GB2281671A - Monopulse receiver - Google Patents

Abstract

In a monopulse radar system, the azimuth angle from a look direction to a target, is found by receiving the target's reply as signals both at 0 DEG elevation above ground, and from a higher elevation. At 0 DEG elevation, both sum SIGMA 1and difference DELTA 1 signals derived; at the higher elevation, a difference DELTA 2 signal is determined. The signals are used to determine azimuth angle, corrected for non-zero target elevation: the ratio DELTA 1/ DELTA 2 of the difference signals is sensitive to target elevation; this is used in combination with the ratio SIGMA 1/ DELTA 1 of signals at 0 DEG elevation to determine (by means of a look-up table or comparable calculation) the target's azimuth correctly, independent of its elevation. <IMAGE>

Description

IMPROVEMENTS RELATING TO RADAR ANTENNA SYSTEMS The present invention relates to a radar antenna system, especially but not exclusively of the phased array type, for use in a monopulse secondary surveillance radar (SSR) system for aircraft detection. Although the invention is described in a specific embodiment of a cylindrical phased-array system, it is applicable to other types of radar antenna, such as conformal and non-planar antennas. The reader is referred to United Kingdom patent GB2219471B and United Kingdom patent application GB2259778A as background.

A conventional radar antenna system suffers beam broadening with elevation angle. The reason for this broadening is that the radar assumes that bearing measurements are made in the horizontal plane even when the incident signal arrives from a significant elevation angle. The effect is to make the aircraft appear at a lesser angle from the antenna's look direction than it actually is.

To illustrate the point, consider an antenna pointing North with an aircraft overhead but slightly to the East. The antenna may detect the aircraft 1" from its look direction, and the radar would determine the azimuth as 10 from North, whereas its true direction is 90" from North when projected to the horizontal plane. In effect, antenna beam broadening for elevation angles up to about 500 may be approximated as a function of the inverse of the cosine of the elevation angle.

For a cylindrical antenna, there is a further source of beam broadening with elevation angle. The curvature of the antenna array introduces extra phase delays, in signals from a distant source to the outer antenna elements in an active set of antenna elements, compared to those at the centre of the active set.

This extra phase delay can be compensated for in the feed system of the antenna. However, a phase compensation which is optimised for signals arriving at zero elevation angle is incorrect, and excessive, for signals arriving from a high elevation angle.

In conventional radar antenna systems of the phased array and monopulse type, there is another way in which the problem of beam broadening is addressed. Measurements of aircraft azimuth are taken in various look directions on either side of the aircraft, and the measurements are averaged in the expectation that errors due to non-zero elevation will cancel. Since smaller azimuth angles are considered to be more accurate, weighted averages are sometimes taken. It is inherently difficult to seek to eliminate systemmatic errors in measurements merely by averaging.

The present invention provides a method of determining azimuth angle of a target relative to a predetermined azimuthal look direction of a monopulse radar antenna system, comprising the steps of transmitting a signal in the look direction, detecting a reply signal from the target and deriving from the reply signal, a received sum signal at predetermined elevation, a received first difference signal at the same elevation, and a received second difference signal at a different elevation, determining a first ratio of relative signal strength of the received sum signal to the received first difference signal, and a second ratio of relative signal strength of the received first difference signal to the received second difference signal, the second ratio being dependent upon target elevation above the predetermined elevation, determining azimuth angle from first and second ratios.

A preferred embodiment of the invention will now be described, by way of example, with reference to the drawings, in which: Figure 1 is a schemmatic diagram of a cylindrical antenna array; Figure 2 is a set of graphs showing relative power of the sum and difference signals as a function of azimuth angle, for various elevations above the horizontal plane; Figure 3 is a representation of a two dimensional look-up table for azimuth angles.

As shown in Figure 1, a preferred antenna system, of the monopulse type, consists of a cylindrical antenna 2 having a set of radiating/receiving antenna elements 4 disposed both circumferentially and vertically. One example look direction (boresight) 6 is shown in Figure 1, although, in use, targets are detected by scanning different radially-spaced look directions.

A sum signal and a difference signal are transmitted in the look direction at 0 elevation to the horizontal plane, as 8 shown by arrow 10 in Figure 1.

A transponder (not shown) mounted on a target 14 provides an appropriate monopulse response detected by the antenna as received sum and difference signals. The monopulse feed network (not shown) and switching matrix (not shown) of the antenna 2 are used to provide a received sum signal and two difference signals. By appropriate adjustment of the phases of signals received by various antenna elements, the first difference signal is calculated as if received from 0 elevation, and the second difference signal is calculated as if received from 50 elevation as shown by arrow 12 in Figure 1.

In the cylindrical antenna 2, when a beam is focussed in a look direction at zero elevation, phase shifts are applied to the signals received by antenna elements 4 positioned towards edges of an active sector of antenna elements 4. The phase shift applied to signals from a selected antenna element 4 is proportional to distance in the look direction from an antenna element in the centre of the active sector to the selected antenna element. Phase shifts required to focus at a positive elevation angle are less, by a factor of cosine elevation angle, than those required to focus at O elevation.

The antenna 2 uses the received sum and difference signals to determine the azimuth angle e of the target 14 from the look direction 6.

