ADS-B provides very high accuracy and also high integrity surveillance information originating from the GNSS (presently GPS) position source. Position accuracy is very much better than any Air Traffic Control (ATC) surveillance radars that are used today. Furthermore, ADS-B provides aircraft ground velocity (i.e. speed and track angle) that is impressively good when compared to ground tracking of radar plot positions. Although there are three possible ADS-B downlink frequencies, in practice the globally common standard uses 1090MHz, sharing the existing SSR/Mode S transponder downlink frequency. Consequently, the ADS-B downlink broadcasts provide a very easy way for a ground system to obtain surveillance information – it just requires ground receivers, most simply with an omni-directional antenna. However, there are some concerns about using ADS-B because the surveillance information is entirely dependent upon the aircraft fit and the external (GPS) position source. Firstly, there are some concerns that an aircraft ADS-B fit is not properly certified or non-compliant to the required ADS-B standard. This concern has not been alleviated by examples of some early aircraft fits that were not properly certified or installed to an earlier ADS-B standard, in particular when the aircraft's inertial reference system (IRS) has been connected as the position source instead of GPS. Secondly, it is potentially easy to generate "spoof" ADS-B information that can be transmitted into an ADS-B ground receiver with the objective of disrupting or deceiving ATC operations. Thirdly, there may be occasions in operation when GPS satellite coverage may be inadequate to provide the required level of GPS integrity information needed for ATC applications.
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