DGPS MAX

1.6 DGPS Service ComparisonAs the DGPS MAX offers the use multiple differential services,questions have been raised in regards to which correction source touse when more than one is available. Ultimately, this is a personalchoice, however, this section provides some information that may helpyou choose which service is most appropriate for you. In the end, itmay be best to try each, if possible, to determine which is mostappropriate.There are four scenarios that may occur for multiple DGPS serviceavailability:! WAAS, OmniSTAR, and beacon are each available! WAAS and OmniSTAR are available! WAAS and beacon are available! OmniSTAR and beacon are availableBeacon signals are not affected by a line of sight. In situations wherethere are tall obstacles that may block the line of sight DGPS signal(both WAAS and OmniSTAR), such as buildings or trees, a beaconsignal may be more desirable. However, the value of our newCOAST technology, discussed in Section 1.8, is that outages of theDGPS signal are less of an influence on reception. This improves therobustness of using a line of sight signal in areas of potentialblockage. If robustness to signal acquisition due to line of sight isconsidered a significant issue, beacon DGPS should be considered inreplace of WAAS or OmniSTAR services.All three services use base stations to calculate GPS correction data.Beacons use a single, local base station for corrections, while WAASand OmniSTAR use a wide area network of stations. If there is asignificant distance to the closest beacon (greater than a 200 - 300hundred miles), this will have an effect on positioning accuracy due todiffering environmental conditions between the remote receiver andbase station (spatial decorrelation). In such a case, if the accuracydegradation is not tolerable, WAAS or OmniSTAR should beconsidered. Both WAAS and OmniSTAR use a network of stations,DGPS MAX Reference Manual 18