Absolute Frequency Wind Measurements for Doppler-Rayleigh Lidar

Abstract

The Doppler-Rayleigh lidar at Arecibo Observatory has been in operation since May 1990 obtaining wind data in the region of the atmosphere between 10 and 60 km altitude where it is difficult to measure atmospheric properties by balloons, satellites, or radars. An injection-seeded Nd:YAG laser coupled with a Fabry-Perot etalon receiver are capable of giving a frequency resolution of about 20 MHz. The etalon is pressure tuned in discrete steps about the center frequency of the transmitter. To get the horizontal component of the wind vector, the laser is pointed 30° off the zenith to the north or south and to the east or west. A zero wind marker for reference is obtained by pointing the lidar vertically. It is assumed that vertical winds are substantially smaller than horizontal and not measurable by the lidar. Long term drifts in the center frequency caused by both drift in the seed laser and in the etalon receiver are compensated for by taking out the drift of the fringes measured in the vertical direction. This uncertainty in frequency drift of the system and the need to depend on the assumption of zero vertical wind velocities are weaknesses in the measurements, and they render the uncertainty of Doppler shift measurements to be much greater than 20 MHz.