GB1605130A - Radar systems using pulse compression - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO RADAR SYSTEMS USING PULSE COMPRESSION (71) We, SIEMENS AKTIENGES ELLSCHAFT, a German Company of Berlin and Munich, Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to radar systems of the type in which pulse-form code words consisting of code elements are transmitted, each code word being made up of sections.

The German Patent Specification No.

1 261 907 describes a radar system, in which, in order to improve the range resolution, pulse compression is utilized by transmitting successively, in each case, two pulse trains in the form of code words in which the distributions of the individual elements are matched to one another in a specific way.

This distribution is so contrived that with sequential numbering of the code words, the even numbered code words yield an autocorrelation function which differs from that of the odd numbered code words and is in antiphase in relation to the prime maximum.

In addition, each second one of the autocorrelation functions formed at reception undergoes a phase reversal, after which a summing of the two autocorrelation functions takes place. As a result, the prime maxima are co-phasally superimposed, whilst the relationship of the even numbered to the odd numbered code words is so contrived that any secondary maxima formed in the pulse compression cancel one another out.

One object of the present invention is to provide a system of this general type in which unwanted influence of the secondary maxima during analysis of echo signals in radar systems employing range channels can be reduced.

The invention consists in a radar system in which pulse-type code words consisting of code elements are transmitted, each code word being made up of two code word sections so chosen that after pulse compression, the autocorrelation functions of the two code word sections produce prime maxima which are identical in amplitude and sign, and respective secondary maxima which may not be equal in amplitude but are of mutually opposite signs, a plurality of individual range channels beng provided, selectively connected via respective range gates and pulse compression being carried out before said range gates, the period of connection of each range channel being substantially equal to the duration of the prime maximum achieved during pulse compression, and each range channel containing a low-pass filter which at least partially suppresses the signal components generated by any secondary maxima in those range channels not then occupied by targets, due to the difference in the signs of said secondary maxima.

Because the secondary maxima, in part at least with alternately positive and negative signs, occur in range channels not occupied by the prime maxima, and have at least approximately similar amplitudes, they are attenuated by the influence of the low-pass filter. A particular advantage is that the additional outlay in circuitry required is negligible, because a low-pass integrating filter is generally already provided in such systems at the range gates, or can be fitted at minimal cost. Therefore, the only additional elements required are devices to produce the coded pulses, and the pulse compression device.

Preferably the composition of the code word sections is so chosen that the secondary maxima completely cancel one another out in those range channels not occupied by targets. In this case, it is not merely a statistically distributed attenuation of the secondary maxima which occurs, but their complete cancellation. Thus, secondary maxima cannot lead to false indication of targets in a range channel.

The invention will now be described with reference to the drawings, in which: Figure 1 is a set of explanatory graphs illustrating the autocorrelation functions of two code word sections; Figure 2 is an explanatory set of pulse diagrams illustrating the composition of a transmitted pulse and of a demodulated echo signal; Figure 3 is a block schematic circuit dia gram of one exemplary embodiment of a radar system constructed in accordance with the invention; Figure 4 is an explanatory set of graphs illustrating the occupation of different range channels during differing radar periods during a typical operating sequence; Figure 5 is a waveform diagram illustrat ing the signal profile in a range channel occupied by a prime maximum; and Figure 6 graphically illustrates the signal profile in a range channel occupied by secon dary maxima.

The top row in Figure 1 shows a code word section A with the assumed distribution : - "- + - +++ - ~ The coding of the individual elements "+" and "-" of this code word section A, can be performed in a manner known per se by frequency-shift or phase-shift modulation.

The autocorrelation function (AKF)A of the code word section A is shown graphically to the right as a function of time forming a wave-train a that exhibits a dear prime maximum and a series of secondary maxima having different signs.

The second row of Figure 1 shows a code word section B having the assumed composition : - "- ++ - +++ +,,.

and yields an autocorrelation function (AKF) B graphically illustrated to the right, which forms a wavetrain b that exhibits a clearly defined maximum and a series of secondary maxima of different signs.

The sequence of the code elements "+" and "-" in the two code word sections A and B is advantageously so chosen that, as the two wavetrains a and b illustrate, the prime maxima are identical in magnitude and sign, whilst the secondary maxima have identical amplitude but opposite signs. Should it happen that the signs of the prime maxima are different and those of the secondary maxima the same, then cancellation can be achieved by inverting one of the two wavetrains. The existence and the structure of code word sections A and B of this kind is described in literature on the subject, see for example in IRE Transactions on Information Theory, 1%1 pages 82-87.

