GB2259778A - Testing radar antenna systems - Google Patents

Abstract

For testing and calibration of a radar antenna array system (12, 16, 6, 4) both in transmission and reception, each array element (4) is provided with a coupler (42). For transmission testing, a test-transmission signal is sampled via the coupler (42) of each active element (4) by a transmission calibration detector (54), the signal passing via a splitter (48) and a switch (52). For testing reception, a test pulse from a calibration test pulse generator (20) is coupled (52, 50, 48) to each active element (4) and is detected by receiver circuitry. In a circular phased array system (Fig 1 not shown) on a tower, RF components are located near the antenna elements (4) to reduce losses and delays and should an element (4) fail to receive, a twin element (4) in a corresponding position is not used. <IMAGE>

Description

IMPROVEMENTS RELATING TO RADAR ANTENNA SYSTh:Ms The present invention relates to a radar antenna system of the phased array type, especially but not exclusively for use in a monopulse secondary surveillance radar (SSR) system for aircraft detection. The reader is referred to United Kingdom patent application GB 2219471A as background. Although the invention is described in a specific embodiment of a circular phased array antenna system, it is applicable to linear or other configurations of such antenna arrays.

Array antenna systems are configured with antenna elements located to provide desired field patterns, usually with directable RF field beams. Each of the antenna elements is fed with RF power at controlled relative levels of phase and amplitude to produce the desired field pattern. The required amplitude and phase distributions of transmitted signals are produced by various forms and combinations of controls and feed networks, which couple a common source of RF power to the individual radiating elements. Signals are received by the same radiating elements and processed in reception circuits involving RF and digital components to determine the location of a target object.

In a radar array antenna system, the multiplicity of transmission and reception paths, and the components which comprise them, present the possibility of variations in performance including drifts, degradations and failures, both of individual components and interacting groups of components. In particular, problems such as power fluctuations, attenuative losses, phase distortions and variations in detector sensitivities, are characteristic of RF equipment. Such variations are often due to temperature variations or component-ageing.

Accordingly, systems for automatic detection of variations in performance, are desirable. This is particularly so in a busy airport for example, where it is necessary to frequently ensure that radar equipment is working correctly if errors and accidents are to be avoided.

A phased array antenna system of the prior art is described in Us 4,639,732. In the disclosure of this document, transmitted signals are sampled and compared with stored error-threshold values to determine whether or not a failure alarm need be sounded. This teaching does not include means for testing or compensating for drifts in the performance of components (especially RF components) of the system, which are so common in practice.

Another problem of previous antenna systems, particularly circular phased array systems, is in transmission of signals.

There are high internal losses of RF power due to attenuation in RF feed circuitry to array elements. Also, there are problems in transmitting a large number of digited signals over long distances whilst maintaining signal synchronism.

Yet another problem is that failure of a single transmission path, resulting in failure of a radiator to transmit, results in inaccuracies in the detection of position of target objects by the radar system. Conventionally, it is difficult to improve the accuracy of such a system simply.

Yet another problem is that towers on which circular phased array systems are to be mounted are of various sizes.

Towers often already exist at a site because they carry primary radar systems.

A radar antenna according to the present invention comprises a radar antenna comprising an array of antenna elements, means for selecting different sets of elements, means for transmitting signals from a selected set of elements and means for receiving replies on a selected set of elements, each element comprising a coupler for sampling a transmission test signal or injecting a reception test signal, the system further comprising transmission test means comprising a transmission test detector which is selectively connectable to at least one coupler of the elements to detect transmission test signals, and reception test means comprising a test signal generator which is selectively connectable to at least one coupler of the elements to inject reception test signals to be received.

A preferred radar antenna comprises a circular array of antenna elements mounted on a tower, and electronic components of which components carrying digital signals are located at the bottom of the tower.

