The Global Positioning System (GPS) is a worldwide radio-navigation system formed from a constellation of 24 satellites and corresponding ground stations. Each satellite continually broadcasts its location in space along with the current time from an internal clock. GPS receivers can determine their position to within a few meters by receiving and analyzing signals transmitted from the satellites. To determine its location, a GPS receiver scans for satellite signals until it has acquired signals from three or more satellites. Two-dimensional locations can be determined by analyzing signals from three satellites, and three-dimensional locations can be determined by analyzing signals from four or more satellites. A GPS receiver determines its location by determining its distance from the GPS satellites based on the received signals and then triangulating these distance measurements. Commercial GPS receivers can determine their locations to an accuracy of 10 meters or less with 95 percent reliability.
Although the current GPS has been successful, it has several shortcomings. For example, GPS satellite signals are subject to errors caused by ionospheric disturbances and satellite orbit discrepancies. Ionospheric and tropospheric refraction can slow satellite signals and cause carriers and codes to diverge. Because ionospheric disturbances vary greatly from location to location, these errors are difficult to correct with civilian-type GPS receivers.
To account for these errors, the FAA developed a Wide Area Augmentation System (WAAS) to improve the accuracy, availability and integrity of the GPS. The WAAS is based on a network of approximately 25 wide area ground reference stations (WRSs) that are linked to cover a service area including the entire U.S. and some areas of Canada and Mexico. Each of the WRSs has been precisely surveyed so that its exact location is known. Signals from GPS satellites are received and analyzed by the WRSs to determine errors in the signals, including errors caused by the ionospheric disturbances described above. Each WRS in the network relays its data to a wide area master station (WMS) where correction information is computed. The WMS calculates correction messages for each GPS satellite based on correction algorithms and assesses the overall integrity of the system. The correction messages are then uplinked to a pair of Geostationary Communication Satellites (GEOs) via a ground uplink system (GUS). The GEOs then broadcast the messages on the same frequency as GPS (L1, 1575.42 MHz) to GPS receivers within the coverage area of the WAAS. GPS receivers may then utilize the WAAS correction data to correct for GPS satellite signal errors caused by ionospheric disturbances and other inaccuracies. The communications satellites also act as additional navigation satellites for the GPS receivers, thus, providing additional navigation signals for position determination.
One type of information that is included in the correction messages from the GEOs is ionospheric correction data. Ionospheric corrections are broadcast for selected ionospheric grid points generally spaced at 5 degree intervals in both latitude and longitude directions. One approach is to store the correction points in a two dimensional array containing a total of 2,592 elements [(360 degrees longitude divided by 5 degrees) times (180 degrees latitude divided by 5 degrees)]. Many GPS receivers, including, for example, GPS receivers used in avionics applications and portable GPS receivers used for recreational and sport applications have limited memory and processing power and therefore cannot quickly and efficiently store and process all 2,592 ionospheric grid point correction elements. Moreover, even for GPS receivers that have sufficient memory to store all the ionospheric grid point correction data, much of the memory required to do so is wasted because only the correction points within a certain distance of a GPS receiver's current location are relevant to the ionospheric conditions at that location.
Accordingly, there exists a need for an improved method and system for receiving ionospheric grid point corrections from WAAS satellites. Moreover, there is a need for such a method and system that benefits from the WAAS data while utilizing a minimal amount of memory and system resources.