JP7745249B2 - Suppression amount determination device, radar receiver, suppression amount determination method, radar reception method, suppression amount determination program, radar reception program - Google Patents

Description

The present invention relates to a suppression amount determination device, a suppression amount determination method, and a suppression amount determination program that determine the amount of suppression required to suppress signals reflected from the sea surface from radar signals received at sea, as well as a radar receiver, a radar reception method, and a radar reception program that use the determined suppression amount to suppress signals reflected from the sea surface.

For safe navigation, marine radars installed on ships monitor obstacles such as nearby ships, land, and navigational markers. The antennas used typically have a fan beam pattern with narrow horizontal directionality and relatively wide vertical directionality, which transmits radio waves while rotating horizontally and receives reflected waves from surrounding objects to determine the distance and direction of the objects. However, the received signals contain not only reflected waves from objects (targets) necessary for safe navigation, but also reflected waves (sea surface reflections) mainly from waves appearing near the ship, which makes it difficult to identify the target.

Sea clutter varies depending on sea conditions such as wind direction and speed, and is generally said to attenuate in proportion to the cube of the distance at short distances and the seventh power at long distances (Non-Patent Document 1). Sensitivity Time Control (STC) has traditionally been widely used as a method for suppressing sea clutter. This process suppresses sea clutter by attenuating the received signal inversely proportional to the power of the distance, in line with the characteristics of sea clutter, which attenuates with distance.

However, as mentioned above, sea clutter is affected by wind direction and speed, and so its behavior varies depending on the direction relative to the radar. Therefore, if uniform attenuation is applied in all directions, events such as the disappearance of targets or residual sea clutter can occur. Non-Patent Document 1 provides a solution to this problem. In Non-Patent Document 1, the sea clutter intensity for each azimuth angle range is estimated from the received signal acquired within a specified azimuth angle range, and the received signal is suppressed accordingly. In addition to extracting features for each azimuth angle range, Patent Document 1 suppresses sea clutter that cannot be completely suppressed using CFAR (Constant False Alarm Rate) processing by determining the amount of attenuation at each sampling point according to distance.

Furthermore, the above processing basically takes advantage of the fact that the sea surface clutter level changes with distance, and applies an attenuation curve (STC curve) according to distance to all received signals. However, in this case, attenuation is applied to the target object as well as the sea surface clutter, which can result in a decrease in identification ability or even loss of signal depending on the size of the target object.

On the other hand, a process called scan-to-scan correlation is also widely used to suppress sea clutter. This is a method of storing multiple sets of previously acquired scan data (i.e., data from one antenna revolution) and suppressing sea clutter in the current data through statistical processing. Sea clutter is generally scattered near the ship, and its location and number tend to change irregularly, so averaging multiple scans can achieve significant sea clutter suppression effects.

JP 2014-25916 A

Tomiji Nirasawa and Hiroshi Okada, "Automated STC of Radar (Lecture at the 51st Lecture Meeting of the Japan Institute of Navigation)," Transactions of the Japan Institute of Navigation, No. 52, pp. 101-108, December 1974.

However, in Non-Patent Document 1, the received signal also contains reflected waves from the target, so there is still a problem in determining the appropriate amount of suppression. Patent Document 1 also has a problem in determining whether reflected waves contained in the received signal are necessary. Furthermore, in the case of inter-scan correlation processing, a large number of scans are required to achieve sufficient suppression, which leads to problems such as poor real-time performance and the tendency to lose fast-moving targets. Furthermore, a direction sensor separate from the radar is required to determine the absolute direction for each scan.
The present invention aims to obtain a suppression amount for appropriately suppressing sea clutter using only radar while maintaining real-time performance.

