GB1571540A - Pulse radar apparatus - Google Patents

Description

(54) PULSE RADAR APPARATUS (71) We, DECCA LIMITED, a British Company, of Decca House, 9 Albert Embankment, London, SE1 7SW, 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 pulse radar apparatus.

According to this invention a pulse radar apparatus has (a) a transmitter for transmitting radio frequency pulses of a predetermined duration and at a predetermined repetition rate, (b) a receiver with a logarithmic video amplifier, (c) means for processing the received logarithmic video signals comprising a video high pass filter having a time constant greater than said radar pulse duration and being arranged for removing the mean level of clutter signals in the logarithmic video signals, an integrating circuit for integrating the lagarithmic video signals to provide backing off signals corresponding to the mean power level of the logarithmic video signals, means for combining the video signals from the video high pass filter with the backing-off signals to back off the remaining clutter signals in the filter video signals and means for prepriming the ingegrating circuit at the beginning of each range scan to set the backing-off signal in accordance with a predetermined sample of the logarithmic signals received in the preceding range scan, (d) means for digitising the processed video in only two levels, (e) a pulse to pulse integrator determining the presence or absence of a processed video digital pulse in each of successive range cells in each range scan and providing a corresponding output pulse only if said pulses are determined in corresponding range cells of two successive range scans, (f) means for storing indications of the presence or absence in successive range cells of output pulses of the pulse to pulse integrator from sucessive individual range scans, (g) cathode ray tube display means having means for providing a constant speed time base trace synchronised with the pulse repetition rate of the radar transmitter, (h) means for reading stored indications from the store at a predetermined rate and applying them as brightness modulation to the cathode ray tube display, and (i) display range selector means switchable to alter the rate of reading out the stored indications or the size of said range cells or both to effect a change in the range-scale of the display.

An example of the invention will now be described with reference to the accompanying drawings in which Figure I is a block diagram illustrating an embodiment of the invention and Figure 2 is a circuit diagram of a preferred form of video processor.

Referring to Figure 1, the illustrated radar apparatus includes an antenna 10 and a transmitter/receiver unit 11. The receiver of the unit 11 includes a logarithmic amplifier well known in the radar art. Logarithmic video signals from the receiver are supplied by a line 12 to a video processor 13 which is also supplied with radar sync pulses from the transmitter receiver unit 11 on a line 14. The operation of the video processor 13 is described in detail below and its effect is to produce processed video on a line 15 in which a major portion of the unwanted clutter signals in the received video is suppressed. The processed video is fed to a digitizer 16 which is effective to digitize the processed video signals into only two levels, i.e. corresponding to the presence or the absence of a signal.The digitizer 16 may comprise merely a high gain amplifier arranged to saturate in response to signals in the processed video exceeding a predetermined threshold level. When below the threshold level, the high gain amplifier provides no output. Thus, the signals from the digitizer on a line 17 are a sequence of pulses of uniform amplitude corresponding to the presence in the processed video on line 15 of signals exceeding a threshold level.

The digitized video on line 17 is supplied to a digital integrator 18. The integrater 18 is a pulse to pulse integrator of a kind known in the radar art which compares the received video signals in successive periods between radar pulses, i.e. successive range scans. The integrator divides each range scan into a predetermined number of range cells and determines whether a video signal is present in each of these cells. The integrator 18 is described as a "two out of two" integrator because it is arranged to provide an output pulse on a line 19 only if it determines the presence of a video digital pulse in corresponding range cells of two successive range scans. The digital integrator thus arranged is very effective at removing from the digitized video spurious signals resulting from the reception of radio frequency pulses transmitted by another radar on the same frequency.By limiting the integrator 18 to "two out of two" operation, the loss of wanted returns corresponding to small fading targets is minimised.

The integrated digital video output of the integrator 18 is written into a digital store 20 under the control of a read/write control unit 21. It will be appreciated that the information rate of the signals on line 19 is dependent on the number of range cells into which each range scan is divided. The integrator 18 is capable of providing 1-bit per range cell. Thus, the read/write control unit 21 is arranged to write the signals on line 19 into the store at a suitable rate so that successive store locations correspond with successive range cells in a particular range scan. The store 20 need have a capacity only sufficient to contain digital indications of the presence or absence of a pulse on line 19 for each range cell in a single range scan of the radar apparatus. In one example, there may be 1,536 range cells in a single range scan.Thus, for a maximum range of the apparatus of 48 nautical miles, the system is capable of distinguishing between targets separated in range by about 1/32 mile. Further, the rate of reading the output from the integrator 18 into the store 20 is then approximately 2.7 MHz.

