Sheehan, Declan Vincent; (1993) Studies of methods of pre-launch testing of satellite radar altimeters. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The radar altimeter operating in a pulse-limited mode has been successful in charting the ocean surfaces of the Earth. The scientific community, in a drive to map rougher terrain, have adopted the same principle. However in order to overcome the problem of slope- induced error, the range window may be widened or narrowed in accordance with the surface roughness. The ERS-1 altimeter included a second range window for operation over ice, but which had to be controlled by macro-command from the ground. The Advanced Terrain-Tracking Altimeter is a prototype altimeter which has an on-board resolution-switching algorithm, allowing the range window to be changed appropriately. This thesis focuses on methods of pre-launch testing of advanced radar altimeters. The early chapters review some of the calibration and testing methods used for the ERS-1 altimeter, presenting a critical assessment of some of the pre-launch methods. The testing procedure for the Adaptive Terrain-Tracking Altimeter is significantly more complex because of the extra resolution-switching algorithm, and a return signal simulator is identified as an essential element in testing the adaptive resolution prior to launch. The core of the thesis therefore describes a novel method of return signal simulation in which sequences of realistic echoes, from all types of surface, are fed in real time to the prototype altimeter, at the appropriate resolution, with the appropriate fading characteristics, and at the appropriate instant in time. Such a simulator is feasible only if the simulated echo is modelled in the deramp domain (i.e range window space) rather than actual delay time. Then the Fourier Transforms of the echoes, rather than the echoes themselves, are calculated at the full pulse repetition frequency and are stored in a memory. The resolution may then be varied by altering the rate at which the echoes are read out of memory. A prototype Return Signal Simulator is built, tested and shown to be capable of testing the Adaptive Terrain-Tracking Altimeter. A test philosophy is defined to assist the testing of the prototype altimeter, which will be undertaken by British Aerospace. A preliminary analysis, using a software implementation of the return signal simulator and realistic echoes, demonstrated that the Model Free Tracker has a superior tracking performance than the generally preferred Offset Centre Of Gravity tracking algorithm. However both algorithms suffer from problems, and these problems are identified. Finally a new approach to the analysis of the effect of chirp phase errors is presented, which leads to a quantitative expression for the height error resulting from chirp phase distortion. Such an approach can be used to apply a correction to the height estimate, unlike previous approaches which could only be used to set a specification for altimeter design.

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