WO1986007467A1

WO1986007467A1 - Multibeam surveillance radar

Info

Publication number: WO1986007467A1
Application number: PCT/GB1986/000347
Authority: WO
Inventors: David Louis Motkin
Original assignee: Plessey Overseas Limited
Priority date: 1985-06-13
Filing date: 1986-06-13
Publication date: 1986-12-18
Also published as: GB8614397D0; GB2177566A; JPS63501443A; EP0229806A1

Abstract

A radar system comprises a transmitter in which each transmitted pulse is divided into a number of sub-pulses; two in the preferred embodiment. The sub-pulses are arranged to have different frequencies (f1, f2) and the azimuth/frequency sensitivity of the horizontal elements of the system antenna causes the sub-pulses to be radiated at different azimuths (22, 24). Thus, each transmitted pulse illuminates simultaneously a number of azimuths corresponding to the number of sub-pulses. In this manner, an increase in the number of pulses directed towards a potential target in each antenna rotation from more than one azimuth direction and within the same pulse repetition interval may be achieved.

Description

MULTIBEAM SURVEILLANCE RADAR

The present invention relates to improvements in or relating to radar systems.

A conventional surveillance radar system transmits a sequence of pulses of radiation each of which is directed towards a particular azimuth direction depending upon the instantaneous position of a mechanically rotating directional antenna. After each of these transmission pulses has been transmitted the radar seeks to receive the pulse radiation reflected from targets which may be at any distance up to a designed maximum range from the radar. The time required for this reception period depends upon the maximum range and is approximately 1 ms for each 150 km.

After this time has elapsed, the next transmission pulse may be transmitted. The time interval between consecutive pulses is termed "the pulse repetition interval". The pulse repetition interval places an upper limit on the rate at which pulses may be transmitted, and, due to the narrow rotating beam, it limits the number of pulses which may be transmitted towards a given potential target in each antenna rotation period. The pulse repetition rate also places a limit on the maximum angular rate at which the antenna may be turned whilst transmitting at least one pulse towards each potential target on every rotation. The present invention seeks to provide a radar system having azimuth diversity whereby an increase in the number of pulses directed towards a potential target in each antenna rotation from more than one azimuth direction within the same pulse repetition interval may be achieved.

Accordingly, there is provided a radar system comprising a transmitter for generating transmission pulses for enabling the generation of radiated beams by an antenna, a signal pulse comprising a number of sub pulses, at least one sub pulse being arranged at a centre frequency which differs from the centre frequency of at least one other .sub pulse for enabling more than one azimuth direction to be illuminated by the radiated beams.

Preferably, each sub pulse is arranged at a centre frequency which differs from the centre frequency of every other sub pulse for enabling a number of azimuth directions corresponding to the number of sub pulse to be illuminated by the radiated beams.

In a preferred embodiment of the invention each transmission pulse comprises two sub pulses.

The transmitter may be arranged to generate two transmission pulses per azimuth beamwidth of the system, each transmission pulse comprises two sub pulses, whereby two groups of two sub pulses are transmitted per azimuth beamwidth and wherein the centre frequencies of the sub pulses are arranged such that the radiated beams are evenly spaced by approximately one quarter of the system azimuth beamwidth.

The present invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 illustrates a general schematic diagram of an azimuth diversity radar system in accordance with the present invention; and.

Figure 2 illustrates a schematic diagram showing how transmitted pulse groups may be arranged in the system illustrated in Figure 1 to provide separated transmitted beams within the azimuth beamwidth of the radar system.

Referring to Figure 1, a radar system 2 comprises a transmitter 4 coupled to one or more rotatable antennas 6, only one of which is shown for clarity. The antenna 6 is used for the transmission of a radiated beam and exhibits the property that the azimuthal direction of the beam of radiation formed by the antenna is dependent upon the frequency of the excitation pulses afforded by the transmitter 4. A frequency selector 8 is coupled to the transmitter 4 for enabling the frequency of the output of the transmitter to be selected according to requirements. A receiver 10 is provided, coupled to the antenna 6. The receiver 10 comprises receive and detection circuits 12, 14 and azimuth determination circuits 16, 18; which are also coupled to the frequency selector 8. It is pointed out that the antenna 6 may be used for transmission and reception, the azimuthal directions of the transmit and receive beams being coincident for any frequency and also dependent on that frequency. An example of such an antenna is one consisting of one or more substantially horizontally mounted slotted linear waveguides, fed from one end. In such antennas the transmission path delay within the waveguide imparts the necessary frequency sensitivity to the beam position in azimuth.

