Note: Descriptions are shown in the official language in which they were submitted.
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Radar Apparatus with Sidelobe Blanking Circuit
The invention relates to a radar apparatus having
a sidelobe blanking circuit, provided with a main channel
comprising a directive antenna, having a main lobe and
sidelobes, and a main receiver; an auxiliary channel
comprising an auxiliary antenna, having a directivity
substantially lower than the directivity of the directive
antenna, and an auxiliary receiver; and comparison and
switch means connected to the main receiver and the
auxiliary receiver for passing signals for further
processing when the signals appear stronger in the main
channel than in the auxiliary channel.
A radar apparatus of this kind is known from the
patent DE 30 28 225 Cl. The known radar apparatus is based
on the principle that the auxiliary antenna gain is greater
than the directive antenna sidelobe gain. Sidelobe
suppression is then obtained to a satisfactory extent,
except for noise sources entering the directive antenna main
lobe flanks. Particularly, if the radar apparatus is
provided with a difference channel for determining the error
angle between a direction of the main lobe and a direction
of a target, a noise source entering this way constitutes a
considerable drawback.
The present invention largely obviates this
problem by actively involving the difference channel in the
sidelobe blanking (SLB) process. It is thereto
characterised in that the radar apparatus is provided with a
difference channel comprising a difference antenna, for
determining an error angle between a direction of the main
lobe and a direction of a target, and a difference receiver;
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and in that the comparison and switch means are designed for
passing signals for further processing when the signals
appear
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stronger in the main channel than in the auxiliary channel
and the difference channel in combination.
From US-A 3,916,408 a radar apparatus provided with a
coherent sidelobe canceller is known having an auxiliary
antenna with a null in the direction of the main lobe of
the directive antenna. This nul prevents clutter returns
from entering the auxiliary channel, which clutter returns
tend to degrade the coherent sidelobe cancelling.
A favourable embodiment of the invention is characterised
in that the comparison and switch means are designed for
passing signals for further processing when the signals
appear stronger in the main channel than in the auxiliary
channel and in the difference channel.
A further favourable embodiment of the invention is
characterised in that a combined channel is provided,
comprising a summation network for summing output signals
of the auxiliary antenna and of the difference antenna, a
combined receiver, connected to the output of the summation
network; and in that the comparison and switch means are
designed for passing signals for further processing when
the signals appear stronger in the main channel than in the
combined channel.
In this embodiment, the combined channel incorporates,
practically unchanged, all characteristics of the
difference channel. Thus, the combined channel may be used
for the determination of error angles; the only difference
being that the sidelobes are raised as it were by combining
them with the output signals of the auxiliary antenna. The
aim is then to allow the antenna gain of 'the combined
antenna to be greater than that of the directive antenna,
except in the central part of the main lobe.
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A still further favourable embodiment of the
invention is obtained for a radar apparatus equipped with an
array of antenna elements from which the directive antenna
is derived for the main channel by means of a main summation
network. This embodiment is characterised in that for a
combined channel, a combined auxiliary antenna and
difference antenna is derived from the array by means of a
collective summation network, that a combined receiver is
provided, connected to the collective summation network, and
that the comparison and switch means are designed for
passing signals for further processing when the signals
appear stronger in the main channel than in the combined
channel.
The invention may be summarized as a radar
apparatus having a sidelobe blanking circuit and an antenna
equipped with an array of antenna elements, comprising: a
main summation network, connected to the array of antenna
elements to realize a directive antenna with a mainlobe and
sidelobes, each having an antenna gain; a combined summation
network, connected to the array of antenna elements to
realize a combined difference antenna/auxiliary antenna with
a difference pattern in an azimuthal direction substantially
corresponding to the main lobe of the directive antenna and
a low directivity auxiliary lobe having an antenna gain that
is substantially higher than the antenna gain of the
sidelobes a direction of which substantially corresponds
with a direction of the low directivity auxiliary antenna; a
main receiver connected to the main summation network; a
combined receiver connected to the combined summation
network; and comparison and switch means connected to
respective outputs of the main receiver and the combined
receiver for passing main receiver output signals that are
stronger than combined receiver output signals.
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The invention will now be further explained with
reference to the following figures of which:
Fig. 1 schematically represents a SLB according to
the state of the art;
Fig. 2 represents antennae diagrams of a directive
antenna, an auxiliary antenna and a difference antenna;
Fig. 3 schematically represents an embodiment of a
SLB according to the invention;
Fig. 4 schematically represents an embodiment of a
SLB with a combined auxiliary channel/difference channel;
Fig. 5 schematically represents a phased array
antenna provided with a SLB according to the invention.
Fig. 1 is a schematical representation of a radar
apparatus provided with SLB according to the state of the
art in which output signals of a main channel, comprising a
directive antenna 1 and a main receiver 2, and output
signals of an auxiliary channel, comprising an auxiliary
antenna 3 having low directivity and an auxiliary
receiver 4, are compared in a SLB-circuit 5 using a
comparator 6 that commands a switch unit 7, such that only
output signals of the main channel are passed that are
stronger than output signals of the auxiliary channel and
that appear at the output 8 for further processing. The
operating principle is such that a jammer penetrating main
receiver 2 via a sidelobe of the directive antenna 1 will
gain strength and penetrate auxiliary receiver 4 via
auxiliary antenna 3 after which switch unit 6 prevents the
noise signal from emerging at the output 8. It will then be
necessary, though, that at least in the direction of the
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jammer, the antenna gain of auxiliary antenna 3 is greater
than the gain of antenna sidelobes of directive antenna 1.
