INSTRUMENT FLIGHT RULES WORKBOOK
In addition to knowing the distance to a satellite, a receiver needs to know the satellite's exact
position in space; this is known as its ephemeris. Each satellite transmits information about its
exact orbital location. The GPS receiver uses this information to precisely establish the position
of the satellite.
Using the calculated pseudo-range and position information supplied by the satellite, the GPS
receiver mathematically determines its position by triangulation. 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 such as distance and bearing to a
waypoint, groundspeed, etc., by using the aircraft's known latitude/longitude and referencing
these to a database built into the receiver.
The GPS constellation of 24 satellites is designed so that a minimum of five are always
observable by a user anywhere on earth. 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
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 non-satellite input source. GPS derived altitude should not be relied upon to determine aircraft
altitude since the vertical error can be quite large. To ensure baro-aiding is available, the current
altimeter setting must be entered into the receiver as described in the operating manual.
RAIM messages vary somewhat between receivers; however, generally there are two types. One
type indicates there are not enough satellites available to provide RAIM integrity monitoring and
another type indicates the RAIM integrity monitor has detected a potential error exceeding the
limit for the current phase of flight. Without RAIM capability, the pilot has no assurance of the
accuracy of the GPS position.
The DOD declared initial operational capability (IOC) of the U.S. GPS on December 8, 1993.
The Federal Aviation Administration (FAA) has granted approval for U.S. civil operators to use
properly certified GPS equipment as a primary means of navigation in oceanic airspace and
certain remote areas. Properly certified GPS equipment may be used as a supplemental means of
IFR navigation for domestic enroute, terminal operations, and certain instrument approach
procedures (IAP). This approval permits the use of GPS in a manner consistent with current
navigation requirements as well as approved air carrier operations specifications.
1-24 INTRODUCTION TO AIRBORNE NAVIGATION AND COMMUNICATIONS
EQUIPMENT AND PRINCIPLES OF OPERATION