Extended ADS (ADS-X) refers to the integration of multilateration techniques into an ADS-B surveillance infrastructure to mitigate the equipage, validation and backup issues surrounding ADS-B and to enable a faster, more comprehensive and more cost-effective ADS-B implementation.

First, ADS-X supports full fleet coverage in advance of equipage. Because multilateration techniques process (Mode A/C/S) transponder signals, the network of ground stations is able to determine the position of aircraft without the need for new avionics. This allows ANSPs to deploy the next generation, low cost technologies without the need for a contentious early mandate for equipment retrofit, while still potentially avoiding the need to replace SSR systems.

The second ADS-B implementation challenge - the need for independent backup and validation of ADS-B self-reported position, is also addressed by the inclusion of multilateration capability in the ADS-B network, as each ADS-B position report is validated in real-time by TDOA triangulation.

The FAA, in its industry briefings, has both confirmed the need for a backup and validation system for ADS-B and has short listed three candidate strategies to address this requirement. The first candidate strategy relies on the continued use of a (reduced) SSR network, which is likely to have a significant impact on the economics of the overall program. The second and third strategies are both based on the use of multilateration as the backup technique.  Furthermore, it is clear that the backup and validation system will not only mitigate the risk of accidentally erroneous self-reported position, but will also address the potential for deliberate spoofing of aircraft position.

“ADS-X integrates MLAT techniques into the existing ADS-B infrastructure to help alleviate equipage, validation, and backup issues.  ADS-X is also cost effective, in that the ground stations process Mode A/C/S transponder signals without the need for expensive avionics upgrades.”

- Mellissa Diamond, Frost & Sullivan Research Analyst