GB1594213A

GB1594213A - Radar/sonar systems

Info

Publication number: GB1594213A
Application number: GB226577A
Authority: GB
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1978-01-09
Filing date: 1978-01-09
Publication date: 1981-07-30

Description

(54) IMPROVEMENTS IN OR RELATING TO RADAR/SONAR SYSTEMS (71) We, THE PLESSEY COMPANY LIM ITED, a British Company, of 2/60, Vicarage Lane, Ilford, Essex, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following state menu: This invention relates to radar/sonar systems and more especially it relates to frequency scanning radar/sonar systems.

The following discussion and description will be directed towards radar systems but it is to be understood that the present invention includes within its scope analogue sonar systems.

An object of the present invention is to provide a radar system in which the effects of scintillation are reduced.

Scintillation effects are caused by changes in the efficiency of a target as a radar reflector. These changes usually stem from the fact that the target is moving and comprises of a plurality of radar reflective surfaces each of which act with varying efficiency in dependence upon the position of the target with respect to a radar transmitter.

The effect of scintillation on a planned position indicator radar display is that the echo signals representing targets fade and loss of detection may result. Scintillation is however frequency dependent and quite small changes in the radar frequency can considerably change the effect of scintillation on a given target echo. It will therefore be appreciated that in a frequency scanning radar system wherein the radar frequency is changed in accordance with the radiated beam angle required, scintillation effects are especially important. In one system for example wherein the beam elevation angle is changed in accordance with frequency and the elevation of a target determined in accordance with the frequency of its echo signal scintillation effects can seriously effect elevation measurements appertaining to that target. This is because the strength of echo signals is frequency dependent.

According to the present invention in a frequency scanned radar/sonar system the component of an echo signal which is contributed by scintillation is determined in dependence upon the rate of change of frequency of that echo signal.

The system may comprise means effective for changing the elevation of the transmitted beam in accordance with transmitted beam frequency and frequency measuring means including a frequency discriminator or the like for determining the frequency of each echo return signal, whereby the elevation of a target is indicated in dependence upon the frequency of an echo from that target.

In the case of a pulsed radar system the frequency modulation imposed by the transmitter is removed by means of a pulse compression filter or similar device in the receiver. The numerous scatterers of the target can cause a distortion in the echo signal spectrum and in general the rate of change of frequency within the distorted echo signal increases when the frequency (or elevation) error increases.

In accordance with one aspect of the invention signals derived from the frequency discriminator are fed to a differentiator arranged to feed a threshold amplifier means whereby signals from the differentiator exceeding a predetermined amplitude defined by a threshold level are recognised as resulting from echo signals in which the rate of change of frequency exceeds a predetermined limit.

According to an alternative aspect of the present invention signals derived from the frequency discriminator or the like are fed to a tapped delay line, output signals from two of the tapping points on the delay line being compared, whereby signals affording a predetermined amplitude difference as measured at the said points are recognised as resulting from echo signals in which the rate of change of frequency exceeds a predetermined limit.

The system may include means whereby signals thus recognised as aforesaid are effectively inhibited so that they are not taken account of in associated signal processing circuits whereby the accuracy of elevation determination in the case of a vertical scanning system is improved.

Alternatively, the system may include means whereby signals recognised as aforesaid are processed by an algorithm in the system 5 computer software. More than one frequency-rate threshold level could be used to give a measure of other possible error and therefore confidence. or otherwise, of the indicated elevation of the target.

An exemplary embodiment of the invention will now be described by way of example with reference to the accompanying drawing which is a block schematic diagram of part of a frequency scanning pulsed radar system utilised to measure elevation.

A frequency scanning radar system utilised to determine the elevation of a target comprises a transmitter which transmits pulsed signals at a nominal frequency of about 3,000 MHz and which vary progressively in frequency by 150 MHz in each vertical scan. The angle of elevation of the radar beam is determined in accordance with the frequency transmitted thus at one elevation limit the frequency is 3,000 MHz and at the other elevation limit the frequency is 3,150 MHz. In this system the angular elevation range scanned with 150 MHz frequency deviation is 30 although it will be appreciated that in other systems alternative angular ranges may be scanned.

