition, the switching timing for switching the beam width of the antenna beam irradiated through the transmitting / receiving antenna 2, the off-nadir angle directed by the transmitting / receiving antenna 2 irradiated with the antenna beam, and the off-nadir angle directed by the transmitting / receiving antenna 2 is controlled. .
[0006]
Next, the operation of the SCAN SAR type synthetic aperture radar apparatus shown in FIG. 9 will be described with reference to FIG.
Note that the synthetic aperture radar apparatus in FIG. 10 exemplifies an apparatus that directs three types of off-nadir angles.
The synthetic aperture radar apparatus provided on the platform is configured so that the antenna beam output from the transmitting / receiving antenna 2 of the synthetic aperture radar apparatus is output to a predetermined observation area of the sub swath 1 at the position 1 on the orbit of the platform. The off-nadir angle of the transmitting / receiving antenna 2 is adjusted.
While the platform moves from position 1 to position 2 on the orbit, the synthetic aperture radar device provided on the platform outputs an antenna beam to the observation region 1 in the sub swath 1. The synthetic aperture radar device creates an image of the observation region 1 based on the pulse echo of the output antenna beam.
[0007]
Further, the synthetic aperture radar apparatus provided in the platform is configured to transmit and receive the antenna beam so that the antenna beam output from the synthetic aperture radar apparatus is output to a predetermined observation area of the sub swath 2 at the position 2 on the platform orbit. Start adjusting off-nadir angle of 2.
While the synthetic aperture radar apparatus is adjusting the off-nadir angle of the transmitting / receiving antenna 2, the platform including the synthetic aperture radar apparatus moves to a position 3 on the trajectory of the platform.
Next, while the platform moves from the position 3 to the position 4 on the orbit, the synthetic aperture radar device provided on the platform outputs an antenna beam to the observation region 3 in the sub swath 2. Then, the synthetic aperture radar device creates an image of the observation region 3 based on the pulse echo of the output antenna beam.
[0008]
Further, the synthetic aperture radar apparatus provided on the platform is configured to transmit and receive the antenna beam so that the antenna beam output from the synthetic aperture radar apparatus is output to a predetermined observation area of the sub swath 3 at the position 4 on the platform orbit. Start adjusting off-nadir angle of 2.
While the synthetic aperture radar apparatus is adjusting the off-nadir angle of the transmitting / receiving antenna 2, the platform including the synthetic aperture radar apparatus moves to a position 5 on the trajectory of the platform.
While the platform moves from the position 5 to the position 6 in the orbit, the synthetic aperture radar device provided on the platform outputs an antenna beam to the observation area 5 in the sub swath 3. The synthetic aperture radar device creates an image of the observation region 5 based on the pulse echo of the output antenna beam.
[0009]
Further, the synthetic aperture radar apparatus provided on the platform outputs the antenna beam output from the synthetic aperture radar apparatus to a predetermined observation region different from the observation region 1 of the subswath 1 at the position 6 on the platform orbit. Thus, the adjustment of the off-nadir angle of the transmission / reception antenna 2 is started.
While the synthetic aperture radar apparatus adjusts the off-nadir angle of the transmitting / receiving antenna 2, the platform including the synthetic aperture radar apparatus moves to a position 7 on the trajectory of the platform.
Thereafter, operations similar to those described above are repeated.
[0010]
[Problems to be solved by the invention]
In the conventional SCAN SAR type synthetic aperture radar device, the pulse repetitive frequency (hereinafter referred to as PRF) cannot be sufficiently high compared to the frequency bandwidth of the echo determined by the main beam width of the antenna. The frequency of an unnecessary signal picked up by the shoulder or side lobe of the pattern main lobe may be folded back, and an ambiguity which is a false image may be generated in the obtained image.
This ambiguity will be described with reference to FIGS. 11A and 11B. The hatched area in FIG. 11A is the power separated from the signal band by oversampling.
Also, the hatched portion in FIG. 11 (b) is the electric power of the ambiguity generated in the desired signal band of the main lobe due to the folding by the PRF.
[0011]
Among these shown in FIG. 11 (b), both ends of the desired signal band where the shoulder portion of the main lobe is folded back have the greatest influence of ambiguity in the azimuth direction, and the image of the observed observation region is shown. The image quality at both ends in the azimuth direction is significantly reduced.
The ratio of the ambiguity signal power to the desired signal power of the main lobe is quantified as an S / A ratio (Signal to Ambiguity Ratio) in the azimuth direction and managed at the time of design.
In the conventional synthetic aperture radar device, in order to improve the S / A ratio, the S / A ratio can be improved by over-sampling by increasing the PRF, or by narrowing the antenna beam width or reducing the side lobe level. I was planning.
However, an increase in the oversampling ratio due to oversampling by raising the PRF causes a problem that the S / A ratio in the range direction deteriorates. In addition, the beam pattern improvement by narrowing the antenna beam width or reducing the side lobe level requires an increase in the antenna aperture diameter, which is particularly realized in a synthetic aperture radar device mounted on a satellite, for example. Have difficulty.
[0012]
As described above, the observation data obtained by observation with the conventional SCAN SAR type synthetic aperture radar device, particularly, the S / A ratio in the azimuth direction is deteriorated, and the image quality at both ends of the observed observation area is remarkably high. There was a problem of lowering.
In addition, the conventional SCAN SAR type synthetic aperture radar device collects and records the data to be recorded on the data recorder mounted on the platform according to the off-nadir angle directed by the antenna or the data to be transmitted to the ground. The amount of data changes in proportion to the number of pixels in the image. For example, the number of pixels in the range direction is expressed by Expression (1).
[Expression 1]