A preferred method of alleviating received signal distortions in a radar antenna system comprising an array of antenna elements including an array segment of active elements, comprises the steps of (i) testing reception by active elements, and if a first radiator does not receive in a predetermined way, (ii) locating a second element substantially equidistant from the middle of the active segment and in juxtaposition to the first element, (iii) not using received signals from the first and second elements until a new array segment is selected.

A preferred method of construction of a radar antenna comprising a circular array of antenna elements mounted on a tower is one in which n banks of N antenna elements are selected, the number of banks n being selected in dependence on the lateral size of the tower, and the banks are connected together on the tower to form the array.

An embodiment of the invention will now be described, by way of example, with reference to the drawings, in which Figure 1A is a schematic diagram of the circularly arrayed radiating columns of the antenna, Figure IB is a schematic diagram of more of the antenna system, Figure 2 is a schematic diagram of a tranmission/ reception (T/R) module and associated circuitry, and Figure 3 is an example sketch graph of amplitudes of beams from an antenna array segment of thirty two radiating columns.

As shown in Figures 1A and 1B, a preferred antenna system for SSR consists of an antenna 2 having a set of circularlyarrayed radiating columns 4. Column radiators 4 which are arrayed circularly are well known in themselves. Each consists of ten or more dipoles and an RF feed system which tailors the amplitude and phase of the signals transmitted by each dipole.

In this embodiment, there are one hundred and sixty columns 4 and thirty two T/R modules 6 which are selectively connected to them by way of multiway switches. The T/R modules 6 are under the control of digital control apparatus 8. The T/R modules 6 provide signals for transmission and are used for received RF signal detection. Thirty two columns 4 are active at any one time for transmission and reception; for example to provide beams in a first azimuthal look direction axis, transmitted signals with appropriate phase and amplitude weightings are directed from T/R modules 6 to columns 4 to produce pairs of beams in look directions which are offset in azimuth relative to their associated look direction axis. To provide beams of a first look direction axis, T/R modules 6 and columns 4 are selected as follows, where modules 6 and columns 4 are numbered individually as shown in Figures 1A and 1B:- T/R Module number Column number 1 1 2 2 3 3 32 32 Similarly beams second look direction axis are provided as follows: T/R Module number Column number 2 2 3 3 31 31 32 32 1 33 Similarly, beams in the third look direction axis are provided as: T/R Module number Column number 3 3 4 4 32 32 1 33 2 34 Beams in subsequent look directions are produced accordingly.

Conventional sum, control and difference pulsed beams are produced and detected as is known in the art. This includes a control pulse being transmitted having a broad beam width and gain which is less than that of the main lobe of the sum beam.

The transponder measures the amplitude of the control pulse and compares it with the amplitude of an interrogative pulse of the sum beam. If the control pulse is stronger, then the transponder decides the signals are from an antenna side lobe direction rather than the main lobe direction, so it does not reply.

Received signals from each active column 4 are amplified and digitised in the T/R modules 6 before being weighted and summed in a digital beam former 10. In the beam former 10, amplitude and phase corrections are made, and I and Q signals (see later) are summed and processed to produce amplitude and phase data signals, as is well known in the art. The resulting data signals are applied to a monopulse plot extractor of known type for determination of the position of the source of the received signal (e.g. a transponder on an aircraft).

Circuitry for transmission and reception is shown in mDre detail in Figure 2. There is a reference oscillator and driver module 12 having an oscillator 14 which selectively provides, via a splitter module 16, a 1030 MHB drive signal to each of the thirty-two T/R modules 6. The driver module 12 also provides an unmodulated 1030MHz reference signal to each T/R module 6 via a second splitter module 15. The unmodulated 1030MHz reference signal is also sent to a transmit calibration detector 54, the functioning of which will be described later.

Each T/R module 6 includes a phase shifter 22 which sets the phase of the RF signal for transmission in order to compensate for the curvature of the array, as well as any phase errors due to non-ideal feeder cables. Each T/R module 6 also has amplitude correction circuitry 24. Using this equipment, amplitude and phase of a signal for transmission from a column is adjusted automatically and digitally, under the control of an external control unit 26. The external control unit 26 is connected to a phase and amplitude control 28 within each T/R module.

