FR2725525A1 - AIR-SOL RADAR TRADE SYSTEM - Google Patents

Abstract

The invention relates to air-to-ground radar tracking systems. It relates to a system in which a comparator arranged very close to the waveguides of a four-element antenna arrangement transmits a signal representative of the time shift of difference signals corresponding to the characteristics of two pairs of shifted beams. in azimuth, the beams of each pair being offset in height. The control signal gives an indication of the inclination of the ground surface. Application to the correction of distance indications given by air-ground radars.

Description

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  soil and cue this integration takes place on the ground without

  if we do not keep separate targets

  large dimension. Consequently, an analogous character

  noise is superimposed on the characteris- ticue rersenée in Figure 1 and introduces an uncertainty on the piace ce

  crossing or overlapping beams. This efe es-

  considered as a residual background noise and constitutes one of the main sources of incursion in the future.

  the case of a radar reverberation of the self.

  FIG. 2a is a nano view of the ground surface illuminated by the incident impulse and it will be noted that the locations corresponding to a cancellation of the echoes of the impulse are on a trace drawn on the ground and

  intersecting the trajectory of the aircraft.

  When the antenna rotates about its axis this

  in relation to the ground or when the soil is inclined trans-

  to the flight path of the aircraft, there is a

  rotation or torsion of line L of zero compression

  indicated me in figu-e 2b. This line is always a line segment but no longer perpendicular to the

  pointing direction of the beam. The penze of the

  The cost of FIG. 1b is then reduced. This svs-

  tem is therefore more sensitive to residual noise and the error

  on the distance increases. In addition, the emolations correspond to

  of a zero echo due to the impulse, from the ground, are distributed in distance and provocuen a c-_

  additional dation of the resion on the distance. 'at.-

  the lowering of the line of sight of the antenna is

  very small and even for small roll angles, the aforementioned effect may increase the error over the distance of

  very important way, for example from about 30 to 200 m.

  The actual roll that the antenna around its roll axis (ie the line of sight) can also be

  the use of a scanning device of oc_-

  Cassegrain, the construction of which is such that the sweeping along a direction which is not included in the planes of azimuth and height provokes a roll of the antenna around its axis of roll. This feature can be avoided by a suitable gimbal arrangement, but some

  very important benefits disappear.

  The correction of the roll of the antenna of an off-axis scan can be ensured by a stabilization

  electronic roll. For this purpose, the four elements of

  are used and each has its own beam characteristic. The four elements are arranged in a square or rectangular arrangement, the beams diverging from the line of sight to give the indicated arrangement.

  in figure la both in height and in azimuth. The

  output signals from the four antenna elements are added

  generated and subtracted in a known manner in a microwave comparator so that they form difference signals in azimuth and height along the "axes of the aircraft", these signals being processed in a transmitter with a resolver of

  pulsed radar giving a high difference signal

  which is stabilized in space.

  When using such an arrangement, errors due to misalignment of the azimuth axis of the antenna and the surface perpendicular to the path

  are due almost entirely to the inclination of the ground.

  The invention relates to a tracking system

  dar in which an indication is drawn from the disposition of the cross-sectional area relative to the plane of height of the antenna and wherein a control signal can be formed to enable correction

  stabilization of the antenna, mechanically or electronically

  nically, or in such a way that it always maintains the separation

  ration of the detached pulse perpendicular to the surface

  transverse to the trajectory, in height.

  More specifically, the invention relates to an air-to-ground radar tracking system which comprises a detached pulse airborne radar comprising a device for forming distance information from the moment of departure.

  the emission of a radar pulse, the antenna arrangement

  not having a beam feature divided in height

  for each of two angular positions distant in azimuth

  mut, each of the characteristics having a recess axis

  directed towards the surface, the system comprising a device

  The control signal is intended to form a control signal representative of the time difference of the difference signals derived from the corresponding characteristics of division of the beams in height, the control signal giving an indication or an order which depends on the inclination of the surface between the beams.

positions intersected by the axes.

