GPS for Dummies.pdf - Engineering Surveyor

56Part II: All About GPSCommercial transportation modelsThese GPS receivers, not designed to be handheld, are installed in aircraft,ships, boats, trucks, and cars. They provide navigation information appropriateto the mode of transportation. These receivers may be part of anAutomated Position Reporting System (APRS) that sends the vehicle’s locationto a monitoring facility.Trimble Navigation (www.trimble.com) is one of the biggest players in thenonconsumer GPS receiver market. If you’re interested in discovering howcommercial and higher-end GPS units work and the features they support,check out the Trimble Web site.How accurate is a GPS receiver?According to the government and GPS receiver manufacturers, expect yourGPS unit to be accurate within 49 feet (that’s 15 meters for metric-savvyfolks). If your GPS reports that you’re at a certain location, you can be reasonablysure that you’re within 49 feet of that exact set of coordinates.GPS receivers tell you how accurate your position is. Based on the quality ofthe satellite signals that the unit receives, the screen displays the estimatedaccuracy in feet or meters. Accuracy depends on Receiver location Obstructions that block satellite signalsEven if you’re not a U.S. government or military GPS user, you can get moreaccuracy by using a GPS receiver that supports corrected location data.Corrected information is broadcast over radio signals that come from either Non-GPS satellites Ground-based beaconsTwo common sources of more accurate location data are Differential GPS (DGPS) Wide Area Augmentation System (WAAS)I cover both DGPS and WAAS in the following section on GPS receiver features.Table 3-1 shows the accuracy you can expect from a GPS receiver. Thesenumbers are guidelines; at times, you may get slightly more or less accuracy.