Airborne Doppler Radar Navigation Corrections

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.

For datasets involving complex terrain, such as that over mountainous areas, an improved corrections algorithm was developed which incorporates digital elevation maps into the corrections calculation. The "GRH" (Georgis-Roux-Hildebrand) method is described in Observation of precipitating systems over complex orography with meteorological Doppler radars: A feasibility study (Georgis, et al. 2000).

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.