  The system may comprise a stabi-

  the height plane in space, the communication signal

  depending on the inclination, in relation to the time

  rice, from the surface, in the direction transversal to the

  trajectory, between said positions of the surface.

  In one variant, the stabilization of the height plan can be modified by the control signal in order to

  that this plane remains perpendicular to the surface between

say positions on it.

  Distance information transmitted by the system

  can be corrected by the control signal in an arrangement

  open loop so that the effect of the trans-

  towards the trajectory be deleted.

  Preferably, the device for forming a control signal comprises a sum and

  difference to which the signals received by an arrangement of

  four-element tenne are transmitted, and which forms a sum signal, a difference signal in azimuth, a signal difference in height, and a nonsense channel signal, the latter

  depending on the inclination of the transverse surface

  the trajectory, between the said positions on the

face.

  Other features and advantages of the invention

  tion of the following description of a

  an air-ground radar system for illustrative purposes.

  and with reference to the accompanying drawings, in which: FIGS. 1a, 1b, 1c and 1d, already described, respectively show the divisional characteristic in height with gradients amoliteness / time

  corresponding to the echoes from the ground in the element

  lower and lower element of the beam, and a graph representing the difference signal as a function of distance and time; FIGS. 2a and 2b already described are plan views of detached pulse beams of FIG. 1,

  intercepting a horizontal ground and a soil with an inclination

  its transverse to the trajectory respectively; FIG. 3a is a view along the line of sight of a detached pulse with four beams approaching an inclined ground;

  FIG. 3b represents waveforms corresponding to

  echo signals transmitted to the four elements of

  tenne, corresponding to Figure 3a; FIG. 4a represents the sections of the four beams at the moment when the line of sight intercepts the inclined ground; FIG. 4b shows graphs indicating the variations of the difference signals of the divided beams

  in height and distant in azimuth, with a graph representing

  feeling the difference between these signals;

  FIG. 5 is a diagram representing a comparison

  pulse generator and indicating the normal signals

  sore entrance and exit; FIG. 6 is a block diagram of an air-ground radar tracking system; and

  FIG. 6a is a diagram showing the orientation

  of a carrier aircraft in relation to the ground surface and representing the various angles treated in the

Figure 6.

  Figures la to 2b having already been described, Figure 3a shows a vertical arrangement of divided azimuthal beam patterns, for example obtained in a system stabilized in space. In this case, when the

  soil is inclined transversely to the trajectory of the

  the four beams come to cut the surface with

  DCBA frequency. (It should be noted that the "beams" do not

  not actually soil because it is not a question of

  radiation, but of the characteristics of the sensiti-

  the reception angle as a function of the angle for each antenna element. Nevertheless, this analogy gives a suitable analysis). The output signals of the antenna elements, if available separately, would be as shown in Figure 3b which indicates the variation of their amplitude.

according to the time.

  Each vertical pair A-C and B-D constitutes a system

  individual beam pattern divided in height as shown

  in Figure 1, the two systems being separated in azimuth.

  Each pair of alternating signals of FIG. 3b thus gives a corresponding difference signal as represented on FIG.

Figure ld.

  The two beam systems divided in height,

  as they are separated in azimuth, give points of

  cut (N in Figure ld) that appear at times

  different because of the transverse inclination to

  jectory. It can be seen that the time difference t between these points is represented approximately by the equation 6t = 4 r tg a. cg 0in / 2 c sin 9 in which r designates the distance to the center of the beam, that is to say the location of intersection of the surface by the line of sight of the antenna, a is the inclination of the ground

  transversely to the trajectory, 0 is the angle of

  divided into azimuth, it is the speed of light, and

  4 is the angle of view of the radar down.

