GB2346262A - Radar - Google Patents

Abstract

A method of positioning a radar beam by monitoring the direction 6 in which the beam is to be positioned, determining the position in which the radar reflector system must be positioned in order to achieve the beam direction in relation to the current radar reflector system position, determining first and second reflector system positions 8, 10 either side of the desired position which can be defined within the limits of accuracy of the reflector positioning system, and moving the reflector system between the first and second positions with the relative times spent in each position being determined by the desired reflector position in relation to the first and second positions whereby to obtain on average a reflector system in the desired position and hence a radar beam pointing in the desired direction. Either a main radar reflector or a sub-reflector of a Cassegrain reflector system may be moved between the two positions.

Description

This invention relates to a radar system.

Accuracy of radar guidance has to satisfy increasingly high requirements, particularly for system$ which are required to track oncoming missiles travelling at high velocity and to guide an"antimissile"also travelling at high velocity to the target.

It is well known to guide a radar beam onto a target by rotating the radar reflector dish in accordance with instructions issued to a servo control mechanism by a monitoring system responsive to the movement of the target in the radar beam. A problem with such arrangement is that the mechanical system for moving the radar dish necessarily moves relatively slowly and because of imperfections in the mechanical system such as back lash in the gearing it is not possible precisely to locate the reflector to the accuracy required.

In a Cassegrain system of reflection, a sub-reflector is provided for receiving microwave radiation from a feed system positioned centrally within the main radar reflector dish, and the radiation is reflected from the sub-reflector onto the main radar dish. This has various advantages, principally that since the feed system is mounted on the reflector dish, it is not necessarx to mount it at a position in front of the reflector dish, with consequent difficulties in engineering the system. With a Cassegrain reflector system, it is possible to adjust the position of the radar beam by moving the subreflector at high speed, of the order of kilohertz. and to position it at an accuracy of a fraction of a degree. Since the bandwidth of the missile guidance system which is guided by the radar system is much lower than that of the radar system, in theory the movement of the Cassegrain sub-reflector should not be apparent to the missile guidance system. However it has been found that because of the high accuracy required in positioning, the degree of accuracy with which the Cassegrain sub-reflector may be moved is not sufficient.

It is an object of the invention to overcome this problem, and in recognising that the speed of mechanical movement of a radar system may be much faster than the time constants of the missile guidance system being guided by the radar system, the present invention provides a method of positioning a radar beam by monitoring the direction in which the beam is to be positioned, determining the position in which the radar reflector system must be positioned in order to achieve the beam direction in relation to the current radar reflector system position, determining first and second reflector system positions either side of the desired position which can be defined within the limits of accuracy of the reflector positioning system, and moving the reflector system between the first and second positions with the relative times spent in each position being determined by the direction of the desired reflector position in relation to the first and second positions whereby to obtain on average a reflector system in the desired position and hence a radar beam pointing in the desired direction.

In a preferred embodiment a Cassegrain reflector system is used wherein the main reflector dish is positioned b means of a low cost mechanical servo mechanism in order to provide coarse beam positioning, and a light weight high speed mechanical servo mechanism is used for moving the Cassegrain sub-reflector in accordance with the invention in order to produce a finer control over the position of the beam.

A preferred embodiment of the invention will now be described with reference to the accompanying drawings wherein: Figure 1 is a schematic view for explanation of the system in accordance with the invention; and, Figure 2 is a block diagram schematic of the radar system in accordance with the invention.

Referring to figure 1 there is shown a radar system in accordance with the invention including a radar reflector 2 arranged to generate a radar beam for guiding a missile 4. The desired path which it is intended to guide the missile is indicated at 6 and the direction of this path can be determined very accurately by electronic techniques. Since it is not possible to control a mechanical servo mechanism for directing the radar dish to the accuracy required to accurately position the radar beam on path 6, it is necessary in accordance with the invention to select two positions 8, 10 either side of path 6 to which the radar reflector system can be moved with the requisite accuracy. For example the angle between positions 8 and 10 may be a small fraction of a degree and their positions are selected as positions which can be defined and achieved from the previous reflector position. As shown, the intended path 6 is much closer to position 8 than to position 10 and hence in accordance with the invention the radar reflector system oscillantes or dithers between the two positions 8,10 at a rate of some kilohertz, with the radar system staying in position 8 three times as long as it stays in position 10, so that on average the radar beam is pointing along the desired path. Since the bandwidth of the missile 4 is much less than the Radar bandwidth, the dithering of the radar beam is not apparent to the missile guidance system.

