GB2073531A - Tracking radar - Google Patents

Abstract

In a low-angle tracking radar frequency diversity is used to reduce or eliminate drastic loss of signal at a particular frequency. The invention proposes a comparison of all the echo returns from a single target to determine whether or not a majority of the returns agree within predetermined limits. If there is such a majority in agreement then only those returns are averaged to determine the output. If the majority of returns do not agree within the predetermined limits then all the returns are averaged.

Description

SPECIFICATION Tracking radar This invention relates to tracking radars of the type employing diversity, e.g. frequency diversity, whereby a target is illuminated at least three times during a single radar scan, the illuminations all occurring during an interval short enough to ensure a substantially unchanged geometrical disposition between the radar and the target.

bin a low angle tracking radar, in the presence of multipath, the geometrical disposition of the target(s) and radar can be such that, at particular critical frequencies, severe reduction in total returned signal can occur. This can be further compounded by an unfortunate fluctuation in effective target echo area, due to target aspect changes brought about by target motion. Such conditions almost inevitably result in temporary loss of reliable tracking.

If however, a frequency diverse radar is employed, it is unlikely that such a drastic loss of signal will be manifest at all frequencies. Indeed, although tracking may well be poor at a particular choice of frequency, at others it may well be unaffected so therefore reliable.

According to the present invention there is provided a radar of the type hereinbefore defined including means for comparing all the different echo returns from a target during a single scan to determine whether there is agreement within predetermined limits between at least a majority of the compared returns and means for averaging only the results of the returns having such agreement.

In a preferred embodiment of the invention there is provided means for averaging the results of all the target returns if the comparing means indicates that at most only a minority of the returns are in agreement within said limits.

In a typical embodiment of the invention, e.g. a tracking radar using an electronically steerable antenna array coupled with array signal processing (ASP) to reduce or eliminate the effect of multi-path signals, it is assumed that frequency diversity is employed, with the radar effectively "looking" at each target 3 times during a single scan. Three adequately spaced r.f. frequencies are used to avoid repeated occurrence of return signal loss at any one particular frequency. The tracking of the radar should be generally well-behaved such that the typical r.m.s. variations in indicated tracking angle are 4 to 5 times less than the deviation of the occasional outlying values to be identified and rejected. The three tracking angles resulting from the 3 "looks" at the different frequencies in substantially the same geometrical situation are compared.If two of the results agree to within an r.m.s. tracking variation factor of, say 3x (representative of typical well-behaved tracking operation), but the third differs from the other two by more than this prescribed value, then the third result is considered likely to be unreliable and is rejected. "Reduced" averaging of the other two values then takes place. In all other cases a simple averaging of the tracking angle over the full three results is effected.

It may be that a return lying just outside the 3 x r.m.s. deviation limit in fact prodyces a less accurate averaging result involving all 3 returns than if it had been marginally closer, thus causing a more useful reduced average instead. Thus in practice it may be advisable to include returns lying in the 3 to 5 x r.m.s. deviation in the reduced averaging process after applying a weighting factorto such returns. The weights to be so assigned should bear some inverse relationship to the effective r.m.s. deviation. Results outside the 5 x r.m.s. deviation would still be totally rejected.

Whilst a minimum of3 echo returns per scan of each target are required the invention can be extended in principle to include more samples, providing such samples are at adequately spaced frequencies, i.e. non-redundant.

1. Aradarofthetype hereinbefore defined including means for comparing all the different echo returns from a target during a single scan to determine whether there is agreement within predetermined limits between at least a majority of the compared returns and means for averaging only the results of the returns having such agreement.

2. A radar according to claim 1 including means for averaging the results of all the target returns if the comparing means indicates that at most only a minority of the returns are in agreement within said limits.

3. A radar according to claim 1 or 2 wherein said predetermined limits are 3 x r.m.s. tracking variation of the majority of the returns.

