FR2661252A1 - Radar initiation system - Google Patents

Abstract

A missile designed to fly just above the sea surface has on board pairs of transmitters (T1 and T2) and receivers (R1 and R2) respectively for transmitting signals and receiving the corresponding returns over four approximately identical segments of space around the line of flight of the missile. A circuit (205) detects the contents of the Doppler modulation of the returns in each segment and produces signals which represent it. A threshold-fixing circuit (213) uses these modulated signals by taking the one relative to the segment most directly opposite the sea in order to create respective threshold signals for comparison with the modulation signals derived from returns coming from the other segments. When a modulation exceeds the corresponding fixed threshold, this indicates that a target has been identified in the corresponding segment and a signal triggering the initiation is created. The amplitude of the threshold signals may depend on the values of parameters such as the Beaufort number, the distance of the missiles from the sea and the angle with which the missile is diving towards the sea.

Description

 The present invention relates to a radar initiation system.

 A missile system has already been proposed in which the missiles fly low over a surface, for example the sea, in order to attack a target close to this surface. A radar initiator on such a missile is subject to a high level of spurious signals, and the initiation threshold should be set to maximize sensitivity to the target, while minimizing sensitivity to spurious.

According to the present invention, there is provided a radar initiator system for use in a missile intended to fly low over a surface, and this system comprises - means for transmitting signals and receiving
corresponding returns, in a plurality of segments
contiguous space, segments that surround the path
of the missile flight, one of those segments facing
more directly to said surface than the others, - a plurality of sem signal processing channels
associated with respective segments in order to
process returns to generate respec signals
tifs indicating the presence (if any) of parasites
and / or dtune target in said segments, - means for drawing, from the indicative signal associated with the
channel corresponding to the aforementioned segment (facing
more directly to the surface), threshold signals
having one or more values related to the level
parasites in this same segment, - comparison means for comparing the signals
threshold thus obtained from said segment to signals
callsigns taken from other segments, and - means for producing a trigger signal
of initiation based on these comparisons.

 In a particular embodiment of the invention, the threshold signals have a relationship with the segments to which they apply, the level of parasites in said first segment which is dependent on the Beaufort number and the height of the missile above the area. In another embodiment, the relationship is also dependent on the angle of incidence formed by the trajectory of the missile on the surface it flies over. In addition, the relationship may depend on the roll angle of the missile, in the case of a missile not stabilized in roll.

Other characteristics and advantages of the invention will appear on reading the detailed description which follows, which aims to show how the invention can be realized, and refers to the drawings, in which - Figure 1 is a schematic illustration of a missile
flying low over the sea, - Figure 2 is a block diagram of an example of
basis of a fi missile initiation system
gure 1, - Figure 3 includes waveforms which
are used to explain the operation of the system, - Figure 4 is the block diagram of a generator
control signal part of the system, - Figure 5 is the block diagram of a variant
of the system, - Figure 6a is the block diagram of a develop
pement of the system of Figure 2, - Figure 6b is the block diagram of an example
circuit for fixing the noise level in
the system of FIG. 6a, FIG. 7 is a diagram intended to explain the
operation of the system of FIG. 6a, - FIGS. 8 and 9 are flowcharts illustrating
operating modes of circuits for fixing
level of parasites likely to form varian
tes to the system of Figure 6a, - Figure 10 is a schematic diagram of another
development of the initiation system, - figure 11 shows diagrams of waveforms
ideal, illustrating the operation of the generator
of Figure 4, and - Figure 12 is the block diagram of a circuit
useful in the case of a roll missile to indicate
what is the quadrant doing facing the sea.

 The present invention relates to a radar initiation system intended to be used in a missile which shaves the sea, in order to attack targets located on the surface of the sea, or close to it. In such a situation, parasites linked to the sea have a significant effect on the initiation system.

 Figure 1 shows a missile 1, stabilized in attitude above the surface of the sea 2. The missile has a cylindrical body on which are arranged two transmitting antennas T1 and T2, on opposite ends of a first diameter of the missile, and two receiving antennas RI and R2 on opposite ends of a second diameter of the missile which is perpendicular to the first. Each antenna has a unidirectional and azimuthal beamwidth (that is to say in a plane perpendicular to the axis of the missile) and at 3dB equal to +500, as well as a small beamwidth in elevation (c ' i.e. in a plane along the axis of the missile), the area inspected by each antenna being tilted forward in the direction of flight of the missile. In this way, the antennas are sensitive to what happens inside a hollow cone located around the axis of the missile, in a known manner. The antennas are of any suitable type known in the art, for example slot waveguide antennas.