After all the range cells of each individual range scan are written into the store 20, the control unit 21 initiates a read out sequence to read the stored indications from the store at a predetermined rate. The read out rate is normally different from the rate of writing the information into the store and may be subject to the control of a display range selector 22.- The display range selector may also be switchable to change the size of the range cells employed by the integrator 18 and thus the rate of writing information into the store, as will become apparent. The stored indications read from the store 20 are feed as a sequence of pulses, corresponding to the presence of signal returns in the received video via a pulse stretcher 23 to a cathode ray tube display 24.A constant speed time base unit 25 is operative to scan the cathode ray across the screen of the display 24 at a constant speed and with a repetition rate which is synchronised with the pulse repetition rate of the radar. The store read/write control unit 21 is arranged to start the read out sequence at the beginning of a time base scan on the display screen. The stretched pulses read out of the store are applied to the cathode ray tube display as brightness modulation so that the trace across the display screen corresponds with a range scan of the apparatus. Normally, the display will be a PPI type display and the trace generated by the constant speed time base unit 25 is arranged to rotate on the display screen in correspondence with the rotation of the antenna 10. Fixed or rotating coil displays may be used.

It will be appreciated that the displayed range on the screen of the display 24 is dependent on the relationship between the speed of the time base 25, which is constant, and the rate at which stored information is read from the store 20. The faster the information is read from the store 20 the greater is the range displayed. Thus, the display range selector may be operative to switch the rate of reading information from the store 20 between appropriate predetermined rates corresponding to desired displayed ranges. In this way range switching is accomplished without the need to increase the time base speed of the display, thereby enabling less expensive circuitry to the employed in generating this time base. The pulse stretcher is useful to brighten the target responses appearing on the display screen and preferably operates so that the amount of stretch is a function of range, the more distant target response pulses being stretched more than the closer targets.

In one arrangement, the constant speed time base is arranged to complete one range scan in a time corresponding to 6 nautical miles of received video, i.e. approximately 72 micro-seconds. If a full 48 nautical miles of stored video is to be displayed during this 72 micro second scan, the information in the store 20 must be read out at a rate of approximately 21.3 MHz. In order to set the displayed range to 24 nautical miles, the display range selector 22 may be effective to reduce this read rate by half, i.e. to approximately 10.7 MHz. However, then only half the total number of range cells will be displayed across the time base scan of the display and the display definition is reduced accordingly. Instead, therefore, the display range selector may be arranged to half the size of each range cell employed by the integrator 18 and double the rate of writing into the store 20.Then the information written into the store will be 1,536 range cells extending over only 24 nautical miles.

The display range is thus 24 miles if the read rate is maintained at 21.3 MHz. Further halfings of the range cells size and corresponding doublings of the writing rate can effect changes in the displayed range to for example 12 and 6 nautical miles.

After 6 nautical miles, corresponding to a writing rate also of approximately 21.3 MHz, it is convenient to select shorter displayed ranges by maintaining the range cell size and the writing rate constant but reducing the read out rate as described above. In this way displayed ranges down to one 1/4 nautical mile can be obtained, this last corresponding to a read out rate of approximately 0.9 MHz.

It can be seen that even with these very short range display settings, the time base speed is still that corresponding to a real time display of 6 nautical miles. Thus, even the short range displays can have the display brightness of a 6 mile display. The brightness of the display may be increased further by repeating the reading sequence immediately after the full set of information to be displayed has been read out so that the same information is read out again and displayed on a subsequent time base scan. There is normally sufficient time between the end of writing a full set of information into the store 20 and the next radar transmitter pulse to read the information from the store more than once.

