GB2273014A - Clutter reduction - Google Patents

Abstract

Clutter is reduced in a marine pulse radar by STC (sensitivity time control - comparison with a variable threshold) 25, FTC (frequency time control - filtering) 26 and subsequent gain control 27. Inputs to section 25, 26 and 27 vary with ambient conditions such as rainfall and sea state; these conditions are input via decoder 40 and are used to select appropriate values of the inputs to sections 25, 26, 27 for each range value. Rather than calculate the inputs to sections 25, 26, 27, these are pre-stored in a high-speed memory 29 for each of a number of ambient conditions; ports 41, 42, 43 then allow a suitable one of these to be output from the memory and used to process the radar video signal. <IMAGE>

Description

CLUTTER CANCELER OF A PULSE RADAR SYSTEM The present invention relates to a video signal processor of a pulse radar, and more particularly to a clutter canceler for stepwise attenuating or efficiently canceling various clutter signal power included in a video signal of a radar receiver.

Radar systems are classified according to use: pulse radars, continuous wave (CW) radars, frequency modulation (FM) radars, phase modulation (PM) radars, etc. The block diagram of a general pulse radar system for navigation, meteorological observation topographical survey, coast defense supervision and vehicle transportation observation is as shown in Fig.1. Here, a pulse modulator 10 generates a repeated train of pulses. A transmitter 11 amplifies a signal according to a train of pulses with a high power and transmits the amplified signal to antenna 14.

A duplexer 12 separates transmission and reception for enabling antenna 14 to be used for both of transmission and reception. Antenna 14 emits the pulses in the air and receives the echoes reflected by the target or ground, sea surfaces, rains, clouds and fogs. The position coordinate of antenna 14 is controlled by position servo-system 22 which is connected between machine unit 13 for driving antenna 14 and display and controller 21. with the azimuth bearing angle on the display being synchronized with the position of antenna 14. The echo signal received via antenna 14 is input to a low-noise RF amplifier 15 through duplexer 12. Low-noise RF amplifier 15 amplifies the received signal while suppressing the noise.A mixer 16 mixes the local oscillator signal from a local oscillator 17 with the received input RF signal to generate an intermediate frequency (IF) signal of 30-120MHz. An IF amplifier 18 is a kind of matched filter and amplifies the signal with an improvement of signal-to-noise (S/N) ratio. Detector 19 generates a video signal from the received IF signal. At this point, the video signal includes the desired signal reflected from a target object, noise and clutter. Clutter also includes the echo signal reflected from ground and sea surfaces, clouds, rains and fogs, etc., and can be classified into sea clutter reflected from sea surfaces, land clutter reflected from ground surfaces and weather clutter (or volume clutter) reflected by rains, clouds, fogs. etc.Such clutter signals can be a useful signal or a unnecessary signal according to purpose of use of the radar. Generally, the clutter is used to name the unnecessary signals. Video signal processor 20 removes the undesired noise and the clutter regarded as unnecessary in connection with the object of use of the radar, and extracts a pure target signal for display. Display and controller 21 shows the target captured by radar on a CRT, and displays every kind of necessary condition.

In addition, the screen condition can be regulated by an operator using a clutter selection regulator.

In Fig.2, video signal processor 20 is illustrated in more detail, showing clutter canceler 23 for canceling the clutter from received echoes and video signal main processor 24 for reducing an extraction level with respect to various noise signals into not more than a certain level and extracting target information. The pulse radar system transmits a transmission pulse and receives its echoes which include the signal reflected by the target and clutter reflected by environmental factors such as ground and sea surtaces, rains, clouds, fogs, etc. Thus, to detect the target accurately. it is necessary to cancel the undesired clutter because the clutter displayed on CRT hinders target detection.

In a conventional method. to facilitate the clutter canceling function of a pulse radar system, the clutter is canceled by selecting a simple fonn of an analog reference model, or as in a method disclosed in U.S. Patent No. 4,837.579, a clutter algorithm is calculated in a system processor, i.e., central processing unit, and the obtained calculation result is applied for each pulse repetition time period (hereinafter referred to as "PRT"). That is, according to Figs.3 and 4 showing the method disclosed in U.S. Patent No. 4,837,579, clutter is calculated by considering a number of variables obtained from a number of radar parameters and various statistic data in a system processor 69.Once a radar operator implements a regulator in the regulation panel according to the operation environment and the display state, system processor 69 starts calculation to derive out a proper algorithm by considering the overall variables.