Even if the code word sections are not so precisely structured that complete cancellation is achieved, there is nevertheless an improvement, because at least some parts of the secondary maxima will mutually compensate.

In Figure 2, row a gives the assumed code word section A, and by way of example a frequency-shift modulation has been assumed, in which a frequency to is assigned to the value "-" and a frequency fl to the value "+". If the transmitted pulse of a radar system is made up of the code elements of row a in this fashion, then it will have a duration TJ as shown in row b, and the duration of any code element will amount to r = rJ/m where m is the number of code elements.

On demodulation of a reflected echo signal such as that shown in row b in a radar receiver, it is necessary to allocate the frequencies to and fl to the values "-" and "+" respectively in a manner known per se, so that a demodulated echo signal as shown in row c is produced. The distribution of the elements in this signal corresponds to the sequence of the "+" and "--" values in row a.

Figure 3 illustrates a radar device which operates with frequency coding as described with reference to Figure 2. A radar antenna RA, preferably a circular-scan antenna, is connected to a transmit/receive switch SE.

The transmitting station has been illustrated in a very simplified fashion, and all those parts not essential to an understanding of the invention have been omitted. A code moduiator CM is provided, to which frequencies produced by two oscillators Go, (fO) and G1, (fl) are applied. By means of a code store and control device CSS, a switch in the code modulator CM is selectively switched as necessary to select the output of the oscillator Go or the oscillator G1. Consequently, wavetrain radar pulses of the kind shown in row b of Figure 2 can be produced. In the code store CSS, the code word sections A and B of Figure 1 are held, possibly together with other pairs of code word sections which possess the properties explained in relation to Figure 1, for their prime and secondary maxima.

The transmitted signal thus produced is transmitted, and after reflection from an object, passes back through the transmit/receive switch SE to a receiver section which has two parallel-connected channels. The receiver section has also been illustrated in very simplified fashion, with all parts not contributing to an understanding of the invention omitted, including the parts for forming the intermediate frequency, etc. In the upper channel, generally in the IF stages, any echo signals are processed which occur at frequency tO (any Doppler shifts must be allowed for by a corresponding bandwidth) of a band-pass filter BFO, which only passes frequencies in a zone around frequency to. An ensuing rectifier GRO produces unipolar negative pulses.

In the lower transmission channel, only those frequencies around the frequency fl are passed, a band-pass filter BF1 being provided for this purpose. An ensuing rectifier GR1 forms unipolar positive pulses. Both signal channels are combined in a device ZS, with the frequencies to having a negative polarity - and those fl a positive polarity "+".

Instead of using oppositely poled rectifiers GRO and GR1, if required an inverter stage can be used in one channel. The output signals thus obtained then correspond to the signal shown in row c of Figure 2.

The demodulator output signals are passed to a compressor device PK to which a reference code is applied from the code store and control device CSS. If a pulse corresponding with code word section A is to be compressed, then by way of reference code, the code word section A is used. The situation is similar when a code word section B is to be compressed. In digital compressor devices, normally the reference codes can be correspondingly switched within the limitations imposed by the particular radar pulse pattern that is used. Should this give rise to any difficulties, then it is equally possible to provide two separate, parallelconnected pulse compressor devices PK, one of which is operative only for the first code word section A, and the other only effective for the second code word section B, i.e. in order to form the autocorrelation function.

The signals obtained at the output of the pulse compressor device PK have a time-based distribution as shown by wavetrains a and b of Figure 1. They are therefore distributed successively amongst a plurality of range channels K1 to Kn, as a function of the time of connection and number of said range channels. The range channels are sequentially connected to the receiver section by a switching device ES, using switching elements ES1 to ESn. As the individual range channels are of identical design, only one is shown in detail, but each contains, in a manner known per se, components such as Doppler filters, amplifiers, etc., which have been indicated by a block DN.There are also provided a low-pass filter TP and a threshold stage SW, and the output of each channel is fed to a switch control AS, by means of which the outputs of the channels K1 to Kn are selectively connected via switch elements, AS1 to ASn, to a common analyser or display device BS, e.g. a display screen. Control of the switching devices ES and AS, as well as the display or analyser circuit BS, and of the transmit/receive switch SE is effected by the code store and control device CSS, which in effect simultaneously constitutes the pulse generator of the radar system, or if the system comprises such a pulse generator, then this is used to synchronise operation of the control device CSS.

