GPS Error Budget

The GPS error budget contains the errors that typically exist when your GPS receiver makes a ranging measurement to a GPS satellite.  Some of these errors can be corrected by your receiver, others cannot.  The values list in an error budget are those that remain after your receiver corrects what it can.
Different error sources are usually binned into different categories, such as Signal-In-Space (SIS), Atmosphere or Receiver.  Here's a typical error budget for GPS:

Error Type Error (meters) Segment
Ephemeris 3.0 Signal-In-Space
Clock 3.0 Signal-In-Space
Ionosphere 4.0 Atmosphere
Troposphere 0.7 Atmosphere
Multipath 1.4 Receiver
Receiver 0.8 Receiver
RSS Total (URE) 6.09
Table 1 - GPS Error Budget.  These errors are one-sigma values

All of these error sources are dynamic and modeling is harder for some, easier for others.  The SIS and atmosphere errors are along the line-of-sight from the receiver's antenna to the GPS satellite.  
  • Satellite ephemeris and clock errors are difficult to model, but the Air Force make sure these errors don't get too large. 
  • The ionospheric error is modeled in your receiver by a half-cosine model developed by John A (Jack) Klobuchar and accounts for roughly 50% of the effect of the ionosphere on the navigation signal. The Ionospheric correction model is specified in the GPS Receiver Interface Standard (IS) IS-GPS-200E. The remaining unmodeled ionospheric error is the largest remaining source of error for single-frequency (civilian) users of GPS.  
  • Tropospheric errors may be modeled in your receiver as well, but no standard algorithm is defined in the IS.  Look at your receiver's documentation, or contact the manufacturer to see if it corrects for tropospheric effects.  The error budget value for tropospheric errors in Table 1 assumes that your receiver does not correct for this.
  • Multipath error results from the signal from a GPS satellite being reflected off of multiple surfaces and then back to the receiver - making the apparent range to the satellite longer.  This values is typical, but may be smaller if you are in an area free of reflective surfaces.  This value may be higher if you are in a city or around other reflecting structures.
  • Receiver errors typically come from noise induced by the signal tracking process.  Note that sources of Radio Frequency interference (RFI) are not listed here and can substantially increase your receiver's noise contribution.  RFI would include interference from potential LightSquared emissions.
 This is one GPS error budget, other error budgets exist as well.