Most corporate pilots know how TCAS operates and what it does, but fewer are familiar with ADS-B. ADS-B works via small onboard transmitter/receivers, which send out brief signal bursts that include the airplane’s identification, GPS position, altitude and current flight profile once per second. These bursts are picked up by all ADS-B-equipped aircraft within line-of-sight range. Each aircraft’s ADS-B computer processes the incoming signals arriving from all other aircraft and presents their locations, IDs, altitudes and profiles on a flight-deck display. The computer also uses the successive bursts from each individual aircraft to derive its track and groundspeed.
The aircraft transmissions are also received by unmanned ground stations, which retransmit them to distant ARTCCs, providing center controllers with a picture of lower-altitude movements below the center’s radar coverage. Conversely, the ARTCC can send weather, traffic and other data to local ground stations, which in turn uplink it to aircraft in their area over the ADS-B datalink channel. For its Alaska Capstone evaluation, the FAA is using the new U.S.-developed Universal Access Transponder (UAT) datalink, while the UPS trial will use the international mode-S link.