SkyView Log Data - indicated altitude
- Thread starter RV6-KPTW
- Start date
I think I'll make this more interesting. There I was, flying fat, dumb, and happy thinking that my indicated altitude reflected what the "ATC system" sees. Yes, transponder check in the last 12 months were fine. I was on a VFR flight 5500 indicated, Baro = 30.38. After landing, FlightAware reported based on ADSB that I was at 5075. I dug into the SkyView log and see density Alt 4991, Pressure Alt 5064, and alt bug (was on autopilot) of 5500. I see that the FlightAware Alt and the SkyView Pressure Alt are pretty close. I always thought that reported altitude was corrected for pressure and so FlightAware should reflect the indicated altitude. What has been the forum experience on this?
thibault
Member
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- Oct