Referring now to Figure 2, there is shown a detailed circuit diagram of the video processor 13. The logarithmic video information from the radar receiver is applied to the processor on a line 30 and is fed immediately to a video high pass filter constituted by a capacitor 31 and a resistor 32. The capacitance and resistance values of the capacitor 31 and resistor 32 are selected so that the time contant of the filter is approximately one order of magnitude greater than the width of the radar pulses of the radar transmitter. This minimises the loss of wanted target responses in the filtered video. The filter is, however, effective to remove the mean level of the clutter signals present in the logarithmic video.The combination of a logarithmic amplifier with a high pass filter is well known as a technique commonly used in constant false alarm rate (CFAR) circuits for removing unwanted clutter.

The logarithmic video on line 30 is also fed directly to an integrator network constituted by a resistor 34 and a capacitor 35.

The integrator network is effective to produce on a line 36 signals corresponding to the mean power level of the logarithmic video signals. These mean power level signals on line 36 are supplied via a potentiometer 37 and a line 38 as backing off signals to one input of a differential amplifier 39. The amplifier 39 has a feed back resistance 40 and a resistance 41 connected in series with its other input and to which is fed the output of the video filter on a line 33. The amplifier 39 operates in the usual way to provide output signals on a line 42 only when the filtered video signals exceed the level of the backing off signal on line 38.

In this way, remaining clutter signals in the filtered video signals can be backed off and it has been found that employing a circuit of this sort causes the backing off level to adapt itself in accordance with the amount of clutter left in the filtered video.

The DC level of the filtered video on the line 33 is clamped at a suitable level by means of a diode 43 connected between the line 33 and the centre point of a potential divider constituted by resistors 44 and 45 connected between a negative rail 46 and a earth. The diode 43 is decoupled by a compacitor 47. The circuit arrangement illustrated is designed for negative going logarithmic video supplied on the line 30 and the diode 43 effectively removes any positive going noise spikes in the filtered video on line 33.

Although the amplitude of the variations in the backing off signal on line 38 can be adjusted by the potentiometer 37, a further potentiometer 48 may be employed to set a maximum limit for the backing off signal.

The potentiometer 48 is connected between the negative rail 46 and earth and the slider of the potentiometer is connected to the line 38 via a diode 49 and a resistor 51. A capacitor 50 decouples the diode 49 and potentiometer 48. Normally, the diode 49 is reversed biassed, but if the backing off signal on the line 38 goes sufficiently negative the diode 49 becomes forward biassed and stops the backing off signal from going even further negative.

The video processor illustrated in Figure 2 also includes a pre-priming circuit. It will be appreciated that the integrator constituted by resistor 34 and capacitor 35 has a predetermined time constant, which must be greater than the pulse width of wanted target pulses in the logarithmic video. Thus, if the capacitor 35 is not pre-primed, at the start of each range scan of logarithmic video signals received, there will be a predetermined settling time before the capacitor 35 charges sufficiently to bring the backing off signal to a suitable level to back off most of the clutter in the received video. This is especially important since the clutter at the start of a range scan is normally at the highest amplitude, requiring the highest level of backing off signal.The pre-priming circuit is effective to give the capacitor 35 a priming charge at the start of each range scan so as to reduce the settling time. The circuit is effective to vary the size of the pre-priming charge in accordance with a sample of the clutter received in the immediately preceding range scan. Radar pulses are supplied to the pre-priming circuit 60 on a line 61. These sync pulses are supplied to delay units 62 to initiate a gating pulse having a width corresponding to the total delay of the units. Logarithmic video fed to the processor on line 30 is fed also on a line 63 to a gate 64. The gating pulse from the delay unit 62 is fed to open the gate 64 to pass a sample of the received video at the start of each radar scan and for a time determined by the delay of the units 62. This sample may be typically from 1 to 5 microseconds long.The total energy in the sampled video is stored in a capacitor 65. The sync pulses are also supplied as the gating signal of a second gate 66 to which is fed the voltage across the capacitor 65. As a result, the output of the gate 66 on a line 67 is a pulse having an amplitude corresponding to the level of the voltage across capacitor 65.

This pre-priming pulse effectively precharges the capacitor 35 by an amount dependent on the energy in the sampled video of the preceding range scan.

