GB2110035A

GB2110035A - Monopulse radar system

Info

Publication number: GB2110035A
Application number: GB08232456A
Authority: GB
Inventors: Dr Frank Robert Conner
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-11-20
Filing date: 1982-11-12
Publication date: 1983-06-08
Also published as: GB2110035B

Abstract

This invention is a description of a tracking radar system which uses a frequency-comparison monopulse technique by means of sum and difference frequencies. A Cassegrain antenna is employed and it consists of a parabolic reflector with a five-horn feed A, B, C, D and E. Two different carrier frequencies (fc + f1) and (fc - f2) are transmitted from transmitter Tx via a duplexer 6 and after reflection from a distant target, the signals received are separated by sideband filters 4, 5, 7 and 8. Each FM receiver 12 or 15 extracts a difference frequency signal (f1 - f2) which yields target information in azimuth and elevation respectively, and an AM receiver 14 extracts a sum frequency signal (f1 + f2) for providing AGC bias and an output for range measurement. Tracking errors due to thermal noise and target glint are reduced by employing a wideband FM technique and a ratio detector (Fig. 3, not shown) in each FM receiver. Target tracking in azimuth, elevation and range is achieved by using closed- loop control systems in two orthogonal planes. <IMAGE>

Description

SPECIFICATION Monopulse radar system I, Dr. Frank Robert Connor, a British subject resident at Flat 3, 10 Avenue Road, London SE25 4EA, do hereby declare this invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:- In tracking radar systems, the amplitudecomparison monopulse technique is generally preferred to the phase-comparison monopulse technique, because of its simpler implementation and better performance. However, the amplitude-comparison monopulse technique is not without its limitations amongst which are radar dependent errors due to thermal noise and target dependent errors due to glint.

In this design which is called a frequency comparison monopulse radar system, errors due to receiver thermal noise and target characteristics are reduced by using sum and difference frequencies and a wideband frequency modulation technique. If a larger frequency deviation of + 100 kHz or more is employed with an FM receiver, amplitude and phase fluctuations due to thermal noise and target glint are greatly reduced. This improves the signal-to-noise ratio of the control system without reducing its bandwidth and so increases boresight precision.

Rapid amplitude changes due to thermal noise are removed by using a ratio detector of novel design which provides its own limiting action as in a conventional circuit. Random target scattering gives rise to target glint which is removed by using two carrier frequencies fairly close to one another but sufficiently far apart to allow for adequate filter separation. The random fluctuations of the two signals are highly correlated and so their amplitude changes tend to cancel out in the differential circuit of the ratio detector or are removed by its limiting action. The remaining phase fluctuations due to thermal noise give rise to some FM which is small compared to the large frequency deviation of + 100 kHz.

The system described here uses a parabolic reflector 1 and a Cassegrain feed. The fivehorn feed 2 shown in Fig. 1 is arranged in the form of a cross with receiving horns A and B providing azimuth control and receiving horns D and E providing elevation control. The centre horn C is used for transmitting and receiving signals and it illumunates the distant target uniformly during transmission. The polar patterns 3 of horns A, B and C are also shown in Fig. 1 and the polar patterns of horns D and E are similar to those of horns A and B respectively. The feed horns 2 are of square cross section for use with circular polarisation.

In Fig. 2, the centre horn C is energised simultaneously with two pulsed signals at frequencies (fc + f,) and (f0 - f2) where fc is a microwave carrier frequency and f1, f2 are two different offset radio frequencies with f1 > f2.

The signals from transmitter 1 3 are fed to a hybrid junction 10 and then to a duplexer 6, prior to radiation from horn C. After reflection from a distant target, the reflected signals are received by pairs of horns A and B or D and E. Suitable sideband filters 4, 5, 7 and 8 are inserted after each receiving horn to ensure that horns A and D are receptive to frequency (fc + f,) only, while horns B and E are receptive to frequency (fc - f2) only. During reception, centre horn C is receptive to signals at both frequencies (fc + f,) and (fc - f2). The signals received by horns A and B or D and E are fed into individual hybrid junctions 9 and 11 which are connected to separate FM receivers 1 2 and 15, one for the azimuth channel and one for the elevation channel respectively.

Each FM receiver consists of a low-noise preamplifier, a downconverter, an IF amplifier with an IF of 10.7 MHz and a ratio detector of novel design. The frequency f, = 10.7 MHz+ 100 kHz= 10.8 MHz and the fre quencyf2= 10.7MHz- 100kHz = 10.6 MHz.

The ratio detector used in each FM receiver is shown in Fig. 3. It consists of two input filters 1 9 and 20 which are tuned to frequencies f, and f2 respectively. The filters are coupled to diodes 21 and 22 which are placed in opposite directions with suitable RC loads as in a conventional diode detector. The large capacitor 27 across the equal resistors 25 and 26 provides the limiting action of the circuit. The input filters respond to amplitude changes which are due to target movement across the boresight axis. These changes are sensed by the equal capacitors 23 and 24 and appear as an output voltage V0 = (V2 - V1)/2 across the output load resistor 28.

If the input filters of the ratio detector are tuned to frequencies f, = 10.8 MHz and f2 = 10.6 MHz respectively, the net output of each FM receiver consists of positive or negative error signals 1 6 and 18 as shown in Fig.

2. The error signals are used to control the servo loops in the azimuth and elevation channels in order to align the centre horn C along the boresight axis. Thus, each FM receiver yields an output which is related to the frequency difference (f1 - f2) for each channel.

In order to operate the receivers independently of target range or signal fading effects, the signals received by centre horn C are fed into an AM receiver 14. This receiver is conventional in design but employs two diode detectors which are tuned to frequencies f, and f2 respectively and they produce a combined output which is related to the sum frequency of (f, + f2). This output 1 7 is used to provide suitable AGC bias for all three receivers and an output signal for range measurements. Hence, if the operating level of the receivers is set for the - 3 dB crossover point of the polar patterns of the receiving horns (when horn C is aligned along boresight), the error signals 1 6 and 1 8 are virtually zero except for some very low level FM noise which is present within the noise bandwidth of the servo loops. Thus, the full benefits of a signal-to-noise ratio improvement attainable in a ivideband FM system can be obtained for a noise bandwidth equal to the servo loop bandwidth employed in each channel.

Claims

1. A radar system wherein means are provided for transmitting two carrier signals simultaneously or for receiving two carrier signals simultaneously.

2. A radar system as claimed in claim 1 wherein the two transmitted signal frequencies are one above and one below a given carrier frequency.

3. A radar system as claimed in claims 1 and 2 wherein the two transmitted frequencies differ in value by a suitable radio frequency.

4. A radar system as claimed in claim 1 wherein a monopulse technique is employed by means of sum and difference frequencies.

5. A radar system as claimed in claim 4 wherein means are provided for using the difference frequency signal for tracking a distant target by means of a closed-loop control system.

6. A radar system as claimed in claims 4 and 5 wherein means are provided for using the sum signal to remove amplitude or frequenct changes in the difference frequency signal and for range measurement.

6. A radar system as claimed in claims 4 and 5 wherein means are provided for using the sum signal to remove amplitude or frequency charges in the difference frequency signal and for range measurement.

7. A radar system as claimed in claims 5 and 6 wherein means are provided for extracting target information in elevation, azimuth and range.

8. A radar system as claimed in claims 5 and 6 wherein means are provided for reducing tracking errors due to thermal noise and target glint by using a wideband FM technique and a ratio detector of novel design.

9. A radar system as claimed in any of the previous claims and substantially described with reference to the drawings accompanying this application.

CLAIMS (14 Feb 1983)