GB2194407A

GB2194407A - Antenna pattern monitor

Info

Publication number: GB2194407A
Application number: GB08620709A
Authority: GB
Inventors: Christmas Bernard Cousins
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1986-08-27
Filing date: 1986-08-27
Publication date: 1988-03-02
Also published as: GB2195509B; GB8620709D0; GB2195509A; GB2194407B; GB8628243D0

Abstract

An antenna pattern monitor for use with a secondary surveillance radar system records the amplitude of the transmitted interrogation signals and uses this data to plot an azimuthal radiation pattern of the transmitting radar antenna over a full 360 DEG . The monitor also transmits secondary radar response pulses back to the interrogating secondary surveillance radar system and records the resulting display, and uses this data to check the operation of the secondary surveillance radar's receiver and data processing circuits.

Description

SPECIFICATION Antenna pattern monitor This invention relates to an antenna pattern monitor. and is particularly applicable to an antenna pattern monitor for monitoring secondary surveillance radars. In a secondary surveillance radar system, the beam pattern of a radar signal transmitted by a radar installation is of great importance. In such a system the transmitted radar signal from a ground antenna is used to interrogate a transponder carried aboard an aircraft and, if the aircraft is in the main boresight direction of the ground radar, the transponder sends a response back to the ground radar station.This response can be modulated to include a code identifying the aircraft and giving its height, and the use of secondary radar in addition to primary radar gives a much more positive indication of the position and course of an aircraft because the aircraft only responds when in the ground radar boresight.

The transmitted radar interrogation signal generally consists of a sequence of three pulses, the amplitudes of the first and second pulses being such that the amplitude of the second pulse is greater than that of the first pulse except for a small angle of direction centred on the radar boresight. The aircraft transponder measures the relative amplitudes of the three pulses and only if the correct relationship is satisfied generally that the amplitude of the second pulse is more than 9 dB less than that of the first pulse indicating that the aircraft is in the boresight direction of the radar, is the transponder enabled to reply. The third pulse is always equal in amplitude to the first pulse and the time delay between the first and third pulses is used to show in which mode any reply should be made.The equal amplitude of first and third pulses also allows the transponder to respond correctly should one set of pulses be too weak to detect, because if only one pulse is detected, it must be a second pulse and if only two pulses are detected they must be a first and third pulse.

Commonly the first and third pulses are transmitted by a 'sum' pattern and the second pulse by a 'difference' pattern.

The amplitude distributions of the beam patterns of the antenna are therefore critical, not only in the boresight direction but over the whole 360 , because as the antenna pattern rotates in a horizontal plane so as to sweep the whole of the airspace under surveillance it is vital that the condition that the amplitude of the second pulse is more than 9dB smaller than that of the first pulse is satisfied only in the boresight direction.

Although all radar transmitters and antennas are thoroughly checked before and during installation, it is possible for minor damage to the antenna or interruption of power to the antenna at certain orientations, due to damaged rotating joints for instance, to alter the antenna pattern whilst in service without this being noticed by the radar operators. The antenna far-field pattern can also be altered by reflection of the transmitted signal from buildings or landscape features, so even if the near-field antenna pattern were to be monitored, there would be no guarantee of a satisfactory far-field antenna pattern. Therefore, a far field radar antenna pattern monitor capable of recording the transmitted patterns as the radar rotates is needed.

This invention provides such a monitor.

According to this invention, a radar antenna pattern monitor includes: a receiver tuned to the frequency of a secondary radar interrogation signal; means for receiving a plurality of sequential radar interrogation pulses forming a single interrogation signal and determining their relative amplitudes; and means associated with said pattern monitor for recording data relating to the relative amplitudes for a plurality of sequentially received interrogation signals, said plurality of signals being indicative of an azimuth radiation pattern of a radar antenna producing the signals.

It would usually be preferred for all parts of this monitor to be together physically at one site. However, it may sometimes be advantageous for the data recording means to be at a site remote from the rest of the monitor, for instance, at the secondary radar interrogation signal transmitter.

The invention will be described with reference to the accompanying figures, where: Figure 1 shows a secondary surveillance radar antenna pattern monitor constructed in accordance with the invention in block diagram form, Figures 2A to 2G show the signals at various points in the monitor shown in Figure 1, Figure 3 shows a graph produced by the monitor shown in Figure 1, Figure 4A shows apparatus connected with an interrogating secondary surveillance radar transmitter cooperating with a monitor as shown in Figure 1 in block diagram form, Figure 4B shows a graph produced by use of the apparatus shown in Figure 3 in cooperation with that shown in Figure 1 and Figure 5 shows an alternative form of secondary surveillance radar antenna pattern monitor constructed in acordance with the invention in block diagram form.