  The determination of the distance r from the axis of

  Therefore, the knowledge of the inclination of the trajectory and the measurement of the offset in time must be known. Apart from the difficulty of measuring the lag in time,

  the foregoing description assumes that the channels A, B, C

  and D can be processed separately (or at least in a fast sequence by a switched receiver). However, it is known that, in any system forming beam differences, the errors introduced are particularly

  sensitive to amplitude and phase shift errors that may

  can not be introduced unintentionally before the

  subtraction, which is why, in all modern systems with detached pulse radar, the beam subtraction process is performed as early as possible in the system, usually

  in a waveguide comparator which is just

  the excitation device of the four horns.

  An operating method that satisfies this

  criterion and allows the use of a comparator of a

  in the fore part of the ap-

  reil is the off-centered sighting method that is well known for locating a target with respect to the axis of

referred.

  The output signals transmitted by a simple two-beam comparator system are first considered when a

  point source runs through the main beam. The functions

  variation of the output signal as a function of the angle corresponds roughly to a single cosine function for the output signal of the sum channel and to a function

  single sinus for the difference channel output signal.

  Given the linearity of the sine function in the crossover region of the beams, the position of a target which is not exactly on the line of sight can be accurately estimated from the sign of the channel signal: ence

  and its relation to the signal of the sum channel, provided that the con-

  usual proportionality, using beam widths, etc. are known. This is a well-established technique, corresponding to an axis measurement

  off center. This principle is applied to the measurement of inclination

  of the soil transversely to the path, in a

  advantageous embodiment of the invention, as follows. Referring to FIGS. 4a and 4b, it will be noted that when the short pulse emitted progresses towards the ground

  tilted, the moment represented on the drawing appears,

  that echoes are received from the point

  which line of sight of the antenna intersects the ground, point O.

  At this moment, the distance from the point O is indicated by the

  normal channel of the channel difference in height, that is to say (A + B) - (C + D), having a cross-check. If the beams divided in height A, C and B, D are treated separately, they present at this time

  sources or targets that are above and below the

  from the point O and which form positive and negative signals respectively, proportional to the

  height of these sources, ie the ground level, above

  above and below this horizontal.

  This arrangement is schematically represented

  by vector signals V1 and V2 of Figure 4a. Lors-

  that the radar pulse travels through the projection of the

  the beam along the ground, the separate signals V1 and

  V2 describe "single-cycle sinusoidal waves" separated

  (representing the characteristics of the difference channel) but shifted in time according to the law in tg a

previously indicated.

  A method for avoiding the complication of the

  sure accurate of the time difference between the signals

  difference (difference) similar to sinusoidal waves

  dales, created separately (and containing some noise), include their electrical subtraction and the use of

  difference between two sinusoidal waves

  time-shifted is a cosine wave, that is,

t

  that is, sin wt - sin L (t - 5t) = 2 cos w (t + -). sin -t-

  This quantity is a cosine wave of amplitude mnt 2 sin-2 Thus, the vector difference of the signals V1 and V2 gives an amount which is proportional to the sine of the shift5 t in time and thus to the sine of half

  the slope of the soil transversely to the trajectory (tg a).

  This quantity also has a sign that depends on the meaning of the

  slope, from right to left or vice versa.

  This relation with the inclination of the ground only applies when the axes of the detached pulse are

  stabilized in space so that the azimuthal axis is

  kept horizontal. In the opposite case, the quantity V1-V2 is connected to the angle of the unstabilized plane of height and the arrangement of the ground transversely to the trajectory (a-3 in FIG. 6a). Thus, in an essentially stabilized system, a measurement of the actual inclination of the ground transverse to the path can be obtained directly, and in a non-stabilized system, a direct measurement of the inclination transverse to the trajectory, with respect to the axes of

  the detached pulse can be obtained directly.

  The difference signal V1-V2, which for the most part practical applications is proportional to the inclination of the ground transversely to the trajectory, can be drawn

  individual beams as previously described. This-

  during, as also indicated above, it is very

  desirable to use a microwave comparator a system location very close to the input so that the

  effects of phase and amplitude errors are minimal.

