In an ADS-B system, velocity vector is determined by the aircraft’s navigation system and broadcast to the ground station. Hence the accuracy of the velocity vector is a property of the aircraft’s navigation system and typically considerably more
Emergency alerting
Update Rate
Multiple Target Resolution
Coverage
Continuity of Service - Continuity is the
the true vector. This leads to a slow detection of an aircraft’s commencement of a turn.
SSR has three reserved Mode A codes to indicate three types of emergency (EMG, RAD, HIJ). SSR has three reserved Mode A codes to indicate three types of emergency (EMG, RAD, HIJ).
SSR update rate is determined by the antenna rotation speed. For Enroute radars this typically is in the range of once per 15 seconds to once per 4 seconds.
A limitation of a Mode A &C SSR is encountered when two aircraft are at similar slant range and azimuth but adequately separated by altitude. In this case the transponder replies from the two aircraft overlap in time. When received by the radar, the radar may not be able to distinguish that there are two replies present. Thus only one aircraft is displayed. Radar sensors typically require at least 1.0 degree difference in azimuth to reliably resolve two aircraft. One degree at a range of 250 NM is more than 4 NM apart. Mode S radars, by use of selective interrogation, avoid this limitation.
SSR coverage is limited by the terrain and buildings around a radar site. The limitations are documented during radar commissioning tests. Controllers use radar with knowledge of its coverage limitations.
SSR failure causes controllers to transition to accurate and responsive than that calculated by radar.
In an ADS-B system equivalent data is transmitted in the status field.
In an ADS-B system, positional data is broadcast twice per second. Thus as many as 7 out of 8 broadcast messages can be lost while maintaining the same update rate as high-update rate radar.
ADS-B does not have a limited resolution capability. Multiple aircraft at exactly the same position will be fully resolved.
ADS-B is also subject to coverage limitations. The limitations will be documented during testing and Controllers will use ADS-B with knowledge of its coverage limitations.
ADS-B failure would also initiate a transition to
probability of a system continuing to perform its function without unscheduled interruptions during the intended period of operation. Continuity is a function of the system design and the reliability of the various elements forming the system. The elements of the ADS-B system are essentially the same as the receiving portion of a radar system plus the source of positional data on board the aircraft.
Reliability
Reliability is a measure of the probability of unexpected loss of the surveillance service. The consequence of the loss may result in reversion to procedural control for either a single aircraft (avionics failure) or for many/all aircraft (sensor system wide failure). Reversion to procedural control for a single aircraft is identical to the treatment of non-ADS-B equipped aircraft.
Planned loss of service has no safety impact since procedures are put in place to mitigate any operational impact before the loss of service occurs. An expected and planned for RAIM outage (where RAIM is available as part of the proposed ADS-B system), is treated as a planned loss of service. This can be achieved if controllers are notified in advance through a RAIM prediction notification as per existing GPS RAIM prediction services.
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