GB2253963A

GB2253963A - Recognition of helicopters by means of a radar installation

Info

Publication number: GB2253963A
Application number: GB8605940A
Authority: GB
Inventors: Albrecht Ludloff; Manfred Minker; Frank Hagedorn
Original assignee: Licentia Patent Verwaltungs GmbH; Telefunken Systemtechnik AG
Current assignee: Licentia Patent Verwaltungs GmbH; Telefunken Systemtechnik AG
Priority date: 1985-04-02
Filing date: 1986-03-11
Publication date: 1992-09-23
Anticipated expiration: 2006-03-11
Also published as: DE3512032C2; FR2683325B1; FR2683325A1; DE3512032A1; GB2253963B; SE8601207D0; BE904538A

Description

Method for the discovery and classification of helicopters by means of a radar installation.

The invention concerns a method for the discovery and classification of helicopters by means of a radar installation.

The discovery of flying targets by means of monitoring radar install ations is based essentially on the evaluation of the radial speed of the flying object relative to the position of the radar. in thar case, the target is picked up for less than about 50 milliseconds during each rotation of the aerial and reflects a series of echo pulseshich are evaluated by the radar signal processing.

When the radial component is absent, as for example during tangential flight, the change in position from aerial rotation to aerial rotation serves as flying target criterion.

In so far as a helicopter carries out similar flying movements, it is discovered and indicated by the radar in the same manner as other flying targets, but is however not without further measures classifiable as a helicopter, i.e. not distinguishable from other moving targets such as for example, rigid wing aircraft.

A property, which is particularly important for fighter helicopters, consists in the possibility of hovering stationarily at only a low height, whereby the radial speed component as well as also a change in position do not apply for the discovery. The problem in such a situation consists in classifying the helicopter in an environment of false targets (fixed target clutter residues, angels, road traffic, rain clutter) as helicopter.

The target dwell time, i.e. the time for which an individual point target is illuminated by the aerial lobe of the radar, is mostly smaller than 50 milliseconds in radar monitoring installations in the interest in the interest of a low circulation time and thereby a high rate of renewal of information so that a detailled evaluation of the spectrum of the target echo or of the temporal course of the echo of the same target is generally not possible.

It is therefore the tak of the present invention to state a method for the improved discovery and recognition of helicopters by means of a radar installation with small target dwell times.

The method according to the invention is described in the patent claim 1. Advantageous refinements and developments of the invention are described in the subclaims.

The invention exploits the recognition that, apart from the spectrally narrow-band echo components from the fuselage of the helicopter, the echo components emanating from the main rotor and the tail rotor are widely distributed spectrally by comparison with the usual pulse repetition frequencies (PRF) of radar monitoring installations. Through the overfolding of the radar echos into the unambiguous Doppler frequency range, the threshold values are exceeded in the majority of the individual filters of the Doppler filter bank and a small scatter of the amplitude values of the output signals of the Doppler filters result for a high percentage of possible situations.

Starting out from this recognition, it is tested according to the invention whether the discovery threshold is exceeded in the majority, preferably in more than 75% of the Doppler filters. In a given case, the width of that amplitude band is determined, which is limited by the amplitude of the greatest and the smallest filter output signal of the individual filter concerned.

According to an advantageous development, the amplitudes of those Doppler filters, which contain the fuselage echo of the helicopter, are not taken into consideration. This is advantageously attained thereby, that the n greatest amplitudes of the output signals of the Doppler filters are not taken into consideration in the determination of the width of the amplitude band, wherein n shall be small compared with the total number N of the Doppler filters and n < N/3 preferably applies.

In this manner, even great amplitudes of possibly arising ground clutter in the fixed target filters let themselves be excluded from the evaluation. For the suppression of ground clutter, there can also be provided a store with resolution-wise or area-cell-wise storage of known ground targets (clutter map). in accordance with which store the output signals of the fixed target filters are in accordance with stored clutter intensity excluded from the evaluation for the helicopter classification.

The decision about the presence of a helicopter target is derived from the ascertained width of the amplitude band. To a first approximation, it then applies that the probability of the presence of a helicopter target is the greater, the narrower is the amplitude band. Apart from the yes/no decision by comparison of the ascertained width of the amplitude.band, it can advantageously be provided to allocate a respective probability value for the presence of a helicopter target to the target reports and to issue or represent this value along with the issue or representation of the target report. The target decision, too, can also be taken from the comparison of the probability value with a threshold value instead of directly from the width of the amplitude band.