The azimuthal space around the missile is divided into quadrants Q1, Q2, Q3 and Q4 by said diameters, and signals are transmitted and received in each quadrant in sequence. This is done for example by - transmitting a pulse from the antenna T1 and receiving
the corresponding return from antenna R1 to cover
quadrant Q1, - send from T2 and receive in R1 to cover Q2, - send from T1 and receive in R2 to cover Q4, and - send from T2 and receive in R2 to cover Q3.

 It will naturally be appreciated that other aerial transmission and reception systems can be used to separate the space into more or less segments, the minimum useful number of segments being two. In addition, the signals do not need to be transmitted and received in the quadrants in sequence, the signals can naturally be transmitted and received simultaneously in pairs of quadrants.

As the missile is stabilized in attitude, quadrant Q1 will always face sea 2, and parasites linked to sea will always be stronger in this quadrant. Quadrants Q2 and Q4 will be subject
at roughly the same level of noise, different from that of Q1, and quadrant Q3 will in turn be subject to yet another level of noise. Quadrant Q1 is used in the embodiments of the invention illustrated in FIGS. 2 and 6 to detect only the level of marine parasites. This detected level is transmitted with an appropriate attenuation to the circuits associated with other quadrants Q2, Q3 and Q4, to ensure that a trip or firing signal is produced only when the return in quadrants Q2, Q3 and Q4 fulfills the following conditions a) the return level is above a fixed level
FL, determined to avoid signal generation
tripping on circuit noise; and b) the return level, which includes a due component
to parasites and a component due to the target, is
greater than a predetermined proportion of para
sites detected in quadrant Q1.

Referring to FIGS. 2 and 3, a control signal generator 200 produces a train of pulses TX, having a form factor of 1: 4 and excites a radiofrequency source 201, which supplies pulses of radiofrequency energy by through a waveguide 202 to a switch TF1. Antennas
TI and T2 are coupled to outputs A and B, to receive alternating radio frequency pulses, as commanded by the control signal TF1 produced by generator 200. A proportion of the energy of each pulse in the waveguide is transmitted via delays (and attenuators) D, to video mixers and amplifiers 203 and 204 connected to the reception antennas R1 and R2 respectively. The ismixer outputs are coupled to identical signal processing channels 205 to 208 via controlled switches 209 to 212.

 The switches 209 to 212 are controlled by the signals RS1A, RS1B, RS2A and RS2B produced by the generator 200 and illustrated in FIG. 3, so that these signals are received by the channels in the sequence 205, 206, 208, 207 to from the quadrants taken in the sequence Q1, Q2, Q4, Q3 specified above.

 The identical channels 205 to 208 each include a Doppler amplifier in series on an envelope detector and an integrator.

Circuits thus described operate in the following manner. We assume that quadrant Q1 is being sampled. Referring to Figures 2 and 3, a TX pulse is sent by the antenna T1 and the corresponding return RX is received after a certain delay at the antenna R1. This RX return will present an overlap with the delayed part, noted
DTX, of the transmitted pulse, and, ideally, the mixer will deliver a video pulse MX whose duration is equal to the interval of common presence of the delayed transmitted pulse DTX and the received pulse RX.

 In practice, the mixer output will include the RX pulse, plus a large component due to direct coupling between the transmitted pulse TX and the receiving antenna R2. The MX pulse will however have a Doppler modulation due to the relative movement between the missile and the target as well as the sea, and can be recognized by the signal processing channel 205 due to this modulation. The output FQ1 of channel 205 represents the level of noise in quadrant Q1.

 The other channels operate in a similar fashion to produce outputs SQ2, SQ3 and SQ4 representing parasites plus a target (if any) in their associated quadrants Q2, Q3 and Q4.

 The output SQ1 represents the noise in the quadrant Q1 and is transmitted to a circuit for fixing the noise level 213, which includes attenuators 214, 215 and 216.