Here, r is the distance between the antenna and the observation point, Δr is the slant range resolution, φ is the radio wave incident angle, and Δθ is the beam irradiation angle in the elevation direction.
From equation (1), the conventional SCAN SAR type synthetic aperture radar device changes the values of r and φ by switching the off-nadir angle directed by the antenna, so the data collected or transmitted by the synthetic aperture radar device The amount changes.
[0013]
In addition, when a planar array antenna is used as the antenna of a conventional SCAN SAR type synthetic aperture radar device, the effective aperture length of the antenna changes depending on the beam angle of the antenna beam emitted from the antenna. The irradiation angle Δθ changes, and the amount of data collected or transmitted by the synthetic aperture radar device changes.
Furthermore, if the slant range resolution Δr is controlled in proportion to Sin (φ) in order to keep the range resolution of the image obtained from the conventional SCAN SAR type synthetic aperture radar device constant, the sub swath width will change. The amount of data collected or transmitted by the aperture radar apparatus varies.
As described above, in the conventional SCAN SAR type synthetic aperture radar device, the amount of observation data collected by the synthetic aperture radar device or transmitted to the ground varies depending on the off-nadir angle directed by the antenna, and is mounted on the platform. There has been a problem that the recording capacity per unit time of the data recorder or the transmission capacity of observation data transmitted from the platform to the ground exceeds the line capacity of the data link.
[0014]
The present invention has been made in view of these problems, and an object thereof is to obtain a SCAN SAR type synthetic aperture radar apparatus capable of obtaining observation data with high image quality.
In addition, the present invention is capable of adjusting the swath width at each of a plurality of off-nadir angles to a constant value even when the off-nadir angles directed by the transmitting and receiving antennas are switched, and recording per unit time of a data recorder mounted on the platform. The object is to obtain a SCAN SAR type synthetic aperture radar device with stable capacity or transmission capacity of observation data transmitted from the platform to the ground.
[0015]
[Means for Solving the Problems]
The synthetic aperture radar device according to the present invention is provided in the moving unit and the moving unit, Transmitting a pulse signal by irradiating an antenna beam to the first and second observation regions to be observed in the first and second predetermined periods through the transmission / reception antenna having a variable directivity direction and the transmission / reception antenna, respectively, Based on the first and second pulse echoes received by the transmitter / receiver, and the transmitter / receiver that receives the first and second pulse echoes, which are pulse signals reflected from the first and second observation regions, respectively. To obtain the first and second observation data, which are the observation data of the first and second observation areas, respectively. The observation section; First and second observations that are part of the first and second observation regions, respectively, and the ratio of the power of the desired signal to the power other than the desired signal in the first and second pulse echoes is equal to or greater than a predetermined value. First and second observation partial data, which are first and second observation data corresponding to the partial areas, are extracted, and the first and second observation data are extracted. A synthesizing unit that synthesizes the observation partial data; The directivity directions in the first and second predetermined periods are the same and constant, and the first and second predetermined periods are overlapped in the direction in which the first and second observation partial regions are parallel to the traveling direction of the moving part. And a control unit configured to be adjacent to each other It is equipped with.
[0016]
Further, the synthetic aperture radar device according to the present invention, The transmission / reception unit transmits a pulse signal by irradiating an antenna beam to the third observation region to be observed during the third predetermined period via the transmission / reception antenna, and the pulse signal reflected by the third observation region The observation unit obtains the third observation data that is the observation data of the third observation region based on the third pulse echo received by the transmission / reception unit, and the synthesis unit A third observation corresponding to a third observation partial region that is a part of the third observation region, and in which the ratio of the power of the desired signal to the power other than the desired signal in the third pulse echo is a predetermined value or more. The third observation part data, which is data, is extracted, and the first to third observation part data are synthesized, and the control unit advances the first and second observation areas and the moving part. Transmitter / receiver and adjacent to each other in a direction perpendicular to the direction Controlling the reception antenna To do.
[0017]
Furthermore, the synthetic aperture radar device according to the present invention is: The control unit controls the transmission / reception unit and the transmission / reception antenna so that the lengths in the direction perpendicular to the traveling direction of the moving unit in each of the first to third observation regions are equal to each other. To do.
[0018]
An observation process design method for a synthetic aperture radar apparatus according to the present invention includes a moving unit and The first observation area, the first observation area, and the movement section of the observation target in the first to third predetermined periods via the transmission / reception antenna provided in the movement section and having a variable directivity direction; Irradiate the antenna beam to the second observation region that has an overlap in the direction parallel to the traveling direction, and the third observation region that is adjacent to the first observation region and the direction perpendicular to the traveling direction of the moving unit. Transmitting and receiving the first and third pulse echoes that are pulse signals reflected by the first to third observation regions, respectively, and the first and second received by the transmitting and receiving unit And an observation unit for acquiring first to third observation data, which are observation data of the first to third observation regions, based on the first to third pulse echoes, respectively. In the observation process design method of the synthetic aperture radar device, Based on the ratio of the power of the desired signal and the power other than the desired signal in the first and second pulse echoes, the superposition ratio, which is the ratio between the first observation region and the superposition part, is calculated. The first step to The wavelength of the antenna beam radiated from the transmitting / receiving antenna, the first antenna opening length that is the length parallel to the traveling direction of the moving part of the transmitting / receiving antenna opening, and the traveling of the moving part of the transmitting / receiving antenna opening Based on the second antenna aperture length, which is the length in the direction perpendicular to the direction, and the distance from the transmit / receive antenna aperture to the first observation region, the moving part of the antenna beam in the first observation region Calculate the irradiation width, which is the length in the direction parallel to the traveling direction of The second step to Based on the overlap ratio and irradiation width, calculate the overlapped part width, which is the length in the direction parallel to the moving direction of the moving part of the overlapped part. And the third step to The time that the moving unit passes through the overlapped part width, the delay time that is the time from when the pulse signal is transmitted from the transmission / reception antenna, reflected by the first observation region, and received by the transmission / reception antenna as the first pulse echo, The third directivity direction, which is the directivity direction of the transmission / reception antenna when the antenna beam is irradiated to the third observation region from the first directivity direction, which is the directivity direction of the transmission / reception antenna, when the single observation region is irradiated with the antenna beam The length of the first predetermined period is calculated based on the switching time, irradiation width, and moving speed of the moving part. And a fourth step.
[0019]
An observation process design method for the synthetic aperture radar device according to the present invention is as follows. The wavelength of the antenna beam irradiated from the transmitting / receiving antenna, the second antenna aperture length, the angle formed by the first directivity direction with the vertical direction, the angle formed by the third directivity direction with the vertical direction, and the directivity direction of the transmit / receive antenna are The length of the first and third observation areas in the direction perpendicular to the traveling direction of the moving part, based on the distance from the opening of the transmission / reception antenna to the first observation area when the direction is one Has a fifth step of setting the values equal to each other Is.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
Embodiment 1 of the Invention
An embodiment of a SCAN SAR type synthetic aperture radar apparatus according to the present invention will be described with reference to FIG.
FIG. 9 is a configuration diagram of the SCAN SAR type synthetic aperture radar apparatus of the first embodiment.
In FIG. 9, 6 is a control part. The control unit 6 of the present invention controls the width of the antenna beam to be irradiated, the antenna beam so that the images of the observation regions adjacent in the range direction are in close contact with each other, and the images of the observation regions adjacent in the azimuth direction overlap. Is controlled to switch the off-nadir angle, which is the depression angle directed by the transmitting / receiving antenna 2, and the off-nadir angle of the transmitting / receiving antenna 2.
The operation of the SCAN SAR type synthetic aperture radar apparatus of the first embodiment is substantially the same as the operation of the conventional SCAN SAR type synthetic aperture radar apparatus shown in FIG.
[0024]
Next, the irradiation position and time sequence of the antenna beam irradiated by the SCAN SAR type synthetic aperture radar apparatus shown in FIG. 9 will be described with reference to FIG.
In FIG. 1, Tci indicates the correlation time. The correlation time Tci is a subswath (hereinafter referred to as the i-th subswath) that is a predetermined observation band in which the transmitting / receiving antenna 2 of the SCAN SAR type synthetic aperture radar apparatus at an arbitrary position is directed at a predetermined off-nadir angle. The time for irradiating the antenna beam is indicated.
Tdi indicates the propagation delay time. The propagation delay time Tdi indicates the time until the transmission pulse irradiated to the i-th sub swath is received by the transmission / reception antenna 2.
Toi indicates the synthetic opening time. The synthetic aperture time Toi is a time during which the transmission / reception antenna 2 is directed to the i-th sub swath.
[0025]
The synthetic aperture time Toi is indicated by the sum of the correlation time Tci and the propagation delay time Tdi.
Tsi indicates a scan switching time. The scan switching time Tsi is a time required to switch the off-nadir angle of the transmission / reception antenna 2 from the i-th sub-swath to the next sub-swath (hereinafter referred to as the (i + 1) th sub-swath).
The (i + 1) th subswath refers to a subswath that is an observation band corresponding to the switched offnadir angle by switching the offnadir angle directed by the transmission / reception antenna 2. In addition, it is said that scanning is performed when these sub-swaths scan the antenna beam.
Tai indicates the scan cycle. This scan period Tai starts observation at the i-th subswath, then switches the off-nadir angle to which the antenna beam is directed, sequentially performs observations at N subswaths, and at all subswaths to which the transmission / reception antenna 2 should be directed. The time until the observation of the i-th sub swath is completed again after the observation of (i) is indicated. That is, it indicates the time required to scan (N + 1) observation areas, which are a part of each sub-swath based on the position of the platform such as a satellite or an airplane, with N sub-swaths that are N observation bands.
[0026]
Lwi indicates the overlap length. The overlap length Lwi is the length of the overlapping portion where two observation regions that are shifted in the azimuth direction with the same sub-swath overlap.
Note that this overlap length does not cause observation leakage between a plurality of observation regions adjacent to each other in the azimuth direction even when the edge of the image is cut off in order to extract an image satisfying a predetermined S / A ratio. Is provided. If the edge of the image is simply cut off, there is a possibility that an observation area having no observation data may be generated between a plurality of observation areas adjacent in the azimuth direction. In order to prevent this observation omission, it is necessary for the control device 6 to sufficiently overlap adjacent observation areas and perform beam scanning in these observation areas.
When combining the observation data of two observation regions that partially overlap each other, the observation data of the overlapping portion where the two observation regions overlap is used by half from the two observation data. That is, half of the observation data of the overlapped portion is deleted from the end of each observation region. In addition, the observation data of the overlapping portions of the two observation regions may be selected by comparing the observation data of each observation region.
[0027]
Next, in relation to the SCAN SAR type synthetic aperture radar apparatus shown in FIG. 1 and the correlation between the irradiation position of the antenna beam and the time sequence of the irradiation switching of the antenna beam, the antenna that the SCAN SAR type synthetic aperture radar apparatus irradiates in particular. A time sequence of beam irradiation switching will be described with reference to FIG.
For example, in the i-th subswath, in order to observe two observation areas shifted in the azimuth direction as two continuous observation areas with no observation omission, the off-nadir angle directed by the synthetic aperture radar apparatus is switched and observed. While the synthetic aperture radar device observes (N + 1) observation areas, the moving distance of the platform such as a satellite provided with the synthetic aperture radar device needs to be within the range of the irradiation width of the antenna beam. .
[0028]
This condition is expressed by Equation (2), and this Equation (2) needs to be satisfied in all sub-swaths (i = 1 to N).
[Expression 2]

