GB1604071A

GB1604071A - Radar system with clutter suppressor

Info

Publication number: GB1604071A
Application number: GB1853078A
Authority: GB
Original assignee: Long M W
Current assignee: Long M W
Priority date: 1977-05-16
Filing date: 1978-05-09
Publication date: 1981-12-02
Also published as: CA1110743A; JPS549596A; DE2821401A1

Description

(54) RADAR SYSTEM WITH AN IMPROVED CLUTTER SUPPRESSOR (71) I, MAURICE WAYNE LONG, a citizen of the United States of America, of 1036 Somerset Drive North West, Atlanta, Georgia, United States of America, do hereby declare the invention for which I pray that a patent may be granted to me, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to a pulsed radar system with an improved clutter suppressor for enhancing echo differences between targets and clutter caused for example by difference in polarization, carrier frequency, pulse length, antenna beamwidth, or antenna pointing direction.

The amplitude characteristics of radar echoes depend on parameters of the radar signals as well as the physical characteristics of the objects from which the echoes emanate. In particular, the echo characteristics depend on polarization, both that used for transmission and that used for reception.

The radar cross section a is a useful quantity for analysing echo strength. This quantity is defined as # = 4#R2 pt where the power density at the target is pt, the power density at the radar is pr and the distance from the target is R.

The terminology aAB is used to denote radar cross section (RCS) for transmit polarization A and receive polarization B. For example, let the symbol x denote a linear polarization in any direction and let y denote a polarization to x. In the case, axy denotes RCS for transmitting any linear polarization and receiving a linear polarization perpendicular to that transmitted. Because of reciprocity, axy = ciyx. It is noted here that received signal power, other factors being equal, is proportional to o.

The reflecting properties of an object depend on various factors including surface shape, size, material, orientation, as well as the frequency and polarization of the radar signals.

Other shape and orientation may cause waves to be reradiated with a polarization different from that incident on the object, i.e., aABi0. Such shapes are said to produce depolarization. Most large objects that have smooth surfaces, either metallic or dielectric, do not cause depolarization. Therefore, there are many targets of interest that cause less depolarization than is caused by most land and sea surfaces.

The concept of using the depolarization effects of clutter to discriminate between the clutter and a target has been discussed in several patents. Apparatus for differentiating between target echoes and clutter echoes based on transmitting a linearly polarized wave, and receiving that polarization and one perpendicular to that polarization has been described in U.S. Patent 2,508,571 issued to E.L. Hudspeth; U.S. Patent 3,566,403 issued to E.G. Hills; U.S. Patent 3,849,780 and issued to F.J. Dynan; and U.S. Patent 3,918,059 issued to D.J. Adrian. Apparatus for differentiating between target echoes and clutter echoes on transmitting a circular polarized wave, and receiving both left-and right-circularly polarized echoes have been described in U.S. Patent 3,403,397 issued to R.C. Harrington and G.R. Latham IV; U.S. Patent 3,614,787 issued to G.E. Hart; and U.S.Patent 3,755,810 issued to G.R. Latham and W.T. Allen.

As reported in the book Radar Reflectivity of Land and Sea, D.C. Heath and Company, Lexington, Mass., 1975, p. 338, by M.W. Long, there are troublesome target-like sea echoes, called sea spikes, that are only slightly depolarized; i.e., the echoes are almost entirely of one polarization. Therefore, the dual polarized radar systems described above are ineffective for suppressing such clutter.

There are two patents by T. Nirasawa and H. Ota that use various parameters, called discriminants herein, for reducing the effects of sea clutter. U.S. Patent 3,893,117 contains claims using two pulse lengths and two beamwidths, and U.S. Patent 4,008,472 contains claims using two polarizations and two carrier wavelengths. The embodiments provide means for comparing the video amplitudes for one value, called operative herein, of a discriminant with another value of the same discriminant. Nirasawa and Ota provide means for gating-off i.e. rejecting the receiver outputs when the ratio of video amplitudes is beyond a preset value and for gating-off the receiver output when that ratio is between two preset values. A limitation of this prior art is that the ratios for clutter can have almost any value and can vary at rates as fast as 200 Hz or more for sea clutter.This means that clutter will often be present in each radar resolution cell used. It should be noted that there are fundamental differences between the gating means used by Nirasawa and Ota and those used in this disclosure for improved suppression of clutter, antenna sidelobes, and interference from external sources. Also, although there are one-antenna embodiments that use pulse length as the discriminant, all other embodiments included by Nirasawa and Ota require the use of two antennas. Further, the embodiments of Nirasawa and Ota limit the signal processing means to those using either the video output of one receiver, the other receiver, or the sum of the outputs of the two receivers.

It is noted that the prior art of Nirasawa and Ota provides for using wavelength as a discriminant without indicating a need to maintain a closeness of frequency so that two simultaneously received video signals will be equal or nearly equal for a target. Further, they use separate transmitters without means for maintaining a frequency difference, and two antennas are used for carrier wavelengths kl and A2. According to Nirasawa and Ota the antennas are identical in construction with each other except that they are designed for the carriers X1 and 2 respectively.

The present invention is based on a different analysis of the relationship between targets and clutter compared with the prior Nirasawa and Ota patent.

The various embodiments of the present invention distinguish targets from clutter by using various discriminants. As used herein the term "discriminant" refers to a radar signal parameter for which different "values", called operatives, of the parameter will give different received video signals. For example, polarization and carrier frequency are each discriminants. "Operatives" thus include specific polarizations, and specific carrier frequencies, and also specific transmit pulse lengths, antenna pointing directions, and antenna beamwidths for example.

According to the present invention there is provided a pulsed radar system including a clutter suppressor comprising a first means for providing at least two different discriminant operatives as hereinbefore defined, a second means for providing at least two received video signals responsive to the different discriminant operatives respectively, comparator means for comparing the magnitudes of a first and a second of said received video signals in such a manner as to provide an output indicative of whether the ratio of said magnitudes falls within at least one predetermined range, the or each said range having a predetermined finite upper limit and a non-zero lower limit, and processing means responsive to said output such as to indicate the presence of a target only in response to the ratio of said signal magnitudes falling within the or one of said predetermined ranges.

Thus, in contrast to the prior Nirasawa and Ota patent, the comparator means of the present invention discriminates between a target and clutter on the basis of the signal magnitude ratio falling within a range (or one of a number of ranges) having predetermined finite non zero upper and lower limits, the range or ranges preferably including unity ratio for normalised signal gains in the video signal channels. The present invention thus utilises the concept that by defining at least one predetermined acceptance range for the ratio of signal magnitudes, the range having a non zero lower limit and a finite upper limit, an improved clutter rejection rate is provided. In accordance with a preferred feature of the present invention it is found that for many targets, an improved clutter rejection rate is achieved by having the range centered on or including the aforesaid unity ratio.

