Atmospheric signal delay affecting GPS measurements made by space vehicles during launch, orbit and reentry

In this thesis, I present neutral atmosphere, ionosphere and total delays experienced by GPS signals traveling to space vehicles during launch, orbit and reentry. I calculate these delays for receivers at 0 km to 1700 km altitude by ray-tracing through the Global Reference Atmosphere Model (1999) and the International Reference Ionosphere (2001). These delays are potentially much larger than those experienced by signals traveling to GPS receivers near the surface of the Earth, but are primarily experienced at negative elevation angles, and are therefore most relevant for space vehicles with limited visibility of GPS satellites and during launch and reentry. I compare these signal delays to the delays predicted by three onboard delay models: the Altshuler and NATO neutral atmosphere delay models, and the Klobuchar ionosphere delay model. I find that these models are inadequate when the space vehicle is in orbit. The NATO model will suffice during the final period of reentry, where it predicts the neutral atmosphere delay to within 1 m of the ray-traced value, but it will not suffice when a satellite is rising or setting. I propose a method to extend the NATO model for receivers at higher altitudes. The Klobuchar model will suffice for most satellites during reentry, but will potentially predict ionosphere delays with errors up to 30 m, and will not suffice when a satellite is rising or setting. I find that a dual frequency GPS receiver will run into many problems if it is designed with nearsurface use in mind. I examine “measure and hold” and “measure and propagate” dual frequency algorithms, which will have errors up to 30 m during orbit and up to 10 m during reentry. I propose a method by which to improve these algorithms for use in a GPS receiver aboard a space vehicle. Thesis Supervisor: Anthony J. Bogner Title: Senior Member of the Technical Staff, Charles Stark Draper Laboratory Thesis Supervisor: Thomas A. Herring Title: Professor of Geophysics [This page intentionally left blank.]

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