Note that L is an antenna opening length that is an opening length in the azimuth direction of the opening of the transmitting / receiving antenna 2.
α is a coefficient determined by a weight for adjusting the opening length in the range direction of the opening of the transmitting / receiving antenna 2.
λ is the transmission wavelength of the antenna beam.
V is the speed of a platform such as a satellite.
R _{Near, i} Is a slant range on the near-range side indicating the shortest distance between the observation area in the i-th sub swath and the opening of the transmitting / receiving antenna 2.
[0029]
Further, the scan cycle Tai is expressed by Expression (3).
[Equation 3]

Note that Tui is a margin time and is necessary for overlapping images in the azimuth direction.
Tpi is the pulse signal length.
That is, the control device 6 determines each parameter so as to satisfy Expression (2) and the margin time Tui> 0.
[0030]
An example in which the S / A ratio is improved by this control will be described below.
If this control for improving S / A is not performed, as shown in FIG. 5, the gain of the antenna beam used for the synthetic aperture processing differs depending on the position in the azimuth direction. There is a tendency for the backscattering coefficient to deteriorate.
The maximum azimuth resolution Δaz at each of the plurality of off-nadir angles is given by equation (6).
[Expression 4]

Maz is the number of multilooks in the azimuth direction.
Further, the overlap ratio γ of the images of the two observation regions shifted in the azimuth direction in a certain sub swath is defined by the equation (7).
[Equation 5]

[0031]
Further, the S / A ratio in the azimuth direction in the observation region observed by the SCAN SAR type synthetic aperture radar apparatus is defined by equation (8).
[Formula 6]

Min {} is a function for obtaining the minimum value of the azimuth angle θ, and the azimuth angle θ has a domain of the formula (9).
Further, the S / A ratio in the azimuth direction of each observation region defined by Expression (8) has the minimum S / A ratio at each azimuth position in each observation region, and the S of the image based on the observation data in each observation region. / A ratio.
Further, δ is given by the equation (10) as an angle for performing the synthetic aperture.
[Expression 7]

