GB2299910A

GB2299910A - Target detection

Info

Publication number: GB2299910A
Application number: GB9507472A
Authority: GB
Inventors: Peter Lintott; Angus Raymond Johnson; Paul George Wright
Original assignee: Commonwealth of Australia
Current assignee: Commonwealth of Australia
Priority date: 1995-04-11
Filing date: 1995-04-11
Publication date: 1996-10-16
Anticipated expiration: 2015-04-11
Also published as: GB9507472D0; GB2299910B

Abstract

A method of target detection in a system employing range cells includes the steps of; (i) comparing 3 the received signal power level of each cell with a first threshold and categorising some of the cells as possibly containing real target returns, (ii) processing 1 the received signal power levels of each cell possibly containing a real target return to correct for non-uniformity of response across the cells, and (iii) comparing 6 the corrected received signal power levels to a second threshold and categorising those cells having a corrected signal power level above the second threshold as containing real target returns and those having a corrected signal power level below the second threshold as not containing real target returns. The double threshold approach enables fewer false alarm responses while maintaining a low level signal detection capability. This is especially useful in skywave over-the-horizon radar. Prevents "scalloping".

Description

IMPROVED TARGET DETECTION This invention relates to a method and apparatus for improved target detection and particularly to a method and apparatus for improved radar target detection.

In systems which operate by sending out an energy pulse, reflecting it from a target and detecting the reflected return the detection of the return is one of the most critical parts of the system. The reflectors producing the return signals are generally referred to as targets.

This detection of return signals is a particular problem in long range radars because of the very low level of the return signal compared to general background noise and the capability to detect return signals only slightly above background noise level is one of the limits on long range radar performance.

In a skywave over the horizon radar system the return signals are formed into a three dimensional array of cells, each cell having a unique range, azimuth and doppler value, the doppler value corresponding to the velocity of the target relative to the radar system.

Conventionally the overall noise level across the array of cells is calculated and a threshold level having a set value over the averaged noise level is calculated. Those cells containing a return signal over the threshold value are classified as containing real target returns, which is to say they correspond to range, azimuth and doppler values having targets within them and the rest are categorised as not containing real target returns or corresponding to range, azimuth and doppler values not having targets within them. The problem with this conventional approach is that there is always a tradeoff between the smallest target return signal which can be detected and the number of false alarms generated by the noise level in a cell being above the threshold.Thus the minimum signal strength returned from a target which can be detected is limited by the danger of overloading the radar system with an excessive number of false alarms.

This invention was intended to provide a method and apparatus for overcoming this problem at least in part.

In a first aspect this invention provides a method of target detection in which received signals are arranged in cells with each cell having a received signal power level associated with it, and including the steps of; (i) comparing the received signal power level of each cell with a first threshold and in dependence on the result of this comparison categorising some of the cells as possibly containing real target returns, (ii) processing the received signal power levels of each cell possibly containing a real target return to correct for non-uniformity of response across the cell, (iii) comparing the corrected received signal power levels of each cell possibly containing real target returns to a second threshold and categorising those cells having a corrected received signal power level above the second threshold as containing real target returns and those having a corrected received signal power level below the second threshold as not containing real target returns.

In a second aspect this invention provides target detection apparatus in which received signals are arranged in cells with each cell having a received signal power level associated with it and including a first comparator for comparing the received signal power level of each cell with a first threshold and in dependence on the result of this comparison categorising some of the cells as possibly containing real target returns, processing means for processing the received signal power levels of each cell possibly containing a real target return to correct for non-uniformity of response across the cell and second comparator means for comparing the corrected received signal power levels of each cell possibly containing real target retums to a second threshold and categorising those cells having a corrected received signal power level above the second threshold as containing real target returns and those having a corrected received signal power level below the second threshold as not containing real target returns.