  It can be shown that this comparator can be made to give the soil tilt signal in the following manner. A feed circuit of four horns with a direct comparator of loose pulses and the associated 90 phase shifter circuits are considered. We

  assume that the four horns 1, 2, 3, 4 (creating

  The four high-frequency beams A, B, C and D) give amplitudes of signals a, b, c and d. Such

  classical comparator of detached pulses at four

  is schematically represented in FIG.

  it can be formed by four waveguides arranged

  a square with 3 dB couplers placed between

  adjacent guides, the phase shifter circuits at 90a being

  in opposite diagonal guides.

  Such a comparator is commonly used simply

  for the formation of a sum signal, a different signal

  -Idence azimuth and signal difference in height, in

  the pursuit of targets with offset of the line of sight.

  The fourth output channel of the comparator is known as the "nonsense channel" because it creates a signal of the form (c + b) - (a + d), that is, representing the difference between the diagonal pairs. This channel usually has a

  termination forming a suitable charge and ignored.

  However, by rearranging the signal of the nonsense channel, it can be noted that it can be presented under the

  form (c-a) - (d-b) so that it contains the signal V1-V2.

  As indicated previously, this signal contains the information on the amplitude and the sign of the inclination of the ground transversely to the trajectory, in a system "stabilized with respect to the terrestrial average". In such a system, this signal is therefore used to correct the electronically stabilized detached pulse axes.

  in a local terrestrial stabilization system and away from

  so that one of the main remaining errors

  in the air-ground radar location is removed.

  Figure 6 shows a system of this type. An antenna 11 with detached four-element pulses is driven, during scanning, by a scanning device control unit 13. The four elements are coupled to a detached pulse comparator 15 shown

  in detail in Figure 5. The output signal from the

  of the comparator is transmitted to a mixer 17 via

  intermediary of a duplexer 19, the difference channels and the channel

  nonsense being directly connected to analog mixers

  guages. The sum channel is connected by the duplexer 19 from a transmitter 21, in a conventional manner, and the mixers 17

  receive a signal from a local oscillator 23.

  The four intermediate frequency output channels

  then reach intermediate frequency amplifiers. In a system stabilized in roll, the signals

  difference in azimuth and height are transmitted to a trans-

  sine-cosine driver 27 which can turn under the control of an input angle control signal 29 so that the

  height plane is oriented, the displacement of the plane corresponds to

  directly to the angular command signal of

  Tree. A roll angle reference source 31, an inertia platform, transmits a signal proportional to the angle 3 (see FIG. 6a) of the antenna height plan.

  and the vertical. This error signal is normally transmitted

  put directly to the sine-cosine transmitter so that the

  plan of height remains effectively vertical and thus inde-

during roll of the aircraft.

  In the embodiment considered, the roll angle error signal is transmitted by a switch

33 of fashion.

  The output signal of the height channel, stabilized

  in roll and from the sine-cosine transmitter, par-

  comes to a detector 35 of phase shift with the height

  sum so that it forms a difference signal as shown in Figure 1d. This signal has a cross or a null value that presents an uncertainty that depends

  the inclination of the soil transversely to the path.

  A narrow pulse marking this crossing is then transmitted by a cross pulse generator 37 which detects the zero crossing. This impulse is transmitted

  to a distance determination circuit 39 intended to

  to know the time elapsed since the emission of the imoulsion, and

  the distance is indicated accordingly.

  The system described so far is classic

  and has the disadvantage due to the inclination of the trans-

  towards the trajectory, as previously described.