Preferably the processing means is responsive to the output of the comparator means but is not otherwise responsive to the video signals.

In the embodiments of the invention described hereinafter, there is disclosed an improved clutter suppressor means that simultaneously compares more than two received video signals. Also, an arrangement is disclosed requiring only one antenna for using polarization, frequency, and beamwidth as discriminants.

In one embodiment, antenna beam pointing direction is used as a discriminant for obtaining improved clutter suppression. Also, the use of small carrier frequency differences as the discriminant is disclosed for obtaining improved clutter suppression and target selectivity. Frequency differences used are typically 10 MHz or less so that the video signals when simultaneously received are decorrelated for clutter but are not decorrelated for targets of interest.

Furthermore, in one embodiment, the transmission of circular polarization and simultaneous reception of two linear polarizations is utilised for the improved detectability of metal diplanes, thereby providing for the improved detectability of diplanar nagivational buoys.

In order that the invention may be more fully understood such embodiments thereof will now be described by way of illustrative example with reference to the accompanying drawings wherein: Figure I is a block diagram of a preferred embodiment of the present invention; Figure 2 is a block diagram of a first alternative embodiment of a signal conditioning system; Figure 3 is a block diagram of a second alternative embodimemt of a signal conditioning system.

Figure 4 is a block diagram of a third alternative embodiment of a signal conditioning system.

Figure 5 is a block diagram of a fourth alternative embodiment of a signal conditioning system.

Figure 6 is a block diagram of a preferred embodiment of two gates in series to form a simple comparator.

Figure 7 is a block diagram of a preferred embodiment of cascaded comparators.

Figure 8 is a block diagram of a preferred embodiment of the present invention that uses a comparator that is comprised of cascaded comparators.

Figure 9 is a block diagram of a sixth alternative embodiment of a signal conditioning system.

Figure 10 is a block diagram of a seventh alternative embodiment of a signal conditioning system.

Figure 11 is a block diagram of an eighth alternative embodiment of a signal conditioning system.

Figure 12 is a block diagram of a ninth alternative embodiment of a signal conditioning system.

The following disclosure describes a radar clutter suppressor and target classifier in which at least two signals of substantial magnitude, resulting from transmit-receive pairs having different combinations of polarizations, are received and a signal is generated based on the ratio of the amplitude of the received signals. Also, the disclosure concerns means to use differences in echo caused by differences in transmitter carrier frequency, pulse length, antenna beamwidth, and beam pointing direction. Moreover, common to the embodiments described herein is the optional use of frequency agile transmitters and receivers. Although frequency agility, per se, is not basic to the various embodiments of this invention, the use of frequency differences as described in the section Comparisons of Frequencies hereinafter.

Figure 1 is a block diagram of a preferred embodiment of the invention. An antenna system 10 and a radar pulsed transmitter-receiver 11 are provided so that there are receiver video output signals responsive to different "values," called operatives, of a discriminant.

The discriminants are polarization, carrier frequency, pulse length, beam width, and antenna pointing direction for pushed radar signals. For example, video 1, shown as block 12, might be responsive to the operative transmit-receive polarization pair Pi and video 2, shown in block 14, might be responsive to the operative transmit-receive polarization pair P2 The amplitudes of videos 1 and 2 are simultaneously compared in the comparator 16 and on the basis of relative amplitude a gate signal 18 is obtained for further signal processing.

In other words, a comparator output 18 is generated only if the ratio of the video 1 and video 2 amplitudes are within a predetermined range of ratios.

By using at least three operatives, three or more video signals can be obtained for signal processing on the basis of ratios. Figure 8 shows a preferred embodiment wherein the amplitudes of four received video signals are compared by a comparator 72. On the basis of various relative magnitudes, a comparator signal 65 is obtained for further signal processing.

The gate signals 18 and 65 may be used directly as radar outputs; they may be processed further and used as radar outputs; or they may be used in the processing of other signals.

Comparisons of polarizations As already mentioned, the strength of radar echo from an object depends on both the transmit and receive polarizations. The terminology "polarization pair" is used to describe both polarizations. For example, transmit horizontal (H) and receive vertical (V), or HV for brevity, in a specific polanzation pair. A first principle of this invention is that the ratio of echo strengths for two polarization pairs depends on the echoing object shape, size, orientation and material of which it consists.

Some of the polarization properties of targets are discussed in the previously referenced book Radar Reflectivity of Land and Sea, and special polarization characteristics that are helpful for understanding the principles underlying the present invention are given below.

Let H, V, L, and R denote horizontal, vertical, left circufar, and right clrcular polarizations.

Because of reciprocity of the transmit and receive paths in air #HV = #VH; #RL = #LR; #HR = #RH; #VL = #LV; etc. It should be clear that if H and V polarizations are transmitted simultaneously, received H and V signal strengths will, in general, depend on the relative phase of the two transmitted waves. However, if #HV = #VH = 0 this relative phase does not affect the strengths of the H and V polarized echoes.

For brevity, the following abbreviations are used: A > > B means A at least 10 times B A # B means A about the same as B A + B means A seldom equal to B.

There are many man-made targets for which the polarization characteristics are like that of a large, smooth metallic surface for which, typically #HH # #VV > > #VH #RL # #HH > > #RR, #RR # #LL #RH # #LH # #LV # #RV # #HH/2 Some natural surfaces can be characterized as large smooth dielectric surfaces. Sea swell is an example. Such surfaces cause little depolarization and #HH is usually equal to or larger than #VV, For these surfaces #HH > > #HV, #VV > > , #HH > > #VV #RH = #LH # #RV = #LV #RH = OHR = #HH, #RV = #VR = #VV The polarization properties of rain can often be characterized by a cloud of spheres.For a sphere and for metal, 3-sided corner reflectors #HH = #VV, #HV = 0, #RL = #HH #RR = #LL = 0 #RH = #RV = #HH, #LH = #LV = #RH The reflectivity properties of metal diplanes (2-sided corners used on navigational buoys) follow: #HH = #VV, #VH # 0, #VH, # #HH #EH # #EV if E represents arbitrary polarizations for which the H and V components are equal.

#RR = #LL, #RL = 0 #RH = #RV = #LV = #LH = #RR/2 The diplane seam orientation affects a for some polarizations even when it is in a plane perpendicular to the radar line-of-sight. Then #HH, #VV, #EH, #EV depend on orientation, but #RH, #RV, #LH, #LV are independent of orientation.