4. A radar according to claim 3 including means for applying to returns lying within the range 3 to 5 x r.m.s. tracking variation of the majority of the returns, when said majority lie within 5 x r.m.s. variation, weighting having an inverse relationship to the amount of such variations, all the returns lying within the 5 x r.m.s. variation limit then being averaged.

5. Atracking radarsubstantiallyas herein described.

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

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Tracking radar This invention relates to tracking radars of the type employing diversity, e.g. frequency diversity, whereby a target is illuminated at least three times during a single radar scan, the illuminations all occurring during an interval short enough to ensure a substantially unchanged geometrical disposition between the radar and the target. bin a low angle tracking radar, in the presence of multipath, the geometrical disposition of the target(s) and radar can be such that, at particular critical frequencies, severe reduction in total returned signal can occur. This can be further compounded by an unfortunate fluctuation in effective target echo area, due to target aspect changes brought about by target motion. Such conditions almost inevitably result in temporary loss of reliable tracking. If however, a frequency diverse radar is employed, it is unlikely that such a drastic loss of signal will be manifest at all frequencies. Indeed, although tracking may well be poor at a particular choice of frequency, at others it may well be unaffected so therefore reliable. According to the present invention there is provided a radar of the type hereinbefore defined including means for comparing all the different echo returns from a target during a single scan to determine whether there is agreement within predetermined limits between at least a majority of the compared returns and means for averaging only the results of the returns having such agreement. In a preferred embodiment of the invention there is provided means for averaging the results of all the target returns if the comparing means indicates that at most only a minority of the returns are in agreement within said limits. In a typical embodiment of the invention, e.g. a tracking radar using an electronically steerable antenna array coupled with array signal processing (ASP) to reduce or eliminate the effect of multi-path signals, it is assumed that frequency diversity is employed, with the radar effectively "looking" at each target 3 times during a single scan. Three adequately spaced r.f. frequencies are used to avoid repeated occurrence of return signal loss at any one particular frequency. The tracking of the radar should be generally well-behaved such that the typical r.m.s. variations in indicated tracking angle are 4 to 5 times less than the deviation of the occasional outlying values to be identified and rejected. The three tracking angles resulting from the 3 "looks" at the different frequencies in substantially the same geometrical situation are compared.If two of the results agree to within an r.m.s. tracking variation factor of, say 3x (representative of typical well-behaved tracking operation), but the third differs from the other two by more than this prescribed value, then the third result is considered likely to be unreliable and is rejected. "Reduced" averaging of the other two values then takes place. In all other cases a simple averaging of the tracking angle over the full three results is effected. It may be that a return lying just outside the 3 x r.m.s. deviation limit in fact prodyces a less accurate averaging result involving all 3 returns than if it had been marginally closer, thus causing a more useful reduced average instead. Thus in practice it may be advisable to include returns lying in the 3 to 5 x r.m.s. deviation in the reduced averaging process after applying a weighting factorto such returns. The weights to be so assigned should bear some inverse relationship to the effective r.m.s. deviation. Results outside the 5 x r.m.s. deviation would still be totally rejected. Whilst a minimum of3 echo returns per scan of each target are required the invention can be extended in principle to include more samples, providing such samples are at adequately spaced frequencies, i.e. non-redundant. CLAIMS

1. Aradarofthetype hereinbefore defined including means for comparing all the different echo returns from a target during a single scan to determine whether there is agreement within predetermined limits between at least a majority of the compared returns and means for averaging only the results of the returns having such agreement.

2. A radar according to claim 1 including means for averaging the results of all the target returns if the comparing means indicates that at most only a minority of the returns are in agreement within said limits.

3. A radar according to claim 1 or 2 wherein said predetermined limits are 3 x r.m.s. tracking variation of the majority of the returns.

4. A radar according to claim 3 including means for applying to returns lying within the range 3 to 5 x r.m.s. tracking variation of the majority of the returns, when said majority lie within 5 x r.m.s. variation, weighting having an inverse relationship to the amount of such variations, all the returns lying within the 5 x r.m.s. variation limit then being averaged.

5. Atracking radarsubstantiallyas herein described.