A Comparison of Cloud Motion Winds from ATS 6 Images with Coinciding SMS 1 Winds

Abstract

A procedure has been developed to accurately measure cloud motion winds from the three-axis stabilized geosynchronous Applications Technology Satellite (ATS 6) without having to rely on its attitude sensor measurements. Attitude changes for this satellite were large enough to result in unacceptably large wind speed errors (?10 m s?1) if not properly accounted for. The attitude-angle telemetry data which accompanied each ATS 6 image did not contain sufficient accuracy to account for the attitude changes as observed in the imagery; consequently, a methodology was developed whereby attitude changes between consecutive images (as a function of scan-line number) were accounted for in wind computations by measurements from the imagery itself by measuring the earth-edge displacements between the successive infrared images. In addition, obvious and distracting image distortions caused by variable horizontal offsets between alternate mirror-scan lines were removed using an image matching procedure. Using an interactive video system, cloud motion winds of low-level cumulus were measured from a time sequence of three ATS 6 images (14 July 1974) in which the earth-edge displacement measurements were used to account for the attitude changes. The winds were then compared to coinciding winds measured from the spin-stabilized Synchronous Meteorological Satellites (SMS 1) images. The generally high quality of SMS 1 wind measurements made them an excellent reference against which ATS 6 winds could be compared. The rms magnitude of the wind vector differences for individual small clouds which were identified in both sets of imagery was 1.6 m s?1, and 1.3 m s?1 for wind vector differences derived from wind field values interpolated to 2° grid points. These values are essentially the same as the reproducibility limits for independently produced SMS 1 wind sets measured from the same imagery; therefore we conclude that it is possible to measure cloud motion winds from a three-axis stabilized synchronous satellite that are comparable in accuracy to those currently derived from spin-stabilized satellites of equivalent resolution.