ADS-B What is all the talk about diversity?

Unlike the FAA in the United States, which over the last two decades invested in ADS-B ground stations to cover the majority of their landmass. Nav Canada invested in Aireon, a space-based ADS-B solution to cope with our vast and sometimes inaccessible geography.

Space-based ADS-B
Space-based ADS-B

This has resulted in most of the general aviation fleet having their transponder antennas on the wrong side of the aircraft, bottom verse top. So, do we just need to relocate the antenna?

It’s not that simple for several reasons. ADS-B ground stations are still located at many Canadian airports, requiring the bottom antenna to be retained. For anyone flying into US airspace only having a top-mounted antenna would not work with a ground-based system. To deal with these issues, transponders need to be diversity models.

Diversity in ADS-B

Diversity in ADS-B refers to having two antennae connected to a single transponder.

The way a single transponder system is connected to two antenna functions is through a technique called Diversity. Each antenna is connected to a dedicated receiver in the transponder. When the system is interrogated, the signal strength measured in each receiver is compared to the other and the one with the strongest signal directs the single transmitter to reply through its antenna only. The requirement for two separate receivers and the comparator circuitry is what increases the cost of diversity transponders by several thousand dollars.

ADS-B transponders are also mandated to continually broadcast the aircraft position and information regardless of interrogation. Diversity transponders alternate their broadcast between the two antennas the Aireon satellite system does not interrogate; it relies on this unsolicited ADS-B Out broadcast.

When can we expect implementation?

Unlike the United States all-or-nothing ADS-B Out equipage deadline on January 1, 2020, Nav Canada is planning a phased implementation. As to when – due to pressure from pilots and operators about the cost of diversity, Nav Canada has delayed implementation and is working with Transport Canada to finalize mandates. Stay tuned for updates.