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 Post subject: Re: Ground Vehicles
PostPosted: Mon Mar 13, 2017 9:45 pm 
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chrisjohnston50 - FlightAware user avatar

Joined: Fri Apr 01, 2016 3:29 am
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Location: Fort Lauderdale
HuffTheWeevilq wrote:
On top of that, does anyone know if there is actually a way to disable ADS-B on an airliner (i.e. a Boeing or Airbus)? It is my impression that when a pilot puts their transponder into STBY mode, ADS-B is still transmitting as usual. Perhaps I am wrong.


On some fleets, the transponder circuit breakers, along with a few other breakers, have color coded collars (usually red) which are pulled on completion of the "SHUTDOWN" checklist.

This is done to de-power certain components if the aircraft is going to be left unattended for an extended period of time powered up.


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 Post subject: Re: Ground Vehicles
PostPosted: Tue Mar 14, 2017 3:58 pm 
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N456TS wrote:
The transponder must be set to GND on the ground. ALT in the air. Standby is pretty much only used in the hangar/times of maintenance. Part of the cert spec is it must automatically change from ALT to GND after landing and GND to ALT when taking off.

So no, pilots shouldn't be putting the transponder to standby unless there is a real reason to do so.


OK, that clarifies things a bit. I've been trying to find some info on this "GND" thing for a while. The ADS-D Decoding Guide http://adsb-decode-guide.readthedocs.io/en/latest/index.html makes no mention of it. So I have two(ish) questions:

What input/sensor does it use for determining GND/ALT? I see a lot of aircraft that jump from GND to 25, 50, 75 feet while taxiing (and sometimes at the gate). Once in great while an aircraft will jump up to cruising altitude while at the gate (and at 0kts). Then there was one time I saw 60000ft at the gate! Clearly the later two are errors (the coding only allows for up to 50175 feet), but I don't see why the first example is reporting a (low) altitude when on the ground. Any thoughts?

Perhaps this is related: How is "GND/ALT" coded in the message? The altitude part can't just be 0 because that actually means -1000 feet. Again, the Decoding Guide doesn't specify.


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 Post subject: Re: Ground Vehicles
PostPosted: Tue Mar 14, 2017 5:18 pm 
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obj - FlightAware user avatar

Joined: Tue Sep 30, 2014 7:14 pm
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There are two different position message types: airborne position and surface position.

Airborne positions include altitude. Surface positions include track heading / speed. When airborne the velocity is in a separate message. The position encodings are slightly different so that surface positions have better resolution but are not globally unique (they decode to one of 8 possible points on the surface of the Earth - the assumption is you know which one is right based on the receiver location)

There is a somewhat complex set of rules for deciding when to do airborne vs surface position reporting that includes e.g. weight-on-wheels sensors (if present); a-priori knowledge of the type of vehicle the transponder is on; current groundspeed, airspeed, and radio height.

The "60000ft at 0kts" may be an artifact of what the FAA data feed does if you're looking at tracklogs rather than direct data you received.


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 Post subject: Re: Ground Vehicles
PostPosted: Tue Mar 14, 2017 5:31 pm 
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chrisjohnston50 - FlightAware user avatar

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N456TS wrote:
The transponder must be set to GND on the ground. ALT in the air. Standby is pretty much only used in the hangar/times of maintenance. Part of the cert spec is it must automatically change from ALT to GND after landing and GND to ALT when taking off.

So no, pilots shouldn't be putting the transponder to standby unless there is a real reason to do so.


Our SHUTDOWN checklist calls for the transponder to be placed in STBY.


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 Post subject: Re: Ground Vehicles
PostPosted: Tue Mar 14, 2017 6:14 pm 
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HuffTheWeevil - FlightAware user avatar

Joined: Tue Jan 06, 2015 1:25 am
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obj wrote:
There are two different position message types: airborne position and surface position.

Airborne positions include altitude. Surface positions include track heading / speed. When airborne the velocity is in a separate message. The position encodings are slightly different so that surface positions have better resolution but are not globally unique (they decode to one of 8 possible points on the surface of the Earth - the assumption is you know which one is right based on the receiver location)

There is a somewhat complex set of rules for deciding when to do airborne vs surface position reporting that includes e.g. weight-on-wheels sensors (if present); a-priori knowledge of the type of vehicle the transponder is on; current groundspeed, airspeed, and radio height.

The "60000ft at 0kts" may be an artifact of what the FAA data feed does if you're looking at tracklogs rather than direct data you received.


Quite intriguing. Thanks for the info.

And the "60000 at 0" was on my own receiver. Only happened for a second or two so I'm assuming just a bug during decoding or something.


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 Post subject: Re: Ground Vehicles
PostPosted: Tue Mar 14, 2017 6:27 pm 
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N456TS - FlightAware user avatar

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chrisjohnston50 wrote:
Our SHUTDOWN checklist calls for the transponder to be placed in STBY.


A real reason to do so?
Maybe it's to help you to not transmit the wrong squawk on next startup if the last pilot didn't clear it out.
But from an FAA (and non-company specifics), there isn't a good reason. They can add to your SHUTDOWN checklist to "rub your belly 3 times counter-clockwise", but it isn't needed for ADS-B to function correctly.


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 Post subject: Re: Ground Vehicles
PostPosted: Tue Mar 14, 2017 7:28 pm 
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Joined: Fri Apr 01, 2016 3:29 am
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N456TS wrote:
chrisjohnston50 wrote:
Our SHUTDOWN checklist calls for the transponder to be placed in STBY.


A real reason to do so?

Maybe it's to help you to not transmit the wrong squawk on next startup if the last pilot didn't clear it out.
But from an FAA (and non-company specifics), there isn't a good reason. They can add to your SHUTDOWN checklist to "rub your belly 3 times counter-clockwise", but it isn't needed for ADS-B to function correctly.


Reason?

I would have to ask the FAA. They are the ones that approve our checklists. No changes can be made to them without their consent.


Last edited by chrisjohnston50 on Thu Mar 16, 2017 1:50 am, edited 1 time in total.

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 Post subject: Re: Ground Vehicles
PostPosted: Wed Mar 15, 2017 10:58 pm 
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N456TS - FlightAware user avatar

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chrisjohnston50 wrote:
Reason?
I would have to ask the FAA.

Ok, approval is not really relevant. They could also approve a list with "count all your fingers before engine start".
Please do ask the FAA. Or your chief pilot. There is no FAR, AIM, nor AC to back up this as some sort of requirement or SOP for all aircraft. It's only company specific. Companies can choose what they wish. (Which is perfectly fine). FAA approval doesn't imply that the FAA promotes or requires that specific task. It only shows that they don't have an issue with you performing the specific tasks and in the order in which the tasks are listed.


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