Referring to Figure 1, radio frequency pulses which will be referred to as R.F. pulses in this description, from a secondary surveillance radar system are received at an antenna 1 and fed to a 3-port circulator 2. The circulator 2 isolates the transmitting and receiving parts of the system; and passes the received pulses through a filter 3 which reduces noise. From the filter 3 the signals pass to a variable attenuator 4 which is adjusted to keep the sig nal strength at a level acceptable to the system.

The R.F. signal is then "mixed down" to an intermediate frequency signal by mixing in a mixer 5 with the output of voltage controlled oscillator 6. The frequency of V.C.0.6 is controlled by receive frequency control 37. This intermediate frequency signal is then amplified by à pre-amplifier 7 and then by log amplifier 8 and is then supplied to the analogue decoder 9.

The signal supplied to analogue decoder 9 would ideally be similar to signal 10 as shown in Figure 2A. However in practice, the signal is likely to be more similar to signal 11 as shown in Figure 2B, the ideally square shape of the pulses being rounded off due to noise and. transmitting system limitations for in stance. The three pulses P1, P2 and P3 will vary in magnitude due to distance from the transmitter, system losses and atmospheric effects, for example, as well as due to the planned variation with angular position. Although- these other effects will not alter the relative sizes of the pulses, they can make it difficult to distinguish a very low signal level pulse from random variations in background n-oise.

These two problems can make it hard to process the signals, because each pulse's am plitude will vary with time as it is received.

The disappearance of low level pulses could, for instance cause confusion as to whether a single low amplitude pulse is a second pulse or a first pulse with the third pulse missing. In order to overcome these problems the ana logue decoder 9 applies the signal to a Schmitt type trigger which produces an output signal 12 at 5V, (see Figure 2C) for all times when the signal 11 is above a threshold value. This threshold can be set low enough that signal 12 is effectively a version of signal 1 1 with the signal level set at 5V. 5 volts was chosen because of its suitability for use in the transistor transistor logic (T.T.L.) circu its used in this system. If another form of system logic were used it would be necessary to change this voltage level.

The signal 12 is fed into the digital decod ing circuit 13. Digital decoding circuit 13 puts signal 12 into a tapped digital delay line. The taps in the delay line are at a plurality of delay times corresponding to the allowed pulse separations in all the different interrogation modes used in S.S.R. systems. Digital decoder 13 thus detects in which mode the received sig nal is by using the pulse timings, lights the -corresponding one of LED's 14-to show which mode has been detected and sends gating pulses corresponding to the timings of the centres of pulses P1 P2 and P3 back to the analogue decoder 9 along lines 15. In Fig ure 2D signal 16 corresponds to the P1 gating pulse in Figure 2E signal 17 corresponds to the P2 gating pulse and in Figure 2F signal 18 corresponds to the P3 gating pulse.

Gating pulses 16, 17 and 18 correspond in time to the centres of pulses PI, P2 and P3 which have been held in an analogue delay circuit in analogue decoder 9. The gating pulses 16, 17 and 18 are used to operate a gate to sample the central portions of pulses P1, P2 and P3, thus sampling the peak of the pulses producing a waveform 19, see Figure 2G.

The analogue decoder 9 then uses the P1 P2 and P3 sample pulses or waveform 19 to produce P1, P2 and P3 signals, the P1, P2 and P3 signals are then sent along lines 20, 21 and 22 respectively to a recording system, a computer 40. Computer 40 is arranged to organise the storage of the three signals in the memory 41, and then correlate the information in memory 40 and produce and display graphical representations of the signals, for instance, a graph of the relative amplitudes of sum and difference signals for a full revolution of the interrogating radar, and to produce printed copies of these graphs. An example of such a graph is shown in Figure 3, although the full 360" pattern is not shown. The graph is of received pulse amplitude against angular position.Each received interrogation pulse sequence is represented by a dot, the dots joining up to form lines 49 and 50. The 'first pulses' 50 and 'second pulses' 49 define the boresight direction 51. However an area 52 could give rise to spurious responses because the second pulse is not greater than the first pulse, in this angular region.

The memory 41 can be of any convenient type but is preferably a semiconductor random access memory (RAM). So that the absence of an interrogating pulse due to the mode inhibit control is not interpreted as an interrogating radar fault the setting of the mode inhibit control 26 is supplied to computer 40 along line 42 and stored in memory 41.