The calculated final output goes to an address decoder and buffer 65 via system bus 68. Address decoder and butter 65 distributes the input to a sensitivity time control (STC) 80 for canceling the surface clutter and a gain compensator 81.

Mean generator 62 temporally stores the data in a memory 71 that system processor 69 supplies, latches the data by the synchronization of a latch clock 63 and converts them into analog form in a D/A convertor 73, and produces the mean signal for canceling the ground and surface clutters from the video signal. A fast time constant (FTC) 61 which cancels the volume clutter reflected by the meteorological factors is a kind of high-pass filter and cancels the clutter at a constant level adjusted by means of a variable resistor. Gain compensator 81 compensates the level according to the reference signal output from reference generator 64 by temporally storing the data supplied from system processor 69 in a memory 75, and latching the data by being synchronized by a latch clock and converting them into analog form in a D/A convertor 77.

As heretofore shown, the conventional clutter canceler plays the role of a thresholder 70 which performs an algorithm function provided by system processor 69. Therefore, whether the data should be supplied at each PRT, or is not supplied at each PRT but is supplied only when the operator regulates, all the algorithm calculations have been performed in the system processor.

This conventional clutter canceler has certain drawbacks. These are degradation of the overall system's efficiency because the system processor performs the clutter processing algorithm to give an output. The conventional clutter canceler cannot be applied in the high resolution radar or color video radar, due to a slow processing speed.

Therefore. it is an object of the present invention to provide a clutter canceler of a pulse radar system capable of storing a pre-calculated algorithm in a high-speed and large-capacity memory, producing an output swiftly in accordance with the input by a regulator. and reliably and promptly processing, to cancel or stepwise attenuate the clutter included in a received radar signal.

According to the present invention there is provided a video processor of a pulse radar system including an antenna, a transmitter-receiver and a display comprising: port selection means for selecting a corresponding port according to a function controlling signal input from a regulator; high-speed and large-capacity clutter algorithm memory means for storing the algorithm selected and pre-calculated according to the port selection means and outputting the resulting data upon being requested; data distribution means for distributing the data input from the clutter algorithm memory and port selection means, to corresponding means; STC means for canceling a ground or sea surface-reflected wave from a received radar signal input according to the output value of the clutter algorithm memory means;FTC means for canceling a meteorology-reflected wave according to the signal from the data distribution means; gain compensation means for compensating by as much as the canceled levels of the STC and FTC means according to the signal input via the data distribution means from the clutter algorithm memory means; and synchronization generating means for extracting a synchronization signal from the received radar signal and synchronizing the clutter algorithm memory means by means of a frequency generator.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: Fig. 1 is a block diagram of a general pulse radar system; Fig.2 is a more detailed diagram of a video signal processor of Fig. 1; Fig. 3 is a block diagram of a conventional clutter canceler; Fig.4 is a more detailed diagram of the clutter canceler shown in Fig.3; Fig.5 is a block diagram of a clutter canceler of a pulse radar according to the present invention; Fig.6 is a more detailed constittitional diagram of the clutter canceler shown in Fig.5; Fig.7 shows sea clutter, wherein sea conditions are at level 5 (typhoon);; Fig.8 shows sea clutter, wherein sea conditions are at level 1 (the calmest state); Fig.9 shows land clutter, wherein ground surface reflection is at maximum level; Fig. 10 shows land clutter, wherein ground surface reflection is at minimum level; Fig. 11 shows rain clutter, wherein rainfall is at maximum level; Fig. 12 shows rain clutters wherein rainfall is at minimum level; Figs. 13A. 13B and 13C show the cases of maximal and minimal clutter corresponding to Figs.7-12; and Figs. 14A through 14E show a method for embodying the clutter canceling algorithm into a memory.

Hereinafter. a detailed explanation of the present invention will follow, with reference to the attached drawings.

Fig. 5 is a block diagram of a clutter canceler in a pulse radar system of the present invention. Signals reflected from ocean waves, rain, snow, clouds, ground surface condition. fog, etc., are included in the video signals received from radar system for ocean navigation and sea coast supervision and ocean monitoring and detection and tracing radars mounted on the airplane, and in the video signals for meteorological observation and ground surface mapping radars.

