Dual frequency radar improves performance of coastal surveillance

Kelvin Hughes has developed radar that combines X-band and S-band for coastal surveillance. The shore-based sensor SBS-900-4 SharpEye radar can be used by any vessel traffic service (VTS) or for marine security and safety requirements. It can be deployed as a single radar site or as part of a radar sensor network, depending on requirements.

The solid state SharpEye technology applies Doppler processing to the radar returns to provide superior target detection, including low-level air targets, even in the harshest of weather conditions. This is achieved with mast-mounted, co-located antennas with a combined turning mechanism and technology to combine returns in both frequency ranges.

Kelvin Hughes sales and marketing director Spike Hughes explained the benefits for coastal surveillance. One of those is the ability to combine X-band and S-band. Others are the processing capability, low energy demand and reduced maintenance requirements. He said: “This costal surveillance and VTS radar solution is significantly differentiated to other types of radar.”

Sales manager Paul Mariner said an advantage of the SBS-900-4 radar was its performance in areas of high rainfall. “The X-band frequency is best suited to providing an acceptable level of range and bearing discrimination. However, its performance in rain cannot match the S-band,” he said.

This is because of the physical properties of rain, such as the reflection and attenuation coefficients of the water droplets, and the degrading effects of these on X-band frequencies. Mr Mariner explained: “In areas experiencing high rainfall, the only previous option was to invest in two separate radar systems including the tower and infrastructure. This enabled the X-band to provide excellent range and bearing discrimination in good weather while providing the option to switch to the S-band when prevailing conditions seriously degraded the X-band performance.”

Another environmental problem with using X-band in tropical regions is the ducting of radio waves in hot weather conditions. Evaporation forms a moisture layer that traps radio waves and creates a duct within which the signal will travel, following the curvature of the Earth, around 50m from the surface. If the radar antenna is above this duct layer, then X-band waves are reflected back to the atmosphere or trapped in the duct. This means targets below the duct can be undetected. However, S-band frequencies are less affected by ducting.

“While not a guaranteed cure, the wide frequency diversity of the two bands offers an increased chance of combatting the problem,” said Mr Mariner. “If the operational requirement cannot accept periods of poor performance, then both X-band and S-band frequencies are required, which results in the need for two antennas. Previously this required separate towers or a larger, heavier tower that could accommodate the turning circle of both the antennas and their weight.”

Kelvin Hughes overcame this challenge by combining the antennas on one turning mechanism. The SBS-900-4 radar has a 5.5m X-band antenna that is located 180 degrees from the 3.9m S-band antenna. The SharpEye X-band and S-band radar use Doppler processing and high resolution pulse compression techniques, while transmitting simultaneously.

“This combination maximises the benefits of X-band resolution and detection performance in clutter with the additional S-band advantages of longer range detection in heavy rain conditions,” Mr Mariner said.

He added: “SharpEye is ideal for VTS and coastal surveillance systems because of its performance and its inherent reliability and low maintenance requirements.” The radar has solid state components that have a longer lifetime than conventional radar equipment. There is no magnetron that other systems have, so there is no need for maintenance. Mr Mariner said the transceiver is designed to be reliable for 150,000 hours – around 17 years – between failures.

“SharpEye uses a patented sequence of coded and varying length pulses to help ensure target fidelity at all ranges,” Mr Mariner explained. “The short pulse sequence comprises a 0.1µs gated continuous wave. There is a medium and a long pulse that contain a non-linear frequency, modulated chirp with a swept bandwidth of approximately 40 MHz. This is to provide excellent range resolution out to the maximum instrumented range whilst maintaining the ability to see small targets at longer ranges.”

The moving target detection techniques place the received radar returns in a bank of narrowband coherently integrated filters to resolve targets from the clutter. “This delivers a significant performance advantage in detecting small targets,” commented Mr Mariner. “This also removes the requirement for operators to continuously adjust radar settings to meet changing environments – such as sea and rain clutter settings – a necessity with conventional radar systems.”

The SharpEye technology can also be tailored for optimum performance in a specific location and environment, said Mr Mariner. For example, a SharpEye VTS radar sited on a river estuary only needs to detect targets at a relatively short range when the radar antenna is being directed across the river, but with a high degree of discrimination.

However, if the antenna is directed down river and out to sea, it needs to detect targets at a much greater range. “SharpEye can be configured to deliver different transmission modes in different scan sectors, to produce the most effective and efficient target detection,” said Mr Mariner.