Referring now to the drawing, echo signals from a radar transmitter (not shown) are received and fed via a low noise amplifier 1 to a mixer 2 which is also fed from a local oscillator 3 the frequency of which is chosen so as to provide an intermediate frequency which varies over the range between 500 and 650 MHz in each scan. Signals at this first intermediate frequency are fed to eight filters only two of which are shown which in combination cover the band between 500 and 650 MHz and which each cover a band of around 25 MHz the filter 4 thus operates at about 500 MHz and has a bandwidth of 25 MHz and the adjacent filter 5 operates at a centre frequency of about 520 MHz and also has a bandwidth of 25 MHz.The outputs of the eight filters are fed each to a mixer, mixers 6 and 7 only being shown, each of which is fed with a different local oscillator frequency so as to provide eight second intermediate frequency output signals of nominally 70 MHz. Thus, the second local oscillator frequency is fed to the mixer 6 on line 8 and to the mixer 7 on line 9.

In order to simplify the following description, circuitry associated with the mixer 6 only will hereinafter be described although it will be appreciated that the other eight mixers such as the mixer 7 will feed similar circuits. The output from the mixer 6 at the nominal IF frequency of 70 MHz is fed via line 10 to a pulse compressor 11 and a log amplifier 12. An IF sample from the log amplifier 12 is fed to a frequency discriminator 13 and via a tapped delay line 14 to an elevation analogue-to-digital converter 15.

The analogue-to-digital converter 15 is clocked with a signal from a video peak log detector 16 which is fed from the log amplifier 12.

The apparatus thus far described is generally well known and commonly utilised in frequency scanning systems. The frequency of each received echo signal is indicated coarsely in dependence upon which of the eight filters an echo signal passes through and fine frequency is determined by the frequency discriminator which is fed from that filter. Thus in the example just before described an echo signal at 500 MHz would pass through the filter 4 and the frequency of that echo signal would be finely determined by the frequency discriminator 13. The output signal from the frequency discriminator will have a particular. frequency dependent amplitude and this amplitude is converted to a corresponding digital signal in the elevation analogue-to-digital convertor 15 which is clocked by a signal provided by the peak log detector 16.In order to arrange that the correct timing relationship obtains between the signal from the frequency discriminator and the peak log detector the tapped delay line 14 is provided which is adjusted to give the appropriate delay.

In order to detect when the rate of change of frequency at the output of the frequency discriminator exceeds a predetermined limit, one of two alternative circuits may be utilised. One of the two circuits is shown within the broken line 17 and the other alternative circuit is shown within the broken line 18.

Referring firstly to the circuit within the broken line 17, an output signal from the frequency discriminator is fed to a differentiator 19 which in turn feeds an amplitude detector 20 or threshold amplifier. If the output signal from the differentiator 19 exceeds predetermined amplitude a signal will be provided on line 21 indicative that an echo signal has been received in which the rate of change of frequency exceeds a predetermined rate which is determined by the amplitude threshold of the amplitude detector 20. The differentiator 19 may be a simple resistor capacitor differentiator.

Referring now to the circuit within the broken line 18 a comparator 22 is used which is arranged to compare the amplitude levels at two points 23 and 24 on the tapped delay line. If the amplitude of the signals measured at these two points exceeds a predetermined level then the reception of an echo signal in which the rate of change of frequency exceeds a predetermined limit is indicated and an output signal will be provided on line 25.

The error signals produced on lines 21 or 25 as the case may be, may be utilised to control the signals on output line 26 which are processed to provide a height indication and to inhibit such signals which result from echo signals in which the rate of change of frequency exceeds the predetermined limit whereby these echo signals are ignored. In this way the adverse effects of scintillation can be reduced, and more accurate elevation measurements may be made which could only otherwise be obtained by reducing the radar beam width to an extent which would not be practicable.

The invention may form part of a so-called 3-D radar system wherein a PPI display is provided and wherein the height of individual targets is determined by a frequency scanning system which determines elevation.

It will be appreciated that various modifications may be made to the arrangement just before described without departing from the scope of the invention and for example the frequency discriminator may alternatively be replaced by other frequency sensing means and in order to suppress noise preceding an echo signal a fast acting gate or pilot tone at the input of the discriminator may be used.

WHAT WE CLAIM IS: 1. A frequency scanned radar/sonar system wherein, the component of an echo signal which is contributed by scintillation is assessed in dependence upon the rate of change of frequency of that echo signal.

