GPS signal propagation is significantly affected by travel through the atmosphere, and such errors are one of the main GPS error factors that Wide Area Augmentation System (WAAS) and other Satellite-Based Augmentation Systems (SBAS) correct for. As GPS signals travel down to the Earth from space, the layers of the atmosphere refracts and slightly delays the signals, particularly within the ionosphere. This delay interferes with the range solutions from the GPS receiver on the ground to the satellite, resulting in positional errors of several meters. WAAS corrects for this by determining how the atmosphere is interfering the signal in a region, and then providing realtime correction data to WAAS-enabled receivers via its own satellites. Similarly, local space conditions—especially solar output—can affect the GPS signal. Major space weather events can and do affect WAAS as well, but the FAA (which operates it) has upgraded and hardened WAAS so it is more robust against solar interference.
However, WAAS does not correct for other common sources of GPS error, such as GPS points collected during a cold start of the receiver. Receivers turned on after being off for several days (or even hours), or moved more than 500 miles use outdated satellite ephemeris data, which initially can cause poor position solutions until the almanac is updated through the GPS signal.
Likewise multi-path error, which is interference caused by signal reflection off surfaces near the receiver, is a common problem as well. It is especially prevalent in urban environments and under thick tree canopies. Since the signal reflecting off a surface can increase the distance from the satellite to the receiver, multi-path errors can affect the accuracy of positions by artificially increasing the pseudo-range. The major sources of GPS positional error are:
- Atmospheric Interference
- Calculation and rounding errors
- Ephemeris (orbital path) data errors
- Multi-path effects