Another advantage of this method of processing the interrogating signals is that all interrogations received are recorded whereas as a conventional transponder rejects all interrogation signals that do not satisfy the conditions for transmission of a response at the receiver stage, these signals could not be recoreded in such a system because they are rejected before they are amplified. It is also useful to be able to check that the geometrical boresight of the interrogating radar antenna is the same as the boresight direction of the radiation pattern, because if it is not, all position information provided by the system will be incorrect. This can be carried out as shown in Figure 4A by fitting a sensor 53 to the interrogating antenna 54 to measure its geometric position, and to give a signal corresponding to this position to a controller 55.

When the antenna 54 passes through a zero position, defined as being when the geometrical boresight is pointed at the distant monitor, the controller 55 sends a signal along line 56 to the interrogating transmitter and in response, the transmitter misses out one interrogating pulse sequence. When the transmitted interrogating pulse sequences are represented graphically as in Figure 4B, the position of the gap 57 left by the missing pulse sequence can be used to find the relative positions of the geometric boresight 58 and radiation pattern boresight 59.

This procedure would only be carried out occasionally because the missed pulse would be assumed to be a fault if it was not expected.

If the relative amplitudes of pulses P1 and P2 are such that a reply should be produced, a signal identifying the interrogation mode is sent along line 23, and an enabling signal is sent along line 24 to reply encoder 25.

It may be wished in practice to only look at some of the modes produced by a multi-mode interrogation system. If this is the case, the mode inhibiting control 26 can be set to stop gating pulses being generated by digital decoder 13 on receiving any single mode or group of modes, preventing them from being recorded or responded to.

Reply encoder 25 produces a response pulse sequence for transmission back to the interrogation system. It may only be wanted to analyse and record the pattern transmitted by a secondary surveillance radar system. In practice however, it will generally be desirable to record the response of the secondary surveillance system to return pulses from transponders. In this case, some means must be provided to record the return signal from the transponder and the interrogating radar display. In order to record the return signal sent by the transponder the reply encoder 25 is linked to the computer 40 by line 44 which carries the return signal sent. In order to record the interrogating radar display the signals sent to the display screen to be observed by the operator can be recorded.Comparison of this recording with a record of the responses sent by the monitor can then be used to measure the performance of the secondary surveillance radar receiving and signal processing systems. This facility is provided in the illustrated monitor by the signals sent to the operator's display in the interrogating radar being placed in memory 41 by the computer 40.

This is represented by lead 42 going to the interrogating radar, but in practice it may be more convenient to use a separate recorder at the distant interrogating radar site and correlate data from this recorder with that in memory 41 later.

In order to make this testing of the receiving and processing systems worthwhile, the reply encoder 25 is capable of producing a wide range of different replies. The reply encoder 25 can reply in any of the aircraft response codes, which code is used is controlled by code selector 27, which can be set to the desired code. The reply encoder 25 can simulate an aircraft at different ranges by delaying its reply for a suitable period, the range selector 28 controls which range is simulated.The reply encoder 25 can also simulate some typical problems encountered by SSR systems; the problem of overlap; where two different aircraft respond when close together and their reply pulse sequences overlap, and pulse missing; where aircraft fail to respond to some interrogation pulses when they are in the boresight of an SSR antenna because their flying attitude temporarily masks the transponder's receiving or transmitting with parts of the airframe, such as wings or tail surfaces for example.

Overlap is simulated by the reply encoder transmitting two reply pulse trains simultaneously, arranging for the individual pulses of the two reply pulse trains to be either overlayed or interleaved. The reply type selector 29 controls whether or not reply overlap is simulated, and if it is, which type of overlap.

Pulse missing is simulated by the reply encoder not responding to some of the plurality of interrogations which will be received by the monitor each time the boresight of the antenna sweeps over it. Which interrogations are responsed to and which are ignored is set by the hit/miss selector 30.

The setting of selectors 27, 28, 29 and 30 will be supplied along line 44 to computer 40 for storage in memory 41.

The selectors 27, 28, 29 and 30 and the mode inhibit control 26 can either be manually controlled or controlled by a computer.

If computer control were to be used the computer 40 would be used to control as well as record the settings used for each response.

The response pulse sequence travels along line 31 to modulator 32 where it is used to modulate the output of voltage controlled oscillator 33. The frequency of VCO 33 is controlled by transmission frequency control 39.

The modulated signal from modulator 32 is then amplified by an amplifier 34 and sent through three port circulator 2 to antenna 1 for transmission to the interrogating system.

The output of VCO's 6 and 33 are sampled by power splitters 35 and 36 respectively.

The signal samples are fed to a timer counter 37 which records the transmission and reception frequencies. If a computer were used For data processing, this information would be supplied to it.

