Flightaware Altitude Reporting

wanttaja

wanttaja

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Ron Wanttaja
Was looking at a track on Flightaware for a recent accident, and it reported altitudes that really didn't match the aircraft's capabilities. How reliable are the altitude reports on Flightaware?

Ron Wanttaja
 
Gps altitude is severely limited, especially with any pressure that highly is different then standard pressure, was watching my student on checkride yesterday doing an rnav with mins at 740, flight aware reported him at 200, pressure was 30.40
 
1anG said:
Gps altitude is severely limited, especially with any pressure that highly is different then standard pressure, was watching my student on checkride yesterday doing an rnav with mins at 740, flight aware reported him at 200, pressure was 30.40
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Thanks, but this was reporting in the flight levels (25,000-35,000) for an LSA. Brand new transponder, just completed certification. Not sure if the ADS-B itself had been tested, yet.

Ron Wanttaja
 
I have seen a 172 at FL370 on FlightAware. I don't know if it's an encoder thing, a 978 vs 1090 thing, or what. Its track filter is not perfect either and will sometimes mix radar and ADS-B data inappropriately.
 
My understanding was that the ADS-B-out reports pressure altitude. I too noticed the discrepancy recently while studying the Bonanza crash in TEX. The reported altitude was too low by 500'. The altimeter setting at the time of the accident was 30.47, which is consistent with the 500' difference.
 
I ran a PAPR on the aircraft in question, and while it mostly passed, it failed on both the barometric and GPS altitude categories. However, I did not actually know the ADS-B equipment installed, and just made a guess.

Ron Wanttaja
 
Flight Aware always shows my altitude off my 2-300 feet.. looks like GPS altitude not pressure.


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Pressure altitude is what is reported by FlightAware. Transponders and ADS-B both transmit pressure altitude. ADS-B also broadcasts geometric altitude which is a form of GPS altitude, but measured to a surface model of the earth. Since pressure altitude is based on a pressure setting of 29.92, if you wish to determine the rough MSL altitude, you need to know the barometric pressure at the time and adjust accordingly for altitudes below the flight levels. Some pressure altitude encoders are to the nearest 100 feet while others are more precise, but regardless, ADS-B can only encode the altitude to the nearest 25 feet.
 
A few days ago I had a Cirrus SR20 come over the house at probably 3000ish that Flightaware reported as 40,800.
 
ADSB report GPS altitude, at least in simple GA units. My wing mounted ADSB out has no pressure sensor or connection to the transponder encoder. I think many of the flight following apps also use ATC derived data if the aircraft is IFR or VFR flight following and in radar contact, at which case would be displaying mode C altitude, which is transmitted as pressure altitude and then converted to local pressure corrections.
 
EdFred said:
I did a flight where FA said I was 200ft below the surface of Lake Michigan.
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I believe that. The GPS in my tablet reports through Garmin Pilot that my house keeps changing altitude. Yesterday it said the house was at -28 feet. Funny, I don't remember Budd Inlet being that high at high tide. The house is around 60 MSL in reality.
 
AlphaPilotFlyer said:
ADSB report GPS altitude, at least in simple GA units. My wing mounted ADSB out has no pressure sensor or connection to the transponder encoder. I think many of the flight following apps also use ATC derived data if the aircraft is IFR or VFR flight following and in radar contact, at which case would be displaying mode C altitude, which is transmitted as pressure altitude and then converted to local pressure corrections.
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If you have a SkyBeacon, it has a built in pressure altitude sensor. Systems such as FlightAware use pressure altitude, they don't convert to local pressure. The whole point of ADS-B, TCAS, and Mode C transponders all use pressure altitude to compare relative altitudes, after all there is no point converting into a MSL value that needs to current and correct altimeter setting in each and every airplane. A pressure altitude needs no pilot input and is reliable.
 
Ghery said:
I believe that. The GPS in my tablet reports through Garmin Pilot that my house keeps changing altitude. Yesterday it said the house was at -28 feet. Funny, I don't remember Budd Inlet being that high at high tide. The house is around 60 MSL in reality.
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FA is using pressure altitude, not MSL altitude. If every airplane reports a common altitude such as pressure altitude, it does not matter if it shows you are at 1000 feet below ground level as long as every other airplane that represents a collision risk is showing a similarly based altitude.
 
John Collins said:
If you have a SkyBeacon, it has a built in pressure altitude sensor. Systems such as FlightAware use pressure altitude, they don't convert to local pressure. The whole point of ADS-B, TCAS, and Mode C transponders all use pressure altitude to compare relative altitudes, after all there is no point converting into a MSL value that needs to current and correct altimeter setting in each and every airplane. A pressure altitude needs no pilot input and is reliable.
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I wondered about that, does the SkyBeacon actually have a pressure sensor? I never found a definitive answer. But FYI, it is acceptable to use geometric (GPS) altitude in UAT ADS-B out systems.
 
jrcox19 said:
I wondered about that, does the SkyBeacon actually have a pressure sensor? I never found a definitive answer. But FYI, it is acceptable to use geometric (GPS) altitude in UAT ADS-B out systems.
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In the SkyBeacon install manual, Section 5.2, TSO Authorization lists all the TSO's that it complies with and includes: Automatic Pressure Altitude Reporting Code-Generating Equipment, TSO-C88b SAE/AS8003. Also in section 6 System Specifications
6.1 System Functionality
skyBeacon is a wing-tip mounted unit that contains a 978 MHz transmitter,
power line transponder monitor, GPS/SBAS receiver, barometric pressure
sensor and altitude encoder, LED position light and LED anti-collision light.
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UAT is no different than 1090ES in terms of what is required for compliance with regulations. In both cases (UAT and 1090ES) 91.227(d) lists the altitude requirements that MUST be broadcast:

(d) Minimum Broadcast Message Element Set for ADS-B Out. Each aircraft must broadcast the following information, as defined in TSO-C166b or TSO-C154c. The pilot must enter information for message elements listed in paragraphs (d)(7) through (d)(10) of this section during the appropriate phase of flight.

(3) An indication of the aircraft's barometric pressure altitude;
(14) An indication of the aircraft's geometric altitude;

So Geometric Altitude must be broadcast along with Pressure Altitude. Geometric Altitude is a form of GPS altitude. Geometric Altitude is not sufficient to be compliant with the regulations and it is not used currently other than as a potential test of reasonableness.
 
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