GAMIT/GLOBK is a comprehensive GPS analysis package developed at MIT and Scripps for the estimation of three-dimensional relative positions of ground stations and satellite orbits. The software is designed to run under any UNIX operating system supporting XWindows. The primary output of GAMIT is a loosely constrained solution (H-) file of parameter estimates and covariances which can be passed to GLOBK for combination of data to estimate station positions and velocities and orbital and Earth-rotation parameters.
UNIT 1 - introduction to GPS
For a short introduction to GNSS, I recommend watching the video below given by Thomas Herring part of the annual SAGE workshops organized by Earthscope.
GPS SIGNAL
Each GPS satellite broadcast signals on at least two carrier frequencies for positioning purposes:
- L1 signal (carrier frequency of 1,575.42 MHz)
Modulated onto the L1 carrier are two pseudo-random noise (PRN) ranging codes and the navigation (broadcast) message. The codes (used to determine the pseudo-ranges) are:
- the 1 millisecond-long C/A-code (chipping rate about 1 MHz);
- the weeklong segment of the P-code (chipping rate about 10 MHz).
The navigation (broadcast) message includes orbital information (ephemeris), the offset $\delta t$ of the satellite clock from the GPS system time, information on the health of the satellite and the expected accuracy of the range measurements (UERE). The message contains also almanac data for other satellites (used by the receiver to determine the location of each satellite). For receivers that track the weeklong P-code, the broadcast message includes a special hand-over word (HOW), that tells the receiver where in the P-code to start searching.