Radar apparatus provided with a coherent clutter map

A radar apparatus provided with transmitter means (1), rotating antenna means (2) and receiver means (3) for the transmission per burst and the processing in a video processor of radar echo signals. The video processor includes moving target detection unit (4) provided with a doppler filter bank, for instance an FFT processor, and slow moving target detection unit (6), provided with coherent clutter maps, one map for each radar transmitter frequency used. The coherent clutter maps are also used for reducing the clutter strength of radar echo signals which are applied to the moving target detection unit (4), by subtracting the coherent clutter strengths stored in the clutter maps from the radar echo signals.

BACKGROUND OF THE INVENTION 
The invention relates to a radar apparatus comprising antenna means, 
transmitter means connected to the antenna means, for generating and 
transmitting pulses, receiver means connected to the antenna means, for 
the reception, per transmitted pulse, of a radar echo signal, a video 
processor connected to the receiver means, comprising a slow moving target 
detection unit provided with a coherent clutter map divided into 
range-azimuth cells. 
What are meant by slow moving targets are targets with radial speeds so low 
that they are not, on the basis of their speeds, discernable from clutter. 
What is meant by a coherent clutter map is a clutter map in which clutter 
information is stored as complex values, representing complex clutter 
strengths. 
Such an apparatus is known from the report "Low Doppler target detection in 
ground clutter" by J. S. Bird, November 1985, Communications Research 
Centre, Ottawa, Canada. According to this report, a coherent clutter map 
will only serve a useful purpose when used in a non-moving antenna, such 
as a step-scan phased array antenna. 
SUMMARY OF THE INVENTION 
The present invention eliminates this drawback and is characterized in that 
the antenna means are mounted for rotation around an axis. 
This is based on the inventive thought that it suffices to design the 
transmitter means such that radar transmissions take place in exactly the 
same directions, from scan to scan. To this end, the invention is further 
characterized in that the antenna means are provided with azimuth-values 
generating means for connection to the transmitter means. Moreover, it is 
characterized in that the transmitter means is provided with 
azimuth-values receiving control means for generating transmitter pulses 
at predetermined azimuth values. 
An advantageous embodiment of the invention is characterized in that groups 
of N mutually coherent transmitter pulses are generated, such that a group 
will substantially coincide with an azimuth cell. This may render the 
radar apparatus suitable for the incorporation of a moving target 
detection unit provided with a doppler filter bank, for instance of FFT 
unit. 
Another advantageous embodiment of the invention is characterized in that 
the control means generates consecutive groups with M (M=1, 2, 3, . . . ) 
different radar frequencies. It will then be required, however, to provide 
the slow moving target detection until with M coherent clutter maps, each 
map on a one-to-one basis added to each radar transmitter frequency. 
If the radar apparatus comprises a moving target detection unit provided 
with a doppler filter bank, the information contained in the clutter maps 
may be used most inventively for substantially eliminating leakage, 
well-known in the art from the zero velocity filter into adjacent filters. 
The invention is thereto also characterized in that the video processor is 
provided with a subtracter circuit connected to the input of the moving 
target detection unit, for reduction of radar echoes supplied to the 
moving target detection unit by coherent clutter signals originating from 
the clutter map.

DETAILED DESCRIPTION OF THE INVENTION 
Radar apparatuses are usually designed for observing moving objects, such 
as aircraft. For this purpose, they are provided with a moving target 
detection unit, in which radar echo signals originating from consecutive 
radar transmitter pulses are compared per range quant. In the event of a 
moving object, the phase of the echo signal in the range quant containing 
the target will continuously vary. These phase variations enable the 
detection of the object. A sophisticated moving target detection unit will 
in most cases be designed as a doppler filter bank operating in 
quadrature, which may be provided with an FFT-unit, which in fact 
determines the doppler frequency for a target. 
In a radar apparatus provided with a moving target detection unit, a 
problem well-known in the art may occur for an aircraft flying 
tangentially, consequently at zero radial velocity. Without any additional 
measures, such an aircraft will disappear from the radar display, since 
consecutive echo signals always possess the same phase. In a search radar 
according to the state of the art, these additional measures involve the 
incorporation of a clutter map. This is a memory field divided into 
range-azimuth cells in which for each range-azimuth value in the 
associated range-azimuth cell the mean radar echo strength, obtained after 
a number of antenna rotations, is stored. This stored mean value, a 
scalar, represents the clutter strength. In each subsequent antenna 
rotation, the clutter strength is updated by means of a recursive filter. 
If a range-azimuth cell contains a target flying tangentially, or a 
hovering helicopter in a range-azimuth cell, the measured radar echo 
strength will generally increase. By continuously comparing the measured 
radar echo strength and the clutter strength, such a stationary or 
slow-moving target may nevertheless be detected. 
For a radar apparatus provided with a rotating antenna, the clutter 
strength is stored as a scalar. Storage as a complex number, which would 
enable a comparison in quadrature of the measured radar echo strength and 
the clutter strength, is believed to operate unsatisfactory. The antenna 
movements cause objects to be measured slightly differently from scan to 
scan, so that a clutter cell has no constant phase. The amplitude is less 
affected by this; by determining the mean value of measured radar echo 
strengths in a clutter cell, it is therefore possible to realize a 
suitable system. 