Some aircraft have polarization characteristics like large, smooth, metallic surfaces plus metal diplanes, as follows: #HH # #VV > > #VH #RL # #RR.

Extended areas of land and sea usually behave as a mixture of randomly oriented infinitesimal dipoles and smooth non-metallic surfaces, as follows: ciHH * ovv + civii, aHH * O, civv * O, aVH * O aRL * aRR + ciLL, (7RL * O, ciRR * O, aLL + O aRH * aRVv aRH * O, ciRV + aLH + aLV, aLH + O, ciLV + O The notation above, e.g. ciHH + ovv, indicates that the various radar cross sections for clutter are seldom equal. In other words, for the example cited the amplitude of HH echo is seldom equal to that the VV echo. Both of these amplitudes generally fluctuate for targets and for clutter. However, the ratio of these amplitudes varies more widely and more rapidly for clutter than it does for man-made targets.

Now, the range of amplitudes for a clutter signal is large. Also, the echo for one polarization usually fluctuates independently of that for another polarization. Thus from each radar resolution cell the ratio of amplitudes of the two video signals will vary from essentially zero to very large values. Clutter echo varies rapidly because it is the composite result of summing echoes from many individual scatterers with phases that are sensitive to the relative positions of the scatterers. Fluctuation rates are usually represented by a continuum of frequencies with a half-power bandwidth that depends on transmitter wavelength. For example, this bandwidth for the sea is typically 50 to 100 Hz for a 3 centimeter wavelength.

Sometimes there are echoes that are exceptionally strong relative to the composite echo from the other scatterers. Such echoes can be caused by a dielectric surface that is smooth and large compared with the radar wavelength. For such a surface there is usually little depolarization of the echo and the echo amplitude for HH polarization will be equal to or exceed that for VV polarization. For example, sea spikes, which are discussed in the book Radar Reflectivity of Land and Sea, are a particular troublesome type of clutter signal, which have a number of target-like characteristics.

The ratio of the amplitudes of two signals is used as the basis for deciding whether or not to process echo signals as targets and thereby deciding on those to be rejected. This ratio can very rapidly for targets and for duffer, but the range of ratio values is normally much larger for clutter for targets. The difference in the possibility distribution of the ratio is basic to this invention. In other words, an enhancement of targets relative to clutter is accomplished through signal processing means by using differences in the frequencies of occurrence of echoes with various received polarization ratios. Further, by choosing certain bands of ratios to process, one can accomplish broad, general classifications for targets and clutter.

The present invention can be used for selecting between broad classes of man-made targets as such buildings, roadways, corner reflectors, and ships. It is also possible to provide for selectivity between broad classes of natural targets such as rain, bodies of water, and types of terrain, and features of the sea surface such as sea swell, white caps, wave fronts, wind slicks, and boundaries between currents. Therefore, the word "target" is used in this disclosure to include both natural and man-made phenomena.

When using polarization as a discriminant, a comparator is used that compares the amplitudes of two or more signals, each of substantial magnitude and resulting from different transmit-receive polarization pairs, to provide an output in response to a ratio of said characteristics to indicate the presence of a target echo. At least two different polarizations are used for the four signals required to provide first transmit, first receive, second transmit and second receive signals. The first and second receive signals provide at least two signal amplitudes of substantial magnitude. The comparator provides an output based on the relative magnitude of the two receive signals and this output is used for or with further signal processing.Thus, the invention uses selected polarizations to simultaneously provide from targets two video signals of substantial magnitude and compares these magnitudes. When the ratio of these magnitudes is within preselected ranges to be discussed in more detail hereinafter, a gate signal is generated to initiate the processing of signals as target signals.

Referring now to Figure 1 for the purpose of discussing the use of polarization as a discriminant, the antenna system 10 and the radar transmitter-receiver 11 are provided so that there are receiver video output signals responsive to target echo strength for at least two different polarization operatives. For example, video 1, shown as block 12, might be responsive to transmitting and receiving horizontal polarizaton and video 2, shown as block 14, might be responsive to transmitting and receiving vertical polarization. Of paramount importance is that the amplitudes of videos 1 and 2 are each of substantial magnitude and that each is responsive to echo strength for different polarization pairs (first transmit receive and second transmit-receive). Neither the transmit nor receive polarizations associated with either video 1 or with video 2 need be the same.However, at least one of the polarizations, for example, transmit polarization for video 1, receive polarization for video 1, transmit polarization for video 2 or receive polarization for video 2, must be different from the other three olarizations used.

The amplitudes of videos 1 and 2 are compared in the comparator 16 and on the basis of relative amplitude a gate signal 18 is obtained for further signal processing. In other words, the video 1 and 2 signals that are each of substantial magnitude are compared and, based on pre-selected ratios of the amplitudes of these signals, a gate signal 18 is obtained. The pre-selected ratios are established by the comparaor 16 plus other such inter-related components as video 1 and 2 circuits, the radar transmitter-receiver 11, the antenna system 10 and the radar target characteristics.

The gate signal 18 may be used directly as a radar output; it may be processed further and used as a radar output; or it may be used in the processing of other radar transmitterreceiver outputs such as those of video 1 and video 2. To illustrate, suppose a target of interest is a sphere, suppose the polarizations are selected as described above, and suppose the radar parameters are chosen, i.e. the gains of the video channels are normalised so that the amplitudes of videos 1 and 2 are equal for a sphere. Suppose the comparator 16 is set so that a gate signal is obtained from the comparator 16 only when videos 1 and 2 are such that the ratio is near unity. Under these conditions and with other approximately selected polarizations, a gate signal 18 would seldom be caused by echo from land or sea.Therefore, spheres and other targets that meet the previously selected polarization characteristics would be displayed as targets and other echoes would be suppressed.

The comparator 16 could, of course, be responsive to more than one band or range of ratios such as that described above as being "near unity". Therefore a gate signal can be provided so as to yield target selectivity based on pre-selected polarization ratios.

It is to be noted that the videos 1 and 2 are only selected to indicate a target when they are of a substantial magnitude. This feature when combined with the ratio comparison, effected by the comparator 16, results in a target only being indicated when the aforesaid ratio of the video signals amplitudes falls within one or more ranges of ratios, each range having a finite upper limit and a non-zero lower limit. In the foregoing example fo a sphere, the normalised videos 1 and 2 thus need to define a ratio which falls within a single pass band centered on unity ratio, if a target is to be indicated. As is indicated hereinafter in more detail with reference to Figure 6, this pass band has a non zero lower limit, for example 0.8, and a finite upper limit of for example 1.25.

