Bjorn’s Corner: Largest navigation change since radar, Part 3

By Bjorn Fehrm

June 29, 2018, ©. Leeham News: Last week we explained ADS-B out, the mandatory equipment needed from 2020 for flying in the US Airspace where a C-type transponder is needed today.

Now we continue with describing the ADS-B in, the listening capability of the system, which is nonmandatory. It offers exciting possibilities, however.

Figure 1. The ADS-B is mandatory in most US airspace areas. Source: FAA

ADS-B in, the icing on the cake

The ADS-B system (Automatic Dependent Surveillance-Broadcast) has transmit and receive channels on both 1090MHz (the 1090ES channel) and 978MHz (the UAT channel). It’s mandatory to transmit ADS-B out on one of these channels by 2020. The receive function, ADS-B in, is optional for both channels.

The 1090ES channel is a reuse of the Mode S transponder’s 1090MHz channel, now with an extended S type message called the ES message. From telling who the aircraft is and a bit about what it does, the ES message is a complete reveal of the aircraft’s 3D position, its velocity and where it’s going.

As the 1090MHz channel is also used for responses to SSR (Secondary Surveillance Radar, the civilian IFF) and TCAS (Traffic Collision Avoidance System), the FAA was afraid ADS-B, in addition, would congest the channel. The result would be a reduced range for the ADS-B and TCAS functions in hot spots like the LA basin, shortly after introducing ADS-B on the frequency.

This is why FAA added an additional channel for the US Airspace (all other countries are OK with the 1090ES capacity for now). It’s a dedicated Datalink channel with high capacity, the 978MHz UAT (Universal Access Transceiver). The high capacity channel is used for several interesting functions beyond ADS-B out, more of later.

Part of the ADS-B system is a ground station chain over which the ATC receives the ADS-B signals. But these stations also transmit information to the aircraft. This is where the in (listening) ability of ADS-B comes into play.

For the ADS-B in on 1090ES, the FAA delivers basic information. It’s the traffic picture the ATC has for the vicinity around the aircraft. The ATC can’t deliver the complete traffic picture it has to the aircraft. It would saturate the channel. A slice of the traffic 30 miles out, +-3,500ft from the aircraft is therefore delivered. And it’s only sent to aircraft which signals it can receive the information (has ADS-B in capacity) in it’s ADS-B out message.

With the sent slice, the aircraft can see on its displays:

  • Any 1090ES out messages from surrounding aircraft received by its 1090ES in function.
  • Any general aviation aircraft sending ADS-B out on 978MHz (ATC relays these signals onto 1090ES)
  • Aircraft covered by ATC radar which doesn’t have a mode S transponder (which therefore can’t be seen by any TCAS the aircraft would have).

With this ATC service called TIS-B (Traffic Information Service -B) an airliner or business jet landing on a secondary airport have the complete traffic picture for the airport. Without TIS-B, Transponder C or A aircraft in the traffic pattern would not be seen.

ATC provides the same service for 978MHz equipped General Aviation aircraft. These can see ADS-B aircraft transmitting on the 978Mhz channel, but no 1090ES aircraft. Most of the time these light aircraft also lack TCAS (TCAS is mandatory for airliners and business jets).

The Traffic Information Service, TIS-B, to the 978MHz aircraft therefore provides:

  • Info on all 1090ES aircraft
  • Any Transponder S, C and A aircraft the ATC is tracking

With this, the traffic picture is complete for the light aircraft as well.

What previously has costed megabucks for a TCAS system, can now be achieved with an iPad and an ADS-B in receiver for less than $1000. More on this and the FAA additional 978MHz services in the next Corner.

3 Comments on “Bjorn’s Corner: Largest navigation change since radar, Part 3

  1. Hello Bjorn,

    Thank you for another excellent series. As someone who enjoys reading about engineering details more than I do reading about politics or manufacturer flame wars, I look forward to your posts.

    From the department of nitpicking: In the second paragraph should “It offers exiting possibilities” be “It offers exciting possibilities”.

      • While I am somewhere in between on tech details and arguing merit of various mfgs (grin) – I too applaud Bjorn as well as this series.

        This is some truly amazing stuff. Having followed its early gestation ala the AK Capstone project, its been fascinating to see it cascade to the world.

        The relatively low cost implementation of it was a real eye opener as well. Shades of NDB.

        Having lived (and flown though only in training) NDB/ADF and it still being viable to the wonders of ADS is something to behold.

        Mostly we used ADF for entertainment listening to radio stations while using VOR navigation, and a real treat with DME.

        In the late 50s we had an NDB co located (some miles apart) with the VORTAC where we lived. One location that was all they had was an NDB approach.

        I sure would not want to fly a rasty approach with a large workload solo on ADF.

        Oddly early training fishing and holding a nailed compass course in rough seas was great training to be able to do a good ADF leg or approach.

        One of my instructors commented he had never seen anyone who could nail a course and hold it that tight.

        You didn’t know my dad and how fanatic he was about courses, but trust me, you learned to not have a weaving wake (or as my wife would say, Dad never let us Tractor the visible fields, a farmers pride and joy was his straight furrows!)

        My poor father had a choice of fishing or steering so he was severely conflicted. He served in the Navy so I strongly suspect (never got to ask) that one of his duties was helm on his ship (he was a machinist mate but they were not fat on crews on his LCL-R)

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