Referring now to figure 2 there is shown a radar system comprising a Cassegrain reflector system having a main parabolic radar dish 2, a feed system 20 mounted at the centre of the parabolic dish and a sub-reflector system 22 mounted at the focus of the dish.

The main reflector dish 2 is provided with a servo control system 24 for controlling the movement thereof and sub-reflector system 22 is provided with a high speed servo mechanism 26 for controlling the movement thereof. Servo mechanism 26 is arranged to move the Cassegrain sub-reflector at a rate of some hertz, and it is sufficiently accurate to position the beam reflected from the main dish to within a small fraction of a degree.

A processing system 30 is provided for analysing reflected signals received at the reflector dish in order to accurately track an object, be it a target or a missile to be guided to the target. System 30 is operative accurately to define a desired beam position for efficient tracking. Instructions are issued on lines 32,34 to servo mechanisms 24 and 26. If the desired beam position is a relative large angular distance from the present position of the reflector beam, then the coarse mechanism 22 is arranged to move the reflector beam to the general direction of the required beam. The position of the main dish is monitored and when dish 2 has reached a defined position, servo mechanism 26 is arranged accurately to move sub-reflector dish between first and second positions either side of the desired position in the manner described above with reference to figure 1.

The present invention has particular application to the method of implementation of a radar field described and claimed in our copending application (F20698). i. e. a method of guiding an object by means of radar information field, wherein a field is created comprising a plurality of lobes closely positioned about a central null in the field, controlling the beam so that the null is directed towards a destination or target, and positioning said object in the null, said object including an antenna and receiver means responsive to the signal strength difference between the lobes of the information field and the null in order to guide the object to the destination or target.

It will be understood that in situations where the beam direction is determined by electronic processing methods, as in F20698 where the null is determined by a ring of side beams, the dithering process in accordance with the invention may be applied to the side beams in order to adjust the position of the central null.

Claims (3)

1. A method of positioning a radar beam by monitoring the direction in which the beam is to be positioned, determining the position in which the radar reflector system must be positioned in order to achieve the beam direction in relation to the current radar reflector system position, determining first and second reflector system positions either side of the desired position which can be defined within the limits of accuracy of the reflector positioning system, and moving the reflector system between the first and second positions with the relative times spent in each position being determined by the direction of the desired reflector position in relation to the first and second positions whereby to obtain on average a reflector system in the desired position and hence a radar beam pointing in the desired direction.

2. A method as claimed in claim 1, employing a Cassegrain reflector system and moving the sub-reflector of the system between said first and second positions.

3. A method as claimed in claim 1 and substantially as described with reference to the accompanying drawings.

3. A method as claimed in claim 1, employing a radar system as claimed in any claim of application (F20698).

4. A method as claimed in claim 1 and substantially as described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows 1. A method of positioning a radar beam by monitoring the direction in which the beam is to be positioned, determining the position in which a main radar reflector or the main radar reflector and a sub-reflector must be positioned to achieve the beam direction in relation to the current positions of the radar reflector or reflectors, determining first and second radar reflector positions either side of the desired position and moving the main reflector, when the main reflector only is used, between the first and second positions with the relative times spent in each position being determined by the desired position of the main reflector, or when the main reflector and the sub-reflector are used, moving the main reflector to a desired position and moving the subreflector between first and second positions with the relative times spent in each position being determined by the desired position of the sub-reflector, whereby, on average the main reflector is, or the main reflector and the sub-reflector are in the desired position and hence pointing a radar beam in the desired direction.

2. A method as claimed in claim 1, employing a radar system as claimed in any claim of application No. 8911380.7.