Absolute Frequency Wind Measurements for Doppler-Rayleigh
Lidar
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 deg 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. We report a
technique by which frequency drifts and the vertical looking zero
Doppler reference can be eliminated by frequency locking the injection
seeding master laser oscillator, and then using it to stabilize the
receiver. This technique could ultimately speed data taking by a
factor of 5 for horizontal wind measurements.
Send e-mail to jfriedman@naic.edu
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