GB2142201A

GB2142201A - Radar system

Info

Publication number: GB2142201A
Application number: GB08317212A
Authority: GB
Inventors: Martin Andrew Fryer; Peter Edward Chadwick
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1983-06-24
Filing date: 1983-06-24
Publication date: 1985-01-09
Also published as: GB2142201B

Abstract

In an early warning system for detecting the presence of aircraft, one or more picket buoys 12 are located in advance of a base station 10 in the direction of expected attack. Each of the picket buoys 12 is arranged to receive radar signals (not neccessarily transmitted from the picket buoy concerned) after reflection from a target 15 and to transmit a signal indicative of detection of a radar signal to the base station 10. The base station 10 is arranged to receive the signal to provide an indication of the presence of an aircraft. The invention also includes a radar picket buoy for use in the system. <IMAGE>

Description

SPECIFICATION Radar system This invention relatestoan early warning radar system for detecting the presence of aircraft and also to a radar picket buoyfor use in such an early warning system.

The term aircraft is intended to cover all types of manned and unmanned airborne craft eg, guided or non-guided missiles, helicopters, balloons and aero planes.

The protection of a fleet of ships at sea from attack bylow flying or surface skimming missiles is extremely difficult in view of the restricted range of radar signals. The horizon for a radar signal under "optical" propagation conditions is 1 .SVFwhere h is the height in feeabove water, and the range is in miles. If we assume a radartransmitting antenna height of 60 feet then the range is 11.6 miles. 150 feet is probablythe maximum, giving a horizon of 18 miles. An attacker travelling at600 mph will take only 108 seconds to travel from the horizon to the ship during which time its presence hasto bedetected and counter measures implemented.This time is often insufficient to enable effective counter measures to be taken.These figures assume smooth sea conditions and zero heightof the attacker. At 700ft attack height, warning range is 33 miles under perfect conditions. Missiles which are radar guided and "locked in" at th is range must be detected early to provide Electronic Warfare measures with Operation time. This invention seeks to provide a system capable ofdetecting the presence of aircraft in which an earlier indication of impending attack is provided.

According to the invention there is provided an early warning system for detecting the presence of aircraft in which one or more picket buoys are located in advance of a base station in the direction of expected attack,the or each ofwhich picket buoys is arranged to receive radar pulses after reflection from a target and to transmit an information signal indicative of detection of a radar signal to the base station, and the base station is arranged to receive the signals to provide an indication ofthe presence of an aircraft.

The information signal may be transmitted via radio e.g. high frequency (H.F.), sonar e.g. audio frequency (A.F.), conductive cable oroptical fibre cable.

If the calculations of the radio range are made assuming smooth sea conditions, then the use of a buoy at radio horizon (which is assumed to be the limit of communication) increases the range by that available from the buoy. However, because of the low information content and thus low data rate required to transfer information from the buoy to the base, weak signals can be used, and in addition, simple aerials for communication at greater heights than is practical for a radar installation may be employed. Thus the communication range will be greaterthanthe horizon range, and in the case ofthe use of H.F. radio with its excellent ground wave communication range, will ailow range improvement. Such a system also improves the performance under heavy sea swell conditions wherein the effective horizon is subject to reduction.

One of the buoys in the system may be arranged to transmita radarsignal for receipt by each ofthe buoys after reflection from an aircraft or alternatively each of the buoys may be arranged to transmit its own radar signal and to receive that signal after reflection from an aircraft. The buoys may be arranged in at least one line.

Each buoy may be arranged to provide a signal related to the receipt time of a radar pulse and the base station may be arranged to process the received signals to provide an indication of the direction of a detected aircraft from the base station. There may be at least three buoys and an indication ofthe position of an aircraft may be determined by triangulation.

The buoys may be arranged in at least 2 spaced lines and the signals from the two lines may be processed to provide heading and speed information relating to a detected aircraft.

The buoys may be anchored to maintain their position or may alternatively be sown at known positions and be free to drift with tidal currents. Where the buoys are free to drift, the processing atthe base station includes a correction to compensate for movement of each buoy in dependence upon predicted tidal flows.

