55 degrees relative to the equator. The orbital radius (i.e.  distance from the center of mass of the earth to the satellite)  is approximately 26,600 km.  each type of receiver installation or use must be understood  by the pilot to avoid misusing navigation information. In all  cases, VFR pilots should never rely solely on one system  of navigation. GPS navigation must be integrated with  other forms of electronic navigation, as well as pilotage  and dead reckoning. Only through the integration of these  techniques can the VFR pilot ensure accuracy in navigation.  Some critical concerns in VFR use of GPS include RAIM  capability, database currency, and antenna location.  With the baseline satellite constellation, users with a clear  view of the sky have a minimum of four satellites in view.  It is more likely that a user would see six to eight satellites.  The satellites broadcast ranging signals and navigation data  allowing users to measure their pseudoranges in order to  estimate their position, velocity and time, in a passive, listen- only mode. The receiver uses data from a minimum of four  satellites above the mask angle (the lowest angle above the  horizon at which a receiver can use a satellite). The exact  number of satellites operating at any one particular time  varies depending on the number of satellite outages and  operational spares in orbit. For current status of the GPS  constellation, please visit http://tycho.usno.navy.mil/gpscurr.  html.  [Figure 16-40]  GPS navigation has become a great asset to VFR pilots  providing increased navigation capability and enhanced  situational awareness while reducing operating costs due  to greater ease in flying direct routes. While GPS has many  benefits to the VFR pilot, care must be exercised to ensure  that system capabilities are not exceeded.  Many VFR GPS receivers and all hand-held units are not  equipped with RAIM alerting capability. Loss of the required  number of satellites in view, or the detection of a position  error, cannot be displayed to the pilot by such receivers.  In receivers with no RAIM capability, no alert would  be provided to the pilot that the navigation solution had  deteriorated and an undetected navigation error could occur.  A systematic cross-check with other navigation techniques  would identify this failure and prevent a serious deviation.  In many receivers, an updatable database is used for  navigation fixes, airports, and instrument procedures.  These databases must be maintained to the current update  for IFR operation, but no such requirement exists for VFR  use. However, in many cases, the database drives a moving  map display that indicates Special Use Airspace and the  various classes of airspace in addition to other operational  information. Without a current database, the moving map  display may be outdated and offer erroneous information  to VFR pilots wishing to fly around critical airspace areas,  such as a Restricted Area or a Class B airspace segment.  Numerous pilots have ventured into airspace they were trying  to avoid by using an outdated database. If there is not a current  database in the receiver, disregard the moving map display  when making critical navigation decisions.  Types of receivers used for GPS navigation under VFR are  varied from a full IFR installation being used to support a  VFR flight to a VFR only installation (in either a VFR or IFR  capable aircraft) to a hand-held receiver. The limitations of  In addition, waypoints are added, removed, relocated, or re­ named as required to meet operational needs. When using  GPS to navigate relative to a named fix, a current database  must be used to properly locate a named waypoint. Without  the update, it is the pilot’s responsibility to verify the  waypoint location referencing to an official current source,  such as the Chart Supplement U.S., sectional chart, or en  route chart.  In many VFR installations of GPS receivers, antenna location  is more a matter of convenience than performance. In IFR  installations, care is exercised to ensure that an adequate  clear view is provided for the antenna to communicate with  satellites. If an alternate location is used, some portion of  the aircraft may block the view of the antenna increasing the  possibility of losing navigation signal.  Figure 16-40.  Satellite constellation.  16-32  VFR Use of GPS  RAIM Capability 