LIDAR measures up conditions for Olympic sailors

LIDAR (light detection and ranging) is the latest tool for Olympic sailors seeking to fill their sails to the max and beat the competition. Researchers at the Ocean University of China have developed a mobile LIDAR system that can be used to provide real-time information on wind conditions at sea, across a wide area¹.

Conventional methods for measuring sea surface winds include anemometers aboard boats and buoys, and Doppler radar techniques. Anemometers only provide localised velocity measurements and therefore it isn’t possible to fully map the winds over a competition area which may span several square kilometres, especially when the conditions can vary from one moment to the next. Doppler radar uses radio waves to take wind velocity measurements over a wide area, but this technique only works well on cloudy days. LIDAR, on the other hand, uses light to measure the movement of wind across vast areas and can operate on the clearest of days.

LIDAR works by emitting light in the target direction and detecting the change in frequency of the backscattered light caused by the movement of the aerosols and molecules from which it is reflected. This is known as the Doppler Effect and is commonly described using the example of an ambulance siren whose pitch changes as it passes the observer. The moving air particles alter the frequency of the light, just as the siren frequency changes according to whether the ambulance is approaching or receding from the observer. Very precise measurements of wind speed can be made by comparing the frequency of the received light with that of the emitted light.

The mobile system built by the Chinese researchers is contained in a small bus and can therefore be easily transported to a competition area. A diode-pumped Nd:YAG laser operating at 532 nm emits light pulses at a frequency of 2.8 kHz. Accurate measurements of wind speed require a highly stable light source, therefore the laser is frequency-locked to minimise drift. The backscattered light is collected using a telescope and then coupled to the frequency discriminating module by fibre optic cable. Operating from a single vantage point on the shore, the system can scan a full 360° with up to 180° of elevation angle and can map an entire competition area every few minutes, with a spatial resolution of 50 to 100 m. The system was successfully demonstrated at the 2007 Qingdao International Regatta and is proposed for use in the upcoming 2008 Olympic games in China.

1) "A high spatial and temporal resolution mobile incoherent Doppler lidar for sea surface wind measurements" by Zhi-Shen Liu et al., Optics Letters, Vol. 33, No. 13, July 1, 2008 p. 1485-1487. doi:10.1364/OL.33.001485