2. A radar/sonar system as claimed in claim 1, comprising means effective for changing the elevation of a transmitted beam in accordance with transmitted beam frequency and frequency measuring means including a frequency discriminator or the like for determining the frequency of each echo return signal whereby the elevation of a target is indicated in dependence upon the frequency of an echo from that target.

3. A radar/sonar system as claimed in claim 2, wherein signals derived from the frequency discriminator are fed to a differentiator arranged to feed a threshold amplifier means whereby signals from the differentiator exceeding a predetermined amplitude defined by a threshold level are recognised as resulting from echo signals in which the rate of change of frequency exceeds a predetermined limit.

4. A radar/sonar system as claimed in claim 2, wherein signals derived from the frequency discriminator or the like are fed to a tapped delay line, output signals from two tapping points on the tapped delay line being compared whereby signals affording a predetermined amplitude difference as measured at the said points are recognised as resulting from echo signals in which the rate of change frequency exceeds a predetermined limit.

5. A radar/sonar system as claimed in claim 3 or claim 4 including inhibit means whereby signals thus recognised as aforesaid are effectively inhibited so that they are not taken account of in associated processing circuits whereby the accuracy of elevation determination in the case of a vertical scanning system is improved.

6. A radar/sonar system as claimed in claim 3 claim 4 or claim 5 wherein the system includes processing means whereby signals recognised as aforesaid are processed by an algorithm in the radar system computor software.

7. A radar/sonar system as claimed in any preceding claim wherein a plurality of frequency-rates of change are detected.

8. A radar system substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

25 as the case may be, may be utilised to control the signals on output line 26 which are processed to provide a height indication and to inhibit such signals which result from echo signals in which the rate of change of frequency exceeds the predetermined limit whereby these echo signals are ignored. In this way the adverse effects of scintillation can be reduced, and more accurate elevation measurements may be made which could only otherwise be obtained by reducing the radar beam width to an extent which would not be practicable.

The invention may form part of a so-called 3-D radar system wherein a PPI display is provided and wherein the height of individual targets is determined by a frequency scanning system which determines elevation.

It will be appreciated that various modifications may be made to the arrangement just before described without departing from the scope of the invention and for example the frequency discriminator may alternatively be replaced by other frequency sensing means and in order to suppress noise preceding an echo signal a fast acting gate or pilot tone at the input of the discriminator may be used.

WHAT WE CLAIM IS: 1. A frequency scanned radar/sonar system wherein, the component of an echo signal which is contributed by scintillation is assessed in dependence upon the rate of change of frequency of that echo signal.
2. A radar/sonar system as claimed in claim 1, comprising means effective for changing the elevation of a transmitted beam in accordance with transmitted beam frequency and frequency measuring means including a frequency discriminator or the like for determining the frequency of each echo return signal whereby the elevation of a target is indicated in dependence upon the frequency of an echo from that target.
3. A radar/sonar system as claimed in claim 2, wherein signals derived from the frequency discriminator are fed to a differentiator arranged to feed a threshold amplifier means whereby signals from the differentiator exceeding a predetermined amplitude defined by a threshold level are recognised as resulting from echo signals in which the rate of change of frequency exceeds a predetermined limit.
4. A radar/sonar system as claimed in claim 2, wherein signals derived from the frequency discriminator or the like are fed to a tapped delay line, output signals from two tapping points on the tapped delay line being compared whereby signals affording a predetermined amplitude difference as measured at the said points are recognised as resulting from echo signals in which the rate of change frequency exceeds a predetermined limit.
5. A radar/sonar system as claimed in claim 3 or claim 4 including inhibit means whereby signals thus recognised as aforesaid are effectively inhibited so that they are not taken account of in associated processing circuits whereby the accuracy of elevation determination in the case of a vertical scanning system is improved.
6. A radar/sonar system as claimed in claim 3 claim 4 or claim 5 wherein the system includes processing means whereby signals recognised as aforesaid are processed by an algorithm in the radar system computor software.
7. A radar/sonar system as claimed in any preceding claim wherein a plurality of frequency-rates of change are detected.
8. A radar system substantially as hereinbefore described with reference to the accompanying drawings.