The suppression amount determination device of the present invention determines a suppression amount for suppressing signals reflected from the sea surface from radar reception signals received at sea. The received signals are signals received for each predetermined direction and indicate intensity at a predetermined distance resolution. The suppression amount determination device includes a mask generation unit, an intra-mask representative feature calculation unit, an intra-mask representative feature recording unit, an azimuth angle range feature tendency calculation unit, a sea surface clutter identification unit, and a specific sea surface clutter suppression amount determination unit. The mask generation unit generates a mask, with a range of distances in which the intensity of the received signal is continuously higher than a predetermined value as one mask. The intra-mask representative feature calculation unit calculates, for each mask, a representative feature amount including a representative tendency value indicating that the received signal within the predetermined mask is likely to be a signal reflected from the sea surface. The intra-mask representative feature recording unit records the representative feature amount for each direction. The azimuth angle range feature tendency calculation unit calculates a tendency value condition, which is a condition for identifying sea surface clutter, based on the representative tendency value for the predetermined direction range. The sea surface clutter identifying unit determines, for each mask of the received signal to be processed, whether the received signal within the mask is a reflection from the sea surface, according to the representative tendency value and tendency value condition of the mask. The specific sea surface clutter suppression amount determiner determines, for each mask of the received signal to be processed, the amount of suppression so as to reduce the received signal of the mask determined by the sea surface clutter identifying unit to be a reflection from the sea surface to below the noise level.

In this invention, sea clutter is identified based on the characteristics of sea surface reflection, and only the identified sea clutter is suppressed. Therefore, the suppression amount determination device of the present invention can obtain the amount of suppression required to appropriately suppress sea clutter using only the radar, while maintaining real-time performance and ensuring target identification.

1 is a diagram showing an example of the functional configuration of a radar receiver 200 having a suppression amount determination device 100 according to the present invention. FIG. 10 is a diagram showing the processing flow of a radar reception method including a suppression amount determination method. 1 is a diagram showing an example of a received signal obtained from a radar. FIG. 4 is a diagram showing an example in which a mask is generated for all indexes with 100 dB or more being "higher than a predetermined value" and the generated mask is added to FIG. 3 . FIG. 5 is a diagram showing an example in which an example in which the width of a mask is used as a representative tendency value is added to FIG. 4 . FIG. 10 is a diagram showing an example of a cumulative probability distribution when the width of a mask (the number of indexes) is used as a representative tendency value. FIG. 5 is a diagram showing an example in which an example in which the distance between adjacent masks is used as a representative tendency value is added to FIG. 4 . FIG. 10 is a diagram showing an example of cumulative probability distribution when the distance to the next farthest mask (number of indexes) is used as a representative tendency value. FIG. 5 is a diagram showing an example in which a value indicating the strength of a received signal within a mask is used as a representative tendency value, and a pair of a representative distance indicating the distance to the mask and the representative tendency value is used as a representative feature amount, added to FIG. 4 . FIG. 10 is a diagram showing an example of a cumulative probability distribution when a value indicating the strength of a received signal within a mask is used as a representative tendency value, and a pair of a representative distance indicating the distance to the mask and the representative tendency value is used as a representative feature amount. FIG. 2 is a diagram showing an example of the functional configuration of a computer.

Embodiments of the present invention will be described in detail below. Components with the same functions will be assigned the same numbers, and duplicate explanations will be omitted.

Figure 1 shows an example of the functional configuration of a radar receiver 200 equipped with a suppression amount determination device 100 of the present invention. Figure 2 shows the processing flow of a radar reception method including a suppression amount determination method. Figure 3 shows an example of a received signal acquired from a radar. The horizontal axis of Figure 3 indicates the index of the radar's reflected signal. The distance indicated by the number is determined by the distance resolution setting. For example, if the distance resolution is 20 m, the number 1000 indicates a distance of 20 km from the radar. Furthermore, if the distance resolution is 0.5 m, the number 1000 indicates a distance of 500 m from the radar. The vertical axis of Figure 3 indicates the strength (Level) of the received signal. The unit of the vertical axis is dB.