  The narrow crossing pulse is also trans-

  set to a gate 41 which is open for the duration of the pulse. The other input of the gate 41 receives the next signal. The intermediate frequency sum signal and the nonsense channel signal are transmitted to a phase shift detector 43 which creates a scaled sum-of-cosine signal version as shown in Figure 4b. The central peak of this signal has an amplitude determined by the difference in time of the two difference signals of which it is compiDosed and thus by the inclination of the ground. The polarity of the signal is determined by the sequence of the right signals

  and difference left of Figure 4b and so by the

  tilting of the ground, from left to right or vice versa. The cosine input signal of the gate 41 is in fact sampled by the narrow pulse of the cross pulse generator so that it forms an output pulse of amplitude and polarity representative of

  the inclination transversely to the trajectory.

  The system has heretofore been described with reference to the operation of a sine-cosine transmitter 27 under

  the control of the roll angle reference source 31.

  Thus, the plane of height of the diagrams of the divided beam is kept vertical and the output pulse of the gate 41 has an amplitude proportional only to the inclination

  8 of the ground, a being reduced to zero. In the absence of stabilization

  roll by the transmitter 27, the output signal

  of the door 41 is proportional to the angle of the plane of

  the antenna and the cross-sectional area

  jectoire, that is to say at the angle (a-3) (see Figure 6a).

  When the mode selector switch 33 is switched to the right accordingly, the pitch plane turns in fact at the angle (a-3), reducing this angle to 0 and making it

  thus the effective plan of height perpendicular to the sur-

  transverse face to the trajectory in the region of the target.

  The distance of the crossing is then corrected to the same value for all the positions transversely to the trajectory, the oblique characteristic represented on the

  FIG. 2 being corrected into a perpendicular characteristic

  as shown in Figure 2a.

* When determining the distance of a

  particular, the operator locks the radar on the

  ble, initially operating in roll stabilized mode using the reference source 31. When the target is obviously acquired, the operator switches to locally stabilized mode on the ground, giving a distance determination

much safer.

  An indication of the transverse inclination to the trajectory may be desirable simultaneously and in addition

  of the clarification of the determination of distance. The ooé-

  ration is performed by a subtraction circuit 45. The roll angle reference source 31 gives a measurement of the angle 3 of the height plane while the door 41 gives a measurement of the angle (a-3). The output signal of the subtraction circuit is therefore the angle a, the inclination of the ground

  transversely to the trajectory. This signal can be displayed

  on an indicator 47 of inclination of the ground.

l4

Claims (5)

  1. Radar air-ground tracking system, of the type   includes an airborne radar with separate pulses, including   a device providing distance information from the moment of emission of a radar pulse and   in which the antenna arrangement gives a characteristic   divided beam in height (figure la) in each   one of two angular positions separated in azimuth, each   one of the features having a cross-cutting axis (OR) directed to the surface, said system being characterized in that it comprises a device (15) for forming a control signal representative of the time shift   (St) difference signals available from the characters   characteristics of the beams separated in height, the control signal giving an indication or order depending on   the inclination of the surface between the positions of the sur-   face which are intercepted by said axes.   2. System according to claim 1, characterized in that it comprises a device (27) for stabilizing the   height plane in space, the control signal de-   during the inclination of the transverse surface   the trajectory between the said positions of the   face intercepted by the axes, relative to the horizontal.   3. System according to claim 2, characterized in that the stabilization of the height plan is modified by   the control signal so that the height plane is   held perpendicular to the surface between the said positions   interception of the surface by said axes.   4. System according to claim 2, characterized in that the distance information transmitted by the system   is corrected by the control signal following an arrangement   open loop so that the effect of any inclination   transverse to the trajectory be deleted.   5. System according to any one of the claims   prior art, characterized in that the device for forming a control signal comprises a sum and difference comparator (15) to which the signals received by a four element antenna arrangement (11) are transmitted.   and which gives a signal sum, a signal difference in azi-   mut, a signal difference in height, and a channel signal   nonsense, the latter being a function of the inclination   naison of the surface transversely to the trajectory, between the positions of interception of the surface by said axes.