These signals are desirable or undesirable in connection with the purpose of radar detection. In case of the undesirable signal, these signal work against detection and capturing ability as in a kind of noise signals. A signal reflected from an ocean surface is called sea clutter, which varies with the height of the waves and is closely related to a wind speed and a radar angle to watch.The strength ac of sea clutter is represented as

where Lp is beam-shape loss, R is the distance to the wave from the antenna, h is antenna height, 6a is the beam azimuth, b is the grazing angle, C is the velocity of light, 7 is the wiclth of pulse and a is the surface clutter scattering coefficient which varies according to wind speed, sea conditions, wavelength of the electromagnetic wave, etc.

Now, for wave conditions of level 5 (typhoon) and level 1 (the calmest state) and the C/N (clutter to noise) level of a radar transmission in case of the X band (8 12GHz) is calculated by applying above equation, and is as shown in Figs.7 and 8, respectively.

Referring to Figs.7 and 8, the horizontal axis represents the range of distance between radar antenna and the retlecting object (unit : the nautical mile), and the vertical axis represents the strength of clutter vs. noise (unit : dB), and the measuring condition is that the antenna is positioned horizontally (Tx elevation = O", Rx elevation = 0"). and the height of antenna is 50 ft. Fig.7 shows the sea clutter for wave condition of typhoon. wherein the strength of clutter decreases exponentially as the distance between antenna and reflecting object becomes farther.

Fig.8 shows the sea clutter for wave condition of the calmest state, wherein the strength of clutter decreases rapidly as the distance becomes farther.

In addition, the strength crc of volume clutter due to rain, snow, clouds and fogs is calculated losing

where z represents the amount of rainfall per hour and determines volume clutter reflectivity yv. and the mutual relationship between them is represented as v(m2/m3) = 5.7 X 10-'4el-6/A4. where X is the wavelength of the electromagnetic wave.

At this point. the standard amount of rainfall applicable all over the world is 1-70mm/H. Also. the C/N domain is calculated by these equations and separated into minimum and maximum levels with a reference of the x band as shown in Figs.ll and 12.

Referring to Figs. 11 and 12, the horizontal axis represents the range of distance between radar antenna and reflecting object (unit : the nautical mile), and the vertical axis represents the strength of clutter vs. noise (unit :dB). At this point, the measuring condition is that the elevation of antenna is 23 . and the height of reflecting object is 8333 ft where rain clouds is located, so that the range of distance becomes greater than any other. Fig. 11 shows the rain clutter for the rainfall amotint ol 70mm/hour wherein the strength of clutter decreases rapidly as the distance between an antenna and the reflecting object becomes farther. Fig. 12 shows the rain clutter for the rainfall amount of 4mm/hour wherein the strength of clutter decreases rapidly and can be seen in a close range.

Further. the strength of land clutter reflected from ground surfaces or land is determined as

where R is distance from antenna to reflecting object, Ba is beam azimuth, b is grazing angle. r is the width of transmitted pulse C is the velocity of light and Lp is beam-shape loss.

Here, the most important variable is scattering parameter e which varies with topology: its mininluin and maximum values being about 0.03-0. 15, corresponding to rocky mountains and mowed grass. Probable maximum and minimum C/N values calculated by the above equation are as shown in Figs.9 and 10, respectively.

In Figs.9 and 10, measuring condition is that antenna height is SOft, the antenna is horizontally positioned (Tx elevation=0", Rx elevation =0"), the horizontal axis represents the range of distance between an antenna and the reflecting object. and the vertical axis represents the strength of clutter vs. noise (unit: dB). Fig.9 shows the land clutter for the maximum reflection (where a=0. 15) from the land wherein the strength of clutter decreases exponentially as the distance becomes farther, and Fig. 10 shows the land clutter for the minimum reflection (where y=0.03) wherein the strength of clutter decreases rapidly and can be seen in a close range.Among the above clutter types, sea clutter and land clutter exist within relatively short range although the range of clutter varies a little according to the type of radar, and their clutters are canceled by STC processing.