In the example of the received signal in Figure 3, there is land at a distance farther than the number 550, and there is sea surface reflection (sea clutter) at a distance closer to the number 550. As can be seen from Figure 3, sea surface reflection is weaker than reflection from land. Furthermore, as shown in Non-Patent Document 1, the strength decreases as the distance increases. In the range where sea surface clutter exists, the strength of the received signal changes at short distance intervals. In this invention, by taking advantage of the fact that sea surface clutter changes at short distance intervals and that the strength decreases as the distance increases, the sea surface reflection contained in the received signal in the direction being measured is identified and the amount of suppression for the identified reflection is determined.

The suppression amount determination device 100 determines the amount of suppression required to suppress signals reflected from the sea surface from radar signals received over the sea. The suppression amount determination device 100 includes a mask generation unit 110, an in-mask representative feature calculation unit 120, an in-mask representative feature recording unit 130, an azimuth angle range feature tendency calculation unit 140, a sea surface clutter identification unit 150, and a specific sea surface clutter suppression amount determination unit 160. The radar receiver 200 includes the suppression amount determination device 100 and further includes a sea surface clutter suppression unit 270. A "received signal" is a signal that indicates the intensity at a predetermined distance resolution received for each predetermined direction. The predetermined direction may be, for example, every 1 degree or every 0.25 degrees. The predetermined distance resolution may be selected from approximately 0.5 m to 30 m, taking into account the distance to be measured. For example, if numbers up to 1000 can be measured, setting the distance resolution to 0.5 m will result in a measurable distance of 500 m. If the distance resolution is set to 30m, the measurable distance is 30km.

The mask generation unit 110 generates a mask (S110) by defining a range of distances where the strength of the received signal is continuously higher than a predetermined value. "Higher than a predetermined value" may mean a value equal to or higher than a threshold value set for all indexes, or a value determined taking distance into consideration (the value decreases with increasing distance). Figure 4 shows an example in which a mask is generated for all indexes with a value of 100 dB or higher defined as "higher than a predetermined value," and the generated mask is added to Figure 3. It can be seen that the mask near "reflections from land, etc." in Figure 3 has a wide width (the width of continuously high intensity), while the mask near "sea clutter" has a narrow width. In other words, many masks are generated near "sea clutter."

The intra-mask representative feature calculation unit 120 calculates, for each mask, a representative feature amount including a representative tendency value indicating that the received signal within a predetermined mask is likely to be a signal reflected from the sea surface (S120). The intra-mask representative feature calculation unit 120 may use the width of the mask, the spacing between adjacent masks, or a value indicating the strength of the received signal within the mask as the representative tendency value. Note that if the representative tendency value is the strength of the received signal within the mask, the representative feature amount is a combination of a representative distance indicating the distance to the mask and the representative tendency value.

The in-mask representative feature recording unit 130 records the representative feature for each orientation (S130). The in-mask representative feature recording unit 130 records predetermined past representative feature for each orientation for use in the processing of the in-azimuth angle range feature tendency calculation unit 140, which will be described later.

The azimuth angle range feature tendency calculation unit 140 calculates the tendency value conditions, which are conditions for identifying sea surface reflections, based on the representative tendency values for a predetermined azimuth range (S140). The "predetermined azimuth range" refers to a range that includes received signals from earlier than the received signal being processed. For example, if received signals are received every 1 degree of azimuth angle, and the "predetermined azimuth range" is set to 10 degrees, then the nine previous received signals can be added to the received signal being processed. This is because sea surface reflections have similar properties within a certain azimuth range. The "predetermined azimuth range" may be 45 degrees or 90 degrees.

If the width of the mask or the distance between adjacent masks is used as the representative tendency value, the "tendency value condition" may be, for example, a condition for determining that the range in which the representative tendency value exists is a high probability of being a reflection signal from the sea surface. If the value indicating the strength of the received signal within the mask is used as the representative tendency value, the "tendency value condition" may be, for example, a condition for determining that the range in which the value obtained by multiplying the representative tendency value intensity by a predetermined function of the representative distance is a high probability of being a reflection signal from the sea surface. If the "function of the representative distance" conforms to equation (4) in Non-Patent Document 1, it may be a function of the cube of the representative distance. However, since some literature uses different functions, the function may be determined as appropriate. The "representative distance" may be determined using a predetermined method as long as it is within the range of distances included in the mask. For example, it may be the center distance within the mask, or the distance with the highest intensity within the mask. The "representative intensity" may be determined based on the intensity within the mask using a predetermined method. For example, it may be the average intensity within the mask, or the highest intensity within the mask.

