Worldwide DGPS Services

OmniSTAR offers GPS correction services that can improve the accuracy of a GPS receiver by more than 100 times. Currently, three levels of service are offered: "VBS" offers sub-meter positioning, "XP" achieves better than 20 centimeters, and "HP" is better than 10 centimeters. These services are available in various combinations in most land areas of the world.
What is a "DGPS" System?
OmniSTAR(tm) VBS is a Differential GPS (DGPS) System. It is capable of improving regular GPS to sub-meter accuracy. GPS computes a user's position by measuring ranges (actually, pseudo-ranges, which are ranges that are derived by an iterative process) to three or more GPS satellites simultaneously. The uncorrected position may be in error by 5 or more meters from the true position.
A DGPS System generates corrections for all non-local GPS errors. This is accomplished by the use of one or more GPS "Base Stations" that measure the errors in the GPS pseudo-ranges and generates corrections. A "real-time" DGPS System not only generates the corrections, but also provides some methodology for getting those corrections to users as quickly as possible. This always involves some type of wireless transmission system. It may employ VHF systems for short ranges (FM Broadcast), low frequency transmitters for medium ranges (Beacons) and geostationary satellites (OmniSTAR) for coverage of entire continents.
The method of generating corrections is similar in most DGPS systems. A GPS base station tracks all GPS Satellites that are in view at its location. The internal software is given the precise surveyed location of the base station antenna, and it can calculate the location in space of all GPS satellites at any time by using the ephemeris data that is broadcast from GPS satellites. From these two pieces of information, an expected range to each satellite can be computed for any time. The difference between that computed range and the measured range is the range error. If that information can be transmitted to other nearby users, they can use those values as corrections to their own measured GPS ranges to the same satellites.