GB2289798A - Improvements relating to radar antenna systems - Google Patents

Abstract

In a circular phased array system which is mounted on a tower, the array is made up of modular banks of elements 4. Accordingly, various sizes of antenna are possible dependent on the number of banks used. As described the invention is applied to a secondary surveillance radar having 32 transmit/receive units 6 each connected to one element of each module. <IMAGE>

Description

IMPROVEMENTS RELATING TO RADAR ANTENNA sYsTEzE 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 221947 it 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 cortbinations 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 performancs, 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 circQitrrto array elements. Also, there are problems in transmitting a large ncrmber 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 imorove the accuracy of such a system simply Yet another problem is that towers on which circular pnased 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 oooprises 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 via a selected set of elements, each element comprising a coupler for sampling a transmission test signal or iniecring a reception testWsignal, 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 earns 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 dnes not receive in a predetermined way, (ii) locating a second element substantially equidistant fran, and on the other side of, the middle of the active segment.

(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 LA is a schematic diagram of the circularly arrayed radiating columns of the antenna, Figure 1B is a schematic diagram of more of the antenna system, Figure 2 is a schematic diagram of a tranmission/ reception (T/R) module anal associated circuitry, and Figure 3 is an example sketch graph of amplitudes of beams from an antenna array secment sòf thirty two radiating columns.

As shaman in Figures lA and 1B, a preferred antenna system for SSR consists of an antenna 2 having a set of circularly- arrayed 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. Thirsty two columns 4 are active at any one time for transmission and reception; for example to provide beams in a first aziituathal 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 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 suttired 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 more 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 MH drive signal to each of the thirty-two T/R modules 6. The driver module 12 also provides an undulated 103CSHz 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-tBo T/R modules 6).

The T/R module 6 has a received- signal branch which includes a band pass filter 36 and law noise amplifier 38 connecting the TIR switch 32 to a miser 40. The received signal which is initially 1090 b is demodlated in the mixer 40 down to 60 }iaz, because the mixer 40 received the 1030 MHz reference signal, via an input port 41, fran the second splitter module 15. this derDdulated signal is passed via an STC variable attenuator (not shown) and irtesmediate frequency (IF) amplifier (not shown) to further mixers (not shown) and a to d converters (not shown). Two digitaltoutput 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 fflxamDle. 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 trrrway 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 save calibration process is repeated for each of the thirty-twc 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 of the modules 6. 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 correctiom 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 sarwhat 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 bE is produced in the test pulse generator 20, which comprise a surface acoustic wave (SAR) 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 thirsty 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. Adjusllrencs~recessary to correct for drift in amplitude and phase characteristics of the receive path are calculated and applied in subset SSR interrogation. The new calibration data is stored and used in the digital ber former 10 for subsequent reception calibration.

Like tr nsmission 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 signaled if the calculated correction factor falls outside this wireow 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 16MHz) over long distances whilst maintaining synchronism between signals.

The RF ccmponents comprise the 1030 MHz 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 . le, shown in Figure 2, is split at the output of the pre-ampli~ier 62 following the mixer 40. The IF signal from each module is transmitted via :a? IF coaxial cables to the bottom of the tuner kere 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.

Comsensation for Effects of Failed Camoonents 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 deterriing the direction of signals received from a target. The plot extractor determines the aziml 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 itasiement 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 niill 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-phace with the sum signal To Thiprove 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 ngghered 1 to 32 being active as shown in Figure 3. A sum beam 68 and difference beam 66 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 rabalanced 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.

Modular 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 thir-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 diameter) 128 18 ft 16 ft 160 22.5 ft 20.5 ft 192 27 ft 25 ft For each size of antenna the flutner of active electronic devices is the same, with 32 T/R modules used in each case.

Claims (3)

CLAIMS:

1. 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.

2. A method according to Claim 1 in which each element of a bank is connectable to one of N transmission/reception feed circuits.

3. A method according to Claim 1 or 2, in which N is 32 and n is an integer between 3 and 5 inclusive.