GPS Solution - About Real-Time Base Station

Since GPS signal errors tend to be quite similar over wide geographic areas, there are obvious advantages to having a single base station serve for any roving stations in that area. Put another way, one can think of very few, if any, reasons for each of 28 GPS users, who are in reasonably close proximity to each other, to collect and rebroadcast identical correction data. This obvious fact, plus the entrepreneurial nature of American society, has produced GPS differential correction services, wherein a user contracts for equipment and the right to receive corrections to the raw GPS signal. Users who are sufficiently close to a U.S. Coast Guard GPS beacon may pick up a signal at no cost besides that of the receiving equipment. Such stations are sometimes referred to as Continually Operating Reference Stations, CORS.

The problem with “centrally located” base stations is that there may not be a signal from one where you are trying to take data. While there are more and more base stations every month, vast gaps exist where their signals do not go. A GPS Solution to this problem is a system in which the broadcast of the correction data comes from a communications satellite. This means that, for a large portion of the Earth’s surface, the user’s receiver is never out of range in the area covered by the satellite, though local topography may block the signal. The satellite of one system that currently employs this method (OMNISTAR) might be as low as 30° above the horizon in northern parts of the United States. The satellite broadcasts a straight-line signal of about 4000 megahertz. The area covered is all of the United States (except parts of Alaska) and parts of Canada and Mexico. Another system that provides a similar service is Racal Landstar. Both systems provide coverage over large parts of the earth.

OMNISTAR has about a dozen base stations located on the periphery of the United States. These stations transmit data regarding the errors in the GPS signals in their areas to a central network control center. These data are then analyzed and repackaged for transmission to the communications satellite. The broadcast from the satellite is such that the data from the several stations can be tailored to the user’s position. How does the OMNISTAR system know where the user is? The user’s GPS unit tells its coordinates to the attached OMNISTAR radio, which then decodes the signal from the communications satellite to provide the proper corrections for the local area. Since real-time differential correction information is useless unless it arrives at the GPS receiver within moments of the time a fix is taken by the receiver, you can see that a lot has to happen in an extremely short amount of time. (A GPS signal code leaves a GPS satellite; it is received by your receiver and by the OMNISTAR base stations; the base stations transmit the signal to a central location where it is processed and sent to the communications satellite, where it is resent to the radio next to your GPS receiver; that radio determines the correction your personal tracking device needs and supplies it. Whew!)

More GPS Tracking Solution at http://www.jimilab.com/ .