The United States Department of Defence (DoD) operates a reliable, 24 hour a day, all weather Global Positioning System (GPS). NAVSTAR (NAVigation Satellite Time And Ranging), the original name given to this geographic positioning and navigation tool, includes an official constellation of 24 satellites (plus active spares) orbiting the Earth at an altitude of approximately 22,000 km. These satellites transmit various coded information to GPS users on different UHF frequencies. This information allows user equipment to calculate a range to each satellite. GPS is essentially a timing system -- ranges are calculated by timing how long it takes for the coded GPS signal to reach the user's GPS antenna. To calculate a geographic position, the GPS receiver uses a complex algorithm incorporating satellite coordinates and ranges to each satellite. Reception of any four or more of these signals allows a GPS receiver to compute 3D coordinates. Tracking of only three satellites reduces the position fix to 2D coordinates (horizontal with fixed vertical). The GPS receiver calculates its position with respect to the phase center of the GPS antenna. The latitude, longitude, and altitude of the antenna are referenced according to the current revision of the World Geodetic System 1984 ellipsoid WGS 84.
Global Navigation Satellite System or 'GLONASS', is a space-based satellite navigation system operated by the Russian Aerospace Defense Forces and offers global coverage. By adding GLONASS, more satellites are available so positions can be fixed more quickly and accurately, especially in built-up areas where the view to some GPS satellites is obscured by buildings. By adding the multi-constellation tracking network utilizing all satellites in view, the result is increased productivity and enhanced workflow in any location, yielding unprecedented reliability. BlueStarGPS supports regional and global satellite systems, including Both GPS and GLONASS.
There are various sources of errors that alter the accuracy of GPS receivers and various ways to compensate for them. The primary sources of errors that now degrade GPS performance include atmospheric errors, geometry of the satellites, multipath, timing and satellite orbit errors.