The embodiments of the present invention may be made with components that are well known to the radar art. The antenna can be of the reflector type, with illumination by a dual linear polarization feed aligned to maximize the reception of a horizontally polarized wave with one feed output and thereby maximize the reception of vertical polarization for the other output. It is noted here that most types of land and sea clutter will be satisfactorily suppressed relative to target echo even if other feed alignment angles are used. However, this disclosure outlines special use of the reception of horizontal and vertical polarizations for the processing of sea spikes and other echoes.

A single antenna can be used for transmitting and receiving a variety of polarizations through use of dual polarization devices called dual mode or polarization diversity couplers.

Details for constructing a dual mode coupler and references for both linear and circular polarization dual-mode devices are given in the paper "Dual-Mode Coupler", IEEE Transactions on Microwave Theory and Techniques Vol. MTT-15, November 1967, pp.

651-652, by R.C. Johnson, F.L. Cain and E.N. Bone. Alternatively, various polymerizations can be obtained by using a separate antenna for each polarization.

"Received polarization 1" is defined as the echo wave polarization to which the amplitude of video 1 is responsive; video 2 is similarly proportional to "received polarization 2." The abbreviations H, V, L, and R will be used to denote horizontal, vertical, left circular, and right circular polarizations.

Figures 2, 3 and 4 give examples of signal conditioning in a signal conditioning system 15 of Figure 1 providing the outputs at video 1 and video 2. In Figure 2, the power from the transmitter 25 is divided in a hybrid junction 24 so that equal H and V signals are transmitted by the H and V antenna, 20. Duplexers 21 and 22 are provided so that return signals may also be received through the H and V antenna system 20. In this configuration, received polarization 11" is H and "received polarization 2" is V. There are applications for which the relative phase 8 is unimportant. However, the apparatus shown is capable of adjusting the phase between the transmitted signals, as is represented in the block diagram at 23, so that if 8 equal 90" the transmitted wave is circularly polarized. An important advantage for transmit circular polarization when receiving H and V polarizations results because aRH = aRV = aLH = aLV for a metal diplane, irrespective of seam orientation.

Therefore, for this choice of polarizations the ratio of the amplitudes of the video 1 and the video 2 signals will not vary with tilt of the diplane.

In Figure 3, the antenna 30 radiates a circular polarization and receives both left-and right-circular polarizations; the relative phase , represented as block 32, is such that after the received signals are combined in the hybrid junction 34 the resultant signals are such that "received video 1" is H and "received video 2" is V. The configurations of Figures 2 and 3 are exemplary of system designs that meet the basic requirements for ratios so that target are enhanced relative to clutter.

Figure 4 illustrates an alternative method of constructing Figure 3. In Figures 3 and 4 the antenna 30 receives L and R. As already noted, for Figure 3 the inputs to the receivers 35 and 36 are proportional to H and V. In Figure 4, signals at intermediate frequency that are proportional to L and R are amplified in 44 and 45 and added with appropriate phase in hybrid junction 47 so that the inputs to 48 and 49 are proportional to H and V. It should be noted that Figures 2, 3 and 4 are each embodiments of signal conditioning system 15 for which the video 1 and video 2 signals are proportional to the echo strengths for H and V polarization. Therefore, the receive polarizations for each of these embodiments are H and V even though the antenna 30 used in Figures 3 and 4 is an L and R antenna.

The operation of a preferred embodiment wherein the relative strengths of video 1 and video 2 are compared when using HH and VV polarizations is described below. By definition, HH means H only is transmitted and H only is received, VV means V only is transmitted and V only is received. Therefore, strictly speaking, HH and VV echoes cannot be obtained simultaneously but they can be obtained in practice by transmitting and receiving H and then transmitting and receiving V within time intervals short compared with the important fluctuation rates of the echoes.

Assume the transmit and receive polarization is switched between H and V on a range-sweep to range-sweep basis, i.e., for one range-sweep transmit and receive polarizations are H (video 1) and shortly thereafter for another range-sweep the transmit and receive polarizations are V (video 2). Then, by conventional radar store and delay techniques the echo signals as a function of range for video 1 can be received and stored, and then compared with those for video 2. Therefore if the transmitter range sweep repetition rate is sufficiently fast, one can obtain in essence a comparison of echo strength for transmit H-receive H with echo strength for transmit V receive V, and this can be accomplished for each radar resolution cell.

There are numerous polarization combinations possible that meet the basic concepts of this invention which can be obtained by switching either the transmit polarization or the receive polarization or switching both transmit and receive polarizations. Examples of wave switching modes are given in Table 1.

TABLE 1 Switching modes Transmit Number of Received Polarization Receivers Polarization Fixed 1 Switched Fixed 2 Switched Switched 1 Fixed Switched 1 Switched Switched 2 Switched Another of several advantages of switching transmitted polarization is now illustrated that combines the use of this invention with a classical use of circular polarization.

Circular polarization has long been used to reduce echo from rain relative to that from certain targets, principally aircraft. Experience has shown that if the same sense circular polarization is received having the same sense as the circular polarization transmitted, the echo from rain is reduced more than that of aircraft. The reason for this is clear when it is recalled that for rain the ratios oRJaRR and oLR/aLL are large but for aircraft those ratios are on the average considerably less than they are for rain.

In order to obtain the aforementioned benefits of circular polarization, assume that one circular polarization is transmitted and received to obtain a video 1 signal. For the next range sweep, assume the other circular polarization is transmitted and received to obtain a video 2 signal. It should be noted that rain echo relative to aircraft (and some other targets) will have been reduced in the video 1 and 2 signals because one signal was obtained from LL echo and the other from RR echo. However, LL echo and RR echo are not identical for all targets. As previously discussed, by radar store and delay techniques the video 1 signals as a function of range can be received and stored, and then compared with those of video 2.By this means land and sea clutter can also be suppressed because the gate signal from the comparator will be obtained more often for aircraft than for land and sea clutter because the ratios of the amplitude of videos 1 and 2 will vary over a smaller range for aircraft than for extended land or sea surfaces. Therefore, video 1 and video 2 will each contain signals for which rain echo will have been suppressed relative to that of aircraft and other similar targets and the gate signal will be obtained more often.for aircraft than for land and sea clutter because the ratio of oRR/aLL will vary over wider ranges for clutter than for aircraft.

One important feature of this invention for suppressing clutter is that of using a predetermined range of ratios, or ranges of ratios, corresponding to those of a desired target or targets to activate the processing of received signals for the radar output. Since the ratios for clutter usually vary over ranges much wider than those for targets, this feature automatically reduces the percentage of clutter signals that are contained in the radar output.