 The attenuators transmit to the comparators 217, 218 and 219 proportions fixed in advance of the level of parasites present in the quadrant Q1. Comparators 217, 218 and 219 also receive signals FL defining levels set in advance to stop triggering on circuit noise, as well as signals SQ4, SQ3 and SQ2 respectively.

The comparators produce firing or tripping signals if the conditions a) and b) indicated above are met.

The circuit described above does not respond to targets present in quadrant Q1. A variant illustrated in Figure 5 can be used to provide a trigger or firing signal in response to a target present in this quadrant. This variant includes a separation circuit 50 which has an input 51 connected to receive the signal
SQ1 produced by channel 205 associated with quadrant Q1. The separator brings the signal SQ1 to an attenuator 52, and to a circuit with a high time constant 53, as well as to the circuit for fixing the noise level 213.

 The output of circuit 53 with a high time constant represents the average level of noise in quadrant Q1 over, for example 10 milliseconds. A target will move rapidly through this quadrant, and so will be able to be detected by the output of the attenuator 52, when this exceeds the output of the circuit 53. A comparator 54 produces a firing signal as a result of the comparison between the output of the attenuator and the output of the circuit 53, as well as with a fixed level FL to avoid the production of the firing signal on a simple circuit noise.

The basic form of the initiation system described above takes the noise level in the signal SQ1, and applies empirically determined fixed attenuations to it to produce the noise levels transmitted to the comparators 217 to 219. The applied attenuation can range from 15 to 20 dB for quadrants Q2 and Q4. More generally, however, the noise levels can be fixed in connection with the signal Q1, as well as in connection with other information such as the state of the sea, the number of
Beaufort, the height of the missile, the angle of incidence of the missile's flight path relative to the sea, and / or other information relating to the attitude and trajectory of the missile.

 The embodiment of FIG. 6 is an initiation system which uses the Beaufort number and the height to fix the levels of parasites.

 This embodiment is the same as that of FIG. 2, except that 10) the channel 205 of FIG. 2 is replaced by a radar altimeter, as described in British patent application 55 243/73, although any other a suitable altimeter can also be used, 20) the control signal generator 200 is modified in a corresponding manner, and 30) the interference level setting circuit comprises a digital processor.

 The production of the TX pulses, the production of the signals SQ2, SQ3 and SQ4, and the operation of the switches TS1A, TS1B as well as 209 to 212 are identical to those of the initiation system a of FIG. 2, and described above in reference to figure 3.

 As described above, when the quadrant Qî is being sampled, the output of the video mixer amplifier 203 comprises a modulated pulse Doppler MX (see FIG. 3), which is transmitted via the switch 209 of FIG. 6A to the radar altimeter 600, which uses it to measure the height of the missile above the sea.

The operating principle of this altimeter is briefly as follows - the duration of the MX pulse depends on the delay between
the TX pulse and the arrival of the corresponding return
laying RX. This duration is therefore dependent on the
distance to the target. The frequency spectrum of
the pulse varies with its duration or with its width
and therefore with distance.

- the altimeter illustrated selects two spectra lines
them in this spectrum, in this example the first and
eighth row, and measures the logarithm of the ratio
of their amplitudes to produce a measure of
distance.

 Referring to Figures 6A and 3, the MX pulse is transmitted to the bandpass filters 602 and 603, tuned to the first and eighth lines of the spectrum.

The filter transmits its lines to mixers 604 and 605, which receive rectangular waves Ref.2 and Rez.1 of frequency equal to the first and eighth lines.

As described above, in practice, the output of the mixer 203 will comprise an important component due to the direct coupling between the transmit and receive antennas, while the desired output MX is a weak signal with Doppler modulation. The outputs of the mixers 604 and 605 are the Doppler modulations of the first and eighth spectral lines, which modulations are amplified and detected before being transmitted to a circuit 606 which forms the logarithm of the ratio of their amplitudes and thus produces a signal LRQ1 representing the distance. The amplified and detected Doppler output of mixer 604 is also used as an SQ1 measurement of the noise level in the quadrant
Q1.

 This noise level SQ1 is transmitted by the noise level fixing circuit to the comparators 217, 218 and 219 with different attenuations, according to the Beaufort number BN, and according to the height.

These attenuations are chosen as follows.