In case of a step-scan phased array antenna, as in the report referred to 
in the introduction, it will be possible, though, to measure the clutter 
strength for each range-azimuth cell in quadrature. Clutter strengths thus 
measured have been found to be fairly constant for a prolonged period of 
time. Particularly strong echoes which may be produced by towers, blocks 
of flats or rock formations hardly vary in time. On the other hand, echoes 
produced by bushes will show far greater variations. In general, a model 
will be attained, in which a range-azimuth cell is provided with a complex 
clutter strength which is assumed to be substantially constant and a 
scalar spread modulus, which is a measure for the constancy of the clutter 
strength. This is illustrated in FIG. 1A, in which a vector A in the 
complex plane represents the complex clutter strength which is assumed to 
be practically constant, whereas spread modulus r defines a disc-shaped 
spread area in the complex plane surrounding the clutter strength. On the 
basis of consecutive measurements and statistical methods well-known in 
the art, the spread modulus r is selected such that the chance of new 
measurements coming within the disc-shaped area determined by r can be 
assessed in advance. The probability of non-occurrence of this event 
corresponds with a false-alarm probability per measurement defined for the 
radar apparatus. 
If a measurement indicates the presence of a new object in a certain 
range-azimuth cell, which object causes an echo characterized by vector B, 
FIG. 1B shows how the resulting vector A+B comes outside the disc-shaped 
area determined by r and will consequently yield a detection. It is noted 
that in a comparison based on a scalar clutter strength, this target would 
not have been detected since the value of modulus A+B does not exceed the 
modulus of A. In fact, the complex clutter strength always yields a better 
result than the scalar clutter strength, although this is particularly the 
case for strong clutter echoes having a low spread modulus r. 
FIG. 2 shows a block diagram of the radar apparatus according to the 
invention. Transmitter means 1 generate transmitter pulses which are 
transmitted by means of the rotating antenna means 2. Echo signals of 
transmitter pulses received by antenna means 2 are fed to receiver means 3 
which apply the received and digitized echo signals to a moving target 
detection unit 4. Transmitter pulses are usually transmitted in bursts of 
N identical pulses, the moving target detection unit comprising an FFT 
processor for detecting comparatively fast moving targets, which 
detections can be presented via a first video output 5 for the tracking 
and display functions. Additionally, the echoes are applied to a slow 
moving target detection unit 6, which compares the echo strengths received 
in a range azimuth cell with a clutter strength stored in clutter map 7. 
Threshold crossings result in detections, presented via a second video 
output 8 for tracking and display functions. The N pulses in a burst are 
not required to be identical. Their relative coherence is sufficient. 
Receiver means 3 may then be arranged such that different transmitter 
pulses generate, as known in the prior art, identical radar echoes at the 
output of receiver means 3. 
As known in the art, the antenna means 2 are equipped with an angle 
indicating device 9, which yields the antenna azimuth values for tracking 
and display purposes as well as with slow moving target detection unit 6 
for addressing clutter map 7. In the radar apparatus according to the 
invention, the azimuth value is also presented to transmitter means 1. 
These means are such designed as to generate transmitter pulses on the 
basis of predetermined azimuth values. In a specific embodiment for 
example, the area of coverage of the radar apparatus is divided into 360 
azimuth sectors of 1 degree and 4096 range quants of 160 m. Each time the 
azimuth value shifts one degree, one pulse may be transmitted, thus 
obtaining echoes for the slow moving target detection unit 6. 
Alternatively each time the azimuth value shifts one degree, a burst of N 
mutually coherent pulses may be transmitted, N for instance corresponding 
to the number of points of an FFT unit in moving target detection unit 4. 
In this case the slow moving target detection unit 6 shall be arranged for 
the processing of a burst. The radar echo signals per range quant may be 
for example averaged over a burst with a weighted sum. This results in a 
reproducible coherent echo strength, in a substantial suppression of 
moving objects and in an improvement of the signal-to-noise ratio. 
For several radar applications, it is required or advisable to use 
different radar transmitter frequencies. One may think in this respect, 
for example to the resolution of velocity ambiguities well-known in the 
art, or to a change in frequency in the event of jamming or interference. 
For a coherent clutter map a jump in frequency is disastrous, in that the 
stored values have little or no significance any longer. This may be 
solved by building a coherent clutter map for each radar transmitter 
frequency. To this end, the radar apparatus shall comprise a link to 
enable the transmitter means 1 to communicate to slow moving target 
detection unit 6 which radar transmitter frequency is employed. The link 
can also be used for the transfer of a sync signal for each transmitted 
radar pulse. 
FIG. 3 presents a block diagram of the slow moving target detection unit 6 
and clutter map 7, the latter possibly consisting of the subclutter maps 
7.1, 7.2, . . . , 7.M, one for each radar transmit frequency employed. 