Another feature important to suppressing clutter is that two transmit-receive polarization pairs can be selected to reject echo from objects that do not depolarize. One such example is that of spikes caused by sea swell for which echo amplitude for HH polarization is equal to or greater than that for VV polarization. Based on the reflecting properties of water, for swells without ripples, HH echo power will usually exceed that for VV by less than a factor of 2, and HH echo power will rarely be a factor of 10 larger than VV echo power. For purposes of implementation, it should be recalled that for a smooth dielectric surface the strength of RH (or LH, BL, HR) echo relative to RV (or LV, VL, VR) echo strength is equal to the strength of HH echo relative to VV echo strength.

Frequency agility For a fixed frequency radar, echo strength fluctuates because of changes in the orientations and relative positions of objects within a radar resolution cell. A change in radar frequency will also change echo strength and therefore rapid changes in frequency (frequency agility) is sometimes used to increase echo fluctuation rates. The book Radar Design Principles, McGraw-Hill Book Comapany, New York, 1969, by F.E. Nathanson outlines certain advantages of frequency agility that are repeated here. By using frequency agility, clutter can be displayed more uniformly and at a level that is weaker than the strong clutter signals that are sometimes present. Further, the probability of target echo strength being at or near a null can be substantially reduced by using frequency agility.It should be apparent that the attributes of frequency agility can be used to advantage with embodiments of this invention.

Comparisons of frequencies This section describes embodiments of the invention that use two or more transmit carrier frequencies for clutter suppression and target classification. Effects of frequency changes on radar echo are described in the previously referenced book by F.E. Nathanson.

Now the amplitudes of two echoes from a target are statistically independent, and therefore unpredictable, if the echoes are for two transmit carrier frequencies that differ by at least Afc, where Af C7Lc (1) Here c is the speed of light, and Le is defined as the effective target length.

Based on theory and experiment, it is known that the echoes from a large number of distributed targets (scatterers) such as rain drops are decorrelated (independent) if the difference in carrier frequency Afe is in accordance with equation 1, where Le is equal to the radial extent (measured along radar beam) of the scatterers that are being illuminated. It follows, therefore, that if the scatterers fill the radar cell of resolution, the effective length Le is equal to the length in range of the radar resolution cell. Since the range resolution for a pulse radar is cut/2, where t is the pulse length, Afc equals lIT if the clutter source fills a radar cell of resolution.

Echo strength for a single sphere is insensitive to small changes in frequency. Therefore, the effective length Le is approximately zero. However, Le for a cluster of many such spheres that are randomly distributed is equal to the smallest of the following: the cluster length (measured along the radar beam) or cut/2.

For a man-made target that does not contain a large number of separate scatterers, Le may be considerably smaller than its physical length measured along the radar beam. For the purposes of analysis here, it is estimated that the echo amplitudes for two carrier frequencies will be nearly equal (fixed target orientation assumed) providing the difference in carrier frequencies is less than #fc = , 20Lp where Lp is the physical length of the target measured along the direfction of the radar beam.

In this invention, a gate signal that permits signals to be processed as target signals is generated only if the relative magnitude of the two video signals resulting from the two transmit frequencies is within a predetermined range of ratios. In other words, for receiving channels of equal gain, video signals are processed as if due to a target only if the relative magnitude of said video signals is within a band of ratios near unity; otherwise, signals do not reach the radar output. Therefore, transmit frequencies are selected so that echo due to area extensive sources will be rejected a higher percentage of time than will echo from desired targets.

Figure 1 is a block diagram of a preferred embodiment of the invention. The antenna system 10 and the radar transmitter-receiver 11 are provided so that there are receiver video output signals responsive to target echo strength for different transmitter frequencies. For example, video 1, shown as block 12, might be responsive to transmitting and receiving a carrier frequency fl and video 2, shown as block 14, might be responsive to transmitting and receiving a carrier frequency f2. It should be recognized here that f1 and f2 might each represent a group of ensemble of transmitter frequencies. For example, fl might be the sequence of frequencies at the output of the frequency agile radar transmitter of block 11, and f2 might be the same sequence shifted by one interpulse period.In other words, the video 2 signals might be responsive to the same transmit frequencies that cause the video 1 signals except that they are delayed by one transmitter interpulse period. For this example, two video signals are obtained simultaneously from blocks 12 and 14 and are each responsive to a different transmit frequency. The delay necessary to obtain the video 2 signals can be accomplished by conventional radar store and delay techniques.

The amplitudes of videos 1 and 2 are simultaneously compared in the comparator 16 and depending on relative amplitude, a gate signal 18 is generated for further signal processing.

In other words, the video 1 and 2 amplitudes are compared and if the ratio of the amplitudes is within a pre-selected range of ratios, a gate signal 18 is generated. The pre-selected ratio is established by the compaaor 16 plus other such inter-related components as the video 1 and 2 circuits, the radar transmitter-receiver 11, and the antenna system 10.

There are many combinations (codes) that can be used for changes (differences) in transmit frequency. Such coding in magnitude of frequency change (difference) can be used for highlighting targets based on effective length L3. For example, suppose that half the frequency differences are of a relatively small value 5f such that: (a) the corresponding video amplitudes within each cell of resolution are decorrelated for clutter and, (b) the corresponding video amplitudes are highly correlated for all targets of interest.Suppose further that half the frequency differences are Af, with Af being larger than of, and with Af such that: (c) the corresponding video amplitudes for large targets, e.g., ships, are on the average decorrelated and, (d) the corresponding video amplitudes for small targets, e.g., buoys, are highly correlated. In the example cited, a comparator signal would: (a) seldom be generated for clutter, (b) be generated approximately half the time for ships and, (c) be generated nearly all the time for buoys.

The discussion above has been based on comparing video signals resulting from transmitting only one carrier frequency at a time. Alternatively, the video 1 and video 2 signals can be obtained also by the simultaneous transmission of two pulses with different carrier frequencies and the simultaneous reception of echoes for those two transmissions.

The frequency changes required can, of course, be obtained by changing the frequency of either one or more transmitters. Figure 5 illustrates an embodiment for which two carrier frequencies are transmitted simultaneously.

For the embodiment of Figure 5 a frequency agile transmitter 25 generates an output with a different frequency fl for each successive range sweep. This output is divided by the hybrid junction 24 into two parts. An output signal of frequency fl reaches the antenna system 10 through a duplexer 21, and an output signal of frequency f2, created by the frequency translator 50, reaches the antenna system 10 through the duplexer 22.

Thus, the antenna system 10 radiates two signals of carrier frequencies f, and f2. It should be understood that in the embodiment of Figure 5 fl and f2 differ by a constant amount determined by the frequency translator 50, and therefore fl and f2 are each changed for successive range sweeps by the frequency agile transmitter 25.