 Referring to FIG. 7, for each Beaufort number, there is a predetermined set of height bands which subdivides the range of heights of the missiles from zero to a maximum. For example, for the Beaufort number 6, the bands H8 to H15 are provided, while for another Beaufort number X, the bands H1 to H7 are provided. Each band is associated with a set of attenuations intended for comparators 217, 218 and 219. The sets of bands of heights form a group 70.

 An example of the interference level fixing circuit of FIG. 6A is illustrated in FIG. 6B.

The attenuation sets associated with all the bands of group 70 are recorded in a read-only memory or ROM 608, in the form of digital words. These words are addressed by an addressing circuit 609, which receives its addresses from an analog-to-digital converter 610 in response to analog signals BN representing the Beaufort number and LRQ1 representing the height of the missile. Signals read from memory
ROM 608 are transmitted via a digital-analog converter 611, to be used to control three analog attenuators 612 to 614 in order to attenuate the signal SQ1 representing the level of noise in the quadrant Q1. The attenuators 612 to 614 transmit the attenuated signals thus obtained from SQ1 to the comparators 217 to 219.

 In another example, the interference level fixing circuit comprises a microcomputer comprising a memory device in which the attenuations associated with group 70 are stored, and addressed by the Beaufort number and by the height.

The general mode of operation of this microcomputer circuit 607 is illustrated in FIG. 8.

 First, the Beaufort number is sent to circuit 607. This is done before the missile is launched. If the number is not known, it is assumed to be equal to Beaufort 6. This allows the selection of the set of bands, and corresponding attenuations associated with this number. Then, the band (H15 for Beaufort 6) corresponding to the maximum height is selected. Thereafter, the actual height is measured using the altimeter 600 and compared to the selected height. If the actual height is different from the selected height, the appropriate band is chosen, with the appropriate attenuations.

 In a development of the system of FIG. 6, the attenuations are fixed according to the height, the Beaufort number, as well as the diving angle (i.e. the angle of incidence of the flight path of the missile towards the sea).

 Referring to FIG. 7, in addition to the storage in circuit 607 of the attenuations associated with a group of sets of bands 70 addressable only by the height and by the Beaufort number, other attenuations associated with groups games such as 71 are also addressable by height and Beaufort number, and are stored so that the different groups can be addressed according to the diving angle. The diving angle DA is transmitted to the circuit for fixing the noise levels from a guide arrangement (not shown) which is carried by the missile (and which is not part of the present invention). In the example of circuit 607 illustrated in FIG. 6B, the ROM 608, which stores the attenuations of groups 70, 71, etc., can also be addressed by a signal which represents DA. In the other example of the circuit 607 making use of the microcomputer, the general mode of operation of the circuit for fixing the noise levels when the diving angle is used is illustrated in FIG. 9.

 First, the Beaufort number is introduced before the missile is launched. If it is unknown, Beaufort 6 is used. All the sets of attenuations appropriate to the Beaufort number, regardless of the diving angle, are then selected, and from these games, the particular game corresponding to an angle of Preselected dive is chosen. The band corresponding to the maximum height is then chosen in this game.

 Then the actual diving angle is measured and compared to the chosen angle. If the actual and chosen dive angles do not match, the appropriate tape set for the actual dive angle is selected, and the appropriate tape for the maximum height is chosen from this set.

 Thereafter, the actual height is measured and compared to the selected height, and the appropriate band for the actual height is chosen.

 The diving angle chosen previously is checked again, and if it corresponds to the actual angle, we continue to choose the bands from the clearance appropriate to this diving angle.

 Otherwise, a new set is chosen, and the band is selected from this new set as described above.

 In principle, the attenuations applied to the signal SQ1 before it is applied to the comparators 217 to 219 can be chosen as a function of other information such as the speed of the missile, or other trajectory information in addition to more information. previously mentioned mations. This requires other sets of attenuations, which must be stored in circuit 607, and suitable means of addressing its attenuations, means which can be defined for example by following the general lines described above.

 The system illustrated in Figure 6, like that in Figure 2, is insensitive to the targets present in Quadrant Q1. However, the variant illustrated in Figure 5 can be used to produce target detection in this quadrant.