The method according to the invention can be enhanced by an additional circulation correlation process. For the suppression of false target reports, which arise through interference with other radar installations or jamming devices, it can be provided to compare target reports from successive radar periods or successive blocks (bursts) of the Doppler processing and to conclude about a genuine target only in the case of two or more azimuthally successive target reports in the same distance ring.

The determination of the width of the amplitude band and the decision about the presence of a helicopter target preferably take place off-line, for which a programmable processor, possibly present for the general target extraction, is advantageously by additional program parts drawn upon also for the classification of helicopters.

The invention is illustrated in the following sill by an example of embodiment with reference to the drawings.

Fig. 1 shows the typical spectrum of a rigid wing aircraft in the form of the signal amplitudes at the output of the filters by the example of a Doppler filter band with 14 individual filters (filters O ... 13).

The threshold is exceeded in only three filters, filter O in addition showing a strong echo which emanates from ground clutter. Since the threshold is exceeded in only 4 filters out of 14 altogether, a "rigid wing aircraft" is decided on.

Fig. 2 shows the spectrum of a helicopter, the cell (fuselage) of which displays the same radial speed as the rigid wing aircraft in Fig. 1.

The ground clutter echo is likewise present. One recognises that the discovery threshold is exceeded in 13 out of 14 filters. Thereby, a first criterion for the recognition of helicopters is fulfilled. If one does not take into consideration the n=4 greatest amplitudes which emanate from the ground clutter and from the fuselage of the helicopter, then there results the amplitude band of the width 0, bounded by the fifth greatest amplitude and the smallest amplitude, shown in Fig. 2. B is compared with a threshold. If O is smaller than this threshold, a helicopter is recognised. The lower possible limit of the amplitude band is the discovery threshold.

It is evident from rig. 2 that the proposed method is applicable to floating (hovering) helicoptars as well as also such disposed in translational flight for the differentiation between helicopters and rigid wing aircraft.

Fig. 3 shows the build-up of the signal processing train of an MTD radar (MTD:= moving target detector) or of a pulse Doppler radar, within which the helicopter recognition is performed.

The intermediate frequency signal is fed to the phase detectors Phl and Ph2, there mixed down into the video position (intermediate frequency = 0) and filtered in the single pulse matched filters Mr. The in-phase signals (I) and the quadrature signals (Q) are scanned in the analog-to-digital converters and converted from analog to digital. They are fed to the Doppler filter bank OF consisting of N filters, here 14. Their output signals are for the saving of hardware fed to a single CFAR detector 8, which works down all filters in time multiplex. On a threshold value being exceeded, a filter report is generated in the formatter F, which report inter alia contains the distance cell, the filter number and the filter output amplitude.

The filter reports buffered in F are fed to the parameter extractor Ex.

This searches - for helicopter recognition - in every distance cell, whether nearly all filters have reported, for example 11 of 14 individual filters. If yes, it calculates the width 8 of the amplitude band as already described and compares the width with a threshold. If B remains below this threshold, a helicopter is recognised and a corresponding report Z is issued. The number n of the greatest amplitudes of the Doppler filter bank, which are to be suppressed, can be predetermined for the extractor by the radar operator for each input.

Claims

1. Method for the discovery and classification of helicopter targets by means of a radar installation with the use of a Doppler filter bank with several individual filters for recognition of fixed targets and of moving targets, characterised thereby, that on the occurrence of threshold values being exceeded in the majority of the individual filters, the width of that amplitude band of the participating filters is ascertained, which is limited by the amplitude of the greatest and the smallest filter output signal, and that a decision on the presence of a helicopter target report is derived from the ascertained width of the amplitude band.

2. Method according to claim 1, characterised thereby,that the width of the amplitude band is ascertained only on threshold values being exceeded in more than 75% of the individual filters.

3. Method according to claim 1 or 2, characterised thereby, that the n greatest amplitudes are not drawn upon for ascertaining the width of the amplitude band, wherein n is substantially smaller than the total number of the individual filters.