The receiver 10 is coupled to a plot extractor 20, the function of which will be described later.

The transmitter 4, in conjunction with the frequency selector 8 is arranged to be capable of transmitting more than one pulse on different frequencies in rapid succession.

The transmitter 4 may be easily implemented using a driven amplifier such as a travelling wave tube, a klyston, a cross-field amplifier or a solid state amplifier. - 5 -

Alternatively, several magnetrons or a very rapid turning magnetron, or any other suitable means, may be employed.

In the following description the term "sub-pulse" is used to describe a pulse transmitted with a particular centre frequency. Where necessary for signal processing purposes, sub-pulses may carry within pulse modulation. The term "pulse group" describes several sub-pulses which, when transmitted, are separated in time by only a small fraction of a pulse repetition interval. Each sub-pulse within a pulse group is transmitted with a different centre frequency. Pulse groups are separated in time by one pulse repetition interval.

In operation, the transmitter 4 generates the transmission pulses of a conventional radar system in the form of groups of sub-pulses, such as the sub-pulses f^ and ±2 shown in Figure 1. The frequencies f^ and f2 are chosen so that the individual radiated beams 22, 24 are conveniently separated in azimuth. Each sub pulse i_ , f₂ excites the antenna 6 to generate one beam. As the sub pulses are at different centre frequencies, the sub pulses cause the radar system 2 to illuminate a number of azimuth directions corresponding to the number of sub pulses having individual centre frequencies; two in the system shown in Figure 1. The azimuth spacing between the contemporaneously illuminating beams 22, 24 may be greater than or less than one azimuth beamwidth and will depend upon the frequency spacing between the sub-pulses. The frequency spacing may be set in the frequency selector 8 to be appropriate to the operational requirement of the radar system 2. For example, a typical slotted waveguide linear array operating near to 3 GHz requires a frequency change of approximatly 50 MHz to move the directional beam radiated by the aerial through one beamwidth in azimuth. During the pulse repetition interval echo signals may be received from targets located in any of the illuminating beams 22, 24. The signals are received by the receiving antenna (which may be identical with the transmitting antenna) because their azimuth of arrival and their frequencies correspond. The signals then pass through the receiver 10 and signal processing system where the identities of signals resulting from the individual sub-pulses are preserved.

To operate with azimuth diversity the receiver 10 is capable of simultaneously receiving sub-pulses on different frequencies and processing these so that the results of detection may be separately ascribed to the different frequencies f^ and f₂. and hence to different azimuths. In the embodiment of Figure 1 this is achieved by the receive and detection circuits 12, 14 and the azimuth determination circuits 16, 18. More than one receiver and signal processing system may be required to preserve the identity of the signals, however other means such as coding the signals may also be possible. After detection, from a knowledge of the frequency of the sub-pulse and of the squint sensitivity of the antenna 6, an azimuth can be ascribed to each target echo. When more than one echo is detected from any target they can be combined in a plot extractor 20 to provide a plot output and hence a better estimate of target position.

Figure 2 illustrates how a two beam azimuth diversity system may be used to double the number of target illuminations per beamwidth and to spread these evenly in azimuth. The pulse group contains two sub-pulses of frequencies fl and f2. In Figure 2 the pulse repetition interval is shown as one half azimuth beamwidth and so two pulse groups are transmitted per beamwidth. The separation between the frequencies fl and f2 is chosen so that the resulting two azimuth beams are separated by one quarter azimuth beamwidth and thus four evenly spaced sub-pulse beams are radiated in every beamwidth. It will be noted that the frequencies of the sub-pulses f^ and f₂, as viewed from a position fixed in space, appear alternately and so the detection advantages of frequency diversity may be claimed. In the example illustrated in Figure 2 the sub-pulse beams are separated by one quarter of the azimuth beamwidth. However, the same effect may be achieved with separation of any odd multiple of one quarter beamwidth. The optimum separation depends upon the number of pulse groups per beamwidth and may also be influenced by other requirements such as stagger within multi target indicator pulse bursts.