According to the method of the present invention, the volume clutter (weather clutter) is canceled by FTC processing, as volume clutter can be generated over all ranges of the radar system. Also, gain compensation is required as much as the level canceled by STC and FTC operations. This clutter cancellation processing algorithm does not need to be calculated and processed by a system processor, as in the conventional method. Instead, the present method is developed in which all possible circtinlstailces are modeled by a statistical method in the steps of 2 up to 512 or above if necessary.These algorithms for the above various circumstances are stored permanently in the higll-speed, large capacity memory device, and any of them can be selected according to the corresponding circumstance to be applied to the signal processing for canceling the clutter.

This method is realized by use of an ultra-higll speed large capacity memory to solve many problems of lowering in performance and processing speed of the system confronted in the conventional method in which the algorithm should be always is calctilated and proceeded by the system processor. This method has more favorable results in the performances of speed, reliability and detection ability than the conventional one. The conventional signal processing techniques using a system processor is limited to less than 32 steps because of the processing capability of the processor. However, the present invention realizes a 256-step or the above signal processing tecllnlqlle.

In Fig. 5, port selector 31 selects one of the STC port leading the signal to STC 25, the gain port leading the signal to gain compensator 27 and the FTC port according to the port selection signal of function regulation signal 32 generated by operating the regulator located in display and controller 21 shown in Fig. 1. Then, the selected port is appliecl with the function selection signal of function regulation signal 32.

Function regulation signal 32 composed of a 4-bit port selection signal and an 8-bit function selection signal can select algorithm of each port up to 256 steps.

The algorithm steps can be extended without limit according to necessity, but generally 64 steps are enough for color methods. However, the algorithm steps can be extended up to 256 steps in preparation of the ultra-high resolution image display technology. STC algolithlll ancl gain compensation algorithm are stored in clutter algorithm memory 29, and the selected algorithm is sent according to the syncllronization signal to data distributor 28 which distributes the data coming from clutter algorithm illenlory 29 and FTC port to STC 25, FTC 26, and gain compensator 27, respectively.STC 25 converts the digital data into the analog data to cancel surface clutter from the radar reception signal. FTC 26 converts the value of data distributor 28 froln digital to analog. and cancels the volume clutter included in the video signal wherein the surface clutter has been canceled in STC 25. Gain compensator 27 compensates the level of video signal as much as canceled in STC 25 and FTC 26. Syncllronization generator 30 extracts a PRT trigger signal from the received radar signal, and generates the address of clutter algorithm memory 29 according to the counter of a freqtiency generator. The memory is accessed in sequence, so that the corresponding address data can be output.

Fig.6 is a detailed block diagram of the clutter canceler of the pulse radar system according to this invention. Antenna 14 shown in Fig. 1 emits waves and receives the signal reflected from a target, and the received signal enters amplitude level regulator 59 via the IF terminal, and is regulated to an appropriate level in which video signal processing is possible, in amplitude level regulator 59, and after which surface clutter components are canceled according to the given algorithm, in surface reflection wave canceler 47. In the signal after canceling the surface clutter components the volume clutters due to rain, snow, clouds, or fog are canceled in weather reflection wave canceler 5 1.Finally, the signal is compensated to a certain level in gain compensator 55 and supplied to a constant false-alarm rate (CFAR) signal processor (not shown). Surface reflection wave canceler 47 is for canceling strong signals reflected from ground surfaces far from or near the antenna, and the strong reflection signals reflected from sea surfaces which vary according to sea surface conditions.

Fig. 13A shows the possible domain of graphs representing the relationship between the range and tle clutter level in the cases of the calmest state of sea condition and the maximum typhoon condition, and Fig. 13B shows the possible domain ill the cases of the illaxinluill and minimuin reflections due to the land condition. The domain is sectioned into a illiiliilltii0l of 32 steps up to a maximum of 256 steps, and stored ill clutter algorithm memory 29. When any algorithm is selected by the regulator located on the front panel of the display unit, the function selection signal is latched in clutter algorithm memory 29 via port selector 31.

Here, 8-bit data stored ill a memory goes tllrougll data distributor 28 and is converted into an analog signal in D:A converter 44. The signal once again goes through function amplifier 45 and function converter 46, and is compared with PRT video signal received simultaneously in surface reflected wave canceler 47 and atteiltiates surtslce reflected signal from PRT video signal.Once a certain level of attenuation is selected by the operator, the algorithm data according to the level goes through data distributor 28, D/A converter 48, signal receiver-and-distributor 49, function switch 50. and weather reflected wave canceler 5 1 at which attenuate weather reflected signal from output signal of surface reflected wave canceler 47.