From time to time a change may be made in the content of the code word sections A and B, to increase security by using other code word sections, C and D, or E and F, and if this change is to be effected, a timer U can be provided which performs this switching operation. Of course, any such modifications must be made both in the modulation stage C, and in the reference code output from the device CSS simultaneously.

In Figure 4, taking an example of 20 range channels K1 to K20, the signal distribution has been graphically illustrated for a situation where alternately, with a constant pulsing frequency, first of all the code word section A of wavetrain e of Figure 1 and then the code word section B of wavetrain b of Figure 1 is transmitted. In row 1 of Figure 4, the first radar period has been illustrated, with the autocorrelation function of the code word section A received as echo signal. It exhibits a clearly defined maximum which, in accordance with the target range and the duration of channel connection, occurs virtually entirely in the range channel K10. Also, a series of secondary maxima occur, which occur in various other range channels. For example, channels K7 and K13 contain relatively strong (negative) secondary maxima.

During the second radar period, the autocorrelation function of the code word section B is received as an echo pulse. In the range channel K10 picking up the actual target, it produces a clearly defined maximum, whilst in the range channels K7 and K13 strong (positive) secondary maxima occur, as shown in line 2 of Figure 4.

During the third radar period, line 3 of Figure 4, the same distributon as that in line 1 appears. During the fourth radar period, in line 4, the same distribution as in line 2 occurs.

From a comparison of lines 1 and 2, it is dear that the prime maxima of the auto correlation functions (AKF) A and (AKF) B have the same sign in channel K10 and there results a target indication of high signal level.

The secondary maxima, which occur in the other range channels, although having similar respective amplitudes, have mutually opposite signs and there is the possibility that by providing suitable circuit elements, the secondary maxima can be made to completely cancel each other out. Even if the composition of the code word sections does not give rise to complete compensation, it will at least produce partial cancellation.

To explain these relationships, reference will now be made to Figures 5 and 6. Figure 5 shows the amplitude distribution for the range channel K10 during a sweep over the target. The shape of the envelope of the individual pulses depends upon the width of the antenna lobe and upon the rate of rotation of the radar antenna RA shown in Figure 3. Within each pulse period T of the radar system, an echo pulse occurs, alternately formed by the code word sections A and B, and with a duration of approximately , because the time of connection of the individual range channels is approximately This is convenient because in autocorrelation, the duration of the prime maximum is also T, which equals the duration of a code element.As the illustration of Figure 4 shows, all the individual pulses stemming from the prime maximum in the autocorrelation functons (AKF) A and (AKF) B, in the range channel K10, have the same sign. Thus, the low-pass filter TP in the range channel K10, produces the envelope of the successive maxima in (AKF) A and (AKF) B, assuming an appropriate time constant and cut-off frequency is set. This signal is relayed as an echo signal to the indicating or analyser circuit BS, because in all circumstances it exceeds the threshold of the respective threshold stage SW.

In Figure 6, the amplitude distribution for a range channel other than K10, has been shown, in this case the range channel K7, by way of example, and it will be seen that during the sweep over the target, alternate negative and positive pulses occur, due to the alternate appearance of the functions (AKF) A and (AKF) B, with respective magnitudes which increase to a maximum and then decrease again. Because of the different signs, the influence of the respective lowpass filter TP of Figure 3, in the particular range channel being considered the individual signal components cancel one another out, at least partially so that the threshold of the threshold stage SW is not exceeded in this case, and the mean value may actually equal zero.Thus, in the range channel occupied by the echo signal of a genuine moving target, an indication with a very high effective signal level is obtained, whilst secondary maxima in other range channels, give rise to no display.

WHAT WE CLAIM IS: 1. A radar system in which pulse-type code words consisting of code elements are transmitted, each code word being made up of two code word sections so chosen that after pulse compression, the autocorrelation functions of the two code word sections produce prime maxima which are identical in amplitude and sign, and respective secondary maxima which may not be equal in ampli tude but are of mutually opposite signs, a plurality of individual range channels being provided, selectively connected via respective range gates, and pulse compression being carried out before said range gates, the period of connection of each range channel being substantially equal to the duration of the prime maximum achieved during pulse com pression, and each range channel containing a low-pass filter which at least partially sup presses the signal components generated by any secondary maxima in those range chan nels not then occupied by targets, due to the differences in the signs of said secondary maxima.