The marine parasite in quadrant Q1 differs from a target by at least the following aspects 1) the parasite first appears on the outer edge
the distance window, as determined by
the delayed pulse received DTX of figure 3, 20) this parasite generally lasts much longer
than a target, except for peaks
high waves, and 30) the mean sea level approaches the missile
at a lower speed than a target.

 Thus, a target will produce a sudden reduction in the distance in quadrant Q1, while the distance to the sea surface changes relatively slowly.

 As a result, the distance signal LRQ1 is sent to the separator 50 of FIG. 5. An output of the separator supplies the circuit with a high time constant 53, while another output supplies the attenuator 52. The comparator 54 produces a firing signal if the output of the attenuator 52 (indicator of a target) exceeds by a predetermined amount the average value of the distance measured over a strong time constant such as 10 milliseconds by the circuit 53 .

In order to have a complete description, an example of the control signal generator suitable for serving as a generator 200 'in Figures 2 and 6 will now be described. It is important that the waveforms used to control emission of the pulses and to mix with the corresponding returns in the mixers 204, 203, as well as in the mixers 604 and 605 have rigorous relative timings. To this end, they are produced in the generator illustrated in FIG. 4 from a common clock 400. If one refers to FIG. 11, and also to FIG. 3, the clock produces a rectangular signal of frequency 8F1 which is used in FIG. 6 as a reference signal Ref2, transmitted to the mixer 604. This signal is transmitted to a frequency divider by 4, denoted 401, to produce a rectangular wave of frequency 2F1 which is transmitted to a entry of an AND 402 door, as well as the entry of a divider by 2.403. The output of the divider 403 is used as a reference signal
Ref. 1, and goes to the mixer 603 of FIG. 6, this signal having a frequency F1. The output of the divider 403 is also transmitted to another input of the AND gate 402 to produce, at the output thereof, the transmitted pulse control signal TX having the frequency F1, but a form factor of 1: 4.

 The output of the clock is also transmitted to a divider circuit by 2, 404 followed by an inverter 405 to produce a signal 4F1 of frequency 4F1, but inverted, or shifted in phase by 1800. This signal 4F1 is then transmitted to a divider-by-circuit 4,406, followed by an inverter 407 and a divider-by-circuit 2,408 to produce the signal TF1 of FIG. 3, which has transitions occurring halfway between the pulses TX. A signal c is determined by a divider by 2, 409 circuit from TF1, and the signals TF1 and c are then supplied by the inverters 410 and 411.

These signals TF1, TF1, c and c are transmitted to four AND gates 412 to 415 as shown in FIG. 4 to supply the signals RS1A, RS1B, RS2A, RS2B used to control the switches 209 to 212.

 The timing of the signals RS1A to RS2B are defined to ensure that the returns of the transmitted pulses TX can be received on the appropriate channels up to the maximum range, assuming that the delays of these circuits D of FIGS. 1 and 6 are short.

 The foregoing description made reference to a missile stabilized in roll or in attitude, in which, in the case of FIG. 6, an altimeter should only be provided on the quadrant Q1 which faces the sea.

 Figure 10 shows the schematic diagram of an initiation system for use in a roll-sensitive missile. In this system, altimeters such as the one illustrated at 600 in Figure 6 are provided in each of the four quadrants.

The system includes a transmit and receive control circuit 100, which includes the antennas
R1, R2, T1, T2, switches TF1, 209 to 212, delays D, video mixers and amplifiers 203, 204, the pulsed radio frequency source 201, and the control signal generator 200 'of FIG. 6.

Switches 209 to 212 transmit pulses
MX to identical circuits 101 to 104 respectively, each of which comprises the filters 602, 603 and the mixers 604 and 605 as illustrated in FIG. 6. The circuits 101 to 104 each deliver Doppler output components representing the amplitudes of the first and eighth lines of the MX pulse spectrum they receive. These components are amplified and detected in pairs of circuits 105, 106, 107 and 108 before being applied to the log ratio circuits 109 to 112 identical to the circuit 606 of FIG. 6.

The signals SQ1, SQ2, SQ3 and SQ4 are taken by the right circuit in each pair of circuits 105 to 108, and transmitted to a logic circuit 113. When the associated quadrant fried facing the sea, the signal SQ1,
SQ2, SQ3 or SQ4 in question represents the level of marine parasites. Otherwise, it represents the level of noise, and possibly with the target, in the direction that this quadrant faces.

