Global Positioning System (GPS) -Cont.

CHAPTER ONE

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

satellite).

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