FR2488999A1 - RADAR COMPRESSION OF PULSES - Google Patents

Abstract

THE INVENTION RELATES TO A PULSE COMPRESSION RADAR. THIS RADAR, WHICH INCLUDES DISTANCE CHANNELS K, K AND IN WHICH THE PULSE COMPRESSION IS CARRIED OUT BEFORE THE DISTANCE GATES, INCLUDES ONE PA ANTENNA, TWO TRANSMISSION CHANNELS WITH BFO, BF1 FILTERS, GRO, GR1 AND DEVICE CORRECTORS PK OF PULSE COMPRESSION WHICH FORM BY SUPERPOSITION OF AUTOCORRELATION FUNCTIONS OF PARTIAL CODE WORDS A, B IN THE COMMAND AND MEMORY DEVICE CSS, A MAIN MAXIMUM, AT THE DURATION OF WHICH IS EQUAL THE CONNECTION DURATION AMONG THE CHANNELS OF DISTANCE IN EACH OF WHICH A TP LOW PASS FILTER ELIMINATES SECONDARY MAXIMA. APPLICATION ESPECIALLY FOR AIRPLANE DETECTION.

Description

The invention relates to a radar in which code words are transmitted in

  pulse form, consisting of code elements each of which is formed by the set of two partial code words chosen so that after pulse compression, the autocorrelation functions of the two partial code words provide identical principal maxima in amplitude and

  sign and secondary maxima of at least approximately

  equally, but of opposite signs.

  German patent application placed in public inspection

  under n01.261.907 has made known the fact of using the com

  pulse pressure to improve the distance resolution of a radar installation. Successive transmission of two pulse sequences is effected in the form of code modes in which the distributions of the individual elements are tuned to one another in a predetermined manner. This distribution is

  in such a way that in the case of continuous numbering

  naked code words, even code words provide a

  autocorrelation function different from the autocorrelation function

  correlation of odd code words whose main maximum is in phase opposition with the main maximum of the autocorrelation function of said odd codewords. Each second autocorrelation function formed during reception is additionally subjected to a phase inversion followed by a summation of the two autocorrelation functions. In this way, the principal maxima are superimposed in phase. The association of the even codewords with the odd code words is realized in such a way that the secondary maxima forming during the

  pulse compression are mutually eliminated.

  The purpose of the invention is to indicate a means of

  reduce the influence of secondary maxima during the ex-

  echo signals in radars with

  distance channels. This problem is resolved in accordance with the

  tion, which is related to a radar of the type indicated above, thanks to the fact that a radar with distance channels is used in a manner known per se, such as radar, and in which pulse compression is performed in advance of distance, that the connection time of a distance channel to the reception part is chosen to be approximately equal to the

  duration of the maximum maximum obtained during pulse compression.

  sionq and that a low-pass filter is provided in a manner known per se in each distance channel which at least partially suppresses the signal components produced by the secondary maxima in non-occupied distance channels.

  targets, based on their different signs.

  Since secondary maxima arrive at least

  partly alternately with positive and negative signs

  in the distance channels not occupied by the principal maximum and have amplitudes of at least

  partially similar, they become weaker as a result of

  fluence of the low-pass filter. On this subject, the particular advantage

  is that the expense necessary for the invention is negligible

  as a low pass filter (low-pass integration filter) is mostly present at remote gates or is kept at a low value. Only are

  therefore necessary in almost

  designed to produce the coded pulses and the

pulse compression.

  According to a preferred variant of the invention, the composition of the partial codewords is chosen so that the secondary maxima are eliminated completely reciprocally in the distance channels not occupied by targets. In this case

  not only a weakening with statistical distribution

  in the case of secondary maxima, but also a complete suppression of the latter. Secondary maxima can not therefore cause any erroneous target indication in

a distance channel.

  As an example-described below and illustrated schematically

  in the annexed drawing only one embodiment of the layout

tif according to the invention.

