GB2045567A - A radar system for determining azimuth and elevation - Google Patents

Abstract

A radar system is designed so that the beamshape varies with elevation. For example, it may be wider at high elevations than at low elevations. As the system scans in azimuth, a target of relatively low elevation will be scanned by a part of the beam having a relatively narrow beam width and at a high elevation will be scanned by a part of the beam having a relatively broad beamwidth. In an amplitude- comparison monopulse radar, circuitry is included for distinguishing between signals occurring from different beamwidths thereby giving an indication of the elevation of the target from which the signals were derived. Systems are already known which provide similar information by producing two or more beams of different elevation angles and comparing the signals received by the different beams. Such systems are however relatively complicated and expensive. <IMAGE>

Description

SPECIFICATION A radar system This invention relates to a radar system which can determine both the azimuth and elevation of an object under observation.

Radar systems are known which scan in azimuth to find the azimuth bearing of an object and which have two or more beams of different elevation angles. The height of the object is determined by comparing the power received in these different beams.

This invention enables a radar system to determine the elevation as well as the azimuth of an object without the need to provide different beams spaced in elevation.

The invention provides radar apparatus adapted to scan through different values of a first directional co-ordinate so that the value of a received signal derived from a fixed object varies as a function of time, the radar being designed such that the said function is different for objects at different values of a second directional co-ordinate, and the radar including means for distinguishing between different signals which vary according to the respective different functions thereby identifying the elevation of the target.

In this way, by using a beam which has different characteristics (e.g. a different beamwidth) for different elevations, it is possible for the apparatus to determine the elevation of a target without the need for two separate beams spaced in elevation.

Normally, the aforementioned first co-ordinate will be azimuth angle and the second coordinate will be elevation angle. This is not, however, essential and any other two coordinates which combine to define the direction of the object can be used.

The "means for distinguishing" preferably includes means for differentiating the signals with respect to time. In this case the said functions must be such that the result of differentiation produces a signal whose length is indicative of the beamwidth and/or whose height is indicative of the beam shape.

In accordance with another aspect of the invention there is provided a radar apparatus adapted to scan an area under observation in a first directional co-ordinate and whose beam shape is different for different values of a second directional co-ordinate so that a signal derived from an object being observed is characteristic of the second co-ordinate of that object, the apparatus including means for recognising the said signal thereby identifying the second co-ordinate of the object.

A particular embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a very schematic block diagram showing a monopulse radar system constructed in accordance with the invention; Figure 2 is a sketch showing the variation with time of the output of the sum, circuit during a radar scan of an object at a relatively low elevation; Figure 3 is a similar to Fig. 2 but for an object at a relatively high elevation; Figure 4 shows the 2nd derivative with respect to time, of the value of E for the low elevation object of Fig. 2; and Figure 5 shows the 2nd derivative with respect to time of E for the high elevation object of Fig. 3; Referring firstly to Fig. 1 there is shown a block diagram of an amplitude-comparisonmonopulse radar.This system includes an antenna (shown only schematically) which rotates in azimuth. The rotation may either involve a mechanical rotation or be effected electronically, as in the case of a static phased array radar. The antenna and/or its associated electronics is designed so that its beamwidth varies (continuously or in steps) with elevation so that the beamwidth is broader at higher elevations than at lower elevations.

The antenna has two adjacent feeds which are illustrated schematically in Fig. 1 by two adjacent dipoles 1 and 2. These are connected to arms 3 and 4 respectively of a hybrid junction 5. The other two arms 6 and 7 of this hybrid junction are connected respectively to sum and difference channels indicated generally by reference numerals 8 and 9. The sum channel comprises a duplexer 10, a mixer 11, an intermediate frequency amplifier 1 2 and an amplitude detector 1 3. The duplexer 10 is also connected to a transmitter 14. The difference channel 9 comprises a mixer 1 5 and an intermediate frequency amplifier 1 6. The two mixers 11 and 1 5 receive signals from a common local oscillator 1 7.

The outputs of amplifiers 1 2 and 1 6 are fed to a phase sensitive detector 18, the output of which is fed to a sum circuit 1 9 which also receives signals from a sweep generator 20.