2. A radar system as claimed in Claim 1, in which the period of connection of each range channel is equal to the duration of one code element.

3. A radar system as claimed in any pre ceding Claim, in which the composition of the code word sections is so chosen that the secondary maxima in those range channels not occupied by targets cancel one another out completely.

4. A radar system as claimed in any preceding Claim, in which means are provided by which associated code word sections are respectively modulated onto two different frequencies and means are provided for filtering and then successively compressing said sections, on reception, to be supplied after combination on a common line, to the range channels.

5. A radar system as claimed in any preceding Claim, in which a timer is provided, which operates at specific time intervals to produce a change in the code word sections transmitted and a simultaneous change in the reference codes applied to the pulse compressor network.

6. A radar system as claimed in any preceding Claim, in which a common code store and control device is provided for transmitter-end coding, receiversnd decoding, and pulse compression.

7. A radar system substantially as described with reference to Figure 3.

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

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. target. The shape of the envelope of the individual pulses depends upon the width of the antenna lobe and upon the rate of rotation of the radar antenna RA shown in Figure 3. Within each pulse period T of the radar system, an echo pulse occurs, alternately formed by the code word sections A and B, and with a duration of approximately , because the time of connection of the individual range channels is approximately This is convenient because in autocorrelation, the duration of the prime maximum is also T, which equals the duration of a code element. As the illustration of Figure 4 shows, all the individual pulses stemming from the prime maximum in the autocorrelation functons (AKF) A and (AKF) B, in the range channel K10, have the same sign.Thus, the low-pass filter TP in the range channel K10, produces the envelope of the successive maxima in (AKF) A and (AKF) B, assuming an appropriate time constant and cut-off frequency is set. This signal is relayed as an echo signal to the indicating or analyser circuit BS, because in all circumstances it exceeds the threshold of the respective threshold stage SW. In Figure 6, the amplitude distribution for a range channel other than K10, has been shown, in this case the range channel K7, by way of example, and it will be seen that during the sweep over the target, alternate negative and positive pulses occur, due to the alternate appearance of the functions (AKF) A and (AKF) B, with respective magnitudes which increase to a maximum and then decrease again. Because of the different signs, the influence of the respective lowpass filter TP of Figure 3, in the particular range channel being considered the individual signal components cancel one another out, at least partially so that the threshold of the threshold stage SW is not exceeded in this case, and the mean value may actually equal zero.Thus, in the range channel occupied by the echo signal of a genuine moving target, an indication with a very high effective signal level is obtained, whilst secondary maxima in other range channels, give rise to no display. WHAT WE CLAIM IS:

1. A radar system in which pulse-type code words consisting of code elements are transmitted, each code word being made up of two code word sections so chosen that after pulse compression, the autocorrelation functions of the two code word sections produce prime maxima which are identical in amplitude and sign, and respective secondary maxima which may not be equal in ampli tude but are of mutually opposite signs, a plurality of individual range channels being provided, selectively connected via respective range gates, and pulse compression being carried out before said range gates, the period of connection of each range channel being substantially equal to the duration of the prime maximum achieved during pulse com pression, and each range channel containing a low-pass filter which at least partially sup presses the signal components generated by any secondary maxima in those range chan nels not then occupied by targets, due to the differences in the signs of said secondary maxima.

2. A radar system as claimed in Claim 1, in which the period of connection of each range channel is equal to the duration of one code element.

3. A radar system as claimed in any pre ceding Claim, in which the composition of the code word sections is so chosen that the secondary maxima in those range channels not occupied by targets cancel one another out completely.

4. A radar system as claimed in any preceding Claim, in which means are provided by which associated code word sections are respectively modulated onto two different frequencies and means are provided for filtering and then successively compressing said sections, on reception, to be supplied after combination on a common line, to the range channels.

5. A radar system as claimed in any preceding Claim, in which a timer is provided, which operates at specific time intervals to produce a change in the code word sections transmitted and a simultaneous change in the reference codes applied to the pulse compressor network.

6. A radar system as claimed in any preceding Claim, in which a common code store and control device is provided for transmitter-end coding, receiversnd decoding, and pulse compression.

7. A radar system substantially as described with reference to Figure 3.