Note that γ is an image overlap ratio (0 ≦ γ <1), and Nb is the number of scans.
[0032]
Next, the S / A ratio at the azimuth position, which is the position in the azimuth direction, of observation data obtained by observation with the SCAN SAR type synthetic aperture radar apparatus will be described with reference to FIG.
It has been theoretically found that the S / A ratio in the azimuth direction of the SCAN SAR type synthetic aperture radar apparatus decreases as it goes from the center of the image to the edge of the image.
In FIG. 3, the vertical axis indicates the value of the S / A ratio.
The horizontal axis indicates the azimuth position where the center of the image is 0, which is the reference value. Since the S / A ratio of the image shows a symmetric value with respect to the center of the image, the horizontal axis in FIG. 3 indicates the S / A ratio from the center of the image to one end of the image. At this time, the azimuth position of the image is shown with 0 at the center of the image and 0.5 at the edge of the image.
[0033]
FIG. 4 shows the relationship between the resolution and the S / A ratio when the oversample ratio that affects the narrowing of the frequency band of the received signal is changed from 1.0 to 1.25. The received signal indicates an echo received by the transmitting / receiving antenna 2 when the antenna beam output from the transmitting / receiving antenna 2 is reflected by the ground surface, the sea surface, or the like. Experiments have shown that the SCAN SAR type synthetic aperture radar apparatus of the first embodiment improves the S / A ratio as the overlap ratio γ is increased as compared to the conventional SCAN SAR type synthetic aperture radar apparatus. Yes.
Note that the left end of FIG. 4 is a case where the overlap ratio γ adjacent to each other does not have a portion where, for example, two observation regions are observed at a certain off-nadir angle directed by the transmitting and receiving antenna 2. This is the S / A ratio in the conventional SCAN SAR type synthetic aperture radar apparatus.
Further, the portion other than the left end is the case where the overlap ratio γ> 0, and shows the S / A ratio in the SCAN SAR type synthetic aperture radar apparatus of the first embodiment. Note that the SCAN SAR type synthetic aperture radar apparatus of the first embodiment is accompanied by a decrease in azimuth resolution when the overlap ratio γ is increased as compared with the conventional SCAN SAR type synthetic aperture radar apparatus.
[0034]
As described above, the synthetic aperture radar apparatus according to the first embodiment partially overlaps two adjacent observation areas observed by the synthetic aperture radar apparatus with the same subswath, and overlaps the two observation areas. The observation data is sorted based on the S / A ratio, which is the ratio of the false image signal power to the desired signal power, and the observation data of these two observation areas are combined into one, so at the end of the two observation areas A high S / A ratio of the observation data of the observation regions superimposed on each other is obtained.
[0035]
Further, the synthetic aperture radar apparatus of the first embodiment switches off-nadir angles directed by the transmission / reception antenna 2 of the synthetic aperture radar apparatus, observes a plurality of observation areas adjacent in the range direction, and In order to create one observation data by combining the observation data of the observation area, the observation data of the two observation areas adjacent in the azimuth direction are combined into one observation data with high quality, and the observation area in the range direction Enlarged observation data can be obtained.
[0036]
Furthermore, the synthetic aperture radar apparatus according to the first embodiment performs a simplified synthesis process in which the observation data of the overlapped portion of the two observation areas that are partially overlapped in the azimuth direction is divided into two to synthesize one observation data. As a result, relatively high quality synthesized observation data can be obtained in a short time.
[0037]
Embodiment 2 of the Invention
Next, an observation process design method according to the present invention will be described.
In the second embodiment, the observation process is designed so that the S / A ratio in the azimuth direction is constant at a plurality of off-nadir angles in the SCAN SAR type synthetic aperture radar apparatus of the first embodiment.
Note that the SCAN SAR type synthetic aperture radar device reduces the azimuth resolution when the S / A ratio in the azimuth direction is improved.
In order to make the S / A ratio in the azimuth direction constant at each of the plurality of off-nadir angles, the overlap ratio γ of the image is made constant at each of the plurality of off-nadir angles.
That is, the image overlap ratios γi and γj may be set so as to satisfy Expression (11). Γi is an overlap ratio of the i-th sub-swath image, and γj is an overlap ratio of the j-th sub-swath image.
[Equation 8]

[0038]
Based on this equation (1), given an overlap ratio γ of images common to each sub-swath, the observation process is determined as follows.
First, the length Lai of the beam irradiation region of the antenna beam irradiated to the i-th sub swath from the transmission / reception antenna 2 is determined by the formula (12) based on the formula (2). The beam irradiation area indicates an observation area. The length Lai of the beam irradiation region indicates the length of the observation region in the azimuth direction.
[Equation 9]

Therefore, based on the overlap ratio γ of the images based on the observation data of each observation region, Expression (13) for deriving the length Lwi of the overlapping portion where two adjacent images overlap is obtained.
[Expression 10]

Then, the margin time Tui, which is the processing time for combining two images overlapping in the azimuth direction with the same subsworth, is the length Lwi of the overlapping portion where the two images overlapping in the azimuth direction overlap. Is a time required for a platform such as a satellite to pass through and is obtained from the equation (14).
[Expression 11]

