A noise radar

(54) (56)

PATENT INVENTOR INVENTOR'S OFFICE NAME

CALLED PUBLICATIONS:

GEC-Marconi Ltd, Stanmore GB William John McClintock, Witham H Albihns Patent Office AB Brusradar

<td>GB</td><td>A</td><td> 1</td><td> 506</td><td> 412</td>

<td>US</td><td>A</td><td> 4</td><td> 328</td><td> 495</td>

<td>US</td><td>A</td><td> 4</td><td> 396</td><td> 916</td>

GB

788 (G01S 13/28), (57)

SUMMARY:

(G01S 9/23), GB A 2,075 (343-17.2), US A 4,338,604 (343-17.2), (343-5) generated a pseudo-random sequence repeated

In a noise radar and used to phase modulate a transmitted signal. The broadcast is interrupted for a number of periods during each pseudo-random sequence, allowing return signal portions from the targets to be received by the same antenna used for broadcast during the interrupt periods. Each transmitted pulse, between successive periods of interruptions, includes a different hunting choice of successive digits in the code, thus increasing apparent randomness of the transmitted code and making detection more difficult.

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The numbers in brackets indicate international identification code, INID code. Letters in clamps indicate international document code.

The present invention relates to a noise radar, comprising a device for generating a code comprising a number sequence. A noise radar is a radar where the transmitted signal is encoded with a random or noise-like code. Such a code may be a long pseudo-random code, such as e.g. is greater than 1000 or preferably greater than 10,000 bits. Noise radar has a number of advantages. It can use a broadband practice guide, which provides good distance resolution and good anti-interference capabilities and low probability of being detected. It can also achieve low ambiguity in determining speed of and distance to a target. Noise radar can also allow a low peak power to be used, further reducing the probability of detection.

In a known noise radar, a continuous transmitted signal in pseudo-random sequences of long duration is usually encoded, phase-coded. In the receiver, the received signal is correlated with a delayed version of the transmitted signal to give an indication of the occurrence and speed of a target at a distance corresponding to the delay. However, noise radar devices have hitherto been little used due to leakage of the transmitted signal directly into the receiver and indirectly from limitations due to unwanted radar echoes for close range of radar work at long distances, as is the case with all radar devices operating with continuous waveform . Also, it is necessary to use separate transmitter and receiver antennas; otherwise the leakage size would be unacceptable.

Noise radar should not be confused with phase-coded pulse radar, where transmitted pulses are phase-coded to allow pulse compression and therefore good distance resolution. In a phase-coded pulse radar, the relative timing of the transmitted and received pulses is used to indicate distance. It is less successful in providing low ambiguity in determining speed of and distance to a target than noise radar is.

It is an object of the invention to provide all the advantages of a noise radar while eliminating the above disadvantages.

The above disadvantages are avoided with a noise radar which has obtained the features specified in claim 1. Further features of the invention are set forth in other claims.

The noise radar of the invention comprises a device for transmitting energy pulses during each sequence, which occupies periods which each span at least one digit of the code with the pulses, which has a regular pulse repetition frequency which is not an even multiple of the code's repetition frequency; a device for changing the pulse repetition rate; and a device for modulating the transmitted energy in accordance with the prevailing output of the code generation device.

By pulsing the transmitted energy in this way, it is possible to receive the returned signal for periods between the pulses, thereby eliminating the above leakage problems. Also, in cases where a pseudo-random code is used, parts of the code supplied to each pulse, if the generator is continuously operated during and between pulses, can be made particularly unpredictable, which reduces the probability of detection. For this purpose, the pulse repetition frequency is preferably not an even multiple of the repetition code of the pseudo-random code.

An option is provided for changing the pulse repetition rate, e.g. between two values, to generate a frequency alternation; or between more than two values. The effect of this is to avoid blind spots at a distance and to further increase the unpredictability mentioned above. The frequency change may be such as to reduce the frequency when the area to be investigated is reduced, and vice versa, to completely remove distance ambiguity.

In the receiver, the code carried by the received signal is preferably correlated with an output of the code generator after first by a delay corresponding to the return and return delay of the transmitted signal. The correlator output signal indicates the presence of a target or part of a target at a distance corresponding to this delay. The delay is preferably varied in steps to make the radar sensitive to different distances at different times.

The invention is described in more detail below with reference to the accompanying drawings, in which Fig. 1 shows a noise radar performed in accordance with the invention; and Figures 2A, 2B and 2C show waveforms at each of three points in Figure 1.