Figure 5 shows an example of representative feature example 1 in Figure 4, where mask width is used as a representative trend value. However, this cannot be added to masks with narrow widths. It is only added to masks with widths that are easily visible. The intra-mask representative feature calculation unit 120 actually calculates representative features for each mask. Figure 6 shows an example of a cumulative probability distribution when mask width (number of indexes) is used as the representative trend value. The horizontal axis represents mask width (number of indexes). The vertical axis represents the proportion (probability) of masks within the mask width when the total number of masks in a predetermined orientation range is used as the parameter. Note that Figure 6 is an example to illustrate the cumulative probability distribution and is not based on the example in Figure 5. Furthermore, other than cumulative probability distributions may also be used. For example, probability density functions, means, variances, etc. may also be used.

If sea surface reflections are characterized by a narrow mask width and a high proportion of sea surface reflection signals in the reflected signals, then a range where the cumulative probability distribution is 90% or less can be determined to be a sea surface reflection. Figure 6 shows the mask width threshold in this case and the range of tendency value conditions that determine whether a signal is a sea surface reflection. However, "90% or less" is an example and can be set as appropriate. Also, while only an upper limit threshold is set, a lower limit threshold can also be set.

Figure 7 shows an example of representative feature example 2, in which the distance between adjacent masks is used as a representative tendency value, added to Figure 4. However, this is not added for masks with narrow spacing. It is only added for masks with a width that is easily visible. The intra-mask representative feature calculation unit 120 actually calculates representative features for each of all masks. Figure 8 shows an example of a cumulative probability distribution when the distance to the next-furthest mask (number of indexes) is used as the representative tendency value. The horizontal axis represents the distance to the mask (number of indexes). The vertical axis represents the proportion (probability) of masks within the distance to the mask when the total number of masks in a predetermined orientation range is used as the parameter. Note that Figure 8 is an example to illustrate the cumulative probability distribution and is not based on the data in Figure 7. Furthermore, methods other than cumulative probability distributions may be used. For example, probability density functions, means, variances, etc. may be used.

If sea surface reflections are characterized by a narrow gap between adjacent masks and a high proportion of sea surface reflection signals in the reflected signal, then a range where the cumulative probability distribution is 90% or less can be determined to be a sea surface reflection. Figure 8 shows the threshold value for the distance to the next-furthest mask in this case, and the range of tendency value conditions that determine whether a reflection is from the sea surface. Note that "90% or less" is an example and can be set as appropriate. Also, while only an upper threshold is set, a lower threshold may also be set. Note that in Figure 7, the distance to the next-furthest mask is used as the gap between adjacent masks, but the distance to the closest mask may also be used, or the sum or average of the distances to both adjacent masks may also be used.

Figure 9 shows an example of a representative feature 3, where the value indicating the strength of the received signal within the mask is used as the representative tendency value, and the representative feature is a combination of the representative distance indicating the distance to the mask and the representative tendency value. Figure 9 also shows an example of a representative feature added to Figure 4, where the distance to the center of the mask is used as the representative distance, and the average intensity within the mask is used as the representative intensity. However, this feature is not added for narrow masks. It is added only for masks with a width that is easily visible. The intra-mask representative feature calculation unit 120 actually calculates representative features for each mask. Figure 10 shows an example of a cumulative probability distribution when the value indicating the strength of the received signal within the mask is used as the representative tendency value, and the representative feature is a combination of the representative distance and the representative tendency value indicating the distance to the mask. The horizontal axis represents the average signal level multiplied by the cube of the distance. The vertical axis represents the proportion (probability) of the number of masks when the total number of masks in a predetermined azimuth range is used as the parameter. Note that Figure 10 is an example to illustrate the cumulative probability distribution and is not based on the example of Figure 9. Also, it is possible to use a distribution other than the cumulative probability distribution. For example, it is possible to use a probability density function, mean, variance, etc.