[0039]
Next, a correlation time Tci indicating a time for irradiating the antenna beam to a certain observation region is determined so as to satisfy Expression (3). If the correlation time Tci is determined, the radio wave delay time Tdi indicating the time until the antenna beam irradiated from the transmitting / receiving antenna 2 is reflected on the ground surface or the sea surface and received by the transmitting / receiving antenna 2 is a predetermined value. It is decided. Further, the scan switching time Tsi for switching the sub swath directed by the transmitting / receiving antenna 2 is determined to a predetermined value depending on the performance of the hardware.
This determines the observation process.
[0040]
The above procedure is summarized as follows.
[Step (hereinafter referred to as A) 1]
From the equation (8), an overlap ratio γ of adjacent images satisfying a required S / A ratio is obtained.
[A2]
From the equation (12), the length Lai of the beam irradiation region of the i-th sub-sose is obtained.
[A3]
From equation (13), the length Lwi of the overlapped portion where adjacent images overlap is obtained.
[A4]
From equation (14), adjacent images are overlapped to obtain a margin time Tui required for the composition processing for compositing into one image.
[A5]
Correlation time Tci satisfying equation (3) is obtained.
[0041]
As described above, the synthetic aperture radar apparatus according to the third embodiment switches the off-nadir angle directed by the transmission / reception antenna 2 included in the synthetic aperture radar apparatus to observe a plurality of observation areas adjacent to each other in the range direction. In order to adjust the overlap ratio of the plurality of observation regions, the observation data of the plurality of observation regions adjacent in the range direction can be combined into one observation data having a uniform S / A ratio.
[0042]
Embodiment 3 of the Invention
Next, another antenna beam observation process design method of the present invention will be described.
In the third embodiment, the observation process is designed so that the noise equivalent backscattering coefficient is constant at a plurality of off-nadir angles in the SCAN SAR type synthetic aperture radar apparatus of the first embodiment.
The SCAN SAR synthetic aperture radar apparatus described in the first embodiment reduces the azimuth resolution and improves the S / A ratio in the azimuth direction.
Therefore, in order to make the noise equivalent backscattering coefficient constant at each of a plurality of off-nadir angles, an overlap ratio γ that is a ratio of an overlapped portion of an observation region and an adjacent observation region with respect to an observation region Must be changed for each of a plurality of off-nadir angles.
[0043]
First, equation (15) shows the radar equation of the SCAN SAR type synthetic aperture radar device.
[Expression 12]

Where σ0 is the noise equivalent backscattering coefficient, Pn is the receiver noise power, Pt is the transmission power, M is the number of integration points, θi is the off-nadir angle of the i-th hit, G (θ) is the antenna gain at the off-nadir angle θ, r (θ) is the slant range at the off-nadir angle θ.
Here, if the number of integration points is sufficiently large and the slant range in the sub swath is regarded as a constant value, Expression (15) is expressed as Expression (16) to Expression (19).
If the number of integration points is sufficiently large, the sum operation can be approximated by integration.
[Formula 13]

[0044]
Thus, in order to set the noise equivalent backscattering coefficient to be equal at each of the plurality of off-nadir angles, the expression (20) based on the expression (16) is constant at each of the plurality of off-nadir angles. The overlap ratio γ of the image may be set.
[Expression 14]

If the overlap ratio γ is determined, the procedure for determining the observation process is the same as in the second embodiment.
Therefore, the procedure for determining the observation process in which the noise equivalent backscattering coefficients of the sub-swaths at each of the plurality of off-nadir angles are equal includes the following steps.
[0045]
[Step (hereinafter referred to as B) 1]
From equation (20), the overlap ratio γ of adjacent images in each sub-sourse is determined.
[B2]
From the equation (12), the length Lai of the beam irradiation region of the i-th subsose is obtained.
[B3]
From the equation (13), the length Lwi of the overlapping portion where adjacent images overlap is obtained.
[B4]
From equation (14), the margin time Tui required for the composition process for overlapping adjacent images and compositing them into one image is obtained.
[B5]
Correlation time Tci is obtained so as to satisfy equation (3).
[0046]
As described above, the synthetic aperture radar apparatus according to the third embodiment switches the off-nadir angle directed by the transmission / reception antenna 2 included in the synthetic aperture radar apparatus to observe a plurality of observation areas adjacent to each other in the range direction. In order to adjust the overlap ratio of the plurality of observation regions, the observation data of the plurality of observation regions adjacent in the range direction can be combined into one observation data having a homogeneous noise equivalent backscattering coefficient.
[0047]
Embodiment 4 of the Invention
Next, another antenna beam observation process design method of the present invention will be described.
In the fourth embodiment, the observation process is designed so that the azimuth resolution and the S / A ratio in the azimuth direction are both constant at a plurality of off-nadir angles in the SCAN SAR type synthetic aperture radar apparatus of the first embodiment. To do.
The SCAN SAR synthetic aperture radar apparatus described in the first embodiment reduces the azimuth resolution and improves the S / A ratio in the azimuth direction.
Moreover, if the observation process shown in Embodiment 2 is used, the S / A ratio can be set to be constant in all sub-swaths.
However, in order to make the azimuth resolution constant in each sub-swath based on each of a plurality of off-nadir angles, it is necessary to set a condition regarding the overlap ratio in the correlation time Tci.
First, Equations (3) and (6) are changed to obtain Equations (21) to (22).
[Expression 15]

[0048]
And based on Formula (21)-Formula (22), azimuth resolution is obtained from Formula (23).
[Expression 16]

