A Method to Locate Airborne Radar Observation Data

Airborne radar as a new meteorological detecting instrument, whose observation data with high spatial and temporal resolution , plays an important role in the research of strong convective weather fine structure and nowcasting. Airborne radar not only observes the weather system flexibly, also can detect some regions that with difficulty from ground-based radar measurement, like sea. Thus, airborne radar makes up the defect. In recent decades, airborne radar devised by NASA and putted on the unmanned aerial vehicle (uav) to observe weather, compared with manned machine, uav has the advantages of security, stability, and the prolonged flight time. But this kind of emerging weather radar is different from the fixed platform of ground-based radar, which carrying platform is moving ,with relative movement to target, and the instability of vehicle itself can also affect the detection position, giving rise to airborne weather radar data processing with more harder than ground radar data processing. In order to apply detection data effectively to atmospheric science, hydrology, nowcasting, and other fields, this paper utilizes airborne radar (named HIWRAP) developed by NASA Goddard Space Flight Center as a research object, then do some researches on its observation data processing. In this study, the coordinate transformation is an important part of radar data pre-processing, and its precision affect the subsequent complexity and accuracy of processing directly .To estimate the observe location precisely is the key of this study.
 First of all, scanning strategy of the airborne weather radar was analyzed, and the location problems involved in radar observation data processing according to observational process are illustrated clearly, namely the coordinate system for observation and data processing is not consistent. Using the coordinate transformation technique to find the actual location of the radar observation data and put all data information into the same coordinate system.
 Secondly, aiming at the problem of locating observation data, this paper analyzes the detection process and clarified relations in relevant spatial locations. The homogeneous coordinate method is used to accomplish the transformation of data coordinate. In order to put the data in the same coordinate system, the intermediate coordinate systems are presented. Then, the process in detail from radar coordinate system to geodetic coordinate system is analyzed, and the location algorithm of data detected by airborne radar is put forward.
 Finally, using the observation data measured by HIWRAP to make simulation and analysis according to the detection process and map the location result. Subsequently, the problem of long time-consuming as well as the position error in this algorithm are analyzed. For the former problem, a method of reducing the dimension matrix is presented to decrease the computing time. For the latter, the larger error in the location caused by the data nonlinear transformation from the space rectangular coordinate system to the geodetic coordinate system. Consequently, the method transformed from space rectangular coordinate into geodetic coordinate system is discussed , and making a comparison in the direct and iterative method. Through the simulation and analysis find that using the iterative algorithm to accomplish coordinate transformation is more accurate than direct algorithm, and less number of iteration can achieve higher accuracy.

transformation formulation:G=Q₈^-1Q₇^-1Q₆^-1Q₅^-1Q₄^-1Q₃^-1Q₂^-1Q₁^-1F

 where，F is a matrix represented the coordinate under radar coordinate system, it consist of range from radar,rotation angle and incidence angle.

Q₁ is a transformation matrix transformed data coordinates from aircraft coordinate system to radar coordinate system, it includes the spatial position of origin of radar coordinate system relative to the spatial position of aircraft particle.

Q₂Q₃Q₄ are transformation matrix transformed data coordinates from aircraft intermediate coordinate system to aircraft coordinate system, it includes the aircraft attitude parameters( roll angle,pitch angle,track angle and heading).

Q₅Q₆Q₇Q₈are transformation matrix transformed data coordinates from space rectangular coordinate system to aircraft intermediate coordinate system, it includes GPS information(aircraft latitude, longitude and altitude) and geometric parameters of the ellipsoid model based on earth.

G is a matrix represented the observation data coordinate under geodetic coordinate system.