The radar echoes for the fl frequencies and those for the f2 frequencies are received by the antenna system 10. Received frequencies fl reach the f1 receiver 26 through duplexer 21 and are detected and processed so that video signals responsive to echoes at those frequencies are obtained at the video 1 output 12; signals responsive to echoes at the f2 frequencies are obtained at the video 2 output 14 after reaching the f2 receiver 27 through duplexer 22 from the antenna system 10. For each range, video 1 and video 2 signals are thus obtained simultaneously. It is apparent that such simultaneously obtained signals can be processed as previously explained with a comparator 16 and signal processor 17 means as illustrated in Figure 1.

Suppose the difference between the f2 and fl frequencies is at least as large as Afc in equation (1) for a given echo source. Then the magnitude of the video 1 signal relative to the magnitude of the video 2 signal is unpredictable. Without target or radar motion, if f1 were not changed the successive fl echoes and successive f2 echoes would be unchanged. It should therefore be clear that the transmitter 25 output frequency f1 must change by at least Afc of equation (1) for assurance that the relative magnitudes of the video 1 and video 2 signals differ a larger percentage of an arbitrary time interval.

Some of the features of the embodiment of Figure 5 will now be explained with a numerical example. Suppose the transmitter pulse length is 1 microsecond and suppose that clutter extends over many radar resolution cells. Then, from material already cited, Le (clutter) = 150 m.

Also assume effective lengths Le of 5 m and 0.5 m for targets. Therefore, the changes in frequency required to assure uncorrelated echo amplitudes are: Afc (clutter) = 1 MHz Afc (5 m target) = 30 MHz Afc (0.5 m target) = 300 MHz Suppose there is no target or radar platform motion, and f2 differs from f1 by 3 MHz.

Then, for both targets the video 1 and video 2 amplitudes will always be nearly equal and unpredictable for the clutter. If the changes in transmitter frequency fl are at least 1 MHz between pulses, the percentage of time that gate pulses are generated by the comparator for clutter will be small compared to such percentages for the targets. If the changes in fl were 30 MHz, the video 1 pulses would be uncorrelated (same for the video 2 pulses) for the 5 m target but correlated for the 0.5 m target. Even so, with f2 less f, equals 3 MHz, the video 1 and video 2 target pulses obtained simultaneously would be nearly equal. It will from the discussion here be apparent that target selectivity can be accomplished by appropriately choosing the frequencies to be used.

It should now be clear that a basic feature of this embodiment of the invention is that of using a predetermined band of ratios, in combination with appropriate carrier frequency differences. This feature reduces the percentage of clutter signals that must be processed.

Use of other discriminants The use of different polarizations and different frequencies has been described for (a) obtaining videos 1 and 2 signals for which the ratio of amplitudes fluctuates widely for clutter and for (b) which this ratio due to a target fluctuates less widely than it does for clutter.

Another method of accomplishing (a) and (b) above is to process video 1 and video 2 signals that are caused by echo from different resolution cells, per se. For example, video 1 might be obtained for transmitter pulses of one length and video 2 for pulses of a different length. Then, by radar store and delay techniques, video 1 can be compared with video 2. In this manner the relative magnitude of two signals from different but overlapping resolution cells can be obtained. Thus, echo from a target that is in both cells can be processed as target echo at the radar output. Another method of comparing different but overlapping resolution cells is to compare video 1 echo for beamwidth 1 and video 2 echo for beamwidth 2. Obviously, various combinations of beamwidths and pulse lengths can be used.

Different but overlapping antenna beam pointing directions can also be used for obtaining different video 1 and video 2 signals. For example, suppose that the beam is scanned continuously in azimuth. Then by radar store and delay techniques, signals due to echo from one beam direction can be compared with the echo from another beam direction.

Then with appropriate time delays for comparative purposes, target echo can be received and compared for different pointing directions.

Figure 2 can be used to illustrate other methods by which video 1 and video 2 can be responsive to different antenna pointing directions. Suppose antenna 20 is replaced by one with two primary feeds that are displaced from the reflector axis so that the antenna beam pointing directions are different for the two feeds. Then, on reception video 1 and video 2 are responsive to different antenna pointing directions. The two feeds can be of the same or different polarizations. Assume that the feeds are for H and V polarizations to suppress sea spikes. Then the phase shifts (2), 23, can be set so that the polarization at the peak of the transmit beam is circular. If feeds of the same polarization are used, the phase shift (a, 23, should be set to maximize the transmit beam.

As previously described for Figure 5, pulses of equal length with carrier frequencies fl and f2 are simultaneously transmitted. Suppose the output pulses of the frequency translator 50 are shortened by a high-speed switch or attenuator that is actuated by the transmitter 25. Such a switch or attenator can be inserted between the frequency translator 50, and the duplexer 22. Then, video 1 and video 2 would be responsive to different transmitter pulse lengths.

Figure 5 can also be used to illustrate a method by which video 1 and video 2 can be responsive to different antenna beamwidths. Suppose the two terminals of antenna 10 represent two primary feeds that illuminate a reflector so that the beamwidths for f1 and f2 are different. Then, video 1 and video 2 are responsive to different antenna beamwidths.

Figure 5 is now used to illustrate the simultaneous use of frequency differences to suppress rain echoes and polarization differences to suppress sea spikes. Both discriminates will contribute toward reducing homogeneous sea clutter. Assume that antenna system 10 is replaced by the H and V antenna 20 of Figure 2. Now, video 1 is responsive to transmit-receive horizontal polarization for carrier frequency fl and video 2 is responsive to transmit-receive vertical polariztion for carrier frequency fl. It should be noted that the polarization pairs HH and VV can be used simultaneously because different frequencies are simultaneously used.

Signal processing Figure 6 illustrates a preferred embodiment of comparator 16 of Figure 1. A1 and A2 represent output amplitudes of videos 1 and 2, shown as 12 and 14 in Figure 1. For the comparator of Figure 6, gates 1 and 2, shown as 55 and 56, are in series so that a comparator output 18 occurs only if both gate 1, 55, and gate 2, 56, pass a signal. To illustrate the performance characteristics of the comparator of Figure 6, assume gate 1 passes a signal 57 if A1 is greater than 0.8A2 and gate 2 passes signal 18 only if (1) signal 57 exists and (2) if A1 is less than 1.25A2. Then, a comparator output 18 occurs only if A1 is greater than 0.8A2 and less than 1.25A2.

