Title Passive radar signal enhancement
Technical field
This invention is related to radar passive systems More particularly, the invention relates to clutter and noise suppression in such systems
Background
Conventional radar systems comprise a collocated transmitter and receiver, which usu- ally share a common antenna to transmit and receive A pulsed signal is transmitted and the time taken for the pulse to travel to the object and back allows the range of the object to be determined
In a passive radar system, there is no dedicated transmitter Instead, the receiver uses third-party transmitters in the environment, and measures the time difference of arrival between the signal arriving directly from the transmitter and the signal arriving via reflection from the object This allows the bi-static range of the object to be determined In addition to bi-static range, a passive radar will typically also measure the bi-static Doppler shift of the echo and also its direction of arrival These measurements allow the location, heading and speed of the object to be calculated In some cases, multiple transmitters and/or receivers can be employed to make several independent measurements of bι- static range, Doppler and bearing and hence significantly improve the final track accuracy
In fig 1 an exemplary example relating to a passive radar system is shown Third party signals, from a passive transmitter 1 1 , such as public domain WCDMA (wideband code division multiplex access), GSM (Global System for Mobile communications), DVB-T (Terrestrial digital video broadcasting), or analogue RF (Radio Frequency) signals that may be received by external users 17 may be used by the passive radar system as an unintended additional use A target 14 is illuminated by signals from transmitter 11 A first antenna 16 of the passive radar system is adapted for receiving reflected signals C from the target 14 A second reference antenna of the passive radar system is adapted to receive signals A transmitted from the third party transmitter Other objects 13 will also be illuminated and contribute with reflected signals B Depending on circumstances, all signals A, B and C may of course be received by both the first and second, reference antenna By rendering the first "target" antenna 16 directional and steerable and the second antenna 15 directional and pointing in a direction towards the third party transmitter 1 1 , cross-talk can be suppressed
A passive radar typically employs the following processing steps, which are identified in the schematic diagram of fig 2 - Reception of the direct signal from the transmιtter(s) and from the surveillance region on dedicated low-noise, linear, digital receivers (not shown)
- Digital beamforming 15 to determine the direction of arrival of signals and spatial rejection of strong ιn-band interference, 18 - 19
- Adaptive filtering to cancel any unwanted direct signal returns in the surveillance chan- nel(s), 20
- Transmitter-specific signal conditioning, 19
- Cross-correlation of the reference channel with the surveillance channels to determine object bi-static range and Doppler, 21
- Detection using constant false alarm rate (CFAR) scheme, 22 - Association and tracking of object returns in range/Doppler space, known as "line tracking", 23, 24, 25
- Association and fusion of line tracks from each transmitter to form the final estimate of an objects location, heading and speed, 26
Prior art document US20080088508 shows a similar passive radar system as shown in fig 2
The following documents and textbooks deals with various aspects of the topic
US2002/0198650 shows a passive coherent location surveillance system comprising various signal processing steps Woodward, P M (1953) "Probability and information theory, with application to radar" England London ISBN 0-89006-103-3
Wikipedia the free encyclopedia "Passive radar" http //en wikipedia org/wιkι/Passιve_radar (2008-10-31) Kingsley, Simon & Quegan, Shaun (1992) "Understanding radar systems" USA Mend- ham SciTech Publishing lnc (1999) ISBN 1-891121-05-7
Bjorn Franzon, Lars Kanderhag, "Konstruktion och Evaluering av DigitalTV-Baserat Pas- sivt Radarsystem", Master thesis at Ericsson Microwave Systam and Chalmers (2006)
Summary
It is a first object of the invention to improve the performance of passive radar systems
The is object is solved by a Processing unit for a radar system adapted to detect targets by means of signal reflections caused by a transmitter , the transmitter being adapted for transmitting radio frequency signals which contain an underlying digital bit stream signal which may be processed according to a radio protocol error correction mechanism util- ized by external receivers
The radar system comprises at least one antenna a target signal path - at least conveying signal contributions as reflected from a potential target - and a reference signal path - at least conveying signal contributions from the transmitter - wherein both respective signal paths comprise analogue to digital converters providing respectively a reference signal and a target signal The reference signal path moreover comprises a replication stage comprising a digital to bit stream converter, an error correction stage and a bit stream to digital converter The error correction stage incorporates a radio protocol error correction mechanism corresponding to the error mechanism used by the external re- ceivers, the bit stream to digital converter providing an improved estimate of the digital data steam underlying the radio frequency signal transmitted from the transmitter
