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UCAR-STARS - New Techniques in Signal Analysis
[Background] [Development
Progress]
Scanning radars and vertically-pointing profilers in operational use for meteorology today utilize Doppler shift information to derive the radial (along-beam) component of motion of the scatterers in the sensor's field of view. That motion typically yields important (but incomplete) information on the wind speed and/or fall velocities of the scatterers. However, Doppler techniques also impose limitations on the measurements they obtain - for example through the 'folding' or aliasing of Doppler velocities - and this aliasing can seriously complicate the analysis of radar data in complex atmospheric circulations. In addition, Doppler techniques yield no measure of the component of velocity transverse to the beam, which in most applications would prove extremely valuable.
The limitations cited above in part motivate longstanding research in the U.S. and elsewhere directed toward development of signal analysis techniques that don't depend upon Doppler shift information. This work has yielded a family of such techniques which generally make use of the correlation function behavior of remotely sensed signals as received at two or more closely spaced antennas (see Figure O1). These techniques are able (with varying success) to derive estimates of the component of scatterer velocity transverse to the radar beam.
RAP scientists A. Praskovsky and E. Praskovskaya have conceived a new approach to the analysis of spaced antenna measurements that utilizes auto- and cross-structure functions to derive scatterer motion information. That approach - termed the Structure Function Analysis of Received Signals (UCAR-STARS) - utilizes the time series of power measurements derived from a radar or other remote sensor. In contrast, the spaced antenna methods that utilize correlation function techniques generally rely upon the time series of both power and phase information from the radar system.
Figure O1: Schematic view of a spaced-antenna radar profiling system utilizing four antennas forming four overlapping sample volumes. Clear-air radar returns critical to profiler sampling result from Bragg scatterers as indicated.
Understanding of the UCAR-STARS approach has advanced significantly in FY 2001 under the developers' efforts to document its derivation, assumptions, methodology and early test performance. Two papers covering the formulation of UCAR-STARS methodology have been submitted to Radio Science, and two more describing its potential application for measurement of the transverse wind component and early verification results are nearing completion. At this early stage, its developers have not yet completed a comprehensive comparison documenting UCAR-STARS performance against that of other spaced antenna techniques. A UCAR/University technical committee chaired by R. Serafin has provided oversight and guidance of recent development work.
Early tests of UCAR-STARS methods have yielded informative results and clear direction for follow-on efforts. Development concepts have been outlined to reduce the 30-60 s sample period typically needed for transverse wind component measurements. These concepts are central to development work in the future. As an offshoot of generalized STARS development, A. Praskovsky derived a simple UCAR-STARS variant for estimating the turbulent energy contained within the radial component of the wind as observed by an airborne radar. Preliminary tests of this methodology and comparisons with in situ turbulence measurements have yielded encouraging results.