If the proportion of signals reflected from the sea surface in the reflected signal is high, then a range of cumulative probability distribution between 5% and 95% can be used to determine that the signal is a reflection from the sea surface. Figure 10 shows the lower and upper thresholds for the value obtained by multiplying the average signal level by the cube of the distance, as well as the range of tendency value conditions for determining that the signal is a reflection from the sea surface. Note that "5%" and "95% or less" are merely examples and can be set as appropriate.

For each mask of the received signal to be processed, the sea surface reflection identification unit 150 determines whether the received signal within the mask is a reflection from the sea surface according to the representative tendency value and tendency value condition of that mask (S150). For example, in the examples of Figures 6, 8, and 10, the received signal within the mask within the range indicated as "tendency value condition" is determined to be a reflection from the sea surface.

The specific sea surface reflection suppression amount determiner 160 determines the amount of suppression for each mask in the received signal to be processed so as to reduce the received signal of the mask determined by the sea surface reflection determiner 150 to be a sea surface reflection below the noise level (S160). In the case of the received signal shown in Figure 3, the amount of suppression should be determined so that it is 80 to 90 dB or less.

In this invention, sea clutter is identified based on the characteristics of sea surface reflection, and only the identified sea clutter is suppressed. Therefore, the suppression amount determination device of the present invention can obtain the amount of suppression required to appropriately suppress sea clutter using only the radar, while maintaining real-time performance and ensuring target identification.

The sea surface clutter suppression unit 270 suppresses the received signal according to the amount of suppression determined for each mask (S270). The radar receiver 200 of the present invention suppresses only clutter from the sea surface, so it can suppress clutter from the sea surface while maintaining target identification.

[Program, recording medium]
The various processes described above can be implemented by loading a program that executes each step of the above method into the recording unit 2020 of the computer 2000 shown in Figure 11 and operating the control unit 2010, input unit 2030, output unit 2040, display unit 2050, etc.

The program describing this processing can be recorded on a computer-readable recording medium. Examples of computer-readable recording media include magnetic recording devices, optical disks, magneto-optical recording media, and semiconductor memory.

This program may be distributed, for example, by selling, transferring, or lending portable recording media such as DVDs or CD-ROMs on which the program is recorded. Furthermore, the program may be stored in a storage device of a server computer, and then distributed by transferring the program from the server computer to other computers via a network.

A computer that executes such a program, for example, first stores the program recorded on a portable recording medium or transferred from a server computer in its own storage device. Then, when executing a process, the computer reads the program stored on its own recording medium and executes the process in accordance with the read program. Alternatively, the computer may read the program directly from the portable recording medium and execute the process in accordance with the program. Furthermore, each time a program is transferred from the server computer to the computer, the computer may execute the process in accordance with the received program. Alternatively, the server computer may not transfer the program to the computer, but may instead execute the process through a so-called ASP (Application Service Provider) service, which realizes the processing function simply by issuing execution instructions and obtaining the results. Note that in this embodiment, the program includes information used for processing by a computer that is equivalent to a program (such as data that is not a direct command to the computer but has properties that define computer processing).

In addition, in this embodiment, the device is configured by executing a specific program on a computer, but at least part of the processing may be implemented in hardware.

