Introduction - Understanding GPS: Principles and Applications - page 13

Safe Flight 21 demonstration projects are in process in several areas within the
United States, including Alaska and the Ohio River Valley.
GPS without augmentation now provides commercial and general aviation
(GA) airborne systems with sufficient integrity to perform nonprecision approaches
(NPA). NPA is the most common type of instrument approach performed by GA
pilots. The FAA has instituted a program to develop NPA procedures using GPS.
This so-called overlay program allows the use of a specially certified GPS receiver in
place of a VHF omnidirectional range (VOR) or nondirectional beacon (NDB)
receiver to fly the conventional VOR or NDB approach. New NPA overlays that
define waypoints independent of ground-based facilities, and that simplify the pro-
cedures required for flight, are being put into service at the rate of about 500 to
1,000 approaches per year and are almost complete at the 5,000 public use airports
in the United States. Other countries are implementing such procedures, and there is
almost universal acceptance of some sort of GPS approach capability at most of the
world's major airports.
In 2003, the FAA declared WAAS operational for instrument flight operations.
WAAS broadcasts on the GPS L1 frequency so that signals are accessible to GPS
receivers without the need for a dedicated DGPS corrections communications link.
The performance of this system is sufficient for NPA and new types of vertically
guided approaches that are only slightly less stringent than Category I precision
approach. Further information regarding WAAS is provided in Chapter 8. Other
SBASs [e.g., EGNOS, Multifunctional Transport Satelllite (MTSAT) Satellite Aug-
mentation System (MSAS), and GPS and GEO Augmented Navigation (GAGAN)]
are being fielded or considered to provide services equivalent to WAAS in other
regions of the world and are described in Chapter 8.
DGPS is necessary to provide the performance required for vertically guided
approaches. Traditional Category I, II, and III precision approaches involve guid-
ance to the runway threshold in all three dimensions. Local area differential correc-
tions, broadcast from an airport-deployed ground-based augmentation system
(GBAS) reference station (see Chapter 8), are anticipated to meet all requirements
for even the most demanding (Category III) approaches. Also, as GALILEO is
deployed, the use of GNSS by aviation for en-route, approach, and landing is
expected to become even more widespread.
1.9.3 Space Guidance
GPS enables various functions for spacecraft applications. These include attitude
determination (i.e., heading, pitch, and roll), time synchronization, orbit determina-
tion, and absolute and relative position determination [19]. The German Space
Agency (DARA) Challenging Microsatellite Payload (CHAMP) has been using GPS
for attitude determination and time synchronization since 2000. In low Earth orbit
(LEO), CHAMP also uses GPS measurements for atmospheric and ionospheric
research and applications in weather prediction and space weather monitoring [20].
Since 1992, the Joint CNES-NASA TOPEX/POSEIDON satellite has used GPS
in conjunction with ground processing for precise orbit determination with accura-
cies on the order of 3 cm [21] to conduct its mission of oceanographic research. The
International Space Station employs GPS to provide position, velocity, and attitude