 Logarithmic ratio circuits 109 to 112 produce signals LRQ1, LRQ2, LRQ3 and LRQ4 which represent the height above the sea when the associated quadrant faces the sea, and otherwise represents the distance from a target if applicable. The signals LRQ1 to LRQ4 are transmitted to the logic circuit 113 and to the distance comparators 114 to 117 which also receive a distance threshold from the circuit 113. The distance comparators produce firing signals according to these distance comparisons .

 The signals SQ1, SQ2, SQ3 and SQ4 are also transmitted to the level comparators 118 to 121, which receive the fixed levels FL to avoid tripping on circuit noise. These comparators transmit the signals to other comparators 122 to 125 which receive from the logic circuit predetermined proportions of the level of interference in the quadrant which is facing the sea at all times. Comparators 112 to 115 also produce the firing signals.

Logic circuit 113 stores in memory
ROM distance threshold and noise attenuation values.

 The parasite attenuation values are addressed as a function of the signals SQ1 to SQ4, of the distance signals LRQ1 to LRQ4, of the pitch, of the roll, of the diving angle, of the speed, of the state of the sea, and the Beaufort number, to the extent that these are available.

 The example in Figure 10 requires the use of altimeters in each of the four quadrants to select the quadrant that faces the sea when the missile is subject to roll.

A variant arrangement for selecting the quadrant which faces the sea is illustrated in FIG. 12. Such an arrangement can be used for example with the system of FIG. 2, by modifying FIG. 2 as follows a) the missile is no longer stabilized b) instead of transmitting only the signal SQ1 to the
noise level fixing circuit 213, all
signals SQ1 to SQ4 are transmitted to a selector
140 (illustrated in Figure 12) which is ordered for
pass the signal from the quadrant facing the
sea to circuit 213, and c) signals SQ1 to SQ4 are also applied to
comparators 141 and 142 as well as switches
143 and 144 illustrated in FIG. 12.

The circuit of figure 12 processes the signals
SQ1 to SQ4 as follows.

10) Use as long an integration time as possible. This ensures that the reports which are deduced therefrom are less disturbed by "radar flashes" coming from the target.

20) The most important quadrants with lg signaur lg, and the second most important signals are determined.

30) The ratio between these signals is obtained, and 40) By using the polar diagram produced from adjacent quadrants, an angle between the quadrants is obtained thereby corresponding to the measured ratio.

 Figure 12 shows an analog representation of the logic that can be used to identify the quadrant, as well as the quadrant offset center angle (starting from analog-digital converters, a logic-digital processor could also be used).

The circuit operates as follows a) the signals SQ1 and SQ3 are compared in the
comparator 141, and switch 143 lets through
the larger of the two, b) the signals SQ2 and SQ4 are compared in the compa
rator 142, and switch 144 lets pass
greater of the two, c) the two signals having passed are compared in the
comparator 145, and the largest is determined. Of
digital indications (larger / smaller) to
from the two comparisons above, as well as
the last one which has just been made, are applied
to a decoder 146, the latter then having information
sufficient to define the direction of the
largest signal, and therefore the quadrant
associated .The decoder acts on the selector 140
to pass the appropriate signal to the circuit
for fixing the parasite level 213 of FIG. 2.

d) the two largest signals are passed to
log amplifiers 147 and 148, and after
these to a 149 difference amplifier. The
output of the latter in turn goes to a circuit not
linear 150, which performs a conversion between the
logarithmic ratio and the angle of deviation of the quadrant
relative to the center.

 By carrying out the comparisons a) and b) on the signals of the opposite quadrants, one ensures that the final ratio is not determined between the signals belonging to opposite quadrants.

 The circuit of figure 12 is also applicable in other situations for example 10) in order to define the direction of target, and thereby to fix a directional machine head in is directionscorrectesen azimuth (for example around the missile axis).

 29) for the purpose of defining the direction of a nearby missile or shell, for the purpose of defining a "miss direction indication or more frequently a miss distance indication, denoted MDI, about a target being towed, for example.