  Figure 1 shows the autocorrelation functions of

two partial codewords.

  FIG. 2 represents the composition of an emitted pulse

and a demodulated echo signal.

  FIG. 3 represents the block diagram of a radar according to FIG.

the invention.

  Figure 4 shows the occupation of different channels

  distance for different periods of the radar.

  Figure 5 shows the variation of the signal in a

  distance channel occupied by a main maximum.

  Figure 6 shows the variation of the signal in a channel

  distance occupied by secondary maxima.

  FIG. 1 shows a partial code word A having the distribution

_- ± _ + + ± --_

  The coding of the individual elements "+" and "-" of this partial code word A can be realized in a known manner by frequency hopping or phase modulation. The autocorrelation function (AKF) A of the partial code word A is represented on the right as a function of time and shows a principal maximum charged and a

  series of secondary maxima of different signs.

  The partial code word B represented on the line b has the composition

_- + ± - + + + +

  and provides an autocorrelation function (AKF) B which also has an accused maximum and a sequence of secondary maxima

different signs.

  The succession of U + code elements. and ".-" of the two partial code words A and B is advantageously chosen so that, as shown in lines a and b, the principal maxima

  equal in size and in sign, while the secondary maxima

  daires have opposite signs and equal amplitudes. Yes

  the signs had to be different in the case of the primary maxima

  In the case of the secondary maxima, it is possible to obtain an inverse suppression of one of the two autocorrelation functions (AKF). The existence and constitution of such partial code words A and B are described in the literature as for example in the article

  IRE Transaction on Information Theory 1961, pages 82-87.

  When partial codewords are not specifically

  in such a way that complete suppression occurs, however, an improvement is still achieved because at least one

  part of the secondary maxima is compensated.

  FIG. 2 shows, by way of example, on the line a for the partial code word A, a jump modulation of frequency that the frequency f 0 is associated with the value "-" while the frequency f 1 is associated with the value "+". When the pulse emitted by a radar is constituted in this way by the assembly of code elements according to the line a, this pulse has the duration T J according to the line b and the duration of an element

  code is t = T, for m code elements.

  When demodulating a reflected echo signal, according to

  line b, in the radar receiver, it is again necessary to

  in known manner, at the frequencies f 0 and f 1, the values 11_n

  and t "+" in order to display the demodulated echo signal

  felt on line c. The distribution of the elements of this signal

  corresponds to the succession of "+" and "-" _ of line a.

  FIG. 3 shows a radar which operates with frequency coding in accordance with that represented in FIG. 2. This radar comprises an antenna RA preferably constituted in the form of a panoramic surveillance antenna and which is connected to a transmit / receive switch SE. The emission side is here represented in a very simplified way and we have deleted all

  parts not necessary for the understanding of the invention.

  The transmitting side of the radar includes a CM code modulator

  which are the frequencies of two GO oscillators (f 0)

  and G 1 (f 1). The CM modulator switch is connected to

  choice of the oscillator GO or the oscillator G 1 by a CSS control device and code memory. The suites of

  frequencies of the type shown in line b of Figure 2 can

  can be produced in this way. The CSS code memory contains the

  partial codewordsA and B of Figure 1 and, if appropriate,

  other pairs of partial codewords that present

  from the point of view of the principal and secondary maxima, the characteristics

  described in this respect with reference to Figure 1.

