How accurate are GPS receivers, and what affects the accuracy ?
The atmosphere, the ionosphere and the position of your receiver could all affect the GPS accuracy. Also buildings or heavy foliage that obstruct the GPS view (line of sight) of the sky may decrease the position accuracy. GPS provides two levels of service — a Standard Positioning Service (SPS) for general public use and an encoded Precise Positioning Service (PPS) primarily intended for use by the Department of Defense. SPS signal accuracy is intentionally degraded to protect U.S. national security interests. This process, called Selective Availability (SA), controls the availability of the system’s full capabilities. The SPS accuracy specifications, given below, include the effects of SA. SPS provides accuracy’s of (for position, the accuracy with respect to geographic, or geodetic coordinates of the Earth) within: 100 meters (2 drms) horizontal 156 meters (2 Sigma) vertical 300 meters (99.99% prob.) horizontal 340 nanoseconds time (95% prob.) SPS Coverage is continuous and worldwid