4. Method according to claim 3, characterised thereby, that n is less than one third of the total number of the individual filters.

5. Method according to claim 3, characterised thereby, that n lies between one and four.

6. Method according to one of the claims 1 to 5, characterised thereby, that the amplitudes of the output signals of the fixed target filters of the filter bank are compared with stored values for fixed targets (clutter map) and the output signals of the fixed target filters are not drawn upon for ascertaining the amplitude band on the presence of a fixed target in the store.

7. Method according to one of the claims 1 to 6, characterised thereby, that the ascertained width of the amplitude band is compared with a predeterminable threshold value and the presence of a helicopter target report is recognised in the case of bandwidths lying below this threshold value.

8. Method according to one of the claims 1 to 7, characterised thereby, that a probability value is allocated to a target report according to the ascertained width of the amplitude band, wherein a higher value corresponds to a narrow amplitude band than to a wide amplitude band.

9. Method according to claim 8, characterised thereby, that the probability value is compared with a predeterminable threshold and the presence of a helicopter target report is recognised only for values lying above this threshold.

10. Method according to claim 8 or 9, characterised thereby, that on the issue or representation of the target report, an information about the allocated probability value is issued or represented therewith.

11. Method according to one of the claims 1 to 10, characterised by an additional circulation correlation process.

12. Method according to one of the claims 1 to 11, characterised thereby, that target reports from successive radar periods or successive blocks of the Doppler processing are compared.

13. A method for the detection of helicopter targets in the midst of other target and clutter returns, the method being substantially as hereinbefore described with reference to and as illustrated by the accompanying drawings.

13. Method according to one of the claims 1 to 12, characterised thereby, that the ascertaining of the width of the amplitude band and the decision on the presence of a helicopter target take place off-line.

Amendments to the claims have been filed as follows 1. A method for the detection of helicopter targets in the midst of other target and clutter returns by means of a radar installation with the use of a Doppler filter bank with a plurality of individual filters for recognition of fixed targets and of moving targets, wherein on the occurrence of threshold values being exceeded in more than half of the individual filters, the width of that amplitude band of at least two thirds of the plurality of individual filters is ascertained, which is limited by the amplitude of the greatest and the smallest filter output signal from those filters which exceed the threshold values, and a decision on the presence of a helicopter target report is derived from tha ascertained width of the amplitude band.

2. A. method as claimed in claim 1, wherein the width of the amplitude band is ascertained only on threshold values being exceeded in more than 75,5; of the individual filters.

3. A method as claimed in either claim 1 or claim 2, wherein said at least two thirds of the plurality is so selected that the n greatest amplitudes are not drawn upon for ascertaining the width of the amplitude band, wherein n is less than one third of the total number of the individual filters.

4. A method as claimed in claim 3, wherein n lies in the range of 1 to 4.

5. A method as claimed in any one of the preceding claims, wherein the amplitudes of the output signals of the fixed target filters of the filter bank are compared with stored values for fixed targets (clutter map) and, on the presence of a fixed target in the store, the output signals of the fixed target are not drawn upon for ascertaining the amplitude band.

6. A method as claimed in any one of the preceding claims, wherein the ascertained width of the amplitude band is compared with a predeterminable threshold value and the prescence of a helicopter target report is recognised in the case of bandwidths lying below this threshold value.

7. A method as claimed in any one of the preceding claims, wherein a probability value is allocated to a target report in dependence on the ascertained width of the amplitude band, wherein a higher value corresponds to a narrow amplitude band than to a wide amplitude band.

8. A method as claimed in claim 7, wherein the probability value is compared with a predeterminable threshold and the presence of a helicopter target report is recognised only for values lying above this threshold.

9. A method as claimed in either claim 7 or claim 8, wherein on the issue or representation of the target report, information on the allocated probability value is either issued or represented therewith.

10. A method as claimed in any one of the preceding claims and comprising the additional step of a scan-to-scan correlation.

11. A method as claimed in any one of the preceding claims and comprising the step of comparing target reports from successive radar periods or successive blocks of the Doppler processing.

12. A method as claimed in any one of the preceding claims, wherein the ascertaining of the width of the amplitude band and the decision on the presence of a helicopter target take place off-line.