Airborne weather Doppler radar data (Doppler velocity) contains both
meteorological signals and aircraft motion.
In order to accurately obtain multiple Doppler wind synthesis, the
aircraft motion must be properly removed from the Doppler velocities.
Errors in the aircraft intertial navigation system (INS) and the radar
pointing angles can be corrected using various methodologies, outlined
briefly here.
The corrections currently calculated and distributed by EOL use the "THL" (Testud-Hildebrand-Lee)
methodology, described in A
Procedure To Correct Airborne Doppler Radar Data For Navigation Errors,
Using
The Echo Returned From the Earth Surface (Testud, et al. 1995).
These corrections are obtained primarily
from calibration legs performed on each research flight, but in some
cases are from other legs. The algorithm uses the fact that the ground
echo return should be flat, and have zero residual velocity. Solving a
system of equations iteratively computes the correction to the INS
parameters and radar pointing angle needed for accurate radial
velocities.
While several of the INS corrections remain stable throughout a
research flight, the ground speed and drift corrections may vary from
leg to leg during
a flight. These corrections are tightly coupled with the tilt angle,
and can be further refined on a leg-by-leg basis using the techniques
defined in Procedures
to Improve the Accuracy of Airborne Doppler Radar Data
(Bosart, et al 2002). The "ETHL" (Extended THL)
method works similarly to the original algorithm by iteratively
reducing ground velocity residuals to zero. In cases where the ground
velocity is not expected to be zero (ie. moving ocean surface), or over
complex terrain, the "BLW" (Bosart-Lee-Wakimoto)
method can be applied, which compares the in situ wind recorded on the
aircraft with the near-aircraft Doppler winds, reducing the discrepancy
between them to produce corrections.
It is strongly recommended that the user ensure
that the ground residual velocities are near zero for the legs of
interest
before performing multiple Doppler analysis. If you have problems or
questions regarding these procedures, please contact Wen-Chau Lee or Michael Bell.