GEOG 862
GPS and GNSS for Geospatial Professionals

Post-computed Ephemerides

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A screen capture of the IGS products table
IGS Product Table

This system is augmented by other tracking networks that produce post-computed ephemerides. Their impetus have been several: the necessity of timely orbital information with more precision than the broadcast ephemeris and the correlation of the terrestrial coordinate systems with the orbital system through VLBI and SLR sites, among others.

Since the monitoring stations are tracking the actual satellites all the time, it is possible this tracking information can be used to produce ephemerides of the actual position of the satellite at the time you were observing it. In other words, given a position on the Earth and given a window of time, it is possible for you to go on the Internet and download from these sites the precise ephemeris, the post-computed ephemeris, of the moment at which you were observing the satellite. It's valuable, of course, because the broadcast ephemeris is not as accurate as the tracking information of the actual satellites you're observing at the time you're observing them. The reason, of course, is because the broadcast ephemeris is predictive. It's uploaded with the idea that it's an approximation of where the settle will be some time later. Much different to have the tracking information of the satellite at exactly the moment that you were tracking it with your receiver.

The following ephemeris categories and the time they are available after tracking (latency) can be found at NOAAs GPS Orbit Data site

Ultra-rapid (iguWWWWD.sp3)
6-hour latency
constrained (no-net rotation, no-net translation) 24-hour file – sp3

Rapid (igrWWWWD.sp3)
13-hour latency
constrained (no-net rotation, no-net translation) 24-hour file – sp3

Final (igsWWWWD.sp3)
12 to 14 day latency
minimally constrained (no-net rotation) 24 hour file - sp3