Radar Positional Data Integrity
discarded.
SSR end to end performance is continually monitored through the deployment of SSR site monitors. If the radar measured position of the site monitor is in error or the sight monitor is not received, alerts are provided to a controller and the individual radar may be removed from service. Position data errors not detected by this check include multipath corruption of position, SSR reflections, diffraction, “beam bending”, Mode C data-link error into Slant Range Correction, incorrect time delay in the transponder and incorrect reply mode from the transponder.
When considering Radar Positional data integrity , apart from the site monitor, there is no integrity monitoring of the positional data from other SSR transponders despite the fact that the positional data reported from other transponders being subject to several environmental effects such as multipath corruption of position, SSR reflections, diffraction (which can cause monopulse processing azimuth errors) and “beam bending”. Reliance is placed on the results of flight testing to provide confidence that the environmental effects, at the time of test, are acceptable. Radars measure range and azimuth separately. Radar azimuth errors easily lead to positional errors in the order of 0.25 – 0.5 Nm (95 percentile) at ranges in excess of 200Nm – which continues to be used for ATC 5Nm separation standards. Experience also indicates that radar data is not tightly constrained within a defined tolerance of truth. The error distribution tends to have large “tails” due to multipath and other reasons. DO260 indicates an In an ADS-B system, ADS-B site monitors can also be deployed. The reception of the site monitor broadcast indicates that the ADS-B ground station can ‘hear’. If the site monitor ADS-B position data that comes from a GPS receiver, GPS performance can also be verified. The integrity of the positional information from the aircraft navigation system must be sufficiently high.
When considering ADS-B data integrity and the use of ADS-B in a manner very similar to ATC radar, it is required that the integrity of the ADS-B data should be equal or better than radar surveillance data. ADS-B integrity is managed in two major ways as follows:
a. The ADS-B standards require the transmission of NUCp and/or NIC/NAC/SIL by the avionics to ADS-B receivers. These receivers are then required to determine whether there is adequate integrity and adequate accuracy for the operational requirement. In the case of ATC radar like services using ADS-B, an integrity level of 10-7 for a position error of < 0.5 Nm is equivalent or better than existing radar systems which clearly have a possible error of
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