It will be realised from the foregoing disclosure that a radar system according to the present invention has several advantages over known systems.

For example, with any particular combination of antenna rotation rate and pulse repetition interval, a more even distribution of pulse energy in azimuth may be achieved, thus facilitating target detection and position estimation.

Moreover, for any particular value of pulse repetition interval, the maximum antenna rotation rate which may be used whilst illuminating each potential target direction with at least one pulse within each antenna rotation may be increased.

Furthermore, because an azimuth diversity radar transmits pulses on more than one frequency during each pulse repetition interval, then the separation of the azimuth directions towards which the pulses are transmitted can be chosen so that, where more than one pulse illuminates a potential target within each antenna rotation period, the illumination of the target will be by pulses of more than one frequency. It will be appreciated, therefore, that the detection sensitivity and advantages normally ascribed to "Frequency Diversity" can be realised.

Although the present invention has been described with respect to a particular embodiment, modifications may be effected whilst remaining within the scope of the invention.

For example, the radar^' system is not restricted to any particular number of sub-pulses within each pulse group, neither is it limited to any particular azimuthal separation between the beams so generated. In the case of radars operating with frequency agility it is permissible for the frequencies of the sub-pulses to have no fixed relationship to each other and to be chosen from all available frequencies within the radar bandwidth using whatever selection algorithm or mechanism as would normally be used in a conventional frequency agile radar. Such an algorithm could be stored in any suitable memory device associated with the frequency selector 8 which would cause the transmitter 4 to generate sub-pulses as determined by the algorithm. The azimuth determination circuits 16, 18 would also respond to the algorithm to enable the azimuth associated with any sub-pulse to be determined.

Additionally, the principles of the present invention may be applied to either two dimensional radars (i.e. those which do not measure target height) including air and surface search radars, or to three dimensional radars; azimuth diversity being of particular value in three dimensional radars because the number of pulse repetition intervals available per azimuth beamwidth at each elevation in such radars is often limited due to the time required to examine several different elevations.

Claims

CLAIMS ;

1. A radar system comprising a transmitter for generating transmission pulses for enabling the generation of radiated beams by an antenna, a signal pulse comprising a number of sub pulses, at least one sub pulse being arranged at a centre frequency which differs from the centre frequency of at least one other sub pulse for enabling more than one azimuth direction to be illuminated by the radiated beams.

2. A radar system according to claim 1 wherein each sub pulse is arranged at a centre frequency which differs from the centre frequency of every other sub pulses for enabling a number of azimuth directions corresponding to the number of sub pulses to be illuminated by the radiated beams.

3. A radar system according to claim 1 or claim 2 wherein a transmission pulse comprises two sub pulses.

4. A radar system according to any one of claims 1 to 3 wherein the sub pulses are arranged to contain information indicative of the azimuth directions illuminated by the radiated beams.

5. A radar system according to any one of claims 1 to 4 comprising a frequency agile radar system wherein the centre frequencies of the sub pulses are arranged to be chosen from the available frequencies within the bandwidth of the radar system in accordance with a selection alogrithm.

6. A radar system according to claim 3 wherein the transmitter is arranged to generate two transmission pulses per azimuth beamwidth of the system, each transmission pulse comprising two sub pulses, whereby two groups of two sub pulses are transmitted per azimuth beamwidth and wherein the centre frequencies of the sub pulses are arranged such that the radiated beams are evenly spaced by approximately one quarter of the system azimuth beamwidth.

7. A radar system according to any one of the preeding claims comprising a receiver for receiving a target echo from any target illuminated by a radiated beam.

8. A radar system according to claim 7 wherein the receiver comprises signal processing means for identifying the azimuth associated with any received target echo.

9. A radar system according to any one of the preceding claims comprising a three dimensional radar system.