The buoys may be arranged to store received information and to transmit that information to the base station only upon receipt of an interrogating radio signal.

The system may include one or more radio relay transmitter/receiverstations, each provided in a buoy located between a picket buoy and the base station to relay the information signals from the buoys.

The invention also includes a radar picket buoy for use in the system which comprises a housing containing a radar transmitter, a radar receiverfor receiving the transmitted signal after reflection from an aircraft, detector means for providing a signal indicative of receipt of the reflection and transmitter means arranged to transmit said signal.

The transmitter means may be a radio transmitter or a sonartransmitter, or may provide an outputfor connection to a conductive cable orto an optical fibre cable for communicating with a base station.

The detector means may provide a signal representative of the time delay between transmission of a pulse and receipt of the pulse after reflection.

The buoy may have an omni directional radar aerial to permittransmission and reception of radar signals in all directions. The buoy may include a store arranged to receive said signal prior to transmission and may include an interrogating receiver responsive to a received control signal to initiate transmission of the stored signal. The detector means may provide the signal representative ofthe time delay in digital form and thetransmitter may be arranged to transmitthe signal byfrequencyshift keying.

In orderthatthe invention and its various other preferred features may be understood more easily, embodimentsthereofwill now be described, byway of example only, with reference to the drawings in which: Figure lisa schematic diagram ofa system operating in accordance with the invention, Figure 2 is a schematic block diagram of a picket buoy constructed in accordance with the invention, Figure 3 is a schematic block diagram of a base station forthe system.

Referring nowto Figure 1 there is shown a capital ship 10 which is to be protected against attack. The ship is provided with a normal radio receiving aerial mast 11. A number of radar picket buoys 12 are sown or anchored in the sea at points remote from the ship 10 but within radio communications distance ofthe radio mast 11. The buoys are positioned between the ship and a direction of expected attack.

The picket buoys 12 are each equipped with radar and radio equipment as will be described later with reference to figure 2. The buoys have a radar aerial 14 for receiving reflected radar signals from a potential attacker 15 and possibly also fortransmitting a radar signal for reflection. The buoys also have a radio aerial and are arranged to transmit radio signals indicative ofthe receipt of a reflected radar signal.

The transmitted radio signal is received by the ship 11 and is processed to provide an indication of impending attack.

One or more radio relay buoys 13 may be positioned between the picket buoys and the ship to extend the range of radio communication between the ship and the picket buoy.

In the simplestform ofthesystem in accordance with the invention a single picket buoy may be employed but preferably one or more lines of picket buoys will be employed between the ship and the direction of potential attack. A line of buoys may extend so as to completelysurround the ship.

One or each ofthe picket buoys may be arranged to transmit a radar signal e.g. radar pulses, carrierwave, or chirp for detection after reflection or alternatively a separate source of radartransmission may be provided on for example another buoy.

The picket buoys may be arranged to provide a signal representative of the actual time of receipt or of the time delay between transmission and reception of a radar pulsefortransmission backto the ship 10.

Such information may be provided in digital form and may be transmitted byfsktransmission.

The information relating to time ortime delay received from the picket buoys is processed in the ship 10 to provide an indication ofthe direction of impending attack, the bearing of an impending attacker and, if more than one line of picket buoys are provided, the speed and heading ofthe attacker. Such processing maybe carried out by any conventional means.

In the case where buoys are sown by for example dropping from aircraft and are nottethered,the processing can be arranged to provide a compensating factor for expected positions of the buoys in dependence upon known tidal flows.

Further details ofthe picket buoy and the receiving station on the ship will now be described with reference to figures 2 and 3.

Referring now to figure 2 a picket buoy comprises a radartransmitter/receiver 16, in this case ofthe pulsed type, which is coupled with the aerial 14which is preferable an omni directional type of aerial to provide hemispherical coverage. A suitable aerial is a monopole type. The radartransmitter receiver is pulsed by a pulse generator 17 to transmit a radar signal from the aerial 14for reflection by an attacker. A reflected signal is received via the aerial 14. The pulse generator 17 also triggers a time delay detector 18 when the pulse is transmitted which detector counts thetimetaken between the transmission ofthe pulse and the receipt of a reflected puls if any. The time delay information is preferably provided in digital form and is stored in a store 19.An interrogating receiver 21 is provided which has a particular address code and enables release ofthe stored information in the store 19 for transmission bythe picket buoy upon receipt of an interrogating signal transmitted from the ship 10.

