14.4 Functional Supplement to Global Satellite Radio Navigation Systems
265
The tropospheric delay depends on the local conditions in which the user is located,
and therefore, the WAAS GEO message does not contain explicit tropospheric corrections and only data are transmitted that allow these corrections to be calculated in
the onboard receiver.
The WAAS system allows the use of up to 52 satellites and has the ability to
transmit corrective information via GLONASS satellites.
Whereas in GBAS systems, the reliability of the information used by the pilot
depends on one ground station; then in SBAS, this reliability will be determined
by the entire network of ground and space objects, which puts forward very strict
requirements for the reliability of SBAS elements.
To ensure communication, received on board the corrections to pseudo-ranges
with the aircraft position, these corrections are transmitted as separate corrections:
• time frequency;
• ephemeris;
• corrections related to the selective access error;
• ionospheric corrections.
The latter are transmitted in the form of a rectangular grid containing about 1000
points.
The elimination of the selective access mode from the GPS signal facilitated the
implementation of the system, since the amount of transmitted information required
to correct the errors caused by this mode is reduced.
The provision of the second frequency to civilian users even more simplifies
the implementation of the system, since the amount of information transmitted over
terrestrial and space data lines is significantly reduced and the function of monitoring
the ionospheric component of the error can be excluded.
One of the most important requirements set by ICAO to the existing and developed
satellite systems of functional augmentation is the requirement of “seamlessness”
during the flight of AV from the service area of one system to the service area of
another, i.e., full compatibility of various systems at the level of aircraft onboard
equipment.
Ground-based augmentation system (GBAS). The ground-based augmentation
system (GBAS) or local control and corrective system (LCCS) is designed to provide
an AV precision landing approach or GLS using GNSS signals. It can also provide
aircraft positioning services within the GBAS ground station coverage area limited
by line-of-sight range (Fig. 14.12).
The GBAS ground station monitors GPS and/or GLONASS signals in the terminal
area and transmits area-specific pseudo-range corrections, integrity parameters, inoperative satellite numbers and final approach segment (FAS) data to AVs in the VHF
band.
The GLS approach is similar to the radio beacon landing system (RBLS) approach.
The difference is that in GLS, the deviation of the AV from the nominal heading and
glide path is determined by comparing the true position of the AV, calculated from
the GNSS / GBAS signals, with the data of the onboard navigation database and the
FAS block.
265
The tropospheric delay depends on the local conditions in which the user is located,
and therefore, the WAAS GEO message does not contain explicit tropospheric corrections and only data are transmitted that allow these corrections to be calculated in
the onboard receiver.
The WAAS system allows the use of up to 52 satellites and has the ability to
transmit corrective information via GLONASS satellites.
Whereas in GBAS systems, the reliability of the information used by the pilot
depends on one ground station; then in SBAS, this reliability will be determined
by the entire network of ground and space objects, which puts forward very strict
requirements for the reliability of SBAS elements.
To ensure communication, received on board the corrections to pseudo-ranges
with the aircraft position, these corrections are transmitted as separate corrections:
• time frequency;
• ephemeris;
• corrections related to the selective access error;
• ionospheric corrections.
The latter are transmitted in the form of a rectangular grid containing about 1000
points.
The elimination of the selective access mode from the GPS signal facilitated the
implementation of the system, since the amount of transmitted information required
to correct the errors caused by this mode is reduced.
The provision of the second frequency to civilian users even more simplifies
the implementation of the system, since the amount of information transmitted over
terrestrial and space data lines is significantly reduced and the function of monitoring
the ionospheric component of the error can be excluded.
One of the most important requirements set by ICAO to the existing and developed
satellite systems of functional augmentation is the requirement of “seamlessness”
during the flight of AV from the service area of one system to the service area of
another, i.e., full compatibility of various systems at the level of aircraft onboard
equipment.
Ground-based augmentation system (GBAS). The ground-based augmentation
system (GBAS) or local control and corrective system (LCCS) is designed to provide
an AV precision landing approach or GLS using GNSS signals. It can also provide
aircraft positioning services within the GBAS ground station coverage area limited
by line-of-sight range (Fig. 14.12).
The GBAS ground station monitors GPS and/or GLONASS signals in the terminal
area and transmits area-specific pseudo-range corrections, integrity parameters, inoperative satellite numbers and final approach segment (FAS) data to AVs in the VHF
band.
The GLS approach is similar to the radio beacon landing system (RBLS) approach.
The difference is that in GLS, the deviation of the AV from the nominal heading and
glide path is determined by comparing the true position of the AV, calculated from
the GNSS / GBAS signals, with the data of the onboard navigation database and the
FAS block.