The ambiguity about which side of the look direction 6 the target 14 lies is resolved by measuring the phase between the received sum and first difference signals.

The beam shapes of the received signals are shown in Figure 2 for six different target elevation angles, 0" to 50 , in 10 steps.

Each graph shows relative signal strength (dB) against azimuth angle e from boresight, i.e. from the look direction. The sum characteristic is shown as a solid line, and the first difference characteristic ("low difference", 0 elevation) is shown as a partly dashed line. It can be seen in Figure 2 how the sum signal broadens with increasing elevation. Figure 2 also shows the second difference characteristic ("high difference"), shown as a partly dotted line. This is the received beam shape assuming the receiver is focussed at 50 elevation.

At significant elevation angles, the azimuth angle e of the target is not accurately determined only from the ratio R1 of signal strengths of the received sum signal to the received first difference signal; see for example how the sum and first difference signals cross at an azimuth angle 0 of 2" at zero elevation, but 7.5 at 50 elevation.

The second difference signal has a higher elevation angle than the first difference signal, so is less sensitive to beam broadening: the sum and second difference signals cross at an azimuth angle 0 of 3.5 at zero elevation, only reducing to 2.5 at 50 elevation.

The second difference signal enables a correction for elevation angle to be made in calculating azimuth 0.

Selecting 0 and 50 as the angles for the elevations of the received difference signals is near optimum, giving good overall performance in terms of beam coverage and sensitivity to target elevation.

Azimuth angle e is determined using a two dimensional look-up table of data stored in the processing electronics of the antenna system. The first dimension of the table is the ratio R1 of the received sum signal strength to the received first difference signal strength. Upon a target being detected, the ratio R1 value is a function of azimuth angle e and elevation of the target. (In prior art systems, the dependence of ratio R1 on target elevation is ignored.) The second dimension of the table is the ratio R2 of signal strengths of the received first difference signal to the received second difference signal. Ratio R2 is dependent upon, and particularly sensitive to, target elevation.

Figure 3 is a graphical representation of the two-dimensional look-up table, showing how azimuth angle e is determined from the ratios R1 and R2. For example, an R1 value of -15dB would indicate a target azimuth angle e of 0.4", if elevation angle were not known and was assumed to be 00. Measuring R2 allows the azimuth angle e to be corrected for non-zero elevation. In the example considered, where R1 is -15dB, an R2 value of -9dB would indicate that the target elevation is 50". In consequence, the azimuth angle to the target relative to the look direction is 1.2" (not 0.4").

Other values of ratios R1 and R2 indicate other elevation angles and azimuth angles.

Although a two dimensional look-up table has been described in a preferred embodiment, other methods of correction are applicable. The curves shown in Figure 2 are linear, at least in the range 0 to 40". Accordingly, in a further embodiment of the invention, a simple arithmetic function is used to provide a correction for elevation to an approximate value for azimuth angle found assuming 0 elevation.

The present invention has the advantage that errors due to non-zero elevation are compensated directly. This is particularly effective in a monopulse system where detecting a second difference signal at a non-zero elevation allows accurate determination of target azimuth angle e. The ratio R2 of the received difference signal strengths is particularly sensitive to elevation.

Claims (9)

1. A method of determining azimuth angle of a target relative to a predetermined azimuthal look direction of a monopulse radar antenna system, comprising the steps of transmitting a signal in the look direction, detecting a reply signal from the target and deriving from the reply signal, a received sum signal at predetermined elevation, a received first difference signal at the same elevation, and a received second difference signal at a different elevation, determining a first ratio of relative signal strength of the received sum signal to the received first difference signal, and a second ratio of relative signal strength of the received first difference signal to the received second difference signal, the second ratio being dependent upon target elevation above the predetermined elevation, determining azimuth angle from first and second ratios.

2. A method according to Claim 1, in which the two different elevations are substantially O from horizontal and substantially 50 from horizontal.

3. A method according to any preceding claim, in which azimuth angle is derived from a two-dimensional look-up table of first and second ratios.

4. A method according to any preceding claim, in which target elevation is determined from the second ratio.

5. A method according to Claim 4, in which an estimate of azimuth angle uncorrected for target elevation is determined from the first ratio, and the actual azimuth angle is derived by applying a simple arithmetic correction dependent upon target elevation.

6. A method according to any preceding claim, in which the radar antenna is a cylindrical antenna having a plurality of circumferentially-spaced look directions for determining azimuth angles of targets.

7. A monopulse radar antenna system operative to determine azimuth angle of a target relative to a predetermined azimuthal look direction, comprising transmission means to transmit a signal in the look direction, receiver means to detect a reply signal from the target and to derive a received sum signal at predetermined elevation, a received first difference signal at the same elevation, and a received second difference signal at a different elevation, means to determine a first ratio of signal strengths of the received sum signal to the received first difference signal, and a second ratio of signal strengths of the received first difference signal to the received second difference signal, the second ratio being dependent upon target elevation above the predetermined elevation, and means to determine azimuth angle from the first and second ratios.

8. A method of measuring azimuth angle of a target as hereinbefore described with reference to the drawings.

9. A monopulse radar antenna as hereinbefore described with reference to the drawings.