Note that the azimuth resolution varies depending on the different sub-swaths in the range direction, and the numerical value of the azimuth resolution obtained from the equation (23) is the maximum value in the sub-swaths and indicates the worst value.
In order to set the azimuth resolution equally in each sub-swath with different off-nadir angles, the azimuth resolution is calculated for each of a plurality of off-nadir angles using Equation (23), and the maximum of the plurality of calculation results is calculated. The value may be adopted as a common azimuth resolution.
Then, in the subswath in which the calculation result is a value different from the maximum value, the azimuth resolution is lowered so as to align with the other subswath.
In this case, if the azimuth resolution is lowered, the margin time Tui in the time sequence increases.
Thus, the observation process in which the azimuth resolution and the S / A ratio in the azimuth direction are both constant at each of a plurality of off-nadir angles is composed of the following steps.
[0049]
[Step (hereinafter referred to as C) 1]
From equation (20), the overlap ratio γ of adjacent images in each sub-sourse is determined.
[C2]
From the equation (12), the length Lai of the beam irradiation region of the i-th sub-sose is obtained.
[C3] a
From equation (13), the length Lwi of the overlapped portion where adjacent images overlap is obtained.
[C4]
From equation (14), adjacent images are overlapped to obtain a margin time Tui required for the composition processing for compositing into one image.
[C5]
From equation (23), the azimuth resolution achievable with each sub-sose is calculated.
[C6]
The maximum value among the azimuth resolutions obtained for each subsworth is adopted as the azimuth resolution of all subsourses.
[C7]
Based on the equation (6), the correlation time Tci is obtained.
[0050]
As described above, the synthetic aperture radar apparatus according to the third embodiment switches the off-nadir angle directed by the transmission / reception antenna 2 included in the synthetic aperture radar apparatus to observe a plurality of observation areas adjacent to each other in the range direction. In order to adjust the overlap ratio of the plurality of observation regions, the observation data of the plurality of observation regions adjacent in the range direction can be combined into one observation data having a uniform S / A ratio and azimuth resolution.
[0051]
Embodiment 5 of the Invention
Next, another antenna beam observation process design method of the present invention will be described.
In the fifth embodiment, the observation process is designed so that the azimuth resolution and the noise equivalent backscattering coefficient are constant at a plurality of off-nadir angles in the SCAN SAR type synthetic aperture radar apparatus of the first embodiment.
The SCAN SAR synthetic aperture radar apparatus described in the first embodiment improves the noise equivalent backscattering coefficient by reducing the azimuth resolution.
Furthermore, if the observation process determined by the procedure shown in Embodiment 3 is used, the noise equivalent backscattering coefficient in all sub-swaths can be set to be constant.
However, in order to make the azimuth resolution constant in each sub-swath based on each of a plurality of off-nadir angles, it is necessary to set a condition regarding the overlap ratio in the correlation time Tci.
[0052]
As shown in the fourth embodiment, in order to set the azimuth resolution equally in each sub-swath with different off-nadir angles, the azimuth resolution is calculated at each of a plurality of off-nadir angles using the equation (23). The maximum value among the plurality of calculation results may be adopted as the common azimuth resolution.
It should be noted that the azimuth resolution is reduced so that the sub-swath whose calculation result is different from the maximum value is aligned with other sub-swaths.
In this case, if the azimuth resolution is lowered, the margin time Tui in the time sequence increases.
From these facts, the observation process in which the azimuth resolution and the S / A ratio in the azimuth direction are both constant at each of a plurality of off-nadir angles includes the following steps.
[0053]
[Step (hereinafter referred to as D) 1]
From equation (20), the adjacent overlap ratio γ in the image of each sub-sourse is determined.
[D2]
From the equation (12), the length Lai of the beam irradiation region of the i-th subsose is obtained.
[D3]
From the equation (13), the length Lwi of the overlapping portion where adjacent images overlap is obtained.
[D4]
From Expression (14), the margin time Tui required for the composition process for overlapping the images and compositing them into one image is obtained.
[D5]
From equation (23), the azimuth resolution achievable with each sub-sose is calculated.
[D6]
The maximum value among the azimuth resolutions obtained for each subsworth is adopted as the azimuth resolution of all subsourses.
[D7]
Based on the equation (6), the correlation time Tci is obtained.
[0054]
As described above, the synthetic aperture radar apparatus according to the third embodiment switches the off-nadir angle directed by the transmission / reception antenna 2 included in the synthetic aperture radar apparatus to observe a plurality of observation areas adjacent to each other in the range direction. In order to adjust the overlap ratio of a plurality of observation regions, the observation data of a plurality of observation regions adjacent in the range direction can be combined into one observation data having a homogeneous noise equivalent backscattering coefficient and azimuth resolution.
[0055]
Embodiment 6 of the Invention
Next, another embodiment of the present invention and another antenna beam observation process design method will be described.
In the sixth embodiment, an array antenna is used for the transmission / reception antenna 2 of the SCAN SAR type synthetic aperture radar apparatus of the first embodiment, and the subswath width, which is the distance in the subswath range direction at each of the plurality of off-nadir angles, is constant. The present invention relates to a control apparatus for controlling an array antenna and irradiating an antenna beam, and a control method therefor.
FIG. 6 shows the configuration of the control device of this synthetic aperture radar apparatus.
In FIG. 6, 7 is a phase shifter and 8 is weighting.
The control device 6 operates the weighting 8 to control the width of the opening diameter of the opening of the transmitting / receiving antenna 2 in the elevation direction. Then, control is performed so as to maintain a constant swath width Sw in each of the plurality of sub-soses.
Further, in FIG. 6, the same or corresponding parts as those in the first embodiment are denoted by the same reference numerals, the description thereof is omitted, and the parts different from those in the first embodiment have been described.
Further, the synthetic aperture radar device has a recording unit (not shown) for recording a reception signal received by the transmission / reception antenna 2, an echo of the antenna beam, and the like.
Further, the synthetic aperture radar device transmits observation data of a plurality of observed areas or a result obtained by combining a plurality of observation data to a base station or the like on the ground surface or the sea surface.
[0056]
A conceptual diagram at the time of observation of the synthetic aperture radar device shown in the sixth embodiment is shown in FIG.
In FIG. 7, 2 is a transmission / reception antenna.
H is the altitude and is the distance from the ground surface, sea surface, etc. to the platform.
ρ is a slant range, and indicates the distance from the opening of the transmission / reception antenna 2 oriented in the range direction, which is a direction perpendicular to the traveling direction of the platform, to a reference point having an observation region to be observed. A certain reference point indicates the center position in the sub swath width in the observed observation area.
θ0 is an off-nadir initial angle, which is an initial value of the off-nadir angle directed by the transmitting / receiving antenna 2.
θ is a beam scanning angle, which is a value obtained by subtracting the off-nadir initial angle from the off-nadir angle directed by the transmitting / receiving antenna 2.
φ is a radio wave incident angle, and indicates the direction in which the antenna beam is irradiated on the ground surface, the sea surface, or the like with respect to the reverse direction of the vertical direction.
At this time, the beam irradiation angle Δθ in the elevation direction of the antenna beam irradiated from the transmission / reception antenna 2 is obtained by Expression (24). In addition, the elevation direction indicates a direction perpendicular to the traveling direction of the platform. The sub swath width Sw is obtained based on the equation (25).
[Expression 17]

D is the effective antenna diameter in the elevation direction, and λ is the wavelength.
[0057]
In this equation (25), the slant range ρ and the radio wave incident angle φ are different depending on whether the SCAN SAR type synthetic aperture radar device is mounted on an aircraft or on a satellite.
In the former case, the slant range ρ and the radio wave incident angle φ are obtained by Expressions (26) to (27).
[Expression 18]

In the latter case, the slant range ρ and the radio wave incident angle φ can be obtained by Expressions (28) to (29).
[Equation 19]