According to a further aspect of the invention there is provided a radar system comprising the processing stage comprising a cross correlation stage, wherein the improved es- timate of the bit stream of the reference path and the bit stream of the target path are fed to the cross correlation stage
According to a still further aspect of the invention there is provided a method for a radar system adapted to detect targets by means of signal reflections caused by a an external transmitter, the external transmitter being adapted for transmitting radio frequency sig- nals which contain a an underlying digital bit stream signal which may be processed according to a radio protocol error correction mechanism utilized by external receivers, the radar system comprising at least one antenna, the method comprising the steps of
- Conveying signal contributions as reflected from a potential target,
- Conveying signal contributions from the transmitter,
- Analogue to digital converting respective signals providing respectively a reference signal and a target signal
- Digital to bit stream converting the reference signal,
- Error correcting the bit stream converted signal, and
- Bit stream to digital converting the error corrected signal wherein the error correction incorporates a radio protocol error correction mechanism corresponding to the error mechanism used by the external receivers, providing an improved estimate of the digital data steam underlying the radio frequency signal transmitted from the transmitter
Further advantages of the invention will appear from the following detailed description of the invention
Brief description of the drawings
Fig 1 shows basic elements of a known passive radar system,
fig 2 shows circuit elements of a known passive radar system,
fig 3 shows circuit elements of an embodiment of a passive radar according to the invention, and
fig 4 shows a flow diagram relating to the embodiment shown in fig 3
Detailed description of preferred embodiments of the invention
In fig 3 an embodiment of a passive radar processing unit 1 according to the invention has been shown
The processing unit 1 may for instance be employed in a radar system as shown in fig 2, whereby elements signals provided by the fig 3 circuit according to the invention are provided to the CFAR detection circuit 22 shown in fig 2 Hence, the fig 3 elements and elements 22, 23 24, 25 and 26 constitute an embodiment of a passive radar system according to the invention
In the preferred embodiment of the processing unit according to the invention shown in fig 3, a reference antenna 15' and a target antenna 15 is provided The reference antenna may preferably be formed as a directional antenna which is directed towards a passive transmitter 1 1 The target antenna 15' may also have directional properties and be adapted to be directed towards a target 14 The signal path of the reference antenna 15 shall be referred to as the reference path 3 while the signal path relating to the target antenna shall be denoted as target signal path 3' It should be noted that instead of two receiving antennas 15, 15', a single antenna could be used, although probably with inferior results
As appears from fig 3, the reference signal path 3 and target signal path 3' share the same elements band pass filter 19, 19' and analogue to digital converters 195, 195' Band pass filters 19, 19' delimit the incoming signals from noise stemming from other RF sources in the environment than the reference transmitter in question
Analogue to digital converters 195, 195' transform the signals into the digital domain sig- nals S1 and SV, respectively
In the reference path 3, a replication stage 200 is provided Here, the digital signal S1 output from the analogue to digital converter 195 of the reference signal path is transformed into a bit stream S2 by digital to bit stream converter 197 which substantially cor- responds to a data bit stream that is basis for the radio frequency signal transmitted from the third party transmitter 11 This bit stream 2 may for instance correspond to a binary bit stream It is understood though, due to signal degradation and multipath fading, that the bit stream S2 is not identical to the bit stream transmitted from the transmitter 11
According to the present invention, a digital error correction algorithm c(m) is performed in error correction stage 199 on the bit stream S2 in the reference signal path 3 Thereby, the received signal S1 is "repaired" and an improved estimate S3 of the originally transmitted bit stream is established
The error correction mechanism c(m) performed in stage 199 should be chosen to correspond to the radio protocol error correction mechanism as used by the transmitter / receiver of the third party system In case the third party transmitter 1 1 is a DVB-T system, a corresponding error correction stage as used in consumer DVB-T receivers according to such a system is incorporated in the error correction stage 199 of the passive radar system according to the invention
Depending on what particular third party transmitter is utilized, a corresponding error correction method is chosen in accordance with error correction mechanism data stored in database 198 Other error correction mechanisms corresponding to those used by other types of transmitters, e g GSM or VVCDMA transmitters are suitably also stored in data base 199 It is moreover foreseen that the external transmitter 11 could be constituted by not a third party but a system inherent component, e g a dedicated transmitter
