GB2143100A

GB2143100A - Radar receiver assembly

Info

Publication number: GB2143100A
Application number: GB08412428A
Authority: GB
Inventors: Charles Nelson Alexander
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1983-05-23
Filing date: 1984-05-16
Publication date: 1985-01-30
Also published as: GB8412428D0; GB2143100B

Abstract

The invention provides a receiver assembly for a bistatic or multistatic radar system and includes an AM detector receiver (1, 2, 3, 4) for receiving direct path and reflected path AM signals from a transmitter or transmitters, and an audio real time frequency analyser (5) for processing output from the analyser, and may further include a recorder (6) in the form of a lofargram display arranged to receive spectral output from the frequency analyser and wherein the display is of frequency versus time and the intensity of the lofargram is proportional to received signal strength. <IMAGE>

Description

SPECIFICATION Radar Receiver Assembly The present invention relates to radar receivers and particularly, though not exclusively, relates to receivers for bistatic and multistatic radar systems for detecting aircraft and missiles at a point remote from a radio frequency (RF) source of the system.

There have been a number of prior proposals for bistatic and multistatic radar systems. An early bistatic radar system is described in US patent No 1,981,884 of November 1 934.

The present invention provides an improved radar receiver assembly for a continuous wave (CW) or pulsed transmission bistatic or multistatic radar system.

According to the present invention a receiver assembly for a bistatic or multistatic radar system includes an AM detector receiver for receiving direct path and reflected path AM signals from a transmitter or transmitters, and an audio real time frequency analyser for processing output from the receiver/s.

Typically the frequency analyser has an operating range of 0--200 Hz, the upper frequency being dependent on the velocity of the detected target.

The spectral output of the frequency analyser may be fed to a lofargram display of frequency versus time wherein the intensity of the lofargram is proportional to signal strength.

A direct coupled amplifier may be connected to receive the AM detector output to extend the frequency range down to dc level.

The receiver may receive direct path and reflected path signals from a plurality of transmitter distributed around the receiver and the analyser may comprise a multi-cell real time analyser with, for example, a frequency resolution of 5 Hz by frequency stacking the receiver outputs and analysing the resulting composite signal over a wide frequency range.

A target signal appears on the lofargram as a continuous line varying in frequency with the sum of the radial velocities of the target with respect to the transmitter and the receiver.

The strength of a typical radar return is given by: p 28ss2 rG 5= \Natts (1) (47t)3R4 where: Pr is the power transmitted, G is the antenna gain, a is a function of the target echoing area, A is the wavelength and R is the target slant range.

In particular embodiments of the invention where the transmitter is closer to the target than the receiver assembly the incident signal path length will be smaller than the reflected path length and there will be a reduction in the attenuation of the received target signal.

An embodiment of the invention will now be described, by way of example only, with reference to the drawings of which: Figure 1 shows, in schematic form, a bistatic radar system and target.

Figure 2 which is also schematic shows a receiver assembly for use in the system of Figures.

Referring to Figure 1, a bistatic radar system includes a ship carried receiver assembly, 1, a buoy carried radio transmitter 2. A target aircraft 3 is also shown. CW transmissions from the buoy follow on direct, or incident, path, Rt, and are reflected from the aircraft to the ship by a reflected path Rr. Signals from the buoy are also received at the ship via a direct path Rd. Thus the ship borne receiver assembly 1 will receive a composite signal which includes signals from paths Rr and Rd. The direct path signal acts as a local oscillator would do in a frequency changing circuit and in beating with the reflected path signal from the aircraft produces a difference signal due to the Doppler shift caused by the motion of the aircraft.

The receiver assembly of Figure 2 includes a single antenna 1 which, in use, receives reflected and direct path signals corresponding to paths R, and Rd of Figure 1. The signals are amplified in an amplifier 2 and AM detected in a detector 3.

Output from the detector 3 is fed via an automatic gain control amplifier 4 to an audio real time spectrum analyser 5. Spectral output from the analyser is then fed to a hard copy intensity recorder to produce a lofargram display. The display shows frequency as a 'y' co-ordinate and time as an 'x' co-ordinate, the intensity of the lofargram being proportional to the signal strength.

Many modifications and variations within the scope of the invention will be apparent to those skilled in the art. For example, the IF bandwidth should be no greater than that required by target Doppler shift and receiver tuning considerations.

Diode demodulation may be used to extract the Doppler beats.

The RF transmission may be pulsed at say 1 second PRF with a 50% duty cycle to reduce transmitter battery consumption.

1. A receiver assembly for a bistatic or multistatic radar system includes an AM detector receiver for receiving direct path and reflected path AM signals from a transmitter or transmitters, and an audio real time frequency analyser for processing output from the receivers.