From equation (25), the sub swath width Sw increases as the off-nadir angle of the beam increases. The off-nadir angle is indicated by the sum of the off-nadir initial angle θ0 and the beam scanning angle θ.
Therefore, according to the procedure shown below, the control device of the synthetic aperture radar device controls the effective diameter D in the elevation direction of the transmission / reception antenna 2 to make the sub swath widths Sw of the different observation regions constant.
[0058]
[Step (hereinafter referred to as E) 1]
A sub swath width Sw in each observation region is set based on the equations (24) and (25).
[E2]
A beam irradiation angle Δθ for realizing the set sub swath width Sw is obtained for each of the plurality of sub swaths based on the equation (25).
[E3]
The effective diameter D in the elevation direction of the transmitting / receiving antenna 2 for realizing the obtained beam irradiation angle Δθ is obtained for each of the plurality of sub-swaths based on the equation (24).
[0059]
As described above, the synthetic aperture radar apparatus of the sixth embodiment adjusts the distance in the range direction of each of the two observation areas adjacent in the range direction, and observes the two observation areas adjacent in the range direction. The recording capacity per unit time to the recording unit provided in the observation unit of the synthetic aperture radar apparatus or the transmission capacity of observation data when transmitting from the observation unit to the ground, for example, can be stabilized.
[0060]
【The invention's effect】
As described above, the synthetic aperture radar device according to the present invention is provided in the moving unit and the moving unit, Transmitting a pulse signal by irradiating an antenna beam to the first and second observation regions to be observed in the first and second predetermined periods through the transmission / reception antenna having a variable directivity direction and the transmission / reception antenna, respectively, Based on the first and second pulse echoes received by the transmitter / receiver, and the transmitter / receiver that receives the first and second pulse echoes, which are pulse signals reflected from the first and second observation regions, respectively. To obtain the first and second observation data, which are the observation data of the first and second observation areas, respectively. The observation section; First and second observations that are part of the first and second observation regions, respectively, and the ratio of the power of the desired signal to the power other than the desired signal in the first and second pulse echoes is equal to or greater than a predetermined value. First and second observation partial data, which are first and second observation data corresponding to the partial areas, are extracted, and the first and second observation data are extracted. A synthesizing unit that synthesizes the observation partial data; The directivity directions in the first and second predetermined periods are the same and constant, and the first and second predetermined periods are overlapped in the direction in which the first and second observation partial regions are parallel to the traveling direction of the moving part. And a control unit configured to be adjacent to each other From the first and second observation data, Extracting first and second observation partial data corresponding to the first and second observation partial areas, respectively, in which the ratio of the power of the desired signal and the power other than the desired signal in the pulse echo is a predetermined value or more In order to synthesize, it becomes easy to obtain images of the first and second observation partial areas obtained based on the synthesized first and second observation partial data with a desired quality.
[0061]
Further, the synthetic aperture radar device according to the present invention, The control unit controls the transmission / reception unit and the transmission / reception antenna so that the lengths in the direction perpendicular to the traveling direction of the moving unit in each of the first to third observation regions are equal to each other. Therefore, the recording capacity per unit time recorded in the recording unit provided in the observation unit or the transmission capacity of the observation data transmitted from the observation unit to the ground, for example, can be stabilized.
[0063]
An observation process design method for a synthetic aperture radar apparatus according to the present invention includes a moving unit and The first observation area, the first observation area, and the movement section of the observation target in the first to third predetermined periods via the transmission / reception antenna provided in the movement section and having a variable directivity direction; Irradiate the antenna beam to the second observation region that has an overlap in the direction parallel to the traveling direction, and the third observation region that is adjacent to the first observation region and the direction perpendicular to the traveling direction of the moving unit. Transmitting and receiving the first and third pulse echoes that are pulse signals reflected by the first to third observation regions, respectively, and the first and second received by the transmitting and receiving unit And an observation unit for acquiring first to third observation data, which are observation data of the first to third observation regions, based on the first to third pulse echoes, respectively. In the observation process design method of the synthetic aperture radar device, Based on the ratio of the power of the desired signal and the power other than the desired signal in the first and second pulse echoes, the superposition ratio, which is the ratio between the first observation region and the superposition part, is calculated. The first step to The wavelength of the antenna beam radiated from the transmitting / receiving antenna, the first antenna opening length that is the length parallel to the traveling direction of the moving part of the transmitting / receiving antenna opening, and the traveling of the moving part of the transmitting / receiving antenna opening Based on the second antenna aperture length, which is the length in the direction perpendicular to the direction, and the distance from the transmit / receive antenna aperture to the first observation region, the moving part of the antenna beam in the first observation region Calculate the irradiation width, which is the length in the direction parallel to the traveling direction of The second step to Based on the overlap ratio and irradiation width, calculate the overlapped part width, which is the length in the direction parallel to the moving direction of the moving part of the overlapped part. And the third step to The time that the moving unit passes through the overlapped part width, the delay time that is the time from when the pulse signal is transmitted from the transmission / reception antenna, reflected by the first observation region, and received by the transmission / reception antenna as the first pulse echo, The third directivity direction, which is the directivity direction of the transmission / reception antenna when the antenna beam is irradiated to the third observation region from the first directivity direction, which is the directivity direction of the transmission / reception antenna, when the single observation region is irradiated with the antenna beam The length of the first predetermined period is calculated based on the switching time, irradiation width, and moving speed of the moving part. Therefore, it is easy to obtain images of a plurality of observation partial areas obtained based on the synthesized plurality of observation partial data with a desired quality.
[0066]
An observation process design method for the synthetic aperture radar device according to the present invention is as follows. The wavelength of the antenna beam emitted from the transmitting / receiving antenna, the second antenna aperture length, the angle formed by the first directivity direction with the vertical direction, the angle formed by the third directivity direction with the vertical direction, and the directivity direction of the transmit / receive antenna are The length of the first and third observation areas in the direction perpendicular to the traveling direction of the moving part, based on the distance from the opening of the transmission / reception antenna to the first observation area when the direction is one Are set equal to each other. Therefore, it is easy to obtain images of a plurality of observation partial areas obtained based on each of a plurality of synthesized observation partial data with a desired quality, and per unit time to be recorded in a recording unit provided in the observation unit. Recording capacity or transmission capacity of observation data transmitted from the observation unit to the ground, for example, can be stabilized.
[Brief description of the drawings]
FIG. 1 is a conceptual diagram showing a time sequence in which a SCAN SAR type synthetic aperture radar apparatus according to the present invention switches an off-nadir angle and switches an observation region.
FIG. 2 is a conceptual diagram showing only a time sequence in which the SCAN SAR type synthetic aperture radar apparatus according to the present invention switches off-nadir angles.
FIG. 3 is a diagram of experimental results showing a correlation between azimuth coordinates and an S / A ratio in the azimuth direction in an image obtained from a SCAN SAR type synthetic aperture radar apparatus according to the present invention;
FIG. 4 is a diagram of an experimental result showing an effect of improving the S / A ratio by the synthetic aperture radar apparatus according to the first embodiment of the present invention.
FIG. 5 is a correlation diagram showing the relationship between the observation point and the observation range of the antenna beam in the CAN SAR type synthetic aperture radar device according to the present invention;
FIG. 6 is a block diagram showing a part of the configuration of a synthetic aperture radar apparatus according to Embodiment 6 of the present invention.
FIG. 7 is a conceptual diagram related to the operation of the synthetic aperture radar device according to the sixth embodiment of the present invention.
FIG. 8 is a conceptual diagram for explaining the S / A ratio in the range direction in a conventional CAN SAR type synthetic aperture radar apparatus.
FIG. 9 is a configuration diagram of a conventional CAN SAR type synthetic aperture radar apparatus.
FIG. 10 is a conceptual diagram related to the operation of a conventional CAN SAR type synthetic aperture radar apparatus.
FIG. 11 is a conceptual diagram for explaining the S / A ratio in the azimuth direction in a conventional CAN SAR type synthetic aperture radar apparatus.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Transmitter / receiver, 2 Transmit / receive antenna, 3 Image reproduction apparatus, 4 Image connection apparatus, 5 Off-nadir angle drive apparatus, 6 Control apparatus, 7 Phase shifter, 8 Waiting.

