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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.
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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 Automated
Flight Service Station. The FAA will provide NOTAMs to
advise pilots of the status of the WAAS and level of service
available.
Function of GPS
GPS operation is based on the concept of ranging and
triangulation from a group of satellites in space which 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
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.
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.
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
　
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