The signal for transmission is amplified then filtered by a band pass filter 30 then passed via a switch 32 for transmission or reception branch connection to a single pole six way switch 34. This allows a signal for transmission to be sent selectively to one of any five columns 4 to which it is connected and correspondingly allows a received signal to pass from the column 4 to reception circuitry. (Of course, all six switch positions are available for 192 column radiators connectable to thirty-two T/R modules 6).

The T/R module 6 has a received signal branch which includes a band pass filter 36 and low noise amplifier 38 connecting the T/R switch 32 to a mixer 40. The received signal, which is initially 1090 mew is demodulated in the mixer 40 down to 60 MHz, because the mixer 40 received the 1030 MHz reference signal, via an input port 41, from the second splitter module 15. This demodulated signal is passed via an STC variable attenuator (not shown) and intermediate frequency (IF) amplifier (not shown) to further mixers (not shown) and a to d converters (not shown). Two digital output signals (I, Q) are produced as is conventional. The signal Q is 90 out of phase with the signal I. The output signals I, Q are input to the beam former 10 as shown in Figure 1B.

Calibration The antenna system is capable of self calibration both during initial installation and periodically in order to compensate for the effect of errors due to temperature changes or ageing for example. Each column 4 of the antenna array 2 has a coupler 42 having a sniffer input 44 to sample the transmitted pulse signal (in transmission calibration) and to inject a test pulse into the column radiator (in reception calibration).

Each sniffer input 44 is connected via coaxial cable 46 to a multiway combiner/splitter module 48. In this embodiment Which has 160 column radiators 4, the combiner/splitter module 48 has 160 ports each connected to an associated column 4, combining to one port 50. The combiner/splitter module 48 and associated connecting cables produce amplitude and phase changes due to their non-ideal behaviour which can be accounted for in equipment calibration. The port 50 is connected selectively by way of a controllable two-way switch 52 between a transmit calibration detector 54 (for transmission calibration) and the calibration test pulse generator 20 (for reception calibration). The switch 52 is under the control of the control unit 26.

Transmission Calibration To calibrate the antenna system in its transmission function, one T/R module 4 at a time is used to generate a transmission signal pulse at 1030 MHz. The pulse is sampled by the coupler 42 in the column 4 which is connected to the T/R module 6 at that time. The sampled signal is input via an input port 46 to the multi-way combiner/splitter module 48 and output via its port 50 and switch 52 to the transmit calibration detector 54.

The transmit calibration detector 54 is a quadrature phase detector with the 1030 MHB unmodulated signal provided directly from oscillator 14 used as the reference signal to its mixer 56. The transmit calibration detector 54 produces analog I and Q output signals which are converted into digital form by respective a to d converters 58, 60. The digital I and Q signals provide a complete measure of the amplitude and phase of the measured transmission signal pulse.

The same calibration process is repeated for each of the thirty-two T/R modules 6 in turn. The measured I and Q values are used to generate amplitude and phase correction factors which are then applied to each the module 18. This calibration is performed for each T/R module in turn, in the dead time prior to an SSR interrogation.

Full transmission paths from RF oscillator 14 to columns 4 are thus calibrated.

The correction factors are also used as part of the Built In Test (BIT). A "window" is placed on the correction factor for each element value. A fault is signalled if the calculated correction factor falls outside this window on three successive calibrations. The transmitter calibration circuit is duplicated and the standby circuit is switched to automatically should the BIT detect a fault.

