Augmentation: GBAS, SBAS and ABAS - ATPL 062 — Radio Navigation

The last lesson ended with an error budget; this one sets about shrinking it and, above all, vouching for it. A raw GNSS (global navigation satellite system) fix is accurate on a good day, but an approach needs more than accuracy: it needs integrity, the guarantee that a bad fix will announce itself. Augmentation supplies both, and the three families are named for where the augmenting happens:

on the ground (GBAS);
from satellites (SBAS);
aboard the aircraft (ABAS).

GBAS: the aerodrome's own correction

The principle of a GBAS (ground-based augmentation system): measure on the ground the errors in the signals transmitted by the GNSS satellites, and relay the measured errors to the user for correction. A reference station at the aerodrome knows its own position exactly, so whatever the satellites' signals get wrong there, they are getting wrong nearby too, and the difference is broadcast. Per the ICAO (International Civil Aviation Organization) GBAS standard, the relay uses a data link in the VHF band of the ILS-VOR systems, 108-118 MHz (VHF: very high frequency; ILS: instrument landing system; VOR: VHF omnidirectional radio range): the corrections travel on the same shelf of the dial as the aids they are replacing. The coverage of a GBAS station is about 20 NM.

What the station sends is a complete approach service: GBAS provides information for guidance in the terminal area, and for three-dimensional guidance in the final approach segment (FAS), by transmitting the FAS data block: the approach's geometry, delivered by radio. One ground station can support all the aircraft subsystems within its coverage, providing approach data, corrections and integrity information for the GNSS satellites in view, via a VHF data broadcast (VDB): one installation, every runway end, every aircraft.

The service is promised inside a defined volume; the minimum software designed coverage area is:

10 degrees on either side of the final approach path to a distance between 15 and 20 NM;
35 degrees on either side of the final approach path up to a distance of 15 NM.

Outside this area the FAS data of the GBAS is not used. Two names close the section:

LAAS (local area augmentation system) — the name sometimes used for a GBAS based on GPS (GPS: global positioning system);
GLS approach (GLS: GNSS landing system) — the name for a GBAS-based approach.

SBAS: the correction from orbit

The principle of an SBAS (satellite-based augmentation system) is the same measurement with a different messenger: measure on the ground the errors in the signals received from the satellites, and transmit differential corrections and integrity messages for the navigation satellites, via satellites of its own. SBAS consists of two elements:

a ground infrastructure of monitoring and processing stations;
communication satellites.

And the use of geostationary satellites enables the messages to be broadcast over very wide areas: one relay hanging over the equator covers a continent.

Two design choices do double duty. First, the frequency band of the SBAS data link is identical to that of the GPS signals: the receiver needs no extra radio, the corrections arrive through the same antenna as the ranging. Second, pseudo-range measurements can be made to the geostationary satellites themselves, as if they were GPS satellites: the messenger doubles as one more ranging source. The results reach the approach plates: SBAS allows the implementation of three-dimensional Type A and Type B approaches, and SBAS is designed to significantly improve accuracy and integrity: on the integrity side, SBAS users are alerted within 6 seconds if a GPS malfunction occurs.

The examples of SBAS, one per region:

EGNOS — the European Geostationary Navigation Overlay Service, in western Europe and the Mediterranean;
WAAS — the wide area augmentation system, in the USA;
MSAS — the multi-functional transport satellite (MTSAT)-based augmentation system, in Japan;
GAGAN — GPS and geostationary earth orbit augmented navigation, in India.

ABAS: the aircraft checks for itself

The third family needs no new transmitter at all. The principle of ABAS (aircraft-based augmentation system): use redundant elements within the GPS constellation, such as a multiplicity of distance measurements to various satellites, or combine the GNSS measurements with those of other navigation sensors, such as inertial systems, in order to develop integrity control. The two branches have names:

Receiver autonomous integrity monitoring (RAIM) — the type of ABAS using only GNSS information, the RAIM of the last lesson;
Aircraft autonomous integrity monitoring (AAIM) — a system using information from additional onboard sensors.

The typical sensors used are the barometric altimeter and the IRS (inertial reference system), the Instrumentation course's inertial navigator: sources whose errors have nothing to do with satellites, which is exactly what makes them good witnesses.

What the exam asks

The three principles, the GBAS numbers, the SBAS examples and the RAIM/AAIM split carry the marks. The overview, following the learning objectives:

Explain the GBAS principle (errors measured on the ground, relayed for correction); state the VHF ILS-VOR-band data link (108-118 MHz) and the ~20 NM coverage

State the terminal-area and FAS services (FAS data block), the one-station-serves-all VDB, the minimum software designed coverage and its outside-use ban, and the LAAS and GLS names

Explain the SBAS principle (ground-measured errors, corrections and integrity via satellites), its two elements, and the geostationary wide-area broadcast

State the GPS-identical data-link band, the pseudo-ranging to the geostationary satellites, the 3D Type A/Type B approaches, the accuracy-and-integrity aim, the 6-second alert, and the four examples

Explain the ABAS principle (redundant constellation elements or combination with other sensors); name RAIM as the GNSS-only type, AAIM as the sensor-aided type, and the barometric altimeter and IRS as the typical sensors

GBAS: the aerodrome's own correction

Check you understood

SBAS: the correction from orbit

ABAS: the aircraft checks for itself

What the exam asks