100 Suppression amount determination device 110 Mask generation unit 120 In-mask representative feature calculation unit 130 In-mask representative feature recording unit 140 Feature tendency calculation unit within azimuth angle range 150 Sea surface clutter identification unit 160 Specific sea surface clutter suppression amount determination unit 200 Radar receiver 270 Sea surface clutter suppression unit 2000 Computer 2010 Control unit 2020 Recording unit 2030 Input unit 2040 Output unit 2050 Display unit

Claims (11)

A suppression amount determination device for determining a suppression amount for suppressing a signal reflected from the sea surface from a radar signal received at sea,
The received signal is a signal received in each predetermined direction and indicates its intensity at a predetermined distance resolution,
a mask generation unit that generates a mask, with one mask being a range of distances where the strength of the received signal is continuously higher than a predetermined value;
a mask representative feature calculation unit that calculates, for each mask, a representative feature amount including a representative tendency value that indicates that the received signal within a predetermined mask is likely to be a signal reflected from the sea surface;
a representative feature recording unit in a mask that records the representative feature amount for each orientation;
an azimuth angle range characteristic tendency calculation unit that calculates a tendency value condition, which is a condition for identifying reflection on the sea surface, based on a representative tendency value in a predetermined azimuth range;
a sea surface clutter identifying unit that determines whether or not a received signal within a mask is a clutter of the sea surface, for each mask of the received signal to be processed, according to the representative tendency value of the mask and the tendency value condition;
a specific sea clutter suppression amount determining unit that determines a suppression amount for each mask of the received signal to be processed so as to reduce the received signal of the mask determined by the sea clutter identifying unit to be a sea clutter to below the noise level;
A suppression amount determining device comprising: 2. The suppression amount determining device according to claim 1,
The suppression amount determining device is characterized in that the in-mask representative feature calculation unit uses a width of the mask as a representative tendency value. 2. The suppression amount determining device according to claim 1,
The suppression amount determining device is characterized in that the intra-mask representative feature calculation unit uses an interval between adjacent masks as a representative tendency value. 4. The suppression amount determining device according to claim 2,
The suppression amount determining device, wherein the tendency value condition is a condition for determining that a range in which a representative tendency value is highly likely to exist is a reflected signal from the sea surface. 2. The suppression amount determining device according to claim 1,
the in-mask representative feature calculation unit determines a value indicating the intensity of the received signal within the mask as a representative tendency value, and determines a pair of a representative distance indicating the distance to the mask and the representative tendency value as a representative feature. 6. The suppression amount determining device according to claim 5,
the tendency value condition is a condition for determining that a range in which there is a high probability that a value obtained by multiplying the representative tendency value intensity by a predetermined function of the representative distance exists is a reflected signal from the sea surface. The suppression amount determining device according to any one of claims 1 to 6,
moreover,
a sea clutter suppression unit that suppresses the received signal according to the amount of suppression. A method for determining a suppression amount for suppressing a signal reflected from the sea surface from a radar signal received at sea, comprising:
The received signal is a signal received in each predetermined direction and indicates its intensity at a predetermined distance resolution,
a mask generation step of generating a mask that is a range of distances where the strength of the received signal is continuously higher than a predetermined value;
a mask representative feature calculation step of calculating, for each mask, a representative feature amount including a representative tendency value indicating that the received signal within a predetermined mask is likely to be a signal reflected from the sea surface;
a mask representative feature recording step of recording the representative feature amount for each orientation;
an azimuth angle range characteristic tendency calculation step for calculating a tendency value condition, which is a condition for identifying reflection on the sea surface, based on a representative tendency value in a predetermined azimuth angle range;
a sea surface clutter identifying step of determining whether or not the received signal within each mask is a clutter from the sea surface, based on the representative tendency value of the mask and the tendency value condition, for each mask of the received signal to be processed;
a specific sea clutter suppression amount determination step for determining a suppression amount for each mask of the received signal to be processed so as to reduce the received signal of the mask determined to be a sea clutter in the sea clutter identification step to below the noise level;
A method for determining the amount of suppression to be performed. The method for determining the amount of suppression according to claim 8,
moreover,
a sea clutter suppression step of suppressing the received signal in accordance with the suppression amount. A suppression amount determination program for causing a computer to function as the suppression amount determination device described in any one of claims 1 to 6. A radar reception program for causing a computer to function as the radar receiver described in claim 7.