This attenuation level is shown in Fig. 13C as calculated by the equation 2, which is about half through one third of the domain of the sea surface reflected wave as shown in Fig. 13A. The signal of which every type of clutter has been canceled to some degree at sUl tace reflected wave canceler 47 and weather reflected wave canceler 51. is compensated in gain colilpeilsator 55 according to output signal of clutter algorithlll memory 29. The output signal of clutter algorithm memory 29 is converted into analog form in D/A converter 52, and proceeds to function amplifier 53 and function converter 54, ultilllately reaching gain regulator 55.The degree of gain compensation is as much as the summing of all levels canceled in surface reflected wave canceler 47 ancl weather reflected wave canceler 5 1. This signal process is carried out for each PRT interval separately, for which the PRT trigger included in a reception signal should be detected and synchronized. That is, the PRT trigger detected in system synchronization signal detector 58 is connected to memory addre. > s generator 57 and synchronized at each PRT.Memory address generator 57 generates a 16-bit address corresponding to the frequency of frequency generator 56. and the address is applied to clutter algorithm memory 29, which can generate the maximum 64k addresses within one PRT period, so that the present clutter canceler can be used for radar systems having short or long PRTs.

Figs. 14A-14E shows a method to embody the clutter algorithm into a memory according to the present invention. Fig. 14A relates to the possible range of surface clutter, and it shows an example of 3n steps wherein algorithm 1 is for the smallest surface clutter and algorithm 32 is for the largest. Fig. 14B relates to a case in which algoritllill ü is selected as a regulator by an operator among the examples of Fig. 14A. The selected algorithm is divided into 512 sample-steps in a PRT period which is accessed sequentially according to addresses generated by the address generator of the synchroilization generator, by which 8-bit data is output.

Here, the sample step is variable as necessary, so that any length (short or long) PRT period can be processed. Fig. 14C shows the PRT period and dynamic range in conilection with Fig. l4B. Fig. 14D shows realization in a memory, of algorithm steps as shown in Fig. 14A, while Fig. 14E is for algorithm 30 of the example shown in Fig. 14B. As above. necessary algorithms can be embodied by necessary steps directly into a high-speed large capacity memory so that the clutter cancellation can be carried without support of system processor.

As shown hereinabove, in the clutter canceler of a pulse radar system according to the present invention, the clutter cancellation algorithm is not executed by the system processor, but instead is processed swiftly according to a pre-calculated result in a Iligll-speed memory, which improves the overall performance bv reducing the load on a system processor. Further, the high-speed memory is easily exchanged, which makes an algorithm possible to be substituted with various other algorithms and can embody the clutter cancellation sampling in up to 2-512 steps. Thtis, tlle present clutter canceler can be adapted to color displays which lprovez detection and capture capability.

While the present invention has been particularly shown and described with reference to particular embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be effected therein without departing from the spirit and scope of the invention as defined by the appended claiins.

Claims (20)

CLAIMS:

1. In a video processor of a pulse radar system having an antenna, a transmitter-receiver and a display, a clutter canceler comprising: port selection means for selecting a corresponding port in accordance with the function controlling signal input from a regulator; high-speed ;Lti0Cl lirge-capacity clutter algorithm Inelilory means for storing an algorithm selected by said port selection means and calculated in advance and outputting the selected algorithm upon request; data distribution inealls for distributing the data input from said clutter algorithm memory means and swd port selection means, to corresponding means;; STC means tor canceling a surface-refected signal in a received radar signal input according to the data output froni clutter algorithm memory gleans; FTC means for canceling a weather-reflected signal in said received radar signal in accordance with the signal being input from said data distribution means; gain compensation means for compensating by as much as the canceled levels of said STC and FTC means in accordance with the signal input from said clutter algorithin illeillory means via said data distribution means; and synchronization generatillg means for synchronizing said clutter algorithm memory means by extracting the synchronization front said received radar sigilal, and having a frequency gellerator.

2. A clutter canceler of a pulse radar system as claimed in claim 1, wherein said clutter algorithm memory means comprises two algorithms, one of which is for canceling surface clutter due to ground surface or sea surfaces, and the other of which is for compensatillg, the gain by as much as the attenuation by said STC and FTC means.

