pages: 1: General & Requirements |
Usage & more Technical Info
There are more and more GPS approaches being added to the IAPs
(Instrument Approach Procedures) on almost a daily basis. At this
time, in real world flying, the first of the precision GPS approaches
are being put into service and tested in the area of Oshkosh, Wisconsin.
There are already of approved non-precision GPS approaches in use
around the US. Some say that the GPS system will mean the end of
the VOR navigation system. Well, that's what "they" said about NDBs
when the VOR was introduced and we've still got NDBs.
From 1-1-22. GLOBAL POSITIONING SYSTEM (GPS)
- The GPS is a United States satellite based radio navigational,
positioning, and time transfer system operated by the Department
of Defense (DoD). The system provides highly accurate position
and velocity information and precise time on a continuous global
basis to an unlimited number of properly equipped users. The
system is unaffected by weather and provides a worldwide common
grid reference system based on the earth fixed coordinate system.
For its earth model, GPS uses the World Geodetic System of 1984
- GPS provides two levels of service: Standard Positioning Service
(SPS) and Precise Positioning Service (PPS). SPS
provides, to all users, horizontal positioning accuracy of 100
meters with a probability of 95 percent and 300 meters with
a probability of 99.99 percent. PPS is more accurate than SPS;
however, this is limited to authorized U.S. and allied military,
federal government, and civil users who can satisfy specific
- GPS operation is based upon 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 course/ acquisition (CA) code,
which contains information on the satellite's position, the
GPS system time, its clock error, and the health and accuracy
of the transmitted data. GPS satellites have very accurate atomic
clocks in order to calculate signal travel time. 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 GPS receiver matches each satellite's CA 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, an ionospheric delay, and time disparities between
the atomic clocks in the satellites and the GPS receiver.
- 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's signal transmits
ephemeris 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 the position information
supplied by the satellite, the GPS receiver mathematically determines
its position by triangulation. The GPS receiver needs at least
three satellites with timing corrections from a fourth satellite
to yield an unaided, unique, and true 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.
- 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 the best four satellites
above its horizon, adding signals from one as it drops signals
from another, to continually calculate its position.
- The GPS receiver verifies the integrity of the signals received
from the GPS constellation through receiver autonomous integrity
monitoring (RAIM) by determining 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 5 satellites
in view, or 4 satellites and baro-aiding to work. 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 solution
equation by using a nonsatellite input source. Baro-aiding uses
the pressure altitude corrected for the local barometric pressure
setting to provide accurate altitude information to the GPS
- The Department of Defense declared initial operational capability
(IOC) of the U.S. Global Positioning System (GPS) on December
8, 1993. The Federal Aviation Administration (FAA) has granted
approval for U.S. civil operators to use GPS equipment as a
primary means of navigation in oceanic airspace and certain
remote areas. GPS equipment may be used as a supplemental means
of IFR navigation for domestic enroute, terminal operations,
and certain instrument approach procedures (IAPs). This approval
permits the use of GPS in a manner that is consistent with current
- GENERAL REQUIREMENTS
- General Requirements: Authorization to conduct any GPS operation
under IFR requires that:
- The GPS navigation equipment used must be approved in accordance
with the requirements specified in TSO C-129, or equivalent,
and the installation must be made in accordance with Notice
8110.47 or 8110.48, the equivalent Advisory Circular or the
Flight Standards/Aircraft Certification (AFS/AIR) joint guidance
memorandum dated July 20, 1992. Equipment approved to TSO
C-115a do not meet the requirements of TSO C-129.
- Aircraft using GPS equipment under IFR must be equipped
with an approved and operational alternate means of navigation
appropriate to the flight. Active monitoring of the alternative
navigation equipment is not required if the installation uses
RAIM for integrity monitoring. For these systems, active monitoring
by the flightcrew is only required when the RAIM capability
of the GPS equipment is lost.
- Procedures must be established for use in the event that
the loss of RAIM capability is predicted to occur. In situations
where this is encountered, the flight must rely on other approved
equipment, delay departure, or cancel the flight.
- The GPS operation must be conducted in accordance with the
FAA-approved aircraft flight manual (AFM) or flight manual
- Aircraft navigating by GPS are considered to be RNAV
aircraft. Therefore, the appropriate equipment suffix must
be included in the ATC flight plan.
- Prior to any GPS IFR operation the pilot should review the
appropriate NOTAMs. NOTAMs will be issued to announce outages.
Pilots may obtain these NOTAMs from FSS briefers upon request.
- Air carrier and commercial operators conducting GPS IFR
operations shall meet the appropriate provisions of their
approved operations specifications.
2: Usage & more Technical Info