Open loop tracking of radio occultation signals from an airborne platform
Abstract
Radio occultation is a remote sensing technique that uses signals from the Global Positioning System (GPS) to determine electron density in the ionosphere, and temperature and water vapor content in the neutral atmosphere. In the lowest part of the atmosphere where there are strong refractivity gradients due to boundary layer structure, the signal may experience rapid phase accelerations, extending beyond the operational range of traditional receiver tracking loops. Open loop (OL) tracking is a method for tracking GPS signals in this situation by replacing a traditional feedback loop with a method that tracks the signal by estimating the deviations from a model-based reference Doppler frequency time history. The open loop (OL) tracking method was originally designed to work using measurements from a stationary receiver, and has been modified to operate using data recorded by a receiver in motion. Occultation data were gathered by the GNSS Instrumentation System for Multistatic and Occultation Sensing (GISMOS) aboard the High-performance Instrumented Airborne Platform for Environmental Research (HIAPER) during flights over the Gulf of Mexico and Southeastern United States in February 2008. Occultations from setting satellites were recorded during five flight days. The OL tracking method was demonstrated on GPS radio occultations recorded during this campaign and was shown to successfully track signals to lower elevations than a high-quality commercial receiver. OL tracking was able to extract continuous phase information down to approximately -3.9, -4.2, and -4.5 degrees elevation relative to the local horizon on three setting occultations observed on 22 February 2008. Finally, the accuracy of OL measurements, averaged over a Fresnel zone, is shown to be on the order of 3 mm/s, which is well within the limit for extracting useful scientific measurements.
Degree
M.S.E.
Advisors
Garrison, Purdue University.
Subject Area
Aerospace engineering|Electrical engineering|Remote sensing
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