In today's congested airspace, aircraft surveillance is a crucial tool for Air Traffic Control (ATC) to keep aircraft safely separated. While land-based radar provides an opportunity for ATC to paint a picture of any airspace user's position, altitude, and velocity, it is limited in features and coverage.
ADS-B and ADS-C each provide ATC with a clearer, more complete view of activity in the airspace of interest. But the difference between -B and -C can be confusing. Let's look at each of these aircraft-based surveillance methods.
ADS-B (
Automatic Dependent Surveillance-Broadcast
) broadcasts the same set of surveillance parameters once a second. The broadcast can be received by any ADS-B receiver within range of the broadcast, and therefore comes with challenges for operators concerned about the privacy of their flight. ADS-B is in use over continental airspace, and has been mandated for use in the US since 01 January 2020. The ADS-B Out mandate in the US requires the use of qualifying ADS-B Out equipment in all Class B and C airspace, as well as in Class A (FL 180 and above) and Class E (FL 100 and above). Privately owned, handheld ADS-B receivers within range of the aircraft broadcast are capable of receiving and displaying the very same data ATC uses to separate traffic in US domestic airspace. Such data is shared with flight data co-ops, which in turn display it publicly, causing privacy concerns for operators. Seeking to address these concerns, the FAA rolled out the Privacy ICAO Address (PIA) program in early January 2020. By using a 3rd-party-provider Flight ID (the FAA-assigned PIA) the program conceals the true identity of an aircraft to privately owned ADS-B receivers. It is important to note that the PIA is limited to US domestic flights on N-registered aircraft (not farther than 12NM offshore nor crossing foreign borders). The PIA must be broadcast from an aircraft's ADS-B, and the operator must file using a 3rd-party-provider Flight ID. When flying outside of the US, the operator must revert to filing with the real aircraft registration and ICAO Aircraft Address, and broadcasting the original ICAO aircraft address from the onboard ADS-B equipment.
ADS-C (
Automatic Dependent Surveillance-Contract
) describes a system delivering custom surveillance data from the aircraft via FANS (Future Air Navigation System) only to ATC recipients. ATC creates and places an ADS contract on a FANS equipped aircraft of interest, and receives the desired set of surveillance parameters, at the desired intervals. A controller may choose to place an ADS contract on an aircraft, asking the aircraft system to deliver only the current flight level, true airspeed, and next FMS waypoint. In addition the controller may choose to only have the surveillance data delivered every seven minutes. ADS-C is entirely customizable to what surveillance parameters a controller wants to receive, and how often the surveillance data should be delivered from an aircraft. ADS-C is in use over remote airspace, where land-based radar or ADS-B are not available to support aircraft separation. ADS-C, along with FANS CPDLC, is mandated for use between flight levels 290 and 410 in the North Atlantic Data Link Mandate (NAT DLM) Area in Phase 2C, effective 30 January 2020. Because of the recent reduction in traffic volume, Gander Oceanic and Shanwick Oceanic have issued a NOTAM suspending the requirement to comply with the NAT DLM for 90 days total, until 30 June 2020.
