In the early 1990s, aircraft operators started moving to satellite-based navigation (GNSS) because of the availability of relatively inexpensive Global Positioning System (GPS) receivers.
Operators used GNSS receivers to fly visual flight rules (VFR) and instrument flight rules (IFR) navigation.
But at the early stage of satellite navigation development, the core-satellite constellations were not developed to satisfy the strict requirements of IFR navigation to fly more precise way.
For this reason, GNSS avionics used in IFR operations should augment the GNSS signal to ensure its accuracy, integrity, continuity and availability.
ABAS
The Aircraft based augmentation was introduced to augment the received GNSS information based on the information available on board to enhance the performance of the core-satellite constellations.
ABAS Avionics equipped in aircraft use one of the following techniques to enhance the performance (accuracy, integrity, continuity, and/or availability) of unaugmented GNSS and/or of the aircraft navigation system:
RAIM, is a technique which uses redundant GNSS information from multiple satellites to asses data integrity using an onboard GNSS receiver with RAIM capability.
Aircraft autonomous integrity monitoring (AAIM), uses information from additional onboard sensors such as the Inertial Navigation System (INS) to calculate the GNSS data integrity.
RAIM
The most common ABAS technique is called Receiver Autonomous Integrity Monitoring (RAIM). RAIM requires redundant satellite range measurements to detect faulty signals and alert the pilot.
RAIM algorithms require a minimum of five visible satellites in order to perform Fault Detection (FD) and detect the presence of an unacceptably large position error for a given mode of flight.
RAIM availability depends on the type of operation; it is lower for the non-precision approach than for the terminal, and lower for the terminal than for en-route. It is for this reason that GPS/RAIM approvals usually have operational restrictions.
GPS/RAIM provided greater benefits at the initial time of the GNSS transition because no additional cost was required to build any ground infrastructure for augmentation.
ABAS techniques were continuously upgraded, even GPS/RAIM allowed in oceanic areas where ground-based navaids are completely impossible to provide navigation guidance.
But in this case, avionics should not only have the ability to detect a faulty satellite (through RAIM), but it should also exclude that satellite and continue to provide guidance. This feature is called Fault Detection and Exclusion (FDE).
FDE uses a minimum of six satellites not only to detect a faulty satellite but also to exclude it from the navigation solution so that the navigation function can continue without interruption.
Under such approval, aircraft carry dual systems and operators perform pre-flight predictions (RAIM Prediction) to ensure that there will be enough satellites in view to support the planned flight.
This provides operators with a cost-effective alternative to inertial navigation systems in oceanic and remote airspace.
AAIM
Another ABAS technique Aircraft autonomous integrity monitoring (AAIM) involves the integration of GNSS with other airborne sensors such as,
Inertial Navigation Systems (INS),
eLORAN Receivers
Automated Celestial Navigation Systems
Distance Measure Equipement
Simple Dead Reckoning
In general, aircraft position is calculated using separate avionics using aircraft-equipped sensors. This calculated data used to identify the position errors from GNSS data
These ABAS concepts, especially RAIM algorithms are extensively used for additional integrity checks during pre-flight planning and all phases of flight to ensure safety.