The operation of the signal processor 17 is not restricted to a specific signal processing technique. For example, the comparaor 16 might provide a train of equal amplitude pulses when the ratio of the amplitudes of videos 1 and 2 is within a preselected range. Those pulses can be further processed in the signal processor 17 or can be used to gate on either, or both, the video 1 and video 2 signals for further processing.

The processing of the comparator pulses instead of the video 1 or video 2 signals can sometimes be advantageous. In that case all signals to be processed, whether from a target or from a spurious source, can be of equal amplitude independent of range. This eliminates the usual requirement for processing extremely wide ranges in signal amplitude resulting, e.g., because echo amplitude for a given target is a strong function of range. Another advantage of processing equal amplitude comparator pulses is that these "clutter" pulses are then never larger than the pulses from a target, even though the actual clutter echo might be much larger than the target echo.

A useful method of signal processing is based on the fact that gate pulses are generated more often for targets than for clutter. Therefore, the gate pulses may be continuously supplied to an integrator in the signal processor 17 and a threshold level following the integrator may be selected so that echo of desired targets is displayed and clutter is rejected.

A number of methods of coding have been outlined for changes in transmit frequency to suppress clutter, and for providing target classification based on effective target length along the radar beam. It will be apparent that the methods can be extended through the use of more complex codes and through the simultaneous use of more than two transmit frequencies.

The cascading of comparator outputs The discussion thus far has been based on comparing two received video signals with one comparator. Alternatively, three or more operatives can be used to obtain three or more different received video signals and each pair of said video signals can be compared with a different comparator. Then, each comparator can be cascaded as illustrated in Figure 7.

Figure 7 is represented simply as comparator 72 in Figure 8. Figures 9--12 show preferred embodiments for signal conditioning means for providing four received video signals, called videos 1 through 4, that have amplitudes A1 through A4.

In Figure 7 Al through A4 represent the amplitudes of received video signals that are compared with comparators C1 through C6. Comparator C1 provides an output signal 60 if the ratio of amplitudes Al and A2 are within a predetermined range of ratios. Comparator C2 generates signal 61 if signal 60 exists and if the ratio of amplitudes A2 and A3 is within some predetermined range of ratios. Similarly, comparator C3 generates signal 62 if signal 61 exists and if the ratio of amplitudes Al and A4 is within a predetermined range of ratios.

By following this reasoning it is finally seen that C6 generates comparator output 65 if signal 64 exists and the ratio of amplitudes A3 and A4 is within a predetermined range of ratios. In other words, comparator C6 output 65 will exist only if all of the various ratios of A1, A2, A3 and A4 are within certain predetermined ratios.

Figure 8 illustrates the use of cascaded comparators that are depicted in Figure 7. Recall that for Al through A4 represent the amplitudes of four different received video signals.

These amplitudes are simultaneously compared in comparator 72, comprising cascaded comparators. On the basis of the various relative amplitudes of A1 through A4 gate signal 65 may or may not be obtained. The gate signal 65 may be used directly as a radar output; it may be processed further and used as a radar output; or it may be used in the processing of other radar transmitter-receiver outputs such as the four received video signals with amplitudes Al through A4.

In Figure 9 the operatives are four carrier frequencies fl through f4 that are transmitted simultaneously and generated by a four frequency transmitter 83a wherein the differences between the frequencies are fixed. This transmitter can be constructed by using one transmitter operating at frequency fl and three frequency translators to generate f2, f3 and f4. The four carrier frequencies are separately received by four receivers 85 through 88.

In Figure 10 the operatives are four antenna pointing directions as is practiced with monopulse radar and described in the book Radar Handbook, McGraw-Hill book Company, New York, 1970, by M.I. Skolnik. The power from the transmitter 83 is equally divided and the phases are corrected with the power divider and phase corrector 90 to provide one transmit beam from antenna 80a, as is practised with monopulse radar. On reception there are four receive beams, one for each feed, and therefore four received video signals, 89a through 89d, are obtained.

In Figure 11 the operatives are two carrier frequencies and two polarizations. H and V polarizations are transmitted and received with antenna 80b. Two transmit carrier frequencies f1 and f2 are simultaneously generated by a two frequency transmitter 83b wherein the difference between fl and f2 is fixed.This transmitter can be constructed by using one transmitter operating at fl and a frequency translator to generate f2. It should be noted that video 1, 89a, is responsive to polarization H and frequency fl; video 2, 89b, is responsive to polarization H and frequency f2; video 3, 89c, is responsive to polarization V and frequency fl; and video 4, 89d, is responsive to polarization V and frequency f2. Phase shift , 92, is provided to permit the transmit polarizations for both f1 and f2 to be circular.

It should be apparent that the H and V antenna, 80b, can be replaced by an antenna with two feeds of one polarization for providing different antenna pointing directions. Then, the phase shift , 92, can be adjusted so that there is one transmit beam direction even though there are two receive beam directions. Also note that the two antenna feeds can have different beamwidths, thereby permitting the use of two antenna beamwidths as operatives.

In Figure 12 the operatives are two polarizations and two antenna beam pointing directions. There are two antenna feeds, 90a and 90b, each for H and V polarizations.

These feeds can illuminate a reflector so that two antenna beam directions are available on reception. Phase shifts IZ)1 and e)2, 92a and 92b, are provided so that the transmit polarizations from both of the feeds, 90a and 90b, are either right circular or left circular.

The transmitter 83 generates power that is equally divided by the hybrid junction 91c and the transmitter power is again equally divided by the hybrid junctions 91a and 91b.

Therefore, each feed 90a and 90b receives equal transmit power. Then, by properly selecting phase shift Q)3, 92c, there will be only one transmit antenna beam pointing drection and the transmit polarization will be either right or left circular. On reception there are two antenna beam directions, one received by feed 1, 90a, and the other received by feed 2, 90b. Also note that both H and V polarizations are received by each antenna feed.

Therefore, through use of four receivers, 85a through 85d, there are four received video signals at videos 1 through 4, 89a through 89d. It can be seen that video 1, 89a, is receptive to H polarization and a first pointing direction; video 2, 89b, is receptive to V polarization and said first pointing direction; video 3, 89c, is receptive to H polarization and a second pointing direction; and video 4, 89d, is receptive to V polarization and said second pointing direction.

Interference and sidelobe suppression The disclosed embodiments contain certain advantages for interference suppression and improving effective antenna patterns. Assume that the system is operating so that radar output is obtained only if videos 1 and 2 are nearly equal. Then, for an extraneous signal to be processed as a target, it must somehow produce nearly equal amplitudes at videos 1 and 2.