Target detection systems employing the invention will now be described by way of example only with reference to the accompanying diagrammatic figures in which: Figure 1 shows a first radar target detection system employing the invention; and Figure 2 shows a second radar target detection system employing the invention; identical parts having the same reference numerals throughout.

It has been realised that in a radar system employing cells one reason for a target detector failing to identify targets is that where the target is not at the centre of the cell in which it is located it may fail to produce a return over the threshold value even though the signal power of the return it generates exceeds the threshold value. This is because the apparent value of the return signal power is reduced because the response of the radar system is generally not uniform across each cell. Scalloping loss is the term used to describe this phenomenon of non-uniformity in the response or sensitivity of the radar system across the ARD cells and generally takes the form of a maximum response or sensitivity at the centre of the cell and a lower response or sensitivity towards the edges.

Referring to Figure 1 the operation of a radar target detection system 1 is described.

The target detection system 1 is supplied with data from a preprocessor 2 in the form of a series of azimuth range and doppler (ARD) cells each of which corresponds to a unique azimuth and range position and doppler value and has a value corresponding to the return signal power found within that cell. The preprocessor 2 carries out various known processes to convert the timing, direction and frequency shift of received signals into azimuth range and doppler range cell data and to remove the effects of noise and clutter, by processes such as noise whitening for example, but its operation will not be discussed in detail.

The data on each cell in tum is supplied to a first thresholding unit 3 which categorises all range cells having a received signal power level above a first threshold as possibly containing a target and passes the identity, return signal power levels and other data, such as signal to noise ratio, associated with each of the range cells having a return signal power level above the first threshold to a scalloping correction unit 5 which calculates the precise values in range, azimuth and doppler within the range cell at which the target must have been to generate the return and corrects the signal power value to eliminate scalloping loss by increasing it to the power value it would have had if the target had been in the centre of the range cell. The identity of each range cell is its unique azimuth range and doppler value.

The scalloping correction unit 5 operates by multiplying the actual signal power level by a factor dependent on location to produce an expected power level which would have been expected to have been received had the target been at the centre of the ARD cell.

The precise dependence of this factor on location is dependent on the parameters of the radar system and the cell size and will have to be calculated on a case by case basis. The range cell identities and their new corrected signal power values are then passed by the scalloping correction unit 5 to a second thresholding unit 6 which compares the signal power levels to a second threshold higher than the first threshold and categorises the ARD cells having a corrected signal power level above this second threshold as containing targets and those having signal power levels below the second threshold as not containing targets and provides information identifying the ARD cells which have been categorised as containing a target as an output along a line 7. This output along line 7 also provides the corrected signal power level and signal to noise ratio associated with each of these ARD cells. This data can be output in any desired format.

The second threshold must be higher than the first threshold but the actual difference between them will depend on the parameters of the radar system.

An alternative form of target identification system is shown in Figure 2. In this case a target identification unit 11 is supplied with pre-processed ARD cell data from a preprocessing unit 2 as in the previous example.

The data on each cell in tum is passed to a first thresholding unit 13 which compares the return signal power level within the cell to a first threshold and to a third, higher, threshold. Those cells having a signal power level below the first threshold are categorised as not containing a target while those cells having a signal power level above the third threshold value are categorised as definitely containing a target and their identification, status and associated data is output along a line 4.

Those cells having a signal level between the first and third thresholds are categorised as possibly containing a target and have their identity and signal level passed to a scalloping correction unit 5.

As in the previous example, the scalloping correction unit 5 calculates the precise values of range, azimuth and doppler within the cell from which the signal has been returned and corrects the signal level to the level it would have had if the signal had been returned from the centre of the ARD cell in order to correct for scalloping loss.

The scalloping corrected data from the scalloping correction unit 5 is then passed to a second thresholding unit 16 which compares the scalloping corrected signal power levels in each cell with a second threshold and categorises those signals above the second threshold as containing targets and those below the second threshold as not containing targets. The second thresholding unit 16 produces data associated with and identifying the ARD cells categorised as containing a target as an output along a line 7.

