RADAR SENSING SYSTEM

Abstract: A sensing system, for example a target sensor for a missile, comprises a radar transmitter in which the transmitter and transmitting antenna are operable to receive the radar return signal and to form in the transmitter an information bearing signal by the combination of transmitted and return signals, the radar receiver apparatus being operable to extract the combined signal from the transmitter. The transmitter can be a Gunn effect or Impatt type of device of which the non-linear impedance characteristic ensures generation of the required combined signal at a bias port of the transmitter. By extracting and making use of this combined signal, the overall system particularly its IF circuitry can be simplified. In effect, the transmitter becomes a common mixer for both the transmitter and receiver.

Description: RADAR SENSING SYSTEM An essential sub-system to many types of guided weapon and munition systems is a sensor within " '.the -missile. This sensor may have to perform the tasks of: target detection / target discrimination, target identification/ direction finding, scanning and homing. For active or semi-active radar used as the sensor, the performance of these f nctionsrequires a transmitter and receiver con¬ sisting of many microwave components, together with IF ' circuitry and signal processing. According to one aspect of the invention, there is provided a target sensing system for installation on board a missile, the system including emission means for emitting electromagnetic radiation towards the target, and signal supply means for supplying a signal to said emission means for causing it to emit said radiation, the emission means and signal supply means being such that radiation reflected back from said target to said emission means produces a return signal which, combined with the transmitted signal, contains information concerning said target, the system further including means for sensing said information. According to another aspect of the invention there is provided a missile comprising a sensing system as described above. For a better understanding of the invention, reference will be made to the accompanying drawing, the single figure of which is a simplified diagram of part of a target sensing system. ' y E I The drawing illustrates an active radar sensor system wherein the number of microwave components might be substantially reduced together with the complexity of the IF.circuitry in relation to that of other systems. The system consists of a suitable radiating aperture (not shown) such as a dish or horn antenna to which is coupled a transmitter of electromagnetic radiation, typically in the 300mm to 1mm wavelength band. This transmitter may be any suitable active device which possesses non-linear relationship between voltage and current in the power supplied to operate the. transmitter- Two suitable, solid state transmitters are transferred electron (Gunn effect) device and the IMPATT type. After transmission, a portion of the electromagnetic radiation will strike an object and some radiation will be reflected back to the antenna. The return signal is detected by the effect produced at the bias ports of the transmitter, caused by the return signal mixing with the transmitted signal in the non-linear impedance of the transmitter. If transmitter and reflecting objects are stationary with respect to each other, this effect to be observed at the bias ports will be a rectified, ά. c. • > ■ voltage. However, if relative motion is present between transmitter and reflector, then an a.c. signal will be observed, the frequency of which will be given by the r.e«lιa β *t.ijo Λ n-,s__τh-,ii rp, x-F =- 2 -i v r¬ ( -■ where v is the relative velocity and λ. is the wavelength. This is usually referred to as the doppler frequency. In practice, an a..c..coupled receiver circuit is proposed as, for a missile or munition type of application, there will be relative motion between sensor and target. In some cases, as, for example, an aircraft target against a sky background, the presence of a doppler frequency can, in itself, be used as the criterion for target detection. However, in the case of a stationary target, for example: a tank against a land background or a ship against a seascape, then the criterion for target detection will be based on the amplitude difference of the doppler signal between the cases when the target is in and out of the antenna beam. In order for the sensing and homing system to better detect, discriminate and identify targets, it is pro¬ posed that the transmitter be capable of producing a variety of waveforms. Simple forms of transmission would be: C. ., pulse, FM-CW, frequency hopping, frequency chirp. It is also proposed that the system be capable of transmitting and receiving linear elliptical polarisations and that these polarisations may be changeable at variable rates. Thus, by choosing the most suitable transmit and receive parameters, the self-oscillating mixer will produce at its bias port, a signal from which may be deduced such information as: target range, relative velocity and acceleration. In addition, the variables such as chirp and polarisation diversity may be used to improve the detection of a target in clutter and to discriminate between different types of target. ' Due to the significant reduction in size and weight afforded by the simplicity of the sensor, it is proposed that the complete transmitter and receiver system, together with doppler amplification and signal processing circuity, may be placed directly within the feed to the antenna. In the case of a front fed dish antenna, this would enable low loss beam scanning to be effected by direct movement of the feed plus sensor assembly. In certain applications,it will be desirable to operate the transmitter in a frequency-agile mode and it is proposed that this be implemented in two ways. Firstly, a single transmitting device may be used and its frequency changed either by varying the conditions of bias or by incorporating an active or passive tuning device coupled to the transmitter resonant circuit. Secondly, two or more transmitting devices, operating at different frequencies may be used. In the case of alternating operation of a multiple device transmitter, it is proposed that a common doppler receiver be used. The active sensor may also be switched to a passive mode, either in a bistatic application wherein a signal is received as the scattered radiation from some physically separated transmitter, or else as a radiometer. In both of these passive cases, the signal extracted from the transmitter (now acting as a local oscillator) is not, primarily, a doppler signal and will be typically of a wide bandwidth, orders of magnitude wider than the doppler spectrum. When operating in this mode, therefore, a wide-band amplifier and detector system 1 may be switched into operation by switch 2. It will be seen that there is described. and illustrated a sensor and homing head system which uses a common device as the transmitter and receiver mixer. The common transmitter is operable in a variety of modulated transmissions to enhance target detection, discrimination and identification and the received target information is extractable from the doppler signal arising within the self-oscillating mixer due to relative movement between sensor and target. Preferably the complete sensor electronics are con¬ tained within the feed of the dish antenna. c: The system of the invention may use multiple transmitters and a common doppler receiver. A.lso, it may involve operation of a self-oscillating mixer in a passive mode with consequent switching of a doppler receiver to a wider bandwidth. Advantageously, derivation of directional information is done by using the complete sensor in a scanning feed, amplitude or phase comparison feed, or by a multi-mode feed, these being incorporated either in a fixed or movable head.

Claims: WE CLAIM: 1. A radar sensing system including signal generating means for forming a radio-frequency transmission signal y transmitting antenna means for receiving said signal and emitting electromagnetic radiation, and receiver means for deriving information about an object upon which said emitted radiation is incident from radiation which is reflected back to the system by the object, characterised in that said transmitting antenna means is also operable to receive said reflected radiation and to pass the resultant return signal back to said signal generating means, in that the signal generating means is operable to combine this return signal with said transmission signal to produce a combined signal containing said information, and wherein said receiver means is connected to said signal generating means and is operable to extract therefrom said combined signal and to derive the object information from that combined signal. 2. A system according to claim 1, wherein said signal generating means comprises a solid-state self-oscillating mixer device having a non-linear impedance characteristic, for example a Gunn effect or IMPATT device. 3. A system according to claim 1 or 2, wherein said signal generating means and said receiver means are located close to said transmitting antenna means within a waveguide feed thereto. 4. A target sensing system for installation on board a missile, the system including emission means for emitting electromagnetic radiation towards the target, and signal supply means for supplying a signal to said emission means for causing it to emit said radiation, the emission means and signal supply means being such that radiation reflected back from said target to said emission means produces a return signal which, combined with the transmitted signal, contains information concerning said target, the system further including means for sensing said information. 5. A missile comprising a sensing system according to any preceding claim ti' ' i *.

Inventors: HOSKING MICHAEL WILLIAM
Applicants: BRITISH AEROSPACE, HOSKING MICHAEL WILLIAM, BRITISH AEROSPACE PUBLIC LTD CO
Application year: 1982
Main classification: G01S7
IPC codes: F41G7, G01S7, G01S13