surveying iii (topographic and geodetic surveys) - Modern Prepper

5-7. Pseudorange. A pseudorange is the time delay between the satellite clock and the receiver clock,as determined from C/A- or P-code pulses. This time difference equates to the range measurement but iscalled a pseudorange since at the time of the measurement, the receiver clock is not synchronized to thesatellite clock. In most cases, an absolute 3D real-time navigation position can be obtained by observingat least four simultaneous pseudoranges.5-8. Carrier Phase Measurements. Carrier frequency tracking measures the phase differencesbetween the Doppler-shifted satellite and receiver frequencies. The phase differences are continuouslychanging due to the changing satellite earth orbit geometry. However, such effects are resolved in thereceiver and subsequent data postprocessing. When carrier phase measurements are observed andcompared between two stations (differential mode), 3D baseline vector accuracy between the stationsbelow the centimeter level is attainable. New receiver technology and processing techniques haveallowed for carrier phase measurements to be used in real-time centimeter positioning.5-9. Broadcast Messages and Ephemeris Data. Each NAVSTAR GPS satellite periodicallybroadcasts data concerning clock corrections, system and satellite status and, most critically, position orephemeris data. There are two basic types of ephemeris data--broadcast and precise.a. Broadcast Ephemerides. Broadcast ephemerides are predicted satellite positions broadcast withinnavigation messages (transmitted from satellites in real time). The ephemerides can be acquired in realtime through a receiver capable of acquiring either a C/A- or P-code. Broadcast ephemerides arecomputed using the past tracking data of satellites. The satellites are continuously tracked by themonitor stations to obtain current data for use in orbit predictions. The data are analyzed by the mastercontrol station, and new parameters for the satellite orbits are transmitted back to the satellites. Thisupload is performed daily, and the new predicted orbital elements are transmitted every hour by thenavigation message.b. Precise Ephemerides. Precise ephemerides are based on actual tracking data that arepostprocessed to obtain more accurate satellite positions. These ephemerides are delayed for processingbut are more accurate than the broadcast ephemerides because they are based on actual tracking data andnot predicted data. Nonmilitary users can obtain this information from the NGS or private sources thatmaintain their own tracking networks. For most survey applications, broadcast ephemerides areadequate to obtain needed accuracies.EN0593 5-4