GPS Satellite Performance

GPS Performance Assessment File (PAF) and Prediction Support File (PSF) data are created daily by AGI and placed on our FTP servers. This plots on this page help to visualize and understand GPS constellation error behaviors. The GPS constellation exhibits errors in its ephemeris (position in space) and clock data that translate into positioning errors in GPS receivers.

These plots represent the PAF and PSF data available from:

For both graphs, the days are listed as JJJ:MM/DD/YYYY, where JJJ is the julian day, MM is the month, DD is the day of the month and YYYY is the year.

Performance Assessment File (PAF) Data

The PAF data is created by gathering the truth ephemeris and clock information (in .SP3 format) for a given day and differencing that data from the propagated broadcast ephemeris and clock information provided in the RINEX navigation file for the same day.  Both the SP3 and the RINEX navigation files are produced by the National Geospatial-Intelligence Agency (NGA) and available here.  The antenna phase center SP3 file is used to calculate the ephemeris and clock truth data.

For the PAF file graph, select which type of data to view, which PRNs to view (selecting multiple allowed) and which year and date to view (selecting multiple dates within a year allowed).  Click the update graphs button to see the data. The initial view of the PAF data contains ephemeris errors for all PRNs for the latest day data is available.  Note that each PRN will have it's own color and symbol regardless of how many PRNs are plotted.  For example, PRN 3 will always be a red triangle .


Prediction Support File (PSF) Data

The plotted values in the PSF graph are seven-day RMS's of the user range error (URE). The UREs are calculated using the Signal-In-Space (SIS) GOSPAR global URE formula, utilizing the data from each PAF file.  The seven-day RMS UREs for a specific PRN are constant for a given day, so the RMS URE values are plotted in a bar graph showing a three-day history of these values for each PRN.  Choose any day for the PSF graph, press the Update Graphs button and a three day history will be plotted.