The ability of providing an improved effective antenna pattern results because the invention involves the comparison of echoes for various polarizations and a different antenna pattern usually exists for each polarization. For example, if the antenna for one polarization has a very narrow main lobe and if for the other polarization the antenna has very weak sidelobes, the resulting system pattern can be one with a very narrow main lobe and very weak sidelobes.

The embodiments of the present invention may be made with components that are well known to the radar art. For increased receiver dynamic range, logarithmic amplifiers can be used. Alternately, linear amplifiers can be used. Suitable comparators, integrators and displays are also available within the present state of the radar art.

WHAT I CLAIM IS: 1. A pulsed radar system including a clutter suppressor comprising a first means for providing at least two different discriminant operatives as hereinbefore defined, a second means for providing at least two received video signals responsive to the different discriminant operatives respectively, comparator means for comparing the magnitudes of a first and a second of said received video signals in such a manner as to provide an output indicative of whether the ratio of said magnitudes falls within at least one predetermined range, the or each said range having a predetermined finite upper limit and a non-zero lower limit, and processing means responsive to said output such as to indicate the presence of a target only in response to the ratio of said signal magnitudes falling within the or one of said predetermined ranges.

2. A radar system according to claim 1 wherein the or one of said predetermined ranges includes the ratio of unity for the magnitudes of the video signals when processed such as to be imparted with substantially the same gain.

3. A radar system according to claim 1 or 2 wherein said discriminant operatives comprise specific polarizations of transmitted and received radar signals.

4. A radar system according to any preceding claim wherein said discriminant operatives include specific carrier frequencies.

5. A radar system according to any preceding claim wherein said discriminant operatives include specific antenna beam pointing directions.

6. A radar system according to any preceding claim wherein said discriminant operatives include specific antenna beamwidths.

7. A radar system according to any preceding claim wherein said discriminant operatives include specific transmit pulse lengths.

8. A radar system according to any preceding claim wherein: said first means provides at least two different operatives of a first discriminant and at least two different operatives of a second different discriminant; and said second means includes means for providing two received signals representing said operatives such that there is a different one of the operatives of each of the discriminants associated with each of said received signals respectively, and means for providing said first and second video signals respectively in response to said two received signals.

9. A radar system according to any preceding claim wherein: said first means provides two different operatives of a first discriminant and two different operatives of a second different discriminant; said second means includes means for providing three received signals representing said operatives such that each of the received signals represents a different combination of said operatives of said discriminants, and means for providing first second and third video signals responsive to said received signals respectively; and said comparator means is arranged to compare the magnitudes of said video signals in pairs, such as to provide an output only when the ratio of the magnitude of each pair of at least two of said pairs falls within at least one of a plurality of said predetermined ranges.

10. A radar system according to claim 8 or 9 wherein the discriminants comprise two of the following: received polarisation, carrier frequency, antenna beam pointing direction, transmit pulse length, antenna beamwidth.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. signals at videos 1 through 4, 89a through 89d. It can be seen that video 1, 89a, is receptive to H polarization and a first pointing direction; video 2, 89b, is receptive to V polarization and said first pointing direction; video 3, 89c, is receptive to H polarization and a second pointing direction; and video 4, 89d, is receptive to V polarization and said second pointing direction. Interference and sidelobe suppression The disclosed embodiments contain certain advantages for interference suppression and improving effective antenna patterns. Assume that the system is operating so that radar output is obtained only if videos 1 and 2 are nearly equal. Then, for an extraneous signal to be processed as a target, it must somehow produce nearly equal amplitudes at videos 1 and 2. The ability of providing an improved effective antenna pattern results because the invention involves the comparison of echoes for various polarizations and a different antenna pattern usually exists for each polarization. For example, if the antenna for one polarization has a very narrow main lobe and if for the other polarization the antenna has very weak sidelobes, the resulting system pattern can be one with a very narrow main lobe and very weak sidelobes. The embodiments of the present invention may be made with components that are well known to the radar art. For increased receiver dynamic range, logarithmic amplifiers can be used. Alternately, linear amplifiers can be used. Suitable comparators, integrators and displays are also available within the present state of the radar art. WHAT I CLAIM IS:

1. A pulsed radar system including a clutter suppressor comprising a first means for providing at least two different discriminant operatives as hereinbefore defined, a second means for providing at least two received video signals responsive to the different discriminant operatives respectively, comparator means for comparing the magnitudes of a first and a second of said received video signals in such a manner as to provide an output indicative of whether the ratio of said magnitudes falls within at least one predetermined range, the or each said range having a predetermined finite upper limit and a non-zero lower limit, and processing means responsive to said output such as to indicate the presence of a target only in response to the ratio of said signal magnitudes falling within the or one of said predetermined ranges.

11. A radar system according to claim 3 wherein said first means includes transmitter

means for transmitting pulsed radar signals of two different polarizations, receiving means for receiving echoes of said different polarizations respectively, and wherein said second means is arranged to provide said first video signal of a magnitude dependent upon the relative magnitude of the transmitted and received radar signals in a first of said polarizations, and said second video signal of a magnitude dependent upon the relative magnitude of the transmitted and received radar signals in the second of said polarizations.

12. A radar system according to claim 11 wherein said polarizations comprise vertical and horizontal polarizations.

13. A radar system according to claim 12 including an antenna arranged to transmit a radar signal having both vertically and horizontally polarized components, said antenna being arranged also to receive horizontally and vertically polarized echoes selectively in separate channels.

14. A radar system according to claim 12 wherein the transmitted radar signal is circularly polarized.

15. A radar system according to claim 3 wherein said first means includes means for transmitting a pulsed radar signal which is circularly polarized in a first sense, receiving means for receiving respectively echoes of the transmitted signal polarised circularly in said first sense and in a second opposite sense respectively, and a hybrid junction arranged to combine the received circularly polarized signals in such a manner as to provide first and second respective orthogonally plane polarized signals, and wherein said second means is responsive to said orthogonally plane polarized signals to provide said first and second video signals respectively.

16. A radar system according to any preceding claim wherein said comparator means comprise a first gate for providing a gate signal only when said ratio of signal magnitudes exceeds a first non zero finite value defined by the first gate, and a second gate for providing said output only in response to both said gate signal and said ratio of signal magnitudes being less than a finite value defined by said second gate.

17. A radar system according to any preceding claim wherein said comparator output is the only signal applied to said processing means which is dependent upon the magnitude of any of the radar signals received by the system.

18. A pulsed radar system including a clutter suppressor and substantially as hereinbefore described with reference to Figures 1 to 4 of the accompanying drawings.

19. A pulsed radar system including a clutter suppressor and substantially as hereinbefore described with reference to Figures 5 to 12 of the accompanying drawings.