  The emission signal thus produced reaches, after reflection

  on an object, via the transmit-receive

  SE to the receiving part of the radar which has two transmission channels connected in parallel. This reception part

  is represented in a very simplified way and has been abolished

  parts that are not necessary for the comprehension

  of the invention, and in particular the formation of the intermediate frequency, etc. In the upper transmission path, echo signals are processed (most of the time in

  intermediate frequency position), which arrive with frequency

  quence f 0 (account must be taken of possible offsets

  Doppler using a corresponding bandwidth). For this-

  a BFO band-pass filter is provided that is only passing in the vicinity of the frequency f 0. A rectifier GRO connected in series with the BFO filter has the role of forming unipolar negative pulses. In the lower transmission channel, only the frequencies close to the frequency f 1 can pass, which is achieved by the bandpass filter BF 1. A GR 1 rectifier connected in series with the bandpass filter BF 1 has for role to form unipolar positive pulses. The two signal paths are combined in an interconnection device ZS so that the frequency f 0 receives the sign "-" and the frequency f 1 receives the sign "+". Instead of

  polarization and GR1 polarization in opposite directions, it is possible

  also provide for an inverter stage. The signals thus obtained correspond to the signal represented on the

line c of Figure 2.

  The demodulated signals arrive at a pulse compression PK device, the reference code of which is taken from the CSS control and code memory device. If such a pulse is to be compressed in accordance with the partial code word A, the partial code word A is also used as the reference code. The same is true when a partial code word B has to be compressed. In the case of digital compression devices, it is normally possible to change the corresponding code code in the course of the radar clock. If this would lead to difficulties, one could also provide two separate devices PK pulse compression, connected in parallel, one of which would have a compression effect only for the first partial codeword A while the other would have a compression effect for the second partial code word B,

  that is, the two devices would therefore form respectively

  the autocorrelation functions of said partial codewords A and B.

  The signals obtained at the output of the compressor device PK

  The pulses have the variation in time which is represented on the lines a and b of FIG. 1. They are thus distributed successively between channels of distance K 1 to K n and this as a function of the connection duration of each channel and the number of its channels. The distance channels are connected

  successively to the reception part by a communication device

  ES through ES 1 to ES n switches. The

  Different distance channels have an identical constitution.

  that and in known manner contain different components, for example Doppler filters, amplifiers, etc., which are represented by the block DN. In addition, a filter has been provided

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  low pass TP and a SW stage with threshold value. On the output side, there is provided a switching device AS, which makes it possible to successively connect the outputs of the channels K 1 to K n with the aid of

  switches AS1 to ASn to an operating device or

  BS, for example a screen. The control of the ES and AS switching devices, the BS display or operating device as well as the transmit / receive switch is performed by

  the control device and code memory CSS which constitutes

  almost simultaneously kills the radar cadence generator

  or, in the case where the cadence generator is present in addition to

  is synchronized by the latter.

  In the case where, from time to time a modification of the partial code words A and B, for example in other partial code words C and D or E and F, is to be carried out, it is possible to provide a generator of time (clock) U that causes this change. Naturally, these same variations must be made simultaneously for modulation (modulation stage

  CM) only for the reference code.

  FIG. 4 shows the variation of the signals for an example of 20 channels of distance K 1 to K 20 when alternately transmitting, with a rate frequency remaining constant, once the partial codeword A represented on the line a of Figure 1, then the partial code word B shown on the line b of Figure 1. On line a of Figure 4 is shown

  the first period of the radar, namely the auto-correlation function

  the partial codeword received as an echo signal.

  presents an accused maximum which is contained practically

  in the distance channel 10 according to the distance from the target and the duration of the connection of the channel. In addition, a series of secondary maxima appear in different other distance channels, for example the K 7 and K 13 channels, which are occupied by secondary maxima.

  (negatives) relatively important.

  During the second period of the radar, the autocorrelation function of the partial code word B is received as an echo pulse. This function provides an accused maximum in the distance channel K 10 containing the target itself,

  while positive (significant) secondary maxima appear

  in the distance channels K 7 and K 13.

  During the third period of the radar represented on line c, the same signal form is obtained again:

  on the line a. In the fourth period of the radar repre-

  on line d, we get the same form of signal that

represented on line b.

