Open loop tracking of rising GPS radio occultation signals

Ulvi Acikoz, Purdue University

Abstract

GPS radio occultation (RO) is a sounding technique in which a GPS signal from an emitting satellite passes through the Earth’s atmosphere before arriving at the receiver. The signal’s velocity and direction of propagation are altered by the refracting medium. The propagation of the signal through the atmosphere produces a bending angle in the ray path, which is related to index of refraction profile, and subsequently to the electron density in the ionosphere and water vapor and temperature in the troposphere. This bending angle is represented in the received signal as a phase and Doppler frequency in excess of the value predicted from the geometric propagation of the signal. In the lower troposphere, the signal is characterized by rapid phase accelerations and rapid fading of signal amplitude due to the refractivity. Such large accelerations cannot be measured accurately with the traditional close loop (CL) receiver in which a local carrier phase is adjusted to minimize the error using a feedback controlled phase lock loop (PLL). To overcome this problem, the open loop (OL) method was developed. In OL method, instead of the feedback control loops, the local carrier phase is derived from a pre-defined model, the predicts the Doppler frequency between the transmitter and the receiver. This thesis extends the OL tracking technique to have the ability of tracking rising GPS radio occultations. Rising occultations present a particular challenge in that the weaker signal with the strongest fading appears before the stronger signal. For rising occultations the backward OL tracking method was developed and implemented to a Linux based software receiver. The method was demonstrated on GPS radio occultations recorded 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 and October 2010. Rising GPS radio occultations were successfully tracked to lower elevations using OL tracking, than was possible with a high-quality commercial receiver. On three rising occultations, OL tracking extracted signal phase down to approximately -4.4, -4.2 and -4.7 degrees elevation. The performance of OL tracking is compared to a survey grade receiver. The error was found to be less than 1 mm/s, which is within the 5 mm/s limit for occultation measurement.

Degree

M.S.E.

Advisors

Garrison, Purdue University.

Subject Area

Aerospace engineering|Atmospheric sciences|Remote sensing

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