{"id":13527,"date":"2017-06-09T08:01:34","date_gmt":"2017-06-09T13:01:34","guid":{"rendered":"https:\/\/wp.stolaf.edu\/it\/?page_id=13527"},"modified":"2025-06-10T08:25:21","modified_gmt":"2025-06-10T13:25:21","slug":"gis-gps-errors","status":"publish","type":"page","link":"https:\/\/wp.stolaf.edu\/lits\/gis-gps-errors\/","title":{"rendered":"Sources of Error"},"content":{"rendered":"

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)<\/a> correct for.\u00a0 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.\u00a0 This delay interferes with the range solutions from the GPS receiver on the ground to the satellite, resulting in positional errors of several meters.\u00a0 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.\u00a0 Similarly, local space conditions\u2014especially solar output\u2014can affect the GPS signal.\u00a0 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.<\/p>\n

However, WAAS does not correct for other common sources of GPS error, such as GPS points collected during a cold start of the receiver.\u00a0 Receivers turned on after being off for several days (or even hours), or moved more than 500 miles use outdated satellite ephemeris data, which initially can cause poor position solutions until the almanac is updated through the GPS signal.<\/p>\n

Likewise multi-path error, which is interference caused by signal reflection off surfaces near the receiver, is a common problem as well.\u00a0 It is especially prevalent in urban environments and under thick tree canopies.\u00a0 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.\u00a0 The major sources of GPS positional error are:<\/p>\n