  When comparing the lines a and b, it turns out that the main maxima of the autocorrelation functions (AKF) A and (AKF B have the same sign in the channel 10 and provide a target indication with a signal level Secondary maxima, which appear in the other distance channels, have values of equal amplitude but of opposite signs.

  and it is possible to reciprocally compensate for these secondary maxima.

  using appropriate control devices. When the composition of the partial codewords does not lead to a complete compensation, at least one deletion occurs

partial.

  To explain these phenomena, reference will now be made to FIGS. 5 and 6. FIG. 5 shows the amplitude distribution for the distance channel K 10 in the case where a target is involved in the radar beam. The shape of the envelope of the individual pulses depends on the width of the antenna lobe and the rotation speed RA of the radar, represented in FIG. 3. During each duration T of a radar pulse period, an echo pulse appears (alternatively partial code word A and partial code word B) whose duration is approximately equal to t. This is achieved by the fact that the connection time of the distance channels is chosen to be approximately equal to the order of magnitude of Z. This is appropriate since in the case of autocorrelation the duration of the principal maximum is also the order derZ (Z7 = duration of a piece of code). As can be seen from the drawing of FIG. 4, all the individual pulses relating to the principal maximum of the autocorrelation functions (AKF) A and (AKF) B have the same sign in the distance channel K 10. The low-pass filter TP in the distance channel

  K 10 thus provides, in the case of a constant time constant corresponding to

  (limit frequency), the envelope of successive maxima of (AKF) A and (AKF) B. This signal is retransmitted as an echo signal to the display or operating device BS since it exceeds in all cases the threshold value of

the SW stage with threshold value.

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  FIG. 6 shows the amplitude distribution for a channel of distance different from the channel K 10. If we assume that it is the distance channel K 7, we see that

  when the radar beam covers a target, it appears

  natively negative and positive impulses, and alternatively once (AKF) A and (AKF) B and whose magnitude increases to a maximum and then decreases. Because of the different signs that they present, the different components of the signal are mutually eliminated, because of the action of the low-pass filter TP shown in FIG. 3 in the distance channel under consideration, and the threshold value of the SW floor

  here the threshold value is not exceeded (average value = zero).

  We thus obtain in the distance channel occupied by a true moving target an indication with a very high useful level while secondary maxima do not lead in others.

  distance channels to no indication.

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Claims (6)

R E V E N D I C A T I 0 N S   A radar in which code-shaped codewords are generated and each of which is formed by the set of two partial codewords selected so that after pulse compression, the autocorrelation functions of the two partial code words provide   principal maxima identical in amplitude and sign and   subximal magnitudes of at least approximately equal amplitude but of opposite signs, characterized in that a radar comprising distance channels (K 1 to K n) and in which, in a manner known per se, radar is used; the compression of pulses is carried out in front of the gates of distance, that the duration of connection of a channel of distance to the part   reception is chosen approximately equal to the duration of the maximum   mum obtained during the pulse compression, and that in each distance channel a low-pass filter (TP) is provided in a manner known per se which at least partially eliminates the signal components produced by the secondary maxima,   in distance channels not occupied by targets (eg   ple K 7), on the basis of their different signs.   2. Radar according to claim 1 characterized in that the connection time of a distance channel is chosen   equal to the duration (Zr) of a code element.   3. Radar according to one of claims 1 or 2 characterized   in that the composition of the partial code words is chosen   so that the maximum values eliminate each other   completely in the distance channels not occupied by ble.   Radar according to any one of claims 1 to 3,   characterized by paired partial codewords   modulate two different frequencies (fO, fl), are   successively on the transmission side after filtering and, after meeting in a common line, are sent by this   last to distance channels (Kl to Kn).   5. Radar according to any one of claims 1 to 4   characterized in that there is provided a time generator or clock (U) which, at predetermined time intervals, causes a modification of the partial code words to be transmitted and a simultaneous modification from code; reference in the 2488999   pulse compression network (PK).   Radar according to any of claims 1 to 5,   characterized in that a control and code memory device (CSS) is provided for encoding the transmit side, reception side decode and pulse compression.