Detection of small targets in good weather is not always challenging, however, in heavy rainfall and rough seas thesituation changes dramatically. In these conditions radar detection of small targets require very high gain antennas with narrow beamwidths and small clutter cells to reduce rainfall and sea clutter return. Parabolic reflector radar antennas include the ability of polarisation switching in X band for improved performance in heavy rainfall and, in combination with frequency diversity transceivers, provide high radar detection capability. Furthermore, for the most demanding weather conditions often found in tropical regions where rainfall can sometimes exceed 40-60mm/hr and temperate latitudes subject to gale force winds, a combined dual X and S band antenna can be used to maintain radar detection by utilising the S band frequency with circular polarisation.
Typically, small radar targets may be only 1m2 or 2m2 radar cross section. These targets have to be detected and tracked simultaneously with the largest of maritime targets ensuring physical separation on the operators display. The ability of a radar system to comply with the IALA Recommendations Advanced Recommendations is also an important consideration.
n the initial phase, the customer needs are assessed for radar detection to ensure compliance with a wide range of target sizes according to the specific requirements.
Radar options for VTS applications
For this market, radar engineers in various organisations had concluded that the most important factors limiting the radar performance of conventional VTS radars was the low gain and resolution of some slotted wave guide (SWG) antennas derived from shipboard navigational radars. By replacing the antenna with a shaped beam, high gain, high resolution, having low side lobes and preferably with additional facilities such as polarisation control and dual band operation, the overall radar performance would be greatly improved. This conclusion is valid irrespective of the type of radar transmitter and receiver, and is especially true for low power marine magnetron systems.
Modern advances in signal processing can also enhance the radar performance in rain or in rough seas, i.e. in clutter; signal processing techniques can do little to improve clear weather radar performance. All such improvements can do is to reduce the degradation of radar performance due to clutter.
Radar options for coastal radar
For sea surface surveillance in clear weather, X band microwave systems are preferred due to the longer range that can be achieved, with better resolution, as compared to S band systems having similar size of antenna and similar transceivers. In heavy rain, and rough seas, the rain clutter reverses this situation as regards target detection, with S band giving the better radar performance; as the rainfall rate increases the detection performance of the S-band remains unaffected whilst X-band starts to degrade. When air targets are to be detected, S band will generally be used to reduce the clutter from rain clouds and due to lower propagation losses. Due to the curvature of the earth, conventional microwave radar is capable of reliable surveillance covering only the radar ‘line-line-of-sight’. This approximates to 15 nautical miles for a small boat, detected by radar at 30 metres height to 80 nautical miles for a large ship, detected by radar at 1,000 metres height.
Selection of the correct radar antenna is paramount for the overall system performance and careful consideration for the antenna during the planning stage of a new installation can be critical. To maximise the performance of a VTS or coastal surveillance application there are several important aspects which need to be considered, these are summarised:
1. Position of primary sensor, the radar antenna to provide maximum coverage range. This should be as high as possible to provide the longest detection range and thereby reduce the number of radar antenna required for the planned scheme.
2. Antenna should have as high gain as possible in order to obtain the maximum power on target. In many situations the short pulse length offered by a high gain antenna will reduce the sea clutter return and improve the target range discrimination.
3. Narrow azimuth beamwidth for better separation of targets in close proximity of each other and target resolution.
4. Given that a high antenna location is advantageous for maximum range if the elevation beam pattern is an inverse cosec2 pattern this will greatly assist in the detection of those targets which are closer to the antenna.
5. Therefore, each coastal surveillance project requires evaluation to consider if an individual or customised system solution is necessary.