|
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.
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.
The OmniSTAR DGPS System was designed with the following objectives: (1) Continental coverage; (2) sub-meter accuracy over the entire coverage area; and (3) a system that automatically corrected a user's GPS position. The first objective dictated that transmission of the corrections had to be from a geostationary satellite.
The methodology developed by OmniSTAR of using corrections from multiple GPS base stations in a user's solution, met the second objective. It was the first widespread use of a "Wide Area DGPS Solution". It is able to use data from a relatively small number of base stations and provide consistent accuracy over very large areas. A unique method of solving for atmospheric delays and weighting of base station data achieves sub-meter capability over the entire coverage area - regardless of the user's location relative to any one base station. This achieves a truly wide-area system with consistent characteristics. A user can take his equipment anywhere within the coverage area (Continent) and get consistent accuracy results, without any intervention or intimate knowledge of GPS or DGPS.
Several GPS Manufacturers currently build models that combine OmniSTAR and GPS in one unit, using a common Antenna. These are high quality GPS receivers that have sub-meter capabilities. All are physically small and can be battery operated. They may be used in Backpack applications or mounted in vehicles or aircraft. These receivers meet the third objective of making the corrections transparent to the user.
The Worldwide OmniSTAR(tm) VBS Network is made up of many permanent Reference Stations (approximately 100 worldwide). These Reference Stations stations track all GPS Satellites above 5 degrees elevation and compute corrections every second. The corrections are in the form of an industry standard message format called RTCM-104, Version II. The corrections are sent to the nearest OmniSTAR(tm) Network Control Center (NCC) via various communication systems. At the NCC these messages are checked, compressed, and formed into packets for transmission up to the OmniSTAR satellite transponders. A packet will contain the latest corrections from each of the area Reference Stations.
All OmniSTAR(tm)-enabled GPS Receivers receive these packets of data from the satellite transponder. The messages are decoded and uncompressed. At that point, the message is an exact duplicate of the data as it was generated at each base station. Then, the atmospheric errors must be corrected. Every base station automatically corrects for atmospheric errors at its location, because it is a part of the overall range error; but the user is likely not at any of those locations, so the corrections are not optimized for the user. Also, the OmniSTAR(tm) system has no information as to each individual's location. If these corrections are to be automatically optimized for each user's location, then it must be done in each user's Omnistar Receiver.
The OmniSTAR(tm) processor uses its location - versus the Reference Station locations, in an inverse distance-weighted least-squares solution. The output of that least-squares calculation is a synthesized RTCM-104 Correction Message that is optimized for the user's location. This technique of optimizing the corrections for each user's location is called the "Virtual Base Station Solution" (VBS). It is this technique that enables the OmniSTAR(tm) user to operate independently and consistently over the entire coverage area without regard to where he is in relation to the base stations.
In most world areas, a single satellite is used by OmniSTAR to provide coverage over an entire Continent - or at least very large geographic areas. In North America, a single Satellite is used, but it needs three separate beams to cover the Continent. The three beams are arranged to cover the East, Central, and Western portions of North America. The same data is broadcast over all three beams, but the user system must select the proper beam frequency. The beams have overlaps of several hundred miles, so the point where the frequency must be changed is not critical. Most recent models will search and select the strongest beam automatically, but older receivers must be manually set to the proper frequency. Islands, Central America (south of Mexico), and South America is covered by a single Satellite (AM-Sat). A single Subscription is available for all the areas covered by this Satellite.
OmniSTAR currently has several high-powered satellites in use around the World. They provide coverage for most of the World's land areas. Subscriptions are sold by geographic area. Any Regional OmniSTAR Service Center can sell and activate subscriptions for any area. They may be arranged prior to traveling to a new area, or after arrival.
Both HP and XP are recent OmniSTAR developments. They were made possible by improvements in GPS receiver hardware and through an extensive OmniSTAR Research and Development program. Both are somewhat similar to the original VBS system, but require dual-frequency GPS receivers and a more extensive and sophisticated Base Station Network, dual-threaded Communications Networks, and unique software.
XP is defined as "better than 20 cms" and HP is "better than 10 cms", 95%CEP. Several GPS manufacturers build OmniSTAR-capable receivers that can run HP or XP, plus VBS as a backup. The selection of XP or HP in a receiver is determined by the "Subscription" sent to a receiver when it is activated for OmniSTAR.
Details of how HP and XP work are proprietary, but if you call us we can supply you with additional information.
Visual
Description
Service
Regions and Coverage Maps | 
Worldwide
DGPS Services