Radar signal processors

Radar signal processor utilizing a technique for detecting targets moving slowly and clutter. The technique comprises dividing the clutter spectrum into a large number of narrow bands and in each band measuring the power ratio between upper and lower side bands. When this ratio exceeds a fixed threshold, the target is declared and, if necessary, its velocity signature can be detailed.

This invention relates to radar signal processors and is concerned with a 
new processing technique for detecting targets moving slowly in a radial 
direction with respect to a radar set, in clutter, and more especially it 
is concerned with the problem of detecting targets on the ground. 
Targets on the ground are necessarily amid ground clutter. They can usually 
be detected only by their movement. Detection is difficult when the 
clutter contains vegetation that moves with the wind especially when the 
target is moving slowly. The problem is particularly severe when the 
targets are moving at about the same speed as the clutter. 
Various attempts have been made to solve the problem by distinguishing 
between objects which are progressing and those which are receding with 
respect to the radar set. These attempts have met with limited success 
mainly because vegetation does not move sinusoidally and its reflecting 
area changes as it moves. Various adaptive systems have been used and 
perhaps the most noteworthy is the Kalmus system wherein the power in 
upper and lower sidebands of an echo signal are compared and a target is 
declared when the ratio exceeds a predetermined threshold. However the 
upper and lower sidebands of the clutter spectrum are not necessarily of 
equal power and so this sytem is not entirely satisfactory. 
According to the present invention we provide a radar system comprising 
means for producing I and Q video signals, filter means for splitting each 
of the I and Q video signals into a plurality of different frequency bands 
so that a pair of I and Q signals are provided for each band, a plurality 
of comparator means each responsive to a pair of I and Q signals so that 
one comparator means is provided for each of said frequency bands, said 
comparator means being operative to compare in each frequency band the 
power in the upper sideband with the power in the lower sideband to 
provide an output signal indicative of the ratio therebetween and a 
plurality of threshold detector means one responsive to each comparator 
means for providing an output signal indicative of the presence of a 
target if a predetermined threshold level is exceeded. 
Thus the signal processing technique utilized in a system according to the 
present invention is to divide the clutter spectrum into a plurality of 
narrow frequency bands, and in each band to measure the power ratio 
between upper and lower sidebands. When this ratio exceeds a predetermined 
threshold level a target is delcared and if needed its velocity signature 
or direction with respect to the radar set can be indicated. 
In an analogue system the filter means may comprise a plurality of 
individual filters so that the filter means comprises a filter bank. 
In a digital system however the filter means may comprise a fast fourier 
transform digital processor. 
The comparator means may each comprise a phase quadrature device to which 
the I signal is fed, a first adder means responsive to the Q signal and 
the output of the phase quadrature device for providing a signal 
characteristic of one sideband signal which is fed to first squarer means, 
a second adder responsive to the Q signal and to the I-signal fed from the 
phase quadrature device via an inverter for providing a signal 
characteristic of the other sideband signal which is fed to second squarer 
means, the first squarer means being arranged to feed via a second 
inverter a third adder fed also from the second squarer thereby to provide 
an output signal from the comparator means.