Preferably the single computer 40 would be used to record the P1, P2, and P3 signals and the replies received at the interrogating system and to control and record the received and transmitted frequencies, and the settings of the mode inhibit, code, type, range and hit/miss controls. These settings of the val- ous codes being altered in some pre-arranged sequence.

Referring to Figure 5, an antenna pattern monitor having an alternative data storage system is shown.

It may be preferred to record and process data at the interrogating radar transmitting site. The antenna pattern monitor could, for instance, be an unmanned permanent installation for monitoring an air traffic control radar, or it could be mounted on a naval helicopter and used for periodic check-ups of a warship's radar systems.

The monitor functions as before, except tha the data along lines 20, 21, 22, 43 and 44 feed into-a short term memory 45 which has enough memory space to store all data associated with one revolution of the interrogation radar. When the antenna pattern monitor is operating, the interrogation radar transmits an interrogation code with some non-standard pulse separation once in each revolution of tht interrogating radar Being of nonstandard pulse separation this is ignored by any other transponder that picks it up. When this code is detected by the digital decoder 13, it sends a pulse along line 46 to short term memory 45.

On receiving this pulse, the short term memory 45 begins feeding data to the reply encoder 25 along line 47. The reply encoder 25 then transmits a series of replies with the data from memory 45 encoded into the aircraft response codes.

The aircraft response codes are received at the interrogation radar site where the data is extracted and processed by a computer system. In normal operation a computer 48 controls the code selector 27, range selector 28, type selector 29 and hit/miss selector 30.

The computer simply goes through a pre-programmed sequence and is not used to store or collate data.

Alternatively, for a land based system, the data could be fed from the monitor to the interrogation radar site along a cable.

In a system where the data from the monitor is fed back to the transmitting site it would be useful to operate the monitor contin uously and use a computer system at the interrogation radar site to continuously compare the system parameters, such as, pulse width, pulse separation, transmitted radiation pattern and the relative positions of geometrical boresight and radiation pattern boresight, with an allowed range of values and to sound an alarm if the system parameters stray outside this range.

Although this system has been described with reference to secondary surveillance radar systems it could be adapted to monitor any type of radar transmitting and receiving system.

Claims

1. A radar antenna pattern monitor including: a receiver tuned to the frequency of a secondary radar interrogation signal; means for receiving a plurality of sequential radar interrogation pulses forming a single interrogation signal and determining their relative amplitudes; and means associated with said pattern monitor for recording data relating to the relative amplitudes for a plurality of sequentially received interrogation signals, said plurality of signals being indicative of an azimuth radiation pattern of a radar antenna producing the signals.

2. A radar antenna pattern monitor as claimed in claim 1 and wherein said data relating to the relative amplitudes for a plurality of sequentially received interrogation signals is recorded for a range of azimuth outside the boresight direction of the secondary radar interrogation system.

3. A radar antenna pattern monitor as claimed in claims 1 or 2 and additionally including: means to produce digital timing signals from the secondary radar interrogation signal; means to use these digital timing signals to extract samples from the plurality of sequential radar interrogation pulses forming a single interrogation signal and means to use these samples to determine the relative amplitudes of the pulses.

4. A radar antenna pattern monitor as claimed in claim 1, 2 or 3 and additionally including means dependent on the relative amplitudes of the sequential pulses forming a single interrogation signal for transmitting a secondary radar response pulse sequence.

5. A radar antenna pattern monitor as claimed in claim 4 where said means for recording data is at a secondary radar interrogating transmitter and data is sent to it encoded in the secondary radar response pulse sequences.

6. A radar antenna pattern monitor as claimed in any preceding claim and additionally including means for recording the transmitted secondary radar response pulse sequence before it is transmitted.

7. A radar antenna pattern monitor as claimed in claim 6 and additionally including means associated with the interrogating radar for recording the secondary radar response pulse sequence after reception by the interrogating radar.

8. A radar antenna pattern monitor as claimed in claim 4, 5, 6 or 7 and additionallyincluding means to transmit a delayed secondary radar response.

9. A radar antenna pattern monitor as claimed in claim 4 or 8 and additionally including means to transmit a plurality of secondary radar response pulse sequences that overlap in time.

10. A radar antenna pattern monitor as claimed in claims 4, 8 or 9 and additionally including means to prevent response to selected ones of a plurality of interrogation signals.

11. A radar antenna pattern monitor as illus trated in Figure 1 of the accompanying drawings and as described with reference thereto.

12. A radar antenna pattern monitor as illustrated in Figure 2 of the accompanying drawings and as described with reference thereto.