M.T.I. radar system

A moving target indication (MTI) radar which includes TACCAR circuitry or similar means for shifting the frequency of the transmitter/local oscillator, includes a main directional antenna, and subordinate antenna elements for detecting returns from sidelobe directions. The signals from the subordinate antenna elements are modulated, and modulated return pulses are identified and employed to eliminate false pseudo moving target signals which would otherwise be received from the side lobes of the main antenna.

FIELD OF THE INVENTION 
This invention relates to Moving Target Indication Radar Systems, with an 
emphasis on Airborne Moving Target Indication radars (AMTI). 
BACKGROUND OF THE INVENTION 
An important aspect of AMTI radar systems performance involves their 
capability of detecting airborne targets against high clutter backgrounds 
and in severe jamming environments; and this capability is a direct 
function of their antenna mainlobe-to-sidelobe response ratio. Since 
targets in the main beam can have extremely small radar cross sections, 
they must compete with clutter returns both in the main beam and in those 
side lobes which might occur in the same range cell as the target. With 
high or medium pulse repetition frequency (PRF) doppler radar 
mechanizations, close-in, strong sidelobe clutter returns can totally 
obscure targets at longer ranges because of their inherent range 
ambiguity. In low PRF systems, where range is unambiguous, the target in 
the main beam needs only to compete with sidelobe clutter which occurs at 
the same range. However, strong fixed sidelobe clutter discrete signals at 
other ranges are frequently detected and will be treated as bona fide 
moving targets because of the AMTI/time averaged clutter coherent airborne 
radar (TACCAR) mechanization typically employed in such radars for 
minimizing beam clutter spread and thus, they will be tracked until their 
lack of target validity is established. This is a significant problem in 
such radars due to these sidelobe discrete signals overloading the track 
processor. 
Incidentally, TACCAR mechanizations are well known, and are discussed for 
example in the following text, entitled "Radar Handbook" by Merrill 
Skolnik, McGraw Hill, 1970, in the chapter entitled "MTI Radar-IF 
Cancellers", see pp. 17-32 to 17-37, and the bibliography on page 17-60, 
in which attention is particularly directed to footnote No. 15, the 
article referenced on page 17-32. 
Briefly, TACCAR, or Time Averaged Clutter Coherent Airborne Radar, involves 
clutter rejection in the main beam of the antenna, through a clutter 
referencing process, which locks the main beam clutter to the pulse 
repetition frequency (PRF) lines of the radar transmitting waveform. This 
is accomplished by driving the coherent or "stable" local oscillator 
(STALO) so as to always maintain the ground clutter spread around the 
pulse repetition lines (representing zero doppler shift). However, this 
has the effect of producing an artificially induced doppler shift in 
returns on the side lobes from stationary targets, and they will now 
appear as moving targets in the radar processor and will be tracked as 
bona fide targets. They will fluctuate tremendously and eventually will 
drop out of the tracker as false targets, but meanwhile they can create a 
serious computer overload situation. Incidentally, the use of the term and 
eventually will "coherent" with respect to the local oscillator is to be 
preferred as compared with the use of the term "stable", as its frequency 
is varied slightly, by the TACCAR process. 
Of course, in the absence of the use of the STALO/TACCAR process, the 
returns from stationary side lobe objects or clutter would be rejected by 
the normal moving target identification (MTI) process which cancels out 
returns which do not shift in position during successive scans, such as in 
ground-based MTI radars. 
Accordingly, one principal object of the present invention is to identify 
and to reject those STALO/TACCAR induced false targets which arise from 
the sidelobe returns from fixed or stationary objects, in airborne MTI 
radars, and also, to apply the basic Sidelobe Discrimination technique to 
other types of radars in which rejection of any sidelobe clutter is 
important, including ground-based systems. 