2. A receiver assembly as claimed in claim 1 for detecting airborne targets wherein the frequency analyser has an operating range of O- 200 Hz.

3. A receiver assembly as claimed in claim 1 or claim 2 further including a lofargram display arranged to receive spectral output from the frequency analyser and wherein the display is of

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Radar Receiver Assembly The present invention relates to radar receivers and particularly, though not exclusively, relates to receivers for bistatic and multistatic radar systems for detecting aircraft and missiles at a point remote from a radio frequency (RF) source of the system. There have been a number of prior proposals for bistatic and multistatic radar systems. An early bistatic radar system is described in US patent No 1,981,884 of November 1 934. The present invention provides an improved radar receiver assembly for a continuous wave (CW) or pulsed transmission bistatic or multistatic radar system. According to the present invention a receiver assembly for a bistatic or multistatic radar system includes an AM detector receiver for receiving direct path and reflected path AM signals from a transmitter or transmitters, and an audio real time frequency analyser for processing output from the receiver/s. Typically the frequency analyser has an operating range of 0--200 Hz, the upper frequency being dependent on the velocity of the detected target. The spectral output of the frequency analyser may be fed to a lofargram display of frequency versus time wherein the intensity of the lofargram is proportional to signal strength. A direct coupled amplifier may be connected to receive the AM detector output to extend the frequency range down to dc level. The receiver may receive direct path and reflected path signals from a plurality of transmitter distributed around the receiver and the analyser may comprise a multi-cell real time analyser with, for example, a frequency resolution of 5 Hz by frequency stacking the receiver outputs and analysing the resulting composite signal over a wide frequency range. A target signal appears on the lofargram as a continuous line varying in frequency with the sum of the radial velocities of the target with respect to the transmitter and the receiver. The strength of a typical radar return is given by: p 28ss2 rG 5= \Natts (1) (47t)3R4 where: Pr is the power transmitted, G is the antenna gain, a is a function of the target echoing area, A is the wavelength and R is the target slant range. In particular embodiments of the invention where the transmitter is closer to the target than the receiver assembly the incident signal path length will be smaller than the reflected path length and there will be a reduction in the attenuation of the received target signal. An embodiment of the invention will now be described, by way of example only, with reference to the drawings of which: Figure 1 shows, in schematic form, a bistatic radar system and target. Figure 2 which is also schematic shows a receiver assembly for use in the system of Figures. Referring to Figure 1, a bistatic radar system includes a ship carried receiver assembly, 1, a buoy carried radio transmitter 2. A target aircraft 3 is also shown. CW transmissions from the buoy follow on direct, or incident, path, Rt, and are reflected from the aircraft to the ship by a reflected path Rr. Signals from the buoy are also received at the ship via a direct path Rd. Thus the ship borne receiver assembly 1 will receive a composite signal which includes signals from paths Rr and Rd. The direct path signal acts as a local oscillator would do in a frequency changing circuit and in beating with the reflected path signal from the aircraft produces a difference signal due to the Doppler shift caused by the motion of the aircraft. The receiver assembly of Figure 2 includes a single antenna 1 which, in use, receives reflected and direct path signals corresponding to paths R, and Rd of Figure 1. The signals are amplified in an amplifier 2 and AM detected in a detector 3. Output from the detector 3 is fed via an automatic gain control amplifier 4 to an audio real time spectrum analyser 5. Spectral output from the analyser is then fed to a hard copy intensity recorder to produce a lofargram display. The display shows frequency as a 'y' co-ordinate and time as an 'x' co-ordinate, the intensity of the lofargram being proportional to the signal strength. Many modifications and variations within the scope of the invention will be apparent to those skilled in the art. For example, the IF bandwidth should be no greater than that required by target Doppler shift and receiver tuning considerations. Diode demodulation may be used to extract the Doppler beats. The RF transmission may be pulsed at say 1 second PRF with a 50% duty cycle to reduce transmitter battery consumption. CLAIMS

3. A receiver assembly as claimed in claim 1 or claim 2 further including a lofargram display arranged to receive spectral output from the frequency analyser and wherein the display is of frequency versus time and the intensity of the lofargram is proportional to signal strength.

4. A receiver assembly as claimed in any of the preceding claims further including a direct coupled amplifier connected to receive the AM detector output and arranged to extend the frequency range of the receiver down to DC level.

5. A receiver assembly substantially as herein described with reference to the drawings.

6. A multistatic radar system including a receiver assembly as claimed in any of the preceding claims.

7. A multistatic radar system substantially as herein described with reference to the drawings.