Comparison is usually performed per radar range quant, per
range-azimuth quant or per range-azimuth-doppler quant,
dependent on the system requirements.
Auxiliary antenna 3 is often referred to in the art as an
omnidirectional antenna, which may be a correct term when a
search radar apparatus is concerned.
For a radar apparatus according to the invention, Fig. 2
represents a possible antenna diagram 9 of a directive
antenna 1, antenna diagram to of an auxiliary antenna 3 and
antenna diagram 11 of a difference antenna, the antenna
gain being plotted as a function of an angle ~p. The antenna
gain of auxiliary antenna 3 shall in all directions be
preferably greater than the gain of the sidelobes 12 of
directive antenna 1.
Fig. 2 shows that noise sources entering a main lobe flank
in the antenna diagram 9 are stronger in the main channel
than in the auxiliary channel and will consequently also
appear at output 8 of the SLH-circuit 5. This phenomenon
may be used to advantage by sources of deliberate
interference, jammers, for degrading the usefulness of a
radar tracker directed at the target. Radar antennas in
general and tracker antennas in particular are often
provided with a difference antenna. According to the
invention this difference antenna can be used to prevent
jamming signals from entering via the flanks. In this
respect it should be noted that in Fig. 2 the top of the
main lobe in antenna diagram 9 projects above the antenna
diagram 1l, as opposed to its flanks. By making the
stipulation that the signal strength is greater in the main
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channel than in the difference channel, damming signals
will be prevented from entering via a main lobe flank.
The directive antenna and the difference antenna can be
5 realised in an embodiment well-known in the art by
positioning two feedhorns side by side. Addition of the
output signals of the gain/feedhorns yields a directive
antenna, subtraction yields a difference antenna.
A possible embodiment of a radar apparatus incorporating
the invention is schematically represented in Fig. 3. In
addition to the main channel comprising a directive antenna
1 and a main receiver 2, and the auxiliary channel
comprising an auxiliary antenna 3 and an auxiliary receiver
4, this embodiment also incorporates a difference channel
comprising a difference antenna 13 and a difference
receiver 14. SLB-circuit 5 is equipped with a second
comparator 15 that compares output signals of the main
channel and the differen°~:e: channel. Furthermore an AND
circuit 16 is provided fs~r combining the output signals of
comparators 6 and 15, such that switch unit 7 passes only
output signals of the main channel that are stronger than
signals both in the auxiliary channel and in the difference
channel. In situations in which the gain of auxiliary
antenna 3 is in all directions smaller than the gain of the
sidelobes of the directive antenna 1, it is possible, in
order to prevent unnecessary changes in the position of
switch means 7 caused by noise or extremely weak signals,
to add a comparator 17 for comparing output signals of the
main channel with a level previously selected by means of
setting 18, the comparators 6 and 15 being enabled via the
output of comparator 17 for output signals of a sufficient
strength.
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If the output signal of difference receiver 14 is used for
determining an error angle, an additional switch unit may
for this output signal be added to SLB-circuit 5 to operate
simultaneously with switch unit 7.
Fig. 4 schematically represents a favourable embodiment of
a radar apparatus provided with a SLB-circuit according to
the invention, in which the output signals of auxiliary
antenna 3 and difference antenna 13 are summed in a
conventional summation network 19. The combined output
signal is subsequently applied to a combined auxiliary
receiver/difference receiver 20 whose output signal is then
compared with the output signal of main receiver 2 using
comparator 15. The output signal of comparator 15 is
subsequently used for changing the position of switch unit
7, such that only output signals of the main channel are
passed that are stronger than output signals of the
combined channel. Comparator 17 may again be added to
prevent switching on noise. A switch unit may also be added
to SLB-circuit 5 for passing the output signal of the
combined auxiliary receiver/difference receiver to be used
for determining an error angle. Apart from an enhanced SLB
performance, a further considerable advantage of said
embodiment is the omission of a receiver.
Fig. 5 schematically represents an embodiment of the
invention to be used in a phased array antenna 21
in which a directive antenna is realised by a main-
summation network 22 through combination of output signals
of antenna elements constituting phased array antenna 21
and a combined auxiliary antenna/difference antenna is
realised by a collective summation network 23, again
through combination of output signals of antenna elements
constituting phased array antenna 21. The collective
summation network 23 may for instance be implemented such
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to form a difference channel well-known in the art to which
are furthermore added the outputs of a limited number of
selected antenna elements which together constitute an
auxiliary antenna. By means of comparator 15, output
signals of main receiver 2 are again compared with output
signals of combined auxiliary receiver/difference receiver
20, comparator 17 being again capable of preventing
switching on noise, particularly if the auxiliary antenna
gain tends to be locally substandard.
The antenna elements contained in a phased array antenna
may be provided with two receiver outputs, for connection
to the main-summation network 22 and to the collective
summation network 23, each having its own phase and/or
amplitude steering circuit for obtaining a directive
antenna and a difference antenna. For obtaining a combined
auxiliary antenna/difference antenna it will with respect
to the phase and/or amplitude steering circuit suffice to
introduce small, for instance random deviations to obtain a
sidelobe level that is at least substantially on all points
higher than the sidelobe level of the directive antenna.
Another possibility is to calculate a suitable phase and/or
amplitude steering circuit from the desired antenna diagram
by means of Fourier transforms. This may be of advantage
when all noise sources are situated in a certain direction,
which enables an optimization of the auxiliary antenna
gain.
The embodiment described on the basis of Fig. 5 has the
advantage that an improved SLB performance is obtained and
furthermore allows the omission of a receiver and a
summation network.