Upon receipt of this interrogating signal the stored information in 19 is released to drive an fsk generator 22 which provides a coded signal for transmission by a radio transmitter 23 and radio aerial 24 for receipt by the ship 10. This system is advantageous in that all buoys may operate on the same radio transmission frequency and can be interrogated selectively in a predetermined sequence.

The blocks indicated within the dotted line are optional in its simplestform the buoy may purely transmit a signal indicative that something has been detected.

Referring now to figure 3 there is shown a block diagram ofthe base station on the ship 10. The aerial mast carries an aerial 31 which receives radio signals from the picket buoys for detection by a radio receiver 32. The signals are transferred to a processor 33 which computes from the received signals the direction of an impending attack. In the case oftime information signals the processor 33 can compute the direction and/or bearing whilst in the case of 2 lines of picket buoys the time information can be used also to provide an indication ofthe speed and heading ofthe attacker. Such computation is carried out by conventional means. The computation may include a compensation factor two correct the computation for variation of the position of buoys which are freely floating in dependence upon predicted tidal flows.

The computed information is transferrndto a display for example an IFF display. The base station is equipped with an interrogating transmitter 35which is arranged to transmit a predetermined sequence of address codes which are provided by an address code generator 36. Each address code is urlique to a particular one of the picket buoys and instructsthe picket buoy to transmit any storedinformation. The address code generator is also coupled to the proces- sor33 in orderthatthe processor can identifythe source of a received signal.

The processing of received information bythe processor 33 may be affected bytriangulation from information received from combinations ofthree or more picket buoys. The processing of information from a multiplicity of picket buoys may be affected by triangulation of information from a permutation of three buoys from the total.

The system of addressing picket buoys in a predetermined sequence has been described with reference to a digital stored information system. It can however be employed in a system where the buoys transmit onlya basic indication that a signal has been detected.

Such a signal may be transmitted directly upon receipt when the buoy is addressed if a signal has been stored orfollowing the next radartransmission pulse or alternatively the addressing may be arranged to initiate thetransmission of an interrogating radar pulse and to initiate radio transmission of a signal representativeofany received reflection of that pulse.

Such arrangements are intended to fall within the scope of this invention.

Although the embodiment described employs transmission of radio between the buoy(s) and the base station, the invention is not restricted to such transmission. Instead ofthe radio transmitter 23 (Figure 2), a sonar transducer may provide an acoustic signal, for receipt by a sonar receiver base base station, which replaces the receiver 32. Such a system is particularly advantageous for use where the base station is on a submarine and enables the presence of aircraft to be detected without surfacing. Alternative possibilities within the scope of this invention are replacement ofthe radio transmitter 23 and radio receiver 32 by means for providing a signal output and for receiving the signal respectively via conductive cables or via optical fibre cables. Such optical cable transmission may be in the visible or infrared spectrum.Suitable transducing means are well known in the art. Such systems could be of special value to more permanent base stations for example on land, where the laying of communicating cables are worthwhile. It will also be appreciated that although the buoys are primarily intended for sowing on the sea, they could also be deployed on land where theycould be mountedordroppedbyparachutefrom aircraft.

It will also be appreciated that the interrogating receivers 21 (Figure 2) and interrogating transmitter 35 (Figure 3) may communicatevia sonar, conductive cable or optical fibre cable instead of by radio.

The system of advanced warning described is relatively cheap to employ and because of the small size ofthe buoys, they are reasonably immune to attack except by anti-radiation missiles.

In a system where a largenumberofbuoysare employed jamming could provide a useful indication of attack.

Claims

1. An early warning system for detecting the presence of an aircraft in which one or more picket buoys are located in advance of a base station in the direction of expected attack, each of which picket buoys is arranged to receive radar pulses after reflection from a target and to transmit an information signal indicative of detection of a radarsignal to the basestation. and the base station is arranged to receivethe signals to provide an indication of the presence of an aircraft.