Radar echoes which are received in an azimuth cell and which originate 
from a burst, are summed per range cell in a summing and weighting unit 10 
provided with a weighting function, for instance a Hamming weighting. To 
this end, summing and weighting unit 10 receives, besides the radar 
echoes, azimuth information for establishing the start of a burst and the 
sync signal for each transmitted pulse. Echo strengths thus determined per 
range-azimuth cell are applied to a comparator circuit 11, to which the 
clutter strengths and spread moduli, associated with clutter map 7.i which 
is related to the relevant transmitter frequency i are presented. For each 
range-azimuth cell, comparator circuit 11 subtracts the clutter strength 
from the radar echo strength, determines the modulus of the difference and 
generates a detection via link 8, if this difference exceeds the spread 
modulus plus a possible additional threshold value in order to realize a 
predetermined false-alarm probability. 
The complex echo strength determined by summing and weighting unit 10 is 
also applied to filter circuit 12, together with the associated complex 
clutter strength and the spread modulus. On the basis of the values 
supplied, filter circuit 12 determines, in a recursive process, new values 
for the clutter strength and the spread modulus, which are subsequently 
stored in clutter map 7.1. For the coherent clutter strength, use may be 
made of a filter according to the formula: 
EQU A.sub.N+1 =.alpha.A.sub.N +.beta.S.sub.N+1 (1) 
where: 
A.sub.N is the clutter strength generated by the clutter map; 
A.sub.N+1 is the new clutter strength to be stored in the clutter map; 
S.sub.N+1 is the echo strength determined by the summing and weighting unit 
10; 
.alpha., .beta. are filter coefficients (usually .alpha.+.beta.=1). 
Depending on various system parameters, the scalar filter coefficients 
.alpha., .beta. are selected such that on the one hand, the filter run-in 
time is kept relatively short as a result of which the clutter map will 
contain relevant data within a relatively short period of time and that on 
the other hand, a stationary object dwelling within a range quant for some 
period of time will not be too quickly considered as clutter. 
Similarly, for the spread modulus, a filter may be used according to the 
formula: 
EQU r.sub.N+1.sup.2 =.alpha.'r.sub.N.sup.2 +.beta.'.vertline.S.sub.N+1- A.sub.N 
.vertline..sup.2 (2) 
where 
r.sub.N is the spread modulus generated by the clutter map; 
r.sub.N+1 is the new spread modulus to be stored in the clutter map; 
.alpha.', .beta.' are filter coefficients (usually .alpha.'+.beta.'=1). 
Depending on various system parameters, the scalar filter coefficients 
.alpha.', .beta.' are once again selected in such a way that on the one 
hand, the filter run-in time will be kept relatively short and that on the 
other hand, an occasional, sharply varying measurement will not too 
seriously disrupt the spread modulus. 
For addressing clutter map 7, address generator 13 receives azimuth 
information from angle indicator 9, the sync signal and the radar 
transmitting frequency presently in use of transmitter means 1. 
In the event of a plurality of radar transmitter frequencies being used, 
consequently a plurality of complex clutter maps being used, the regular 
use of each range-azimuth cell for each frequency, enabling the clutter 
strengths and spread moduli to be updated, is a precondition for the 
proper functioning of the radar apparatus. Measurements have shown clutter 
strengths and spread moduli measured by an earth-fixed radar apparatus 
under standard conditions to remain valid for at least five minutes. 
Within this period all range-azimuth cells for all frequencies have to be 
updated i.e. used. 
The weighted sum determined in the summing and weighting unit 10 on the 
basis of the radar echoes, gives a certain degree of suppression for 
moving targets. Additional measures are nevertheless required to prevent 
moving targets from penetrating into the filter circuit 12 via summing and 
weighting unit 10, thereby modifying the contents of the clutter maps. 
These additional measures comprise an inhibit input 14 mounted on filter 
circuit 12, which is connected to output 5 of the moving target detection 
unit 4. This prevents a clutter strength being modified by a moving target 
from being stored in the clutter maps. 
A moving target detection unit processes the echoes of a burst by the 
application, per range quant, of a doppler filter process, for instance an 
N-point FFT. The output signals of the filters -N/2, . . . , -1, +1, . . . 
, N/2-1 are thresholded at the output of a doppler filter bank, and a 
threshold crossing yields a detection. The output of the zero velocity 
filter generates signals for clutter and is therefore not used. An problem 
well-known in the art is that extremely strong clutter echoes may 
unintentionally yield a threshold crossing in one of the other filters as 
a result of the usually non-optimal performance of a doppler filter. A 
most advantageous embodiment of the radar apparatus according to the 
invention obviates this problem by applying the coherent clutter strengths 
stored in the clutter maps. FIG. 4 shows how the received and digitized 
echo signals of a burst are applied to a subtracter circuit 15, each echo 
signal per range quant being reduced by the complex clutter strength 
stored for that range quant and the radar transmitter frequency used. Echo 
signals thus modified are applied to moving target detection unit 4. 
Subtraction of known strong clutter echoes prior to doppler filtering 
prevents the occurrence of the above-mentioned unintentional threshold 
passings. The threshold values applied may even be considerable lowered, 
as a result of which the sensitivity of the radar apparatus may increase.