The lines 4 and 17 supply the data identifying and associated with the range cells categorised as containing a target to a data combiner 18 which generates an output supplying the identity and associated data of all of the range cells containing a target along aline9.

The second threshold used by the second thresholding unit 16 would generally be equal to the third, higher, threshold used in the first thresholding unit 13 but could be intermediate the first and third threshold values used by the first thresholding unit 13. The second threshold will always be greater than the first threshold.

Comparing the examples it will be clear that there are a number of ways of combining the various elements described other than those illustrated in the figures. For example in systems where the first thresholding unit compares the cell signal power levels to a single threshold it is clearly possible to either set a high threshold and categorise all signal levels above the threshold as containing targets and reexamine all cells below the threshold or set a low first threshold and categorise all the cells having a signal power level below that threshold as definitely not containing a target and re-examine all cells having a received signal power level above the threshold.

Although the results above are discussed above in terms of a radar system it will be clear that the same technique could be used in similar systems detecting energy reflected from remote targets such as seismic surveying or sonar systems, for example.

Claims

1. A method of target detection in which received signals are arranged in cells with each cell having a received signal power level associated with it, and including the steps of; (i) comparing the received signal power level of each cell with a first threshold and in dependence on the result of this comparison categorising some of the cells as possibly containing real target returns, (ii) processing the received signal power levels of each cell possibly containing a real target return to correct for non-uniformity of response across the cell, (iii) comparing the corrected received signal power levels of each cell possibly containing real target returns to a second threshold and categorising those cells having a corrected received signal power level above the second threshold as containing real target returns and those having a corrected received signal power level below the second threshold as not containing real target returns.

2. A method as claimed in claim 1 in which, in step (i), cells having a received signal power level below the first threshold are categorised as possibly containing real target returns.

3. A method as claimed in claim 1 in which, in step (i), cells having a received signal power level above the first threshold are categorised as possibly containing real target returns.

4. A method as claimed in any preceding claim in which, in step (i), a third threshold is also used and cells having a received signal power level between the first and third thresholds are categorised as possibly containing real target returns.

5. A method as claimed in any preceding claim in which the correction for non-uniformity in step (ii) is carried out by calculating the position within the cell at which a target would have had to have been in order to generate the received signal and then multiplying the received signal power level by a factor dependent on this position.

6. Target detection apparatus in which received signals are arranged in cells with each cell having a received signal power level associated with it and including a first comparator for comparing the received signal power level of each cell with a first threshold and in dependence on the result of this comparison categorising some of the cells as possibly containing real target returns, processing means for processing the received signal power levels of each cell possibly containing a real target return to correct for non-uniformity of response across the cell and second comparator means for comparing the corrected received signal power levels of each cell possibly containing real target returns to a second threshold and categorising those cells having a corrected received signal power level above the second threshold as containing real target returns and those having a corrected received signal power level below the second threshold as not containing real target returns.

7. Apparatus as claimed in claim 6 in which the first comparator categorises the cells having a received signal power level below the first threshold as possibly containing real target returns.

8. Apparatus as claimed in claim 6 in which the first comparator categorises the cells having a received signal power level above the first threshold as possibly containing real target returns.

9. Apparatus as claimed in any one of claims 6 to 8 in which the first comparator compares the received signal power level of each cell with a first threshold and a third threshold and categorises those cells having a received signal power level between the first and third thresholds as possibly containing real target returns.

10. Apparatus as claimed in any one of claims 6 to 9 in which the processor corrects for non-uniformity by calculating the position within the cell at which a target would have had to have been in order to generate the received signal and then multiplying the received signal power level by a factor dependent on this position.

11. Apparatus for target detection substantially as shown in or as described with reference to Figure 1 of the accompanying figures.

12. Apparatus for target detection substantially as shown in or as described with reference to Figure 2 of the accompanying figures.