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3. GPS operation is based on the concept of
ranging and triangulation from a group of satellites in
space which act as precise reference points. A GPS
receiver measures distance from a satellite using the
travel time of a radio signal. Each satellite transmits
a specific code, called a coarse acquisition (C/A)
code, which contains information on the satellite’s
position, the GPS system 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
2/19/04 AIM
Air Navigation Aids 1−1−25
time, the distance traveled by the signal can be
computed from the arrival time.
4. The GPS receiver matches each satellite’s
C/A code with an identical copy of the code contained
in the receiver’s database. By shifting its copy of the
satellite’s code in a matching process, and by
comparing this shift with its internal clock, the
receiver can calculate how long it took the signal to
travel from the satellite to the receiver. 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. Pseudo−range is subject to several error
sources; for example: ionospheric and tropospheric
delays and multipath.
5. 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.
6. 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, ground speed,
etc., by using the aircraft’s known latitude/longitude
and referencing these to a database built into the
receiver.
7. 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).
8. 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 5 satellites
in view, or 4 satellites and a barometric altimeter
(baro−aiding) to detect an integrity anomaly. For
receivers capable of doing so, RAIM needs
6 satellites in view (or 5 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. To ensure that baro−aiding is available, the
current altimeter setting must be entered into the
receiver as described in the operating manual.
9. 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.
10. The DOD declared initial operational
capability (IOC) of the U.S. GPS on December 8,
1993. The 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 en route, terminal
operations, and certain instrument approach procedures
(IAPs). This approval permits the use of GPS
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