At a general level, marine radar systems typically comprise a rotating radar antenna scanner that transmits electromagnetic pulses during rotational 360° sweeps (scans) at a number azimuthal directions in the sweep, determined by an azimuthal sampling rate. For each pulse in an azimuthal direction the echo return signal is received and sampled at a number of ranges from the scanner. The resultant digital radar video stream generated by the scanner is then typically signal processed to detect targets for presenting on a radar display, such as a Plan Position Indicator (PPI).
In recent years it has become practical to improve the azimuth resolution of radar using signal processing techniques commonly referred to as ‘beam sharpening’. Beam sharpening in the context of this specification is considered to be any process that significantly improves target discrimination in azimuth.
One well-known beam sharpening method is to use deconvolution. In radar, the rotating antenna sweeps over targets and mathematically this can be described as the convolution of the target pattern and the antenna pattern. The resulting signal is then deconvolved to recover the original target pattern. The deconvolution process may be described in the azimuth spectral domain as applying a filter with an inverse frequency response to the antenna. In theory the original target pattern can be perfectly recovered. However, in practice the multi-element antennas typically used for radar applications have a low pass response with zeros in the stopband. The inverse response therefore has points of infinite gain that cannot be implemented. In practice, the inverse response must be modified in some manner. Typically the inverse antenna response is limited to a practical level of maximum gain through the use of an appropriate low pass filter, and an example of a possible filter response relative to the associate antenna and inverse antenna response is shown in FIG. 1.
GB2449171 discloses one form of deconvolution-based beam sharpening which involves iteratively carrying out the steps of filtering the incoming signal and then clipping values of the filtered signal which are below a baseline level, for example.
Beam sharpening has the advantage of better discriminating targets in azimuth and is very beneficial to navigation and collision avoidance in general but does have downsides in the following respects:                i) Detail is increased everywhere in the radar image so it can be more difficult to interpret for novice users and in rough sea conditions on small craft. This includes areas where detail does not add much value such as within extensive land targets.        ii) Targets at close range can become difficult to see on the display because of the geometric compression a PPI display inherently causes.        iii) Frequency Modulated Continuous Wave (FMCW) radars are capable of detecting targets down to a range of several meters. At such ranges a much greater proportion of targets become azimuth extensive. A complex target, such as a sailboat, at these ranges may produce multiple discriminated returns. In this case discriminating features within the target is of no navigational value.        iv) A PPI produces inherent visual distortion of the radar image, being highly compressed in azimuth at close range progressing to being highly expanded at long range. For radars with wide azimuth beamwidth such as radomes, this causes increased error between the chart and radar image when overlaid.        
In this specification where reference has been made to patent specifications, other external documents, or other sources of information, this is generally for the purpose of providing a context for discussing the features of the invention. Unless specifically stated otherwise, reference to such external documents is not to be construed as an admission that such documents, or such sources of information, in any jurisdiction, are prior art, or form part of the common general knowledge in the art.
It is an object of the invention to provide an improved radar beam sharpening system method which at least goes some way to addressing some of the above disadvantages of beam sharpening, or which at least provides the public with a useful choice.