GPS Precision point solution

Project name:		GPS Precision point solution
Language:		VB6, (also an ANSI C maybe available if required)
Target platform:		PC (C version would be suitable for porting)
Abstract:		This software module processes raw GPS data using the precision SP3 and IONEX files from the JPL (Jet Propulsion Laboratory) in the USA, to calculate the antenna position with a greater precision than is possible using the standard broadcast data.
Key features and advantages:		The software is a complete solution, it automatically downloads the data that is required from the internet, fits high accuracy polynomials to the data, and uses this information to calculate a PVT (position/velocity/time) solution.

Full Description

• Download the required information from the internet
• Convert the downloaded (ASCII) files into a internal database format
• Use the new information to calculate a PVT solution

Download the required information from the internet

There are two types of file required for this phase, firstly the SP3 files which contain the satellite orbit and clock corrections, and the IONEX files which contain the information about delays through the ionosphere. The files can be found from a number of different sources on the internet and at a number of different precision levels depending on how old the data is, SP3 files are available in real time, after 24 hours and 7 days at increasing levels of accuracy. The availability of IONEX data is similar. The download mechanism provides a very simple definition file which details where to download the files from, with the provision to list a number of sources in case one of the servers is not available. Once the data is downloaded, the files are de-compressed and stored locally ready for use.

Convert the downloaded (ASCii) files into a internal database format

Once the data files have been downloaded from the internet and stored locally, they need to be converted into a useful format so they can be used in the GPS solution. The downloaded data is an ASCII table of information, so the first step of the process is to convert the data into an internal binary array. The more complex part of the process is to fit high order mathematical functions to the data to interpolate between points with the maximum accuracy. Fitting the correct functions to the data is an essential part of the PPP process, and this has been extensively validated against other data.

Use the new information to calculate a PVT solution

Once the high accuracy data has been loaded and interpolation functions fitted, it can be used as part of the PVT solution. To enable this functions are available for:

1. FindPrecisionVerticalIonoDelay
   Inputs: the signal piece point, time, Ionex Grid Database()
   Outputs: Vertical delay of the signal through the ionosphere
2. FindIonospherePiecePoint
   Inputs: Satellite azimuth, Satellite elevation, Users position
   Outputs: the signal piece point, the slant factor (this is the multiplier for the vertical delay)
3. GetPreciseSvPos
   (The code also include corrections for the offset of the GPS satellite antenna phase centre relative to it’s centre of mass.)
   Inputs: SV index, time, transmission time
   Outputs: SV Position, SV Clock Error, Sp3Database(), relativistic correction

These functions can typically be used within a standard GPS PVT solution to replace similar function calls that use the standard broadcast information.