Navigation

Defense (DOD) is responsible for operating the GPS satellite constellation and monitors the GPS satellites to ensure proper operation. 3 3 0 ° The status of a GPS satellite is broadcast as part of the data message transmitted by the satellite. GPS status information N 3 6 33 E is also available from the U.S. Coast Guard navigation 30 21 information service at (703) 313-5907 or online at www. W 51 42 12 S navcen.uscg.gov. Additionally, satellite status is available through the NOTAM system. The GPS receiver verifies the integrity (usability) of the signals received from the GPS constellation through receiver autonomous integrity monitoring (RAIM) to determine if a satellite is providing corrupted information. At least one satellite, in addition to those required for navigation, must be in view for the receiver to perform the RAIM function; 3 thus, RAIM needs a minimum of five satellites in view or 3 0 ° four satellites and a barometric altimeter (baro-aiding) to detect an integrity anomaly. For receivers capable of doing N 3 6 33 E so, RAIM needs six satellites in view (or five satellites with 30 21 baro-aiding) to isolate the corrupt satellite signal and remove W 51 42 12 S it from the navigation solution. Baro-aiding is a method of augmenting the GPS integrity solution by using a nonsatellite input source. GPS derived altitude should not be relied upon 3 4 0 to determine aircraft altitude since the vertical error can ° b e be quite large and no integrity is provided. To ensure that a rin baro-aiding is available, the current altimeter setting must be g to entered into the receiver as described in the operating manual. s ta tio n RAIM messages vary somewhat between receivers; however, generally there are two types. One type indicates that there are not enough satellites available to provide RAIM integrity Figure 16-39. ADF tracking. monitoring and another type indicates that the RAIM integrity monitor has detected a potential error that exceeds the limit for the current phase of flight. Without RAIM capability, the The GPS is a satellite radio navigation and time dissemination pilot has no assurance of the accuracy of the GPS position. system developed and operated by the U.S. Department of Defense (DOD). Civilian interface and GPS system status Selective Availability is available from the U.S. Coast Guard. Selective Availability (SA) is a method by which the accuracy It is not necessary to understand the technical aspects of of GPS is intentionally degraded. This feature is designed GPS operation to use it in VFR/IFR navigation. It does differ to deny hostile use of precise GPS positioning data. SA was significantly from conventional, ground-based electronic discontinued on May 1, 2000, but many GPS receivers are navigation and awareness of those differences is important. designed to assume that SA is still active. Awareness of equipment approvals and limitations is critical to the safety of flight. The baseline GPS satellite constellation consists of 24 satellites positioned in six earth-centered orbital planes The GPS navigation system broadcasts a signal that is used with four operation satellites and a spare satellite slot in by receivers to determine precise position anywhere in the each orbital plane. The system can support a constellation world. The receiver tracks multiple satellites and determines of up to thirty satellites in orbit. The orbital period of a GPS a pseudorange measurement to determine the user location. satellite is one-half of a sidereal day or 11 hours 58 minutes. A minimum of four satellites is necessary to establish an The orbits are nearly circular and equally spaced about the accurate three-dimensional position. The Department of equator at a 60-degree separation with an inclination of 16-31