It is noted in passing that sidelobe discrimination methods have previously 
been proposed, in conventional directional antenna receiving systems, see 
U.S. Pat. No. 4,266,226, granted May 5, 1981, by way of example. However, 
the system disclosed in this patent does not address the radar system's 
problems. Normally, in conventional radar systems which do not use 
TACCAR/STALO mechanizations, the returns from fixed objects picked up by 
the sidelobes, could be eliminated by conventional moving target 
identification techniques, provided they are not of large amplitudes. In 
accordance with the present invention, radar systems utilizing TACCAR or 
similar processes are faced with the unusual situation that sidelobe fixed 
target returns are converted into spurious apparently moving targets; and 
the present invention involves the elimination of this problem. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a radar system which may include 
the STALO/TACCAR implementation as discussed above provides a coded 
modulation on the side lobe signals, and selectively inhibits any returns 
(targets) which carry this modulation. 
In one preferred embodiment, auxiliary antennas are provided to receive 
returns from side lobe orientations, and modulation is applied to the 
signals received on these antennas. The main lobe signal return is fed 
through delay circuitry to a selective inhibit circuit, which is activated 
when modulated return signals are received on the auxiliary antennas, 
thereby eliminating the false sidelobe signals which would otherwise 
appear as real targets. 
The modulation may be introduced in any suitable manner, but a diode 
disabling switch operated at the desired modulation frequency has been 
employed effectively. 
Other objects, features, and advantages of the invention will become 
apparent from a consideration of the following detailed description and 
from the accompanying drawings.

DETAILED DESCRIPTION 
Referring more particularly to the drawings, FIG. 1 is a diagrammatic view 
of an Airborne Early Warning (AEW) aircraft 12 having a radar dome 14 on 
patrol duty for the U.S. Customs Service to prevent contraband goods from 
being smuggled into the United States. Shown in FIG. 1 are a number of 
off-shore drilling platforms or oil rigs 16. Now, with the radar system 
mounted in the aircraft 12 in operation, its antenna array is rotated so 
that its main beam 18 might be directed due east, while subordinate side 
lobes could be directed as indicated at 20 and 22, for example. 
As noted above, the intruding targets in the main beam may well have an 
extremely small radar cross section, and it is important that they be 
detected against clutter returns, which may be from very prominent and 
sizeable objects, such as the oil rigs 16. In order to make the moving 
targets more readily identifiable in the presence of main-beam clutter, 
radar systems have in recent years included "Time Averaged Clutter 
Coherent Airborne Radar" (TACCAR) mechanizations, which lock the main beam 
clutter to the pulse repetition frequency lines of the radar transmitted 
waveform in the frequency domain. This is accomplished by driving or 
slightly shifting the frequency of the so-called "STALO" or "Stable Local 
Oscillator" so as to always maintain the ground clutter spread about the 
PRF lines (zero doppler shift). As the antenna rotates, the STALO is 
continuously controlled as a function of aircraft heading velocity and 
antenna pointing angle so that the clutter is shifted to the zero doppler 
point, or line. 
In FIG. 2, the doppler spectrum of the airborne moving target indication 
radar is shown, with the TACCAR process in operation. The distance of the 
target from the pulse repetition frequency or PRF lines 26 and 28, or the 
FFT (Fast Fourier Transform) cell number, indicates the speed of the 
target. The airborne moving target indication response envelope is 
indicated by the dashed line 30, and a typical target might be indicated 
by the pulse 32. 
However, as a result of the TACCAR process, fixed objects, such as the oil 
rigs 16, as shown in FIG. 1, may appear to be moving targets, when a 
return arrives from one of the side lobes. Such a return is indicated by 
the pulse 34, as shown in FIG. 2. In view of the fact that some of the 
objects, such as the huge oil rigs shown in FIG. 1, may have a very 
substantial radar cross-section for reflecting signals back to the radar, 
the sidelobe signals may be of substantial magnitude. Further, if there 
are a substantial number of these false sidelobe returns which appear to 
be bona fide moving targets, the computer processor may be overloaded, and 
unable to properly handle valid incoming targets. 
The present invention involves the identification of such spurious signals, 
and the early discrimination or rejection of them, in a manner to be 
discussed below. 