2. Asystemasclalmedinclaim 1,wherein one of the buoys is arranged to transmit a radar signal for receipt by each ofthe buoys after reflection from an aircraft.

3. A system as claimed in claim 1, wherein each of the buoys is arranged to transmit its own radarsignal and to receive that signal after reflection from and aircraft.

4. Asystem as claimed in any one of the preceding claims in which the buoys are arranged in at least one line.

5. Asystem as claimed in anyone of the preceding claims, wherein each buoy is arranged to provide a signal related to the receipt time of a radar pulse and the base station is arranged to process the received signals to provide an indication of the direction of detected aircraftfromthe base station.

6. A system as claimed in claim 4, wherein there are at least three buoys and an indication ofthe position of an aircraft is determined by triangulation.

7. Asystem as claimed in claim 5 or 6, wherein the buoys are arranged in at leasttwo spaced lines and the signalsfromthetwo lines are processed to provide heading and speed information relating to a detected aircraft.

8. Asystem as claimed in any one ofthe preceding claims, wherein the buoys are anchored to maintain their position.

9. A system as claimed in any one of claims 1 to 7, wherein the buoys are sown at known positions but are free to drift with tidal currents.

10. Asystem as claimed in claim 5,6 or7, wherein the buoys are sown at known positions but are free to drift with tidal currents and the processing at the base station includes a correction to compensate for the movement of each buoy in dependence upon predicted tidal flows.

11. Asystem as claimed in anyone of the preceding claims, wherein the buoys are arranged to store received information and to transmit that information to the base station only upon receipt of an interrogating signal.

12. Asystem as claimed in any one of the preceding claims in which the information signals from the buoys are relayed by one or more receiver/ transmitter stations each provided in a buoy located between a picket buoy and the base station.

13. Asystem as claimed in any one of the preceding claims, wherein the information signal is a radio signal.

14. Asystem as claimed in any one of claims 1 to 12, wherein the information signal is transmitted in the form of a sonar signal.

15. Asystem as claimed in any one of claims 1 to 12, wherein the information signal is transmitted via a conductive cable to the base station.

16. Asystem as claimed in any one of claims 1 to 12, wherein the information signal is transmitted via an optical cable to the base station.

17. An early warning system, for detecting the presence of aircraft substantially as described herein with reference to the drawings.

18. A radar picket buoyfor use in the system as claimed in any one ofthe preceding claims, the buoy comprising a housing containing a radar transmitter, a radar receiver for receiving the transmitted signal after reflection from an aircraft, detector means for providing a signal indicative of receipt ofthe reflection and transmitter means arranged to transmit said signal.

19. A buoy as claimed in claim 18,wherein the transmitter means is a radio transmitter.

20. A buoy as claimed in claim 18, wherein the transmitter means is a sonartransducer.

21. Abuoyasclaimed inclaim 18,whereinthe transmitter means has an outputforconnection to a conductive cable for communicating with a base station.

22. Abuoyasclaimed in claim 18,whereinthe transmitter means provides an optical signal output suitablefortransmission via an optical cable.

23. A buoy as claimed in anyone of claims 18to 22 having an omnidirectional radar aerial to permit transmission and reception of radar signals in all directions.

24. A buoy as claimed in any one of claims 18 to 23 including a store arranged to receive said signal prior to transmission.

25. A buoy as claimed in claim 16, including an interrogating receiver responsive to a received control signal to initiate transmission ofthe stored signal.

26. A buoy as claimed in any one of claims 1 8to 25 wherein the detector means comprises a time delay detector which provides the signal in aform repre sentativeofthetimedelaybetweentransmission of a pulse and receipt of the pulse after reflection.

27. A buoy as claimed in claim 26, wherein the detector means provides the signal representative of thetime delay in digital form.

28. A buoy as claimed in claim 27,whereinthe transmitterisanfsktransmitterwhich is keyed bythe digital signal to transmit the signal by frequency shift keying.

29. A radar picket buoy substantially as described herein with reference to Figure 2 ofthe drawings.