3. A clutter canceler and of a pulse radar system as claimed in claim 1, wherein said STC: means comprises D/A conversion means for converting the digital data input from said data distribution means into analog form, function amplification means for amplifying the analog signal of said D/A conversion means. function conversion means for adjusting the output signal of said function amplification means to be located ill the range of the clutter cancellation algorithm, and surfacereflected wave cancellarion means for canceling the surface clutter Ill said received radar signal according to the output of said function conversion means.

4. A clutter canceler of a pulse radar system as claimed in claim 1, wherein said FTC means e:omprise.s D/A conversion means for converting the input digital data from said data distribution means into analog form, signal reception and distribution means for appropriately amplifying the analog signal of said D/A conversion means, and function switch means for switching the output of said signal reception and distribtition means to be in an algorithm range, and weather-reflected wave cancellation mealls for canceling the weather clutter in said received radar signal according to the output of said function switch means.

5. A clutter canceler of a pulse radar system as claimed in claim 1, wherein said gain compensation means comprises D/A conversion means for converting the digital data input from said data distribution means into analog form, function amplification means for amplifying the analog signal of said D/A conversion means, function conversion means for adjusting the output signal of sid function amplitication means to be located in the range of the gain compensation algorithm, and gain regulation illealls for compensating the gain according to said function conversion meals.

6. A clutter canceler of a pulse radar system as claimed in claim 1, wherein said synchronization generating means comprises synchronization signal detection means for detecting a PRT trigger in said received radar signal, frequency generating means for oscillating to synchronize said sampling steps in accordance with said trigger signal, and address generating means for generating the address of said clutter algorithm memory according to the output signal from said frequency generating means.

7. A clutter canceler of a pulse radar system as claimed in claim 1, wherein said port selection means comprises an STC port for selecting the data of said STC means according to said regulator, a gain port for selecting the data of said gain compensation means, an FTC port for selecting the data of said FTC means, and a decoder for selecting said ports according to said regulator selection.

8. A clutter cailceliilg method in a pulse radar system comprising the steps of: selecting a clutter canceling algorithm corresponding to said sea and weather conditions observed by a liver; extracting the clutter canceling algorithm corresponding to one algorithm selected by said user alllollg a plurality of clutter canceling algorithms corresponding to said degree of sea and weather conditions and having been calculated and stored in advance; and canceling tile clutter included in a radar signal received according to said extracted clutter canceling algorithm.

whereby high-speetl processing ot clutter canceling is carried otit by use of a clutter canceling algorithm pre-calculated according to a degree of sea and weather conditions.

9. A clutter canceler of a pulse radar system substantially as hereinbefore described with reference to the accompanying drawings.

10. A clutter canceling method in a pulse radar system substantially as hereinbefore described with reference to the accompanying drawings.

11. Signal processing apparatus for processing received signals reflected from an object, arranged to cancel clutter or unwanted reflections from the received signals by processing thereof, comprising means for storing data defining at least two different clutter processing algorithms corresponding to different ambient conditions, means for inputting control data relating to ambient conditions, and means for selecting one of said stored predetermined algorithms in dependence upon said control data and for executing said selected algorithm.

12. Apparatus according to claim 11, in which each of said algorithms comprises a time sequence of thresholds applied to said received signal.

13. Apparatus according to claim 12 in which said time sequences define a threshold which decays over time.

14. Apparatus according to any of claims 11-13 in which said received signal comprises reflections of a transmitted pulsed signal.

15. Apparatus according to claim 14 in which said algorithm is executed once every pulse repetition period.

16. Apparatus according to claim 15, in which period.

16. Apparatus according to claim 15, in which the selection of said algorithms is determined in accordance with the frequency of the signal.

17. A radar processing circuit comprising means for processing a received signal in accordance with a time series of value stored in a look-up table.

18. Apparatus according to claim 17, in which said values are a threshold, and said series decays over time.

19. Apparatus according to claim 17 or claim 18, in which there are provided a plurality of said lookup tables each corresponding to a processing algorithm to be applied in different ambient (e.g. climactic or terrain) conditions.

20. Pulsed radar signal processing apparatus for removing clutter, comprising operator input means enabling an operator to input climactic data and/or terrain data, a plurality of predetermined clutter reduction profiles each corresponding to different climactic and/or terrain conditions and means for selecting one of said profiles in accordance with said input.