Claims (5)

A moving part;
A transmission / reception antenna provided in the moving unit and having a variable directivity direction;
Via the transmission / reception antenna, the first and second observation regions to be observed are irradiated with an antenna beam for the first and second predetermined periods, respectively, and a pulse signal is transmitted. A transmission / reception unit that receives the first and second pulse echoes, which are pulse signals reflected respectively in the two observation regions;
An observation unit for acquiring first and second observation data, which are observation data of the first and second observation regions, respectively, based on the first and second pulse echoes received by the transmission / reception unit ;
The first and second observation regions that are a part of each of the first and second observation regions, wherein a ratio of the power of the desired signal and the power other than the desired signal in the first and second pulse echoes is a predetermined value or more. Extracting the first and second observation partial data, which are the first and second observation data respectively corresponding to the second observation partial area, and combining the first and second observation partial data;
The directivity directions in the first and second predetermined periods are set to be equal and constant, and the first and second predetermined periods are parallel to the traveling direction of the moving unit in the first and second predetermined periods. A synthetic aperture radar apparatus including a control unit configured to be adjacent to each other with overlapping portions in the direction . The transmission / reception unit transmits a pulse signal by irradiating an antenna beam to the third observation region to be observed in the third predetermined period via the transmission / reception antenna, and the pulse signal is transmitted in the third observation region. Receive a third pulse echo that is the reflected pulse signal,
The observation unit acquires third observation data that is observation data of the third observation region based on the third pulse echo received by the transmission / reception unit,
The synthesis unit is a part of the third observation region, and a third observation partial region in which a ratio of the power of the desired signal and the power other than the desired signal in the third pulse echo is a predetermined value or more. The third observation data corresponding to the third observation data is extracted, and the first to third observation data are synthesized,
The control unit controls the transmission / reception unit and the transmission / reception antenna so that the third observation region is adjacent to the first and second observation regions in a direction perpendicular to a traveling direction of the moving unit. The synthetic aperture radar apparatus according to claim 1. The control unit controls the transmission / reception unit and the transmission / reception antenna so that the lengths of the first to third observation regions in the direction perpendicular to the moving direction of the moving unit are equal to each other. The synthetic aperture radar apparatus according to claim 2 . A moving part ;
A transmission / reception antenna provided in the moving unit and having a variable directivity direction;
Through the transmission / reception antenna, the first observation region to be observed is overlapped in the first to third predetermined periods, respectively, in the direction parallel to the traveling direction of the first observation region and the moving unit. A pulse signal is transmitted by irradiating an antenna beam to an adjacent second observation region and a third observation region adjacent to the first observation region and a direction perpendicular to the traveling direction of the moving unit. Transmitting and receiving units for receiving first to third pulse echoes that are pulse signals reflected in the first to third observation regions, respectively;
An observation unit that acquires first to third observation data, which are observation data of the first to third observation regions, based on the first to third pulse echoes received by the transmission / reception unit, respectively. In the observation process design method of the synthetic aperture radar device,
Based on the ratio of the power of the desired signal and the power other than the desired signal in the first and second pulse echoes, a first calculation of a superposition ratio that is a ratio between the first observation region and the superimposition portion And the steps
A wavelength of an antenna beam emitted from the transmission / reception antenna, a first antenna opening length that is a length of the opening of the transmission / reception antenna in a direction parallel to a traveling direction of the moving unit, and an opening of the transmission / reception antenna. second antenna aperture length is the length of a direction perpendicular to the direction of movement of the moving part, and also preparative distance from the opening of the receiving antenna to the first observation region, of the antenna beam A second step of calculating an irradiation width that is a length in a direction parallel to the traveling direction of the moving unit in the first observation region ;
Based on the superposition ratio and the irradiation width, a third step of calculating a superposition part width that is a length of the superposition part in a direction parallel to the traveling direction of the moving unit ;
The time for the moving part to pass through the overlapped part width, the time from when the pulse signal is transmitted from the transmission / reception antenna, reflected by the first observation region, and received by the transmission / reception antenna as the first pulse echo When the antenna beam is applied to the third observation region from the first directivity direction, which is the directivity direction of the transmission / reception antenna when the antenna beam is applied to the first observation region. The length of the first predetermined period is calculated based on the switching time, which is the time required for switching to the third directivity direction, which is the directivity direction of the transmission / reception antenna, the irradiation width, and the moving speed of the moving unit. An observation process design method for a synthetic aperture radar apparatus having a fourth step. The wavelength of the antenna beam emitted from the transmitting / receiving antenna, the second antenna aperture length, the angle formed by the first directivity direction with the vertical direction, the angle formed by the third directivity direction with the vertical direction, and the transmit / receive antenna Based on the distance from the opening of the transmitting / receiving antenna to the first observation region when the directivity direction is the first directivity direction, the first direction perpendicular to the traveling direction of the moving unit 5. The method of designing an observation process for a synthetic aperture radar device according to claim 4, further comprising a fifth step of setting the lengths of the third observation region and the third observation region equal to each other .

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