FIG. 3 discloses a typical AMTI radar system, modified to implement the 
principles of the present invention. More specifically, the rotodome 42 
includes the principal directional array antenna 44 and the auxiliary 
antennas or antenna elements 46. Additional antennas 48 may also be 
employed. The radar system, as shown in FIG. 3, includes the TACCAR 
control signal on lead 50. The local oscillator, which is driven by or 
controlled by the TACCAR signal, is included in circuit block 51. The 
remainder of the radar system is devoted to the digital signal processing 
portion of the sidelobe inhibit process. 
The signals from the auxiliary antennas 46, which include pickup from the 
sidelobe directions, are routed to the modulator 58, and the modulated 
signals are coupled to the main signals from the directional antenna 44 in 
the directional coupler 60. Signals from the directional coupler 60 are 
converted to digital form in the analog to digital converter 61, and are 
directed and to the AMTI digital delay line 64. The inhibit/reject circuit 
66 checks all digital signal words received to determine whether or not 
they are modulated, and therefore are coming from the sidelobes, or not. 
If modulation is indeed detected, then the "SIDELOBE INHIBIT" circuit 66 
is actuated, and the pulse is blocked from the radar processing circuitry 
and the radar display indicated by the block 68. This action would 
correspond to the elimination of the pulse 34 of FIG. 2, while a true 
target pulse 32, which would not be accompanied by modulation, would be 
allowed to pass through the "SIDELOBE INHIBIT" circuit 66. The modulation 
frequency of the diode switch included within the circuit 58 is controlled 
by the modulator 70. In one typical installation, the frequency of 
modulation, or of switching of the diode switch associated with the 
auxiliary antennas or antenna elements might be at an IF frequency of 
approximately 30 megahertz, while the operating frequency of a typical low 
PRF pulse doppler radar system might be in the order of 400-450 megahertz, 
and the pulse repetition rate might be in the order of 300 per second. A 
conventional frequency swept pulse or "chirp" can be employed. The use of 
digital coded pulse compression is the preferred method as the signals are 
readily coded and encoded in the more typical signal processing elements 
of current radars such as are depicted in FIG. 3. As noted above, FIG. 3 
is a somewhat complete block circuit diagram of an AMTI radar system 
equipped with sidelobe rejection arrangements of the present invention. In 
general, those skilled in the radar art will readily recognize the circuit 
blocks from the legends included therein. The sidelobe inhibiting or 
rejection function is accomplished in block 66. 
Referring now to FIG. 4 of the drawings, the result of modulation is shown 
graphically. More specifically, the return from the mainlobe direction, as 
indicated at 82, is virtually free of modulation, while returns on the 
sidelobes, as indicated at 84 and 86 may be readily identified by the hash 
or modulation, which is included in the return signals (either amplitude, 
phase or digitally coded). 
For completeness, FIGS. 5 and 6 show one configuration of a radome which 
may be employed in the implementation of the present invention. More 
particularly, the radome 14 may include the multi-element directional 
antenna array 92, and the auxiliary antennas or antenna elements 94. The 
signals from the auxiliary antennas 94, together with the signals from the 
pylon antennas are modulated, as shown in FIG. 3 of the drawings. FIG. 6 
shows the radome 14 with its supporting legs 96, and the additional fixed 
auxiliary antennas 98. 
In conclusion, it is to be understood that the foregoing detailed 
description and the accompanying drawings relate to one illustrative 
embodiment of the invention. Other arrangements may be employed without 
departing from the spirit and scope of the invention. Thus, by way of 
example and not of limitation, other modulation arrangements may be 
employed, and the spurious modulated signals may be eliminated at other 
points in the system instead of in the RF section of the radar system. The 
input from the auxiliary antenna elements need not be coupled to the input 
from the mainlobe antenna, but may be sampled for modulation directly. 
Further, instead of using the delay circuitry, as shown at 64 in FIG. 3 of 
the drawings, the modulated pulses may be detected in the RF stages of the 
system and may be eliminated at a subsequent point in the data processing 
circuitry indicated at block 68. Also, other radar signal processing 
techniques may cause stationary target reflections from sidelobes to 
appear as moving targets; and the present invention would also be 
applicable to any such systems. Accordingly, the present invention is not 
limited to the arrangements precisely as shown in the drawings, and as set 
forth hereinabove in the Detailed Description.