The output E of the sum circuit 1 9 gives an output which varies as shown in Fig. 2. It indicates the azimuth angle of a target under observation and is fed to a suitable display.

This display also receives range information from the amplitude detector 1 3.

Referring now to Fig. 2 it will be seen that the signal E comprises pulses whose amplitudes first increase in accordance with a curve E = a (t - t2)2 where t is time and a, is a constant dictated by the particular elevation.

When time t3 is reached the pulses start decreasing in amplitude in accordance with the curve E = a (t - t4)2.

The signal E is also fed to a differentiator 21 which twice differentiates the signal E with respect to time and squares the resulting differential so as to produce an output signal (Ë)2: this being shown in Fig. 4.

Because the rises and falls in amplitude are in accordance with a square law: (E)2 is constant during the time interval t2 to t4. (E)2 is therefore a substantially rectangular pulse as shown in Fig. 4.

Figs. 2 and 4 relate only to signals received from an output at a given elevation angle. At a higher elevation angle the signals E vary as shown in Fig. 3.

In Fig. 3 the envelope of the signals E rise in amplitude between time tl and t3 in accordance with a curve E = a2 (t - t1)2 and fall in amplitude between times t3 and t4 in accordance with the curve E = a2 (t - t5)2. The value a2 is a constant dictated by the particular elevation angle.

The pulses shown in Fig. 3 result in an output (E)2 from the differentiator as shown in Fig. 5. This output consists of a rectangular pulse of duration t5 - t and height 4a22. This is higher and of longer duration than the pulse shown in Fig. 4 because the target is at a higher elevation.

The height 4a is less for lower elevation angles and greater for higher elevation angles.

The time t3 remains constant for all elevation angles. It depends, of course, on the azimuth angle of the target.

The signal (E)2 is fed to a pulse height discriminating circuit (not shown) and the output of the latter, which represents elevation angle, is supplied to the display system.

In the particular embodiment of the invention which has been described, although the beamwidth becomes broader at higher elevations, the value of E always varies in accordance with a square law so that (E)2 is a rectangular wave. In alternative embodiments it would be possible to construct the systems so that E varies in accordance with the nth power (where n is an integer higher than two).

The differentiator must then be designed to produce an output signal proportional to the nth differential of E with respect to time.

It should also be observed that the time t, becomes earlier for higher elevation angles and t5 becoms later for higher elevation angles. The time t2 remaining constant for all elevation angles. It depends, of course, on the azimuth angle of the target. Thus, in an alternative form of the invention, if sufficient resolution were available, the signal (E)2 could be fed to a pulse length discriminating circuit, instead of a pulse height discriminating circuit.

Claims (7)

1. Radar apparatus adapted to scan through different values of a first directional co-ordinate so that the value of a received signal derived from a fixed object varies as a function of time, the radar being designed such that the said function is different for objects at different values of a second directional co-ordinate, and the radar including means for distinguishing between different signals which vary according to the respective different functions thereby identifying the elevation of the target.

2. Radar apparatus according to claim 1 in which the first co-ordinate is azimuth angle and the second co-ordinate is elevation angle.

3. Radar apparatus according to claim 1 or 2 whose beam shape is different for objects at different values of the said second coordinate and in which the said means for distinguishing is operative to distinguish between different beam shapes.

4. Radar apparatus according to claim 3 in which the means for distinguishing includes means for differentiating the signals derived from an object at a particular elevation and in which the said functions are such that this differentiation produces a differential signal whose height and/or duration is characteristic of the beamshape at the said particular elevation.

5. Monopulse radar apparatus according to any preceding claim in which the said received signal is the difference between sum and difference signals.

6. Radar apparatus adapted to scan an area under observation in a first directional coordinate and whose beamshape is different for different values of a second directional coordinate so that a signal derived from an object being observed is characteristic of the second co-ordinate of that object, the apparatus including means for recognising the said signal thereby identifying the second co-ordinate of the object.

7. Radar apparatus substantially described with reference to the accompanying drawings and substantially as illustrated therein.