Referring now to FIG. 1 a radar I.F. signal is fed to a pair of phase 
sensitive detectors 1 and 2 which are fed with an intermediate frequency 
reference oscillator signal from an oscillator 3. The oscillator 3 feeds 
the phase sensitive detector 1 directly and the phase sensitive detector 2 
via a phase quadrature network 4. Thus baseband I and Q video signals are 
provided at the output of the phase sensitive detectors 1 and 2 on lines 5 
and 6 respectively. The signals 5 and 6 are fed to filter banks 7 and 8 
respectively which each include a number of bandpass filters 9, 10, 11, 12 
and 9a, 10a, 11a and 12a. Although in the drawing each filter bank is 
shown to include five filters very many more filters may be in practice 
provided. The filters 9, 10, 11 and 12 are arranged to occupy adjacent and 
overlapping bands so as completely to cover the required bandwidth. In 
practice each filter may cover a bandwidth of 10 Hz and a total bandwidth 
of around 400 Hz may be covered with a centre frequency at around 200 Hz. 
Although the frequency is substantially baseband the lowest frequency of 
interest in practice might be about 20 Hz. Corresponding pairs of output 
signals from the filters are fed each to a power difference circuit such 
as the power difference circuits 13 and 14 shown. Thus the power 
difference circuit 13 is fed from the filters 10 and 10a and the power 
difference circuit 14 is fed from the filters 12 and 12a. Thus one power 
difference circuit will be provided for each pair of filters and each 
power difference circuit will be fed with I and Q signals in a very narrow 
frequency band. The power difference circuit, which will be later 
described in more detail with reference to FIG. 3, is operative to compare 
the power in the upper sideband with power in the lower sideband to 
provide a positive or negative signal output depending upon which power is 
greater. Each power difference circuit is arranged to feed a threshold 
detector only one of which referenced 15 is shown. The threshold circuit 
provides an output signal if the input signal from its associated power 
difference circuit 14 exceeds a predetermined threshold level. In practice 
the threshold detectors are bipolar devices such that an output signal is 
provided if the input signal exceeds the threshold level in a positive or 
in a negative direction. Thus an output signal from each threshold 
detector is provided if a target is indicated within the bandwidth of its 
associated filter and the polarity of the output signal will indicate in 
which direction the target is moving i.e. towards or away from the radar 
set. It will be appreciated that the speed in a radial direction with 
respect to the radar set will be also indicated in dependence upon which 
filter receives the signal providing an output indication. Thus the 
nominal radar frequency might be equivalent to the 200 Hz centre frequency 
and signals appearing in the sidebands will correspond to doppler shifted 
signals such that the further such signals are away from the 200 Hz 
nominal centre frequency then the greater will be their radial speed 
relative to the radar set. 
In order to provide for digital processing an arrangement is provided as 
shown in FIG. 2 wherein corresponding parts carry the same numerical 
designation as FIG. 1. In FIG. 2 the I and Q signals on lines 5 and 6 
respectively are fed to analogue to digital converters 16 and 17 which are 
arranged to feed faster fourier transform processors 18 and 19 
respectively. The fast fourier processors 18 and 19 operate to provide a 
similar result to the filter banks 8 and 9 as shown in FIG. 1 and produce 
a corresponding number of frequency output signals wherein I and Q signals 
are provided for each of a number of frequency bands. As described with 
reference to FIG. 1 corresponding signals I and Q in each band are fed to 
power difference circuits 20, 21 and 22 only three of which are shown and 
these circuits are arranged to feed bipolar threshold detectors 23, 24 and 
25. It will be appreciated however that the power difference circuits and 
bipolar threshold detectors are provided for each pair of frequency output 
signals from the processors 18 and 19. The circuit as just before 
described with reference to FIG. 2 operates in a precisely analogous 
manner to the analogue processor described with reference to FIG. 1. The 
power difference circuit 13, 14 or 20, 21, 22 may be constructed as 
described with reference to FIG. 3. 
Referring now to FIG. 3 the I signal from a filter bank and the 
corresponding Q signal are fed to a pair of adders 26, 27, the I signal 
being fed via a phase quadrature device 28. The output of the phase 
quadrature device 28 is fed directly to the adder 26 with a Q signal and 
to the adder 27 via a phase inverter 29 the adder 27 being fed also with a 
Q signal. Signals corresponding to the lower and upper sideband 
respectively are thus provided on lines 30 and 31 which are fed to 
squarers 32 and 33 to provide on output lines 34 and 35 signals 
corresponding to the power in the upper and lower sideband respectively. 
The signal on line 34 is fed via a phase inverter 36 to a summing device 
37 which is fed also via the line 35 to provide an output signal on line 
38. Thus it will be appreciated that the sense of the signal on the line 
38 will indicate the direction of movement of the target with respect to 
the radar set. Signals on the line 38 are fed to an associated bipolar 
threshold detector such as the detector 15 or the detectors 23-25 one of 
which is provided for each pair of filters. Bipolar threshold detectors 
are well known and may be fabricated in any conventional manner. 
The performance of a radar set using the new technique has been calculated 
using target and clutter data believed to represent all types of ground 
clutter and all likely meteorological conditions. In clutter where the 
target and clutter spectro overlap a radar using the new technique can be 
expected to detect a moving man with a sub clutter visibility of -45 dB. 
The corresponding figure for a radar with Kalmus filtering would be about 
-30 dB and for a pulse doppler radar very much less.