2.2.12. SELECTIVE AVAILABILITY (SA)
The US Department of Defense introduced an artificial error (Selective Availability) into satellite data to downgrade the accuracy of a GPS position to 100 m for civilian users. Without SA, the accuracy of GPS position can reach down to 10 m.
On May 1st 2000, the US Department of Defense switched SA off after an announcement of the US president Bill Clinton.
6 It is different from the Estimated Position Error (EPE) calculated by FM computers. EPE is the drift of
the FM position.
To make the most of the possible accuracy of the GPS system, the GPS sensor must include the SA Awareness function. It takes into account that SA is off and provides more realistic estimates of HFOM and HPL. At the time of writing the brochure, Thalès MMR only is capable of SA Awareness function. Other GPS sensors installed on AIRBUS aircraft assume that SA is always on: the accuracy of the GPS position is always downgraded on a conservative basis.
2.2.13. NAVIGATIONAL UNCERTAINTY CATEGORY (NUC)
DO-260 defines NUC to characterize both the accuracy and the integrity of ADS-B data. NUCP is relative to the position, and NUCR to the velocity. The higher the NUC, the higher the quality of ADS-B data.
NUCP may be derived from HFOM or HPL as per DO-260. However, as illustrated in Figure 2-5, it is preferable to derive NUCP from HPL to take into account a potential satellite failure (as HFOM assumes there is no satellite failure). Transponders proposed on AIRBUS aircraft derive NUCp from HPL first, then from HFOM is HPL is not available.
2.2.14. NAVIGATION INTEGRITY CATEGORY (NIC)
DO-260A segregates the accuracy and the integrity of ADS-B data, and defines the NIC instead of the DO-260 NUC integrity information. NIC is derived from HPL and defines the same circle as HPL does. HPL is a radius whereas NIC a category.
2.2.15. NAVIGATIONAL ACCURACY CATEGORY (NAC)
The DO-260A NAC takes the place of the DO-260 NUC accuracy information, and is derived from HFOM. NAC defines the same circle as HFOM does. HFOM is a radius and NAC is a category.
NACP is relative to the position and NACV to the velocity.
2.2.16. SURVEILLANCE INTEGRITY LEVEL (SIL)
SIL is the probability that the current position is outside the circle defined by NIC.
The higher the NIC, NAC and SIL, the higher the quality of ADS-B data.
Figure 2-6 illustrates the differences between HFOM, HPL, NIC, NAC, and SIL.
2.2.17. ADS-B CONTROLS AND INDICATIONS
For the General Aviation community, transponder and ADS-B transmitter are most of the time separate units. Consequently, pilots can independently operate transponder and ADS-B transmissions. For the Air Transport Aviation community, transponder and ADS-B transmitter are merged in a single unit: the transponder Mode S capable of ADS-B OUT.
Per design, it had been elected to make ADS-B transmissions totally transparent to the flight crew on AIRBUS aircraft. Consequently, there are neither new controls nor new indications related to the ADS-B OUT transmissions addressed to the flight crew. The main benefits are the avoidance of heavy modifications (e.g. no new connections with other systems, no change on ATC/TCAS panel) and a minimum impact on cockpit procedures.
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