AIM COM 5.3.1 Aircraft-Based Augmentation System (ABAS)
RAIM and FDE functions in current IFR-certified avionics are considered ABAS. RAIM can provide the integrity for the en route, terminal, and NPA phases of flight. FDE improves the continuity of operation in the event of a satellite failure and can support primary-means oceanic operations.
RAIM uses extra satellites in view to compare solutions and detect problems. It usually takes four satellites to compute a navigation solution, and a minimum of 5 for RAIM to function. The availability of RAIM is a function of the number of visible satellites and their geometry. It is complicated by the movement of satellites relative to a coverage area and temporary satellite outages resulting from scheduled maintenance or failures.
If the number of satellites in view and their geometry do not support the applicable alert limit (2 NM en route, 1 NM terminal and 0.3 NM NPA), RAIM is unable to guarantee the integrity of the position solution. (Note that this does not imply a satellite malfunction.) In this case, the RAIM function in the avionics will alert the pilot, but will continue providing a navigation solution. Except in cases of emergency, pilots must discontinue using GNSS for IFR navigation when such an alert occurs.
A second type of RAIM alert occurs when the avionics detects a satellite range error (typically caused by a satellite malfunction) that may cause an accuracy degradation that exceeds the alert limit for the current phase of flight. When this occurs, the avionics alerts the pilot and denies navigation guidance by displaying red flags on the HSI or CDI. Continued flight using GNSS is then not possible until the satellite is flagged as unhealthy by the control centre, or normal satellite operation is restored.
Some avionics go beyond basic RAIM by having an FDE feature that allows the avionics to detect which satellite is faulty, and then to exclude it from the navigation solution. FDE requires a minimum of 6 satellites with good geometry to function. It has the advantage of allowing continued navigation in the presence of a satellite malfunction.
Most first generation avionics do not have FDE and were designed when GPS had a feature called SA that deliberately degraded accuracy. SA has since been discontinued, and new generation SBAS-capable receivers (TSO-C145a/C146a) account for SA being terminated. These receivers experience a higher RAIM availability, even in the absence of SBAS messages, and also have FDE capability.
AIM COM 5.3.1 Aircraft-Based Augmentation System (ABAS)
RAIM and FDE functions in current IFR-certified avionics are considered ABAS. RAIM can provide the integrity for the en route, terminal, and NPA phases of flight. FDE improves the continuity of operation in the event of a satellite failure and can support primary-means oceanic operations.
RAIM uses extra satellites in view to compare solutions and detect problems. It usually takes four satellites to compute a navigation solution, and a minimum of 5 for RAIM to function. The availability of RAIM is a function of the number of visible satellites and their geometry. It is complicated by the movement of satellites relative to a coverage area and temporary satellite outages resulting from scheduled maintenance or failures.
If the number of satellites in view and their geometry do not support the applicable alert limit (2 NM en route, 1 NM terminal and 0.3 NM NPA), RAIM is unable to guarantee the integrity of the position solution. (Note that this does not imply a satellite malfunction.) In this case, the RAIM function in the avionics will alert the pilot, but will continue providing a navigation solution. Except in cases of emergency, pilots must discontinue using GNSS for IFR navigation when such an alert occurs.
A second type of RAIM alert occurs when the avionics detects a satellite range error (typically caused by a satellite malfunction) that may cause an accuracy degradation that exceeds the alert limit for the current phase of flight. When this occurs, the avionics alerts the pilot and denies navigation guidance by displaying red flags on the HSI or CDI. Continued flight using GNSS is then not possible until the satellite is flagged as unhealthy by the control centre, or normal satellite operation is restored.
Some avionics go beyond basic RAIM by having an FDE feature that allows the avionics to detect which satellite is faulty, and then to exclude it from the navigation solution. FDE requires a minimum of 6 satellites with good geometry to function. It has the advantage of allowing continued navigation in the presence of a satellite malfunction.
Most first generation avionics do not have FDE and were designed when GPS had a feature called SA that deliberately degraded accuracy. SA has since been discontinued, and new generation SBAS-capable receivers (TSO-C145a/C146a) account for SA being terminated. These receivers experience a higher RAIM availability, even in the absence of SBAS messages, and also have FDE capability.