The above described video processor is especially useful in reducing the displayed clutter in a marine radar apparatus. In rough sea conditions, sea clutter can completely mask nearby targets on the radar display. Manual controls for reducing clutter by raising the detection threshold of the apparatus can reduce the clutter allowing hidden targets to be located but have the drawback of reducing the threshold at other parts of the display where the clutter is less serious. As a result, weaker targets are sometimes lost. The adaptive operation of the above described video processor enables the threshold level to be altered automatically during each range scan to provide a substantially constant low level of back ground clutter and provide simultaneously the maximum available sensitivity of the radar apparatus where this can be used.

Figure 2 also shows differential high gain amplifier 70 with the processed video supplied to one input of the amplifier on a line 71 and a DC reference level applied on the other input on a line 72. This amplifier 70 may constitute the digitizer 16 of Figure 1.

The gain of the amplifier 70 is made sufficiently high so that any signal in the processed video on line 71 which exceeds the threshold level on line 72 causes the output of the amplifier 70 on a line 73 to saturate. The level on the line 72 may be adjustable to discriminate against weak signals in the processed video. The output on line 73 comprises a series of pulses of uniform amplitude corresponding to signal pulses in the processed video which exceed the reference level.

It will be appreciated that the amplifier 70 is not essential. The amplifier 3 of the video processor may be made with sufficiently high gain so that its output on line 42 saturates whenever the filtered video on line 33 exceeds the backing-off signal level set on line 38. Then, the signals on line 42 will comprise a series of pulses of uniform amplitude which may be considered as video digitised in only two levels. In this case a D.C. component should be added to the adaptive backing-off signal on line 38 to provide a function equivalent to the reference level on line 72.

WHAT WE CLAIM IS: 1. Pulse radar apparatus having (a) a transmitter for transmitting radio frequency pulses of a predetermined duration and at a predetermined repetition rate, (b) a receiver with a logarithmic video amplifier, ampliser;eans for processing the received logarithmic video signals comprising a video high pass filter having a time constant greater than said radar pulse duration and being arranged for removing the mean level of clutter signals in the logarithmic video signals, an integrating circuit for integrating the logarithmic video signals to provide backing off signals corresponding to the mean power level of the logarithmic video signals, means for combining the video signals from the video high pass filter with the backing-off signals to back off the remaining clutter signals in the filtered video signals and means for prepriming the integrating circuit at the beginning of each range scan to set the backing-off signal in accordance with a predetermined sample of the logarithmic signals received in the preceding range scan, (d) means for digitising the processed video in only two levels, (e) a pulse to pulse integrator determining the presence of absence of a processed video digital pulse in each of successive range cells in each range scan and providing a corresponding output pulse only if said pulses are determined in corresponding range cells of two successive range scans, (f) means for storing indications of the

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

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. start of each range scan of logarithmic video signals received, there will be a predetermined settling time before the capacitor 35 charges sufficiently to bring the backing off signal to a suitable level to back off most of the clutter in the received video. This is especially important since the clutter at the start of a range scan is normally at the highest amplitude, requiring the highest level of backing off signal. The pre-priming circuit is effective to give the capacitor 35 a priming charge at the start of each range scan so as to reduce the settling time. The circuit is effective to vary the size of the pre-priming charge in accordance with a sample of the clutter received in the immediately preceding range scan. Radar pulses are supplied to the pre-priming circuit 60 on a line 61.These sync pulses are supplied to delay units 62 to initiate a gating pulse having a width corresponding to the total delay of the units. Logarithmic video fed to the processor on line 30 is fed also on a line 63 to a gate 64. The gating pulse from the delay unit 62 is fed to open the gate 64 to pass a sample of the received video at the start of each radar scan and for a time determined by the delay of the units 62. This sample may be typically from 1 to 5 microseconds long. The total energy in the sampled video is stored in a capacitor 65. The sync pulses are also supplied as the gating signal of a second gate 66 to which is fed the voltage across the capacitor 65. As a result, the output of the gate 66 on a line 67 is a pulse having an amplitude corresponding to the level of the voltage across capacitor 65. This pre-priming pulse effectively precharges the capacitor 35 by an amount dependent on the energy in the sampled video of the preceding range scan. The above described video processor is especially useful in reducing the displayed clutter in a marine radar apparatus. In rough sea conditions, sea clutter can completely mask nearby targets on the radar display. Manual controls for reducing clutter by raising the detection threshold of the apparatus can reduce the clutter allowing hidden targets to be located but have the drawback of reducing the threshold at other parts of the display where the clutter is less serious. As a result, weaker targets are sometimes lost. The adaptive operation of the above described video processor enables the threshold level to be altered automatically during each range scan to provide a substantially constant low level of back ground clutter and provide simultaneously the maximum available sensitivity of the radar apparatus where this can be used. Figure 2 also shows differential high gain amplifier 70 with the processed video supplied to one input of the amplifier on a line 71 and a DC reference level applied on the other input on a line 72. This amplifier 70 may constitute the digitizer 16 of Figure 1. The gain of the amplifier 70 is made sufficiently high so that any signal in the processed video on line 71 which exceeds the threshold level on line 72 causes the output of the amplifier 70 on a line 73 to saturate. The level on the line 72 may be adjustable to discriminate against weak signals in the processed video. The output on line 73 comprises a series of pulses of uniform amplitude corresponding to signal pulses in the processed video which exceed the reference level. It will be appreciated that the amplifier 70 is not essential. The amplifier 3 of the video processor may be made with sufficiently high gain so that its output on line 42 saturates whenever the filtered video on line 33 exceeds the backing-off signal level set on line 38. Then, the signals on line 42 will comprise a series of pulses of uniform amplitude which may be considered as video digitised in only two levels. In this case a D.C. component should be added to the adaptive backing-off signal on line 38 to provide a function equivalent to the reference level on line 72. WHAT WE CLAIM IS:

1. Pulse radar apparatus having (a) a transmitter for transmitting radio frequency pulses of a predetermined duration and at a predetermined repetition rate, (b) a receiver with a logarithmic video amplifier, ampliser;eans for processing the received logarithmic video signals comprising a video high pass filter having a time constant greater than said radar pulse duration and being arranged for removing the mean level of clutter signals in the logarithmic video signals, an integrating circuit for integrating the logarithmic video signals to provide backing off signals corresponding to the mean power level of the logarithmic video signals, means for combining the video signals from the video high pass filter with the backing-off signals to back off the remaining clutter signals in the filtered video signals and means for prepriming the integrating circuit at the beginning of each range scan to set the backing-off signal in accordance with a predetermined sample of the logarithmic signals received in the preceding range scan, (d) means for digitising the processed video in only two levels, (e) a pulse to pulse integrator determining the presence of absence of a processed video digital pulse in each of successive range cells in each range scan and providing a corresponding output pulse only if said pulses are determined in corresponding range cells of two successive range scans, (f) means for storing indications of the

presence or absence in successive range cells of output pulses of the pulse to pulse integrator from successive individual range scans, (g) cathode ray tube display means having means for providing a constant speed time base trace synchronised with the pulse repetition rate of the radar transmitter, (h) means for reading stored indications from the store at a predetermined rate and applying them as brightness modulation to the cathode ray tube display, and (i) display range selector means switchable to alter the rate of reading out the stored indications or the size of said range cells or both to effect a change in the range-scale of the display.

2. Apparatus as claimed in claim 1 including a pulse stretcher for stretching the duration of pulses representing the indications read from the store, these stretched pulses being applied as brightness modulation to the display.

3. Apparatus as claimed in claim 2 wherein the pulse stretcher is arranged to stretch the pulses by amounts which are a function of the ranges of the respective targets indicated by the pulses, so that more distant target response pulses are stretched more than closer targets.

4. Apparatus as claimed in any preceding claim wherein the display range selector means is arranged to alter the size of said range cells to effect range scale changes between the longer displayed ranges of the apparatus. and to alter the reading rate from the store to effect changes between the shorter displayed ranges.

5. Apparatus as claimed in any preceding claim wherein a diode is connected between the output of the video high pass filter and a reference line to clamp the base level of the filtered video signals at a predetermined level and remove any noise spikes in the video extending below said base level.

6. Apparatus as claimed in any preceding claim, wherein said prepriming means is arranged to set the backing-off signal to a level dependent on the total received video energy in a predetermined time sample at the beginning of the preceding range scan.

7. Pulse radar apparatus substantially as hereinbefore described with reference to the accompanying drawings.