Referring to the figures, a control 1 comprises a continuously running clock signal core 1A which operates at a frequency of 100 MHz and continuously drives code generators 2 and 23 for pseudo-random code until ordered to reset. A reset signal is applied to line 3 at regular time intervals determined by a clock register IB. The reset signal acts immediately to reset the code generator 2 and after a delay applied to the IC to reset the code generator 23. The delay applied to the IC determines the distance continuously during inspection. The pseudo-random code generator 2 generates a code shown very schematically in Figure 2A, where the bit rate is shown to be greatly reduced for reasons of clarity. This code is presented on line 4 of a two-phase modulator 5, where it modulates a carrier frequency signal from a radio frequency oscillator 6 to produce on line 7 a two-phase modulated signal. This is amplified at 8 and upconverted or converted to an output signal by mixing at 9 with a local oscillator signal generated from 10 via a divider 11. The upper sideband from mixer 9 is allowed to pass a filter 12 and is reinforced at 13 prior to transfer to a gate 14, where it is modulated by pulses appearing on line 14A from a clock register ID in control 1. Gate 14 thus generates pulses, as shown in FIG. 2B, where the times when phase changes occur are indicated by the vertical lines. The output of gate 14 is passed through a circulator 15 to an antenna 16, from which it is transmitted to an area under inspection. From a target or multiple targets in the range, reflected signals are received by the same antenna and passed through the circulator 15 to a second gate 17. This is operated by gate pass signals on a line 17A fed by the clock register ID via a variable delay 1E, the latter being selected to set the receiver is capable of being operated between and not during transmitted pulses. The gate 17 is thus caused to pass the received signal during reception periods P, as shown in Fig. 2C. The output of gate 17 is amplified at 18 and downconverted or converted to an intermediate frequency by mixing at 19 with the local oscillator signal. The lower sideband is passed through a filter 20 and is reinforced at 21 to a suitable level for correlation in a mixer 22A.

The second pseudo-random code generator 23, which is identical to the generator 2, is clocked by the same clock signals as those which clock the code generator 2. of the reset signal on line 24, which is delayed relative to the time when generator 2 starts. This delay is applied at 1C and selected to equal the return and return times of the signal to be transmitted to and received from a target at a distance selected at that time for inspection.

The output of the pseudo-random code code generator 23 is used at 26 to bi-phase modulate a signal from the oscillator 6, which signal has been shifted in frequency at 25 with a variable value set by a control signal on the line 25A depending on the speed of the target at that time. is desirable to inspect. This signal can be supplied from a computer or could be set manually in a simple version of the invention.

The output of modulator 26 is mixed at 22A with the return signals from 21. The output of mixer 22A is passed through a low pass filter 22B and the return portions from successive pulse periods are integrated at 22C. The integrator 22C may be a bandpass filter with a bandwidth, which is a fraction of the integration time. Alternatively, it can be a digital, fast Fourier transform device. Components 22A, 22B and 22C form a correlator by which an output waveform is generated at 22D, the magnitude of this output waveform indicating the strength of the return signal; and its frequency components indicate the Doppler components of the received signal.

In the system shown, it should be noted that the ambiguity of the distance determination is dictated by the length of the code and not by the pulse repetition frequency. To reduce the area ambiguity even more, more than one pseudo-random code generator of different types can be arranged at 2 and 23, respectively, which are arranged to operate in sequence and be constructed to generate codes of the same or different lengths . The same effect can be achieved by using sequentially different output points on the shift register of the pseudo-random code generator but in this case the code lengths will always be the same. In this specification, the term pseudo-random sequence is considered to include a sequence of various pseudo-random sequences, e.g. generated in one of the ways specified in the previous paragraph.

In theory, it would be possible to use only one code generator instead of the two shown at 2 and 23 in Fig. 1. A digital delay would then be necessary to connect the output of the single code generator to modulator 26. Such an arrangement would be cumbersome. in practice when using existing technology, as a delay of perhaps 10,000 digits may be required.

An alternative embodiment of the invention includes a number of correlators and modulators of the same kind as those shown at 22 and 26. These are arranged to receive inputs from and 22 via dividers. The inputs to the various modulators are taken from a shift register which receives the output from the code generator 23 for pseudo-random code. The effect of this is to provide simultaneous output signals from each correlator representing targets in different distance cells, either directly following one another or at a distance.

It should be noted that the choice of periods when the gates 14 and 17 are open has been made solely for the purpose of preventing leakage of the transmitted signal into the receiver and to prevent returning signals from being received from unwanted radar echoes from nearby objects. These pulse periods are of no significance when determining distances in contrast to the pulses generated in a phase-coded pulse radar. The radar according to the invention, as illustrated in FIG. 1, can be considered as a continuous interrupting wave radar.

An improvement in performance can be achieved by stopping the generation of the pseudo-random sequence during the periods when transmission is not taking place. This enables the pseudo-random sequence to be spread over a greater number of pulses without repetition: This results in an unequivocal improvement. Furthermore, the entire code generated at 23 at 22 is usefully correlated with the received signal; this improves spacing side performance.

If it is desirable to follow a target moving rapidly toward or away from the radar, the Doppler offset provided at may not compensate for the large bandwidth of the received code compression or code expansion signal due to the target's movement. This problem can be solved by expanding or compressing the code generated at 23, using a slightly different signal frequency to clock the pseudo-random code generator 23. This makes the duration of the code sequence shorter or longer. The control 1 receives the target speed and distance information necessary to initiate a target tracking sequence on line 27 either from the information at 22D or from a separate monitoring radar.

While the illustrated embodiment of the invention utilizes two-phase modulation, other forms of modulation, such as quadrature phase modulation, may alternatively be used. In a system using quadrature phase modulation, each code generator 2 and 23 would be replaced by twin code generators to provide two-digit words defining the phase modulation supplied by circuits equivalent to 5 and 26.

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