|
Study on accuracy of GPS for its application in SAR interferometry
Parag S. Narvekar, K. S. Rao
Center for Studies in Resources Engineering
Indian Institute of Technology Bombay
parag_narvekar@yahoo.com
Abstract
One of the main applications of GPS is to find out the exact location of any point of interest almost anywhere on the globe. For some of the application this information is needed very precisely like for SAR Interferometry. Space borne synthetic Aperture Radar (SAR) interferometry is an exciting and powerful technique that has gained prominence in recent years through the ERS-1 satellite mission. Its potential is illustrated by recent examples of application to detection of centimetre-level surface displacements and production of wide area digital elevation models (DEMs), studies are going on to improve the accuracy to millimeter level. The position information obtained from the GPS can be used for improving the accuracy of ground control points in SAR system. Also the heights at particular locations can be compared with the heights of same locations in DEMs and can be used to improve the accuracy of SAR Interferometry. Thus there is need for obtaining GPS measurements at very high accuracy. In the present work the various sources of inaccuracies in GPS are studied e.g. Multipath, Receivers Noise, Clock error etc. A special study is done on the Atmospheric effects on signals using data of atmospheric parameters (pressure, temperature, humidity etc.) obtained from Indian Meteorological Department Pune and an attempt is made to find out in what extent it can affect the results.
Introduction
GPS is a satellite navigation system designed to provide instantaneous position, velocity and time information almost anywhere on the globe at any time and in any weather condition. Position information i.e. latitude longitude and height obtained from GPS can be compared with the DEMs generated by SAR Interferometry. GPS can be used to provide the Ground Control Points for SAR system and also to study the atmospheric effects on SAR Interferometry by comparing the Atmospheric delay in both the systems. Thus GPS can be used for improving accuracy in SAR Interferometry and there is need to estimate these errors and to bring them at high level of accuracy so that it can be compared with SAR System. Also a recording of a position is not of much use if it is not accompanied by some form of error estimate. There are various sources of inaccuracies are involved in Global Positioning System measurement, some of them are as follows Satellite clocks, Orbit error, Receivers Noise, Multipath, Intentional degradation of the satellite signal — Selective Availability (SA) (The government turned off SA in May 2000), Anti-spoofing (AS), Satellite geometry/shading, Ionospheric Delay, Tropospheric Delay etc.
Theory
There are two types of service available to GPS users, the SPS and the PPS.
SPS
– Standard Positioning Service is the positioning accuracy that is provided by the GPS measurements based on the single L1 frequency C/A code.
PPS – Precise Positioning Service is the highest level of dynamic positioning accuracy that is provided by GPS measurements based on the dual frequency P-code.
The signals, which are generated from a standard frequency of 10.23 MHz, are L1 at 1575.42 MHz and L2 at 1227.60 MHz and are often called as the
carriers. These signals are effected by various errors, some of them are discussed below and methods to tackle this problem are given. Atmospheric errors are discussed separately in Results and Discussion.
Selective Availability (SA)
SA
essentially consists of two different components, known as dither and epsilon. Dither is an intentional manipulation of the satellite clock frequency resulting in the generation of the carrier waves and the codes with varying wavelengths. In other words, under SA, the distance between each C/A code chip will be variable, and no longer the designed 293m (chip length). The replica code generated within the receiver will still assume the chip length to be 293m and pseudorange measurements are based on this. Typical pseudorange errors for satellites with SA imposed are +/-100m.
|
|
