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I am reading this: https://mode-s.org/decode/ehs/bds50-track-n-turn.html

I am wondering the Track angle which ADS-B In recieve is really the track angle (derived from line joining the previous and current point) or the heading angle (or its body angle)?

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  • $\begingroup$ The document you linked says "true track angle". What exactly is unclear to you? $\endgroup$
    – Bianfable
    Feb 4, 2020 at 10:17
  • $\begingroup$ @Bianfable I am just wondering how this "True Track Angle" is being produced, whether they are derived from GPS track angle or from instrumentation IMU heading. Or whether they are filtered through Kalman Filter and combined from various sources $\endgroup$
    – user1538798
    Feb 5, 2020 at 1:38

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It is the true track angle.

  • True means with respect to the geographic North (as opposed to the magnetic North)

  • Track angle means the direction of motion in the horizontal plane (as opposed to heading, which means the direction of the projection of the primary body axis in the horizontal plane).

Track angle can be derived from a line joining successive position points, but that is not typically how its done. GPS commonly uses a combination of Kalman filtered change of position combined with (Kalman filtered) velocity derived from Doppler shifts.

For ADS-B In, the track angle is just what is reported by ADS-B Out. It does not involve processing successive position reports.

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  • $\begingroup$ Thanks for your answer! Was confused over the difference heading angle and track angle. Seem like they mean the same in this case $\endgroup$
    – user1538798
    Feb 5, 2020 at 1:23
  • $\begingroup$ If I'm not mistaken, the heading angle can differ from the track angle if the plane has to compensate for the wind to fly in a certain direction, e.g. crabbing in a windy landing. $\endgroup$
    – zymhan
    Feb 5, 2020 at 20:06
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The link you provided appears to be an article combining ADS-b with Enhanced Mode-S. It has three sections and an intro. The “Track Angle” and “Roll Angle” information is in the Enhanced Mode-S section, and NOT the ADS-b section.

Are you asking about the data displayed for the other aircraft displayed on your ADS-b in? The above answer sums it up.

Additionally, ADS-b does not transmit attitude and heading information to other ADS-b receivers and ground stations. Any AHRS data calculated by an ADS-b unit is only for the use of the aircraft in which it is being used (ownship). Also, that info is not considered accurate for flight and navigation purposes. It can not be used except as a supplementary reference. Legally, it can not even be used as a backup to your TSOed, panel-mounted instruments. Although, in an emergency, something is better than nothing.

The information that ADS-b transmits is based on GPS data. GPS would give you the aircraft’s position (including altitude) and track (including trends) regardless of its heading and attitude. It transmits this information to aircraft’s airborne receivers and ground stations for rebroadcast automatically, without being interrogated.

An Enhanced Mode-S Transponder, when interrogated, transmits its information to ATC’s secondary surveillance radar. Along with its normal Mode-S information, Enhanced Mode-S can transmit specific Downlink Aircraft Parameters (DAP) when requested. This information is provided by various aircraft sensors. Some of these DAPs are:

  • roll angle
  • true track angle
  • ground speed
  • track angle rate
  • true airspeed
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  • $\begingroup$ Hi, you are right in that I am asking more about Enhanced Mode-S. Is it by regulation and law that all aircraft must transmit ADS-B or ADS-B Enhanced Mode-S. Also you have mentioned "that info is not considered accurate for flight and navigation purposes", is it due to ADS-B being a delayed information or? $\endgroup$
    – user1538798
    Feb 5, 2020 at 1:34
  • $\begingroup$ The information that I mentioned that is not intended for flight nor navigation is the AHRS information that you can get from some ADS-b in devices. Those devices can give you heading, roll, and pitch. But, it is not a certified source of this information. ADS-b out does not have that info by itself. $\endgroup$
    – Dean F.
    Feb 5, 2020 at 2:16

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