Search Lidar for the detection of small sea-surface objects (1996 – 2006)
Light Detection And Ranging (LIDAR) technique uses light pulses to measure the angle, distance and reflection (like radar uses microwaves). In 1982, TNO converted an old laser system produced by the Optische Delft into a mini-lidar system. The mini-lidar was used amongst other experiments for atmospheric aerosol measurements.
Coastal surveillance sensors and ships near the coast have to detect small sea-surface objects. These objects have a low radar cross-section and a low velocity which makes them hard to detect by radar. Stealth constructions will reduce radar reflections of such objects even further. Typical objects include jet skis, small boats, periscope tubes, wreckage, man overboard, swimmers, illegal immigrants, drug transport and asymmetric threats. In 1996, TNO started to investigate whether lidar could be used to detect small sea-surface objects and whether we could build the prototype of a wide area scanning system, or in other words, a Search Lidar.
The search lidar principle
A Search Lidar is based on a laser that sends a pulse of infrared light over the water’s surface. Objects at the water surface reflect this light pulse while the water surface itself has a negligible reflection. Experiments in 1996 and 2000 at Meetpost Noordwijk confirmed the low sea reflection.
The reflected light is received by a sensitive detector. The time interval between sending the pulse and receiving the reflection determines the distance to the object (time-of-flight). The principle of a search lidar consists of a scanning laser beam with a relatively narrow beam in combination with an optical receiver aligned with the laser beam. Scanning is done in azimuth and elevation to cover the sea surface around the system. Search lidars are intended for the detection of targets and the measurement of their position. Speed and heading can be obtained from multiple measurements. It is possible that search lidars can also be used for the classification or identification of non-cooperating long-range targets.
Search lidar performance
Lidar measurements by TNO at the coast of the Netherlands have shown a very good signal-to-clutter ratio concerning buoys several kilometres from the shore. The results show that due to the low sea-surface clutter, a search lidar can detect small sea-surface targets. A plot of the lidar reflections is shown below. Experiments over the years 1996 to 2006 have confirmed the low sea reflection. Even at high wind speeds around 15 m/s (7 Beaufort), the sea reflection disturbances remain low.
During a lidar experiment, the position and reflection of a buoy at a distance of 9.3 km were successfully measured. The buoy could not be observed with the naked eye and only slightly using a binocular. With the lidar, the buoy could be detected with a signal-to-noise ratio of about 140.
A screenshot of the waveform recorder is shown below. The gradual decrease in the signal represents the atmospheric backscatter, attenuated by the geometric effect and the atmospheric transmission losses. Due to the application of an analogue logarithmic amplifier, the large dynamic range of the signal could be covered within a single waveform.
The encouraging lidar results led to the development of a search-lidar demonstrator which features a high pulse rate laser at a wavelength of 1.5 microns, making it eye-safe. The system was successfully tested near the Dutch coast. The search time is around three seconds for a semi-circular scan of the coast with a pulse rate of 2 kHz. This search time is sufficiently low to be applied to coastal surveillance.
Heuvel, J.C. van den, Bekman, H.H.P.T., Putten, F.J.M., Cohen, L.A. (2007), Detection of Small Sea-Surface Targets with a Search Lidar, Proceedings SPIE, Turner, M.D., Kamerman, G.W., Laser Radar Technology and Applications XII, 11 April 2007, Orlando, FL, USA.