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.

Transitioning from Radar to ADS-B

What is ADS-B you ask;

  • Automatic in the sense that it operates without external stimulus (unlike radar)
  • Dependent in the sense that the surveillance information is derived from onboard systems
  • Surveillance in the sense that it’s primarily intended to provide surveillance information to other parties
  • Broadcast because it transmits to all listeners without knowing who those listeners are, there is no two-way communication

The capability of an aircraft to transmit ADS-B signals is called ADS-B OUT, the capability of an aircraft to receive ADS-B signals is called ADS-B IN. The Garmin GTX-335 is an example of an ADS-B OUT only transponder, while the GTX345 is ADS-B OUT and ADS-B IN.

ADSB-how-it-works for ADS-B - Transitioning from Radar
ADSB-how-it-works – from FAA

ADS-B data contains information about the identity, position, altitude, and velocity of the aircraft. The position and velocity information is derived from GNSS (GPS).

ADS-B is a surveillance technology whereby the aircraft broadcasts information about its identity, position, altitude, and velocity of the aircraft to any interested party. The position and velocity information is derived from GNSS (GPS).

TCAS

ADS-B is not intended to replace TCAS, although in the future it will augment TCAS. The TCAS algorithm currently only uses distance and altitude to calculate whether there is a conflict and to determine the best conflict resolution strategy. With the more accurate ADS-B position available to the system as well, the number of TCAS interrogations can be reduced (the surveillance radio frequencies are getting congested in some areas) and the performance of TCAS can be improved. A new standard for this hybrid approach was published in 2013.

It may also be possible to have a passive TCAS-like system that does not require active interrogation but is purely depending on ADS-B. Currently, a technical standard for a new Airborne Collision Avoidance System (ACAS, the generic name for TCAS) is being developed in a joint RTCA / EUROCAE committee (RTCA SC-147 / EUROCAE WG-75), which will take advantage of more data offered by ADS-B. This new standard will eventually replace TCAS II.

ATC

The purpose of ADS-B is not to replace ground-based ATC. It will change the way ATC is done. ADS-B IN will improve the situational awareness of pilots; they will have a display of accurate positions of other aircraft. New procedures will allow pilots to maintain separation from other aircraft in marginal Visual Meteorological Conditions (VMC), where they currently would often lose sight of other traffic. More advanced usage of ADS-B will be flight deck based interval management (FIM) where ATC will be able to instruct aircraft to ‘follow that plane XX seconds behind for landing on runway YY’.

ADS-B is not a replacement for all radars, although it will allow the number of radars to be reduced. For remote areas that currently do not have radar coverage because of the associated high costs, ADS-B will be a cost-effective alternative.

The display of ADS-B IN requires a compatible cockpit monitor. The Garmin GTN series of navigators can display ADS-B targets blended with TCAS whereas older models of navigators and MFDs will only display TCAS traffic. The Garmin GTX-345 contains a built-in Bluetooth transmitter that can broadcast ADS-B traffic information to a handheld cockpit display such as an IPAD.

Canadian Maintenance Organizations (AMO’s) are approved under a bilateral agreement between the FAA and Transport Canada to complete ADS-B installations and return n-registered aircraft to service.

Rocky Mountain Aircrafts Avionics team can keep you equipped to stay airborne in 2020. Contact Us.