Reception Calibration In a somewhat analogous manner to transmission calibration, each receiver path from column radiator 4 through to the digital beam former 10 is tested and calibrated. A test pulse signal at 1090 MHZ is produced in the test pulse generator 20, which comprise a surface acoustic wave (SAW) oscillator. The test pulse signal is selectively injected into each column 4 by way of the switch 52 and combiner/splitter module 48. The test pulse enters each associated T/R module 6 and is detected to provide digital I/Q outputs as when the T/R module 6 is operative in an SSR interrogation. All thirty two T/R modules 6 are tested for reception at one time.

The I, Q outputs are passed to the beam former 10 where phase and amplitude data are compared with original calibration data. Adjustments necessary to correct for drift in amplitude and phase characteristics of the receive path are calculated and applied in subsequent SSR interrogation. The new calibration data is stored and used in the digital beamer former 10 for subsequent reception calibration.

Like transmission calibration factors, the reception calibration factors are also used as part of the BIT. A "window" is placed on the correction factor for each element value, and a fault is signalled if the calculated correction factor falls outside this window on three successive calibrations. The calibration oscillator is duplicated and switched automatically should the BIT detect a fault.

By the above process the full transmitter path and the receiver path through each module are tested for source to antenna column. The frequency of the calibration checking is prior to each SSR interrogation or at any desired interval, for example every hour or every day.

A drift in calibration should give rise to small changes in the corrections needed. However if a large change is required, this is an indication of a component failure. If the failure indication occurs for three consecutive repeat calibrations, then a failure is reported. Three indications are required rather than one to protect against the fault flag signal being sent as a result of transient external interference.

Location of Electronic Components All RF components are placed at the top of a tower on which the antenna is mounted, in order to minimise path lengths and consequential losses in signal power to the antenna columns 4. In consequence ranges of more than 200 miles are readily achieved.

All IF components and signal processing digital circuitry are located in the equipment room below, near to the plot extractor. Thus path lengths for digital signals are also shortened. This minimises the problems of the prior art of transmitting a large number of digital signals (e.g. 16 per radiating column at 16my;) over long distances whilst maintaining synchronism between signals.

The RF components comprise the 1030 MH oscillator and driver module 12, and components along the transmission path and RF front end of the reception path, in each T/R module 6.

The T/R module, shown in Figure 2, is split at the output of the pre-amplifier 62 following the mixer 40. The IF signal from each module is transmitted via IF coaxial cables to the bottom of the tower where the remainder of the module is situated. The calibration system described above will compensate for amplitude losses and phase delays in these IF coaxial cables.

Compensation for Effects of Failed Components The radar system is of a monopulse type in which signals received by radiators are processed into 'sum' and 'difference' receive beams. Specifically, in the receive mode, signals received from a certain look direction by a selected set of radiators 4 are connected through T/R modules 6 to the digital beamer former 10. The received signals there are reduced to sum and difference signals, and fed to the plot extractor. The plot extractor of the radar system has conventional sum and difference inputs, for determining the direction of signals received from a target. The plot extractor determines the azimuthal deviation of the target direction from the look direction boresight by measurement of the relative signal strengths in the sum and difference channels.

The plot extractor includes a circuit for determining the range of the reply signals by measurement of the elapsed time interval between a radar interrogation signal transmission, and a received reply. The determined range appears as an output.

The plot extractor also includes a circuit for decoding information, such as elevation or identity, which may be encoded by a target in its reply signal.

For a difference beam, the destructive interference of signals causes a sharp null signal in the look direction of the active antenna portion. The difference signal in the null is 900 out of phase with the sum beam.

A failed column radiator 4 or associated T/R module 6 introduces errors in measurements of bearings of transponders, because field patterns are distorted. In particular, the sharp null becomes less deep and the difference signal in the null becomes in-phase with the sum signal. To improve accuracy the equivalently placed radiator 4 on the other side of the active portion of the array is also switched off.

Consider as an illustrative example, columns numbered 1 to 32 being active as shown in Figure 3. A sum beam 66 and difference beam 63 results.

