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 information is data linked form the aircraft to the ground. Mode S ADS-B is an inherent part of the Mode S SSR system and hence has the same data-link characteristics as Mode S radar. Many of today’s radars, which provide satisfactory service, use only Modes A & C. It is useful to compare the integrity of the Modes A&C and Mode S data-links. The Mode A (identity) and Mode C (pressure altitude) data is sent as a frame comprising 12 bits of information. The data is not protected by any parity, cyclic redundancy check (CRC) or any other inherent error detection mechanism. Radars attempt to detect transmission errors by checking for consistency of Replies from multiple interrogations. Error in the received identity (Mode A) could mislead either the controller or the automation system as to the aircraft’s identity. Error in the received Mode C code could mislead the controller on aircraft altitude and lead to error in slant range correction and hence the aircraft position is incorrectly displayed. These errors are often seen in an operational environment. The Mode S transmission of data (including ADS-B squitter messages) uses a very robust 24-bit error detection algorithm for integrity check. The integrity check is applied at the radar ground station. If the check fails the data is considered unreliable and is Data-Link Integrity. Both SSR and ADS-B
 
 

 
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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