Using Curve Fitting Method to Improve the Precision of GPS Satellites Position Computed by Almanac Data
Date Issued
2009
Date
2009
Author(s)
Tseng, Yen-Pin
Abstract
In general , the GPS satellite positions computed from Almanac Data are not accurate enough. Their error are about 1~3 km within 1 week. How to improve the position errors is the main object of this thesis..irst we get the Almanac of the 1st day. Then we compute the satellite positions of the next days base on the Almanac(inaccurate satellite position). Also , we compute these ones based on the Ephemeris(accurate satellite position). Let the satellite position errors , are obtained by taking the difference between them. Some interesting trends are found . First , the errors of the first day are about 2~3 km. They will converge slowly to the minimal values about 1km at day4. Then , the errors will diverge. ased on the characters of the position errors , we also find that although the position errors during day1~4 are relative smaller , they are not regular. During day5~8 , the position errors are bigger but the regularity is better . We use the property of the trends , two algorithms to improve the satellite position errors computed by Almanac are developed . In Algorithm I a curve fitting method is used to establish the model parameters of the position errors of day1 This model is extrapolated to day2 for computing the satellite position errors . The estimated satellite positions of day2 are obtainded by adding the day2 positions by Almanac and the errors.n Algorithm II , a curve fitting method is used to establish the best model parameters of the position error of day5. This model is extrapolated to day 66 for computing the satellite position errors. The estimated satellite positions of day6 ard obtained by adding the day6 positions by Almanac and the errors. n the simulation result we can find out whether we use Algorithm I or Algorithm II, Almanac+Error Model Estimation , the position error can be less than 500m , which is better than just by Almanac , also is better than the Almanac which is 4 days ago, the position error is about 1km, to improve the position error of satellite position.
Subjects
GPS
curve fitting
Ephemeris
Almanac
satellite orbit
Type
thesis
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