If column 8 as shown in Figure 3 fails, by switching off its twin, column number 25, the distortion of beams relative to the desired look direction is reduced. This is because total signals received by respective halves of the active array segment are rebalanced in magnitude. As before, the difference null is deepened and it becomes 900 out of phase with the sum signal.

Testing of columns and T/R modules is undertaken automatically as described earlier, to decide whether or not components have failed, as well as for calibration purposes.

WIodular Structure The phased array antenna system is modular and sets of 32 radiating columns can be added or subtracted to change the size of the antenna. Each of the thirty-two new cloumns 2 is then connected to a corresponding one of the thirty two T/R modules 6.

This aspect is of particular value where the antenna is to be fitted to an existing tower the dimensions of which are fixed.

The following table shows the dimensions of antennas with different numbers of columns: Columns Outside Diameter Inside Diameter (tower diairter) 128 18 ft 16 ft 160 22.5 ft 20.5 ft 192 27 ft 25 ft For each size of antenna the number of active electronic devices is the same, with 32 T/R modules used in each case.

Claims (15)

CLAIMS:

1. A radar antenna comprising an array of antenna elements, means for selecting different sets of elements, means for transmitting signals from a selected set of elements and means for receiving replies on a selected set of elements, each element comprising a coupler for sampling a transmission test signal or injecting a reception test signal, the system further comprising transmission test means comprising a transmission test detector which is selectively connectable to at least one coupler of the elements to detect transmission test signals, and reception test means comprising a test signal generator which is selectively connectable to at least one coupler of the elements to inject reception test signals to be received.

2. A radar antenna according to claim 1, in which the transmission test detector detects signals from each of a number of selected elements in turn and the test signal generator provides signals to be received to a plurality of couplers substantially simultaneously.

3. A radar antenna according to claim 1 or claim 2, in which the transmission test means comprises means to calculate transmission correction factors and servo-control means to adjust signals for transmission in dependence on the values of the transmission correction factors.

4. A radar antenna according to any preceding claim in which the reception test means comprises means to calculate reception correction factors, and means to adjust received signal values in dependence on the values of the reception correction factors.

5. A radar antenna according to claim 3 or claim 4 in which if a correction factor is outside of a predetermined range for a plurality of consecutive tests, a flag signal indicates a fault.

6. A radar antenna according to any preceding claim in which transmission and reception tests are made prior to each operational transmission of the radar.

7. A radar antenna according to any of claims 1 to 5, in which transmission and reception tests are undertaken regularly.

8. A radar antenna according to any of claims 1 to 7, in wnich the different sets of array elements have different look directions.

9. A radar antenna comprising a circular array of antenna elements radiators mounted on a tower, and electronic components of which components carrying digital signals are located at the bottom of the tower.

10. A method of alleviating received signal distortions in a radar antenna system comprising an array of antenna elements including an array segment of active elements, comprising the steps of (i) testing reception by active elements, and if a first element does not receive in a predetermined way, (ii) locating a second element substantially equidistant from the middle of the active segment and in juxtaposition to the first element, (iii) not using received signals from the first and second elements until a new array segment is selected.

11. A radar antenna comprising an array of antenna elements, means for selecting different sets of elements, the different sets having different look directions, means for transmitting signals from a selected set of elements and means for receiving replies on a selected set of elements, having means for alleviation of received signal distortions according to the method of claim 10.

12. A radar antenna according to claim 11, in which reception by radiators is tested, each radiator comprising a coupler for sampling a reception test signal, the system further comprising a reception test signal source which is connectable to the coupler of at least one element.

13. A method of construction of a radar antenna comprising a circular array of antenna elements mounted on a tower in which n banks of N antenna elements are selected, the number of banks n being selected in dependence on the lateral size of the tower, and the banks are connected together on the tower to form the array.

14. A method according to claim 13 in which each element of a bank is connectable to one of N transmission/reception feed circuits.

15. A method according to claim 13 or 14, in which N is 32 and n is an integer between 3 and 5 inclusive.