Figure 16-39.  ADF tracking.  The GPS is a satellite radio navigation and time dissemination  system developed and operated by the U.S. Department of  Defense (DOD). Civilian interface and GPS system status  is available from the U.S. Coast Guard.  It is not necessary to understand the technical aspects of  GPS operation to use it in VFR/IFR navigation. It does differ  significantly from conventional, ground-based electronic  navigation and awareness of those differences is important.  Awareness of equipment approvals and limitations is critical  to the safety of flight.  The GPS navigation system broadcasts a signal that is used  by receivers to determine precise position anywhere in the  world. The receiver tracks multiple satellites and determines  a pseudorange measurement to determine the user location.  A minimum of four satellites is necessary to establish an  accurate three-dimensional position. The Department of  Selective Availability (SA) is a method by which the accuracy  of GPS is intentionally degraded. This feature is designed  to deny hostile use of precise GPS positioning data. SA was  discontinued on May 1, 2000, but many GPS receivers are  designed to assume that SA is still active.  The baseline GPS satellite constellation consists of 24  satellites positioned in six earth-centered orbital planes  with four operation satellites and a spare satellite slot in  each orbital plane. The system can support a constellation  of up to thirty satellites in orbit. The orbital period of a GPS  satellite is one-half of a sidereal day or 11 hours 58 minutes.  The orbits are nearly circular and equally spaced about the  equator at a 60-degree separation with an inclination of  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  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  pilot has no assurance of the accuracy of the GPS position.  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;  thus, RAIM needs a minimum of five satellites in view or  four satellites and a barometric altimeter (baro-aiding) to  detect an integrity anomaly. For receivers capable of doing  so, RAIM needs six satellites in view (or five satellites with  baro-aiding) to isolate the corrupt satellite signal and remove  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  to determine aircraft altitude since the vertical error can  be quite large and no integrity is provided. To ensure that  baro-aiding is available, the current altimeter setting must be  entered into the receiver as described in the operating manual.  Defense (DOD) is responsible for operating the GPS satellite  constellation and monitors the GPS satellites to ensure proper  operation.  The status of a GPS satellite is broadcast as part of the data  message transmitted by the satellite. GPS status information  is also available from the U.S. Coast Guard navigation  information service at (703) 313-5907 or online at www.  navcen.uscg.gov. Additionally, satellite status is available  through the NOTAM system.  16-31  Selective Availability  330 °  33  30  24 21  15$^{12}$ 6 3  W  S  E  N  330 °  33  30  24 21  15$^{12}$ 6 3  W  S  E  N  340° bearing to station 