Differential GPS - Understanding GPS: Principles and Applications - page 432

8.6.1.2 SBAS
ICAO has developed standards [48] for two types of code-based DGPS systems for
civil aircraft navigation applications. This section will describe SBAS systems, and
the following section will describe GBAS systems.
An SBAS is a WADGPS system that provides differential GPS corrections and
integrity data using GEOs as the communications path. A unique feature of SBASs is
that they provide DGPS data, using a signal broadcast directly at the GPS L1 fre-
quency, that can be used for ranging. The goal of SBASs is to meet navigation system
requirements for civil aviation from the en-route phase of flight through vertically
guided precision approach. A number of SBASs had been implemented or were
planned at the time of this writing [49]. These include the WAAS within the United
States, the EGNOS within Europe, the Multifunctional Transport Satellite
(MTSAT)-based MSAS within Japan and Southeast Asia, and the GPS and GEO
Augmented Navigation (GAGAN) system in India.
History
As discussed in Chapter 7, RAIM or DGPS are required to provide the necessary lev-
els of integrity to GNSS to support air navigation. In the early 1980s, a concept of
providing integrity data for GPS over a GEO communications link using a signal on
the GPS L1 frequency emerged. This concept was referred to as a GPS integrity chan-
nel (GIC) [50]. In 1989, Inmarsat began test transmissions of GPS-like spread spec-
trum signals through a geostationary satellite over the Atlantic Ocean to prove the
feasibility of using a navigation repeater to transmit pseudorandom-coded spread
spectrum ranging signals. The test results indicated that transmitting these signals
through geostationary satellites was possible [51]. In the same time frame, organiza-
tions including Inmarsat and RTCA's Special Committee 159 (SC-159) began estab-
lishing a signal format for GIC, which later evolved into SBAS. In the 1990s, SBAS
programs were well underway within the United States, Europe, and Japan.
Inmarsat on their own initiative included navigation transponders on the five
Inmarsat-3 satellites that were launched from April 1996-February 1998. In
November 1999, Japan attempted to launch its own SBAS GEO for MSAS, but
experienced a setback when the satellite, MTSAT-1, had to be destroyed following a
launch failure. In August 2000, the U.S. FAA's WAAS system, using two of the
Inmarsat-3 satellites—AORW and POR—was declared to be continually available
for nonsafety applications. In July 2003, WAAS was commissioned for safety of life
services. At the time of this writing, the EGNOS system was operational with three
GEOs and anticipating commissioning for safety of life operations; the MSAS
ground segment was complete, the replacement satellite, MTSAT-1R, was success-
fully launched in February 2005; and GAGAN was in an advanced stage of
development with operations expected to commence within the next several years.
SBAS Requirements
ICAO requirements for SBAS and GBAS for en-route through category I precision
approach operations are shown in Table 8.3 [48]. Most researchers believe that cat-
egory I requirements cannot be met by SBAS in the near term (before the population
of the GPS constellation with L5-capable satellites). New classes of less stringent
vertically guided approaches, referred to as GNSS approach and operations with