Claims (11)

 of initiation based on these comparisons.  derived from other segments, and - means for producing a trigger signal  threshold thus derived from this-segment at indicative signals  values related to the noise level in this segment, - comparison means for comparing the signals of  threshold signals having one or more  segment. which faces more directly to the surface,  indicative of which of the channels is associated with said-  possible present in the segments, - means for determining from said signals  indicative of possible pests and / or a target  returns in order to generate respective signals  signal associated with the respective segments, to process  the others, - a similar plurality of processing channels  facing more directly to said surface than  one of these segments  contiguous space, segments surrounding the tra  corresponding returns according to a plurality of segments  I. Radar initiation system intended to be used in a missile intended to fly low over a surface, characterized in that it comprises - means for transmitting signals and receiving  2. System according to claim 1, characterized in that each respective processing channel comprises means for generating a modified signal indicative of the interval of common appearance of a transmitted signal and the corresponding received signal, and means for generate a signal indicative of a Doppler modulation of this modified signal, and thereby to represent possible parasites and / or a possible target manifesting in the respective segment.  3. System according to claim 2, characterized in that the means for generating a signal signal doline Doppler modulation comprise a Doppler amplifier circuit for receiving the modified signal and an envelope detector circuit as well as an integrator circuit arranged in series with this one.  4. System according to one of claims 1 to 3, characterized in that the means for deriving said threshold signals comprise a plurality of attenuator circuits for receiving the signal obtained on the specific segment facing the surface, and the attenuate by respective fixed and preset proportions, in order to generate the threshold signals applied to the comparison means  5. System according to claim 4, characterized in that the attenuator circuit attenuates the signal, derived from said specific segment, in respective proportions which depend on the segment to which the corresponding threshold signal refers.  6. System according to one of claims 1 to 3, characterized in that means for deriving said threshold signals comprise an attenuator circuit for receiving the indicative signal derived from the particular segment facing more towards the surface, as well as for receiving one or more parameters including the Beaufort number, the distance of the missiles with respect to said surface, and the diving angle of the missile, this attenuating circuit reacting to a control signal received by attenuating the indicative signal by respective proportions , thereby generating said threshold signals, each of these proportions depending on the value of the parameter represented by said control signal relating to the signal to which the corresponding threshold signal refers.  7. System according to claim 6, characterized in that the attenuator circuit comprises means for memorizing signals defining discrete bands of missile distance values corresponding to different Beaufort numbers, as well as processing means which react to the signals applied, indicative of a number of Beaufort and the distance of the missiles, by selecting from the storage means the band which corresponds to said number and to said distance, and in .. the application of appropriate mitigation.  8. System according to one of claims 6 and 7, taken in dependence on claim 2, characterized in that the means for generating signals indicative of a Doppler modulation of a modified signal, derived from said particular segment facing at the surface, comprise means for generating signals indicative of the Doppler modulations of two discrete spectral lines of said modified signal, and means for comparing said signals derived from these spectral lines, in order to generate an output signal indicative of the distance from the missile with respect to the surface, and by the fact that the signals, indicative of interference, derived from only one of said spectral lines, as well as the output signal in question, indicative of the distance, are applied to said attenuating circuit.  9. System according to claim 8, characterized in that the means for generating signals indicative of a Doppler modulation of a modified signal derived from each of the segments comprise - means for generating first signals respec  indicative of a Doppler modulation of each  of the two spectral lines of the corresponding signal  modified, each of these first signals being indica  tif of the level of parasites in the corresponding segment  dant, and - means for comparing said first signals  in order to generate a second signal indicative of the  missile distance to an object from which  comes the corresponding return, and by the fact that the system also comprises means for receiving a first signal and the second derivative signal for each of the segments, these means responding to the flight attitude of the missile for using the first and second signals coming from that of the segments which is more directly opposite said surface, in order to generate from this respective threshold signals intended to be compared with the first remaining signals.  10. System according to one of claims 1 to 9, characterized in that it comprises means for averaging the signals, derived from said segment facing the surface, during a time interval fixed in advance, in order to generate an average signal, and means for producing an indication whenever the amplitude of one of said signals, derived from that particular segment facing the surface, exceeds the amplitude of the averaged signal, in order to thereby indicate the presence of a target in this particular segment.  11. System according to one of claims 1 to 10, characterized in that it comprises two transmission means and two reception means arranged alternately around the missile body in order to define four substantially identical segments.