The improved estimated bit stream S3 is converted back into a digital format S1__C which corresponds to the digital bit format SV provided by A/D converter 195' in target signal path 3' The improved reference signal S1_C and the target signal S1 ' are fed to cross correlation stage 21 of the passive radar system according to the invention
In other words there is provided a Processing unit 1 for a radar system 1 adapted to de- tect targets 14 by means of signal reflections caused by a transmitter 11 , the transmitter 11 being adapted for transmitting radio frequency signals A which contain an underlying digital bit stream signal S1 which may be processed according to a radio protocol error correction mechanism c(m) utilized by external receivers 17
The radar system 11 comprises at least one antenna 15, 16, a target signal path 3' - at least conveying signal contributions as reflected from a potential target 14 - and a reference signal path 3 - at least conveying signal contributions from the transmitter 11 - wherein both respective signal paths comprise analogue to digital converters 195, 195' providing respectively a reference signal S1 and a target signal SV The reference signal path 3 moreover comprises a replication stage 200 comprising a digital to bit stream converter 197, an error correction stage 199 and a bit stream to digital converter 201 The error correction stage incorporates a radio protocol error correction mechanism c(m) corresponding to the error mechanism used by the external receivers, the bit stream to digital converter 201 providing an improved estimate S1__C of the digital data steam S1 underlying the radio frequency signal transmitted from the transmitter 1 1
According to a further aspect of the invention there is provided a radar system comprising the processing stage 1 comprising a cross correlation stage 21 , wherein the improved estimate S1__C of the bit stream of the reference path 3 and the bit stream SV of the target path 3' are fed to the cross correlation stage 21
It is important to get as good a reference signal as possible to be able to detect reflections from airborne objects According to the invention, the information stored in the transmitted signal is reproduced into a copy that is as close the original signal sent by the transmitter as practical possible The invention is particular effective when huge amount of digital information is transferred Such signals can be used with advantage by the passive radar system according to the invention because such signals have a high bandwidth which gives good resolution at long distances and which results in easier detection of targets When the reference signal is rendered very close to the original version sent out by the transmitter, targets are shown with more effect and with a higher precision, which again means that signals are less prone to get confused with noise The result of utilizing the invention can be likened with having a cable with direct connection to the transmitter It is noted that the transmitter could even be satellite borne A plot of auto correlation of the reference signal would decrease faster by increasing distance, which probably would reduce the amplitude of the distance side lobes The only reflections in distance would be created by the signal itself
Another advantage is that decoding the bit stream can be carried out by components that already exist in large scale on the marketplace
In fig 4, according to a still further aspect of the invention there is provided a method for a radar system 1 adapted to detect targets 14 by means of signal reflections caused by a an external transmitter 11 , the external transmitter 11 being adapted for transmitting radio frequency signals A which contain a an underlying digital bit stream signal which may be processed according to a radio protocol error correction mechanism utilized by external receivers 17, the radar system 11 comprising at least one antenna 15, 16, the method comprising the steps of
- Conveying signal contributions as reflected from a potential target 114,
- Conveying signal contributions from the transmitter 111 ,
- Analogue to digital converting respective signals 1195, 1195' providing respectively a reference signal S1 and a target signal S1 '
- Digital to bit stream converting 1197 the reference signal S1 ,
- Error correcting 1 199 the bit stream converted signal, and
- Bit stream to digital converting the error corrected signal 1201 wherein the error correction incorporates a radio protocol error correction mechanism c(m) corresponding to the error mechanism used by the external receivers, providing an improved estimate S1_C of the digital data steam S1 underlying the radio frequency signal transmitted from the transmitter 11 The figures contain the following abbreviations
ANT antenna
BPF band pass filter
A/D analogue to digital converter
D/BITST digital to bit stream converter
ERR_CORR error correction
BITST/D bit stream to digital converter
X__CORR cross correlation stage
RADAR RAdio Detection And Ranging
TV Television
GSM Global System for Mobile communications
WCDMA Wideband Code Division Multiplex Access
DVB-T Digital Video Broadcasting - Terrestrial