aircraft receiver/processor can accurately measure the time  each signal takes to arrive at the receiver and, therefore,  determine aircraft position. The control element consists of a network of ground-based  GPS monitoring and control stations that ensure the accuracy  of satellite positions and their clocks. In its present form, it  has five monitoring stations, three ground antennas, and a  master control station. The user element consists of antennas and receiver/processors  on board the aircraft that provide positioning, velocity,  and precise timing to the user. GPS equipment used while  operating under IFR must meet the standards set forth in  Technical Standard Order (TSO) C-129 (or equivalent); meet  the airworthiness installation requirements; be “approved” for  that type of IFR operation; and be operated in accordance with  the applicable POH/AFM or flight manual supplement. An updatable GPS database that supports the appropriate  operations (e.g., en route, terminal, and instrument  approaches) is required when operating under IFR. The  aircraft GPS navigation database contains WPs from the  geographic areas where GPS navigation has been approved  for IFR operations. The pilot selects the desired WPs from  the database and may add user-defined WPs for the flight. Equipment approved in accordance with TSO C-115a, visual  flight rules (VFR), and hand-held GPS systems do not meet  the requirements of TSO C-129 and are not authorized for  IFR navigation, instrument approaches, or as a principal  instrument flight reference. During IFR operations, these  units (TSO C-115a) may be considered only an aid to  situational awareness. Prior to GPS/WAAS IFR operation, the pilot must review  appropriate NOTAMs and aeronautical information. This  information is available on request from an flight service  station (FSS). The FAA does provide NOTAMs to advise  pilots of the status of the WAAS and level of service  available.  GPS operation is based on the concept of ranging and  triangulation from a group of satellites in space that act  as precise reference points. The receiver uses data from a  minimum of four satellites above the mask angle (the lowest  angle above the horizon at which it can use a satellite). The aircraft GPS receiver measures distance from a satellite  using the travel time of a radio signal. Each satellite transmits  a specific code, called a course/acquisition (CA) code, which  contains information about satellite position, the GPS system  Aircraft using GPS navigation equipment under IFR for  domestic en route, terminal operations, and certain IAPs,  must be equipped with an approved and operational alternate  means of navigation appropriate to the flight. The avionics  necessary to receive all of the ground-based facilities  appropriate for the route to the destination airport and any  required alternate airport must be installed and operational.  Ground-based facilities necessary for these routes must also  be operational. Active monitoring of alternative navigation  equipment is not required if the GPS receiver uses RAIM for  integrity monitoring. Active monitoring of an alternate means  Generally, there are two types of RAIM messages. One  type indicates that there are not enough satellites available  to provide RAIM and another type indicates that the RAIM  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. time, and the health and accuracy of the transmitted data.  Knowing the speed at which the signal traveled (approximately  186,000 miles per second) and the exact broadcast time,  the distance traveled by the signal can be computed from  the arrival time. The distance derived from this method of  computing distance is called a pseudo-range because it is not  a direct measurement of distance, but a measurement based  on time. In addition to knowing the distance to a satellite, a  receiver needs to know the satellite’s exact position in space,  its ephemeris. Each satellite transmits information about its  exact orbital location. The GPS receiver uses this information  to establish the precise position of the satellite. Using the calculated pseudo-range and position information  supplied by the satellite, the GPS receiver/processor  mathematically determines its position by triangulation  from several satellites. The GPS receiver needs at least four  satellites to yield a three-dimensional position (latitude,  longitude, and altitude) and time solution. The GPS receiver  computes navigational values (distance and bearing to  a WP, groundspeed, etc.) by using the aircraft’s known  latitude/longitude and referencing these to a database built  into the receiver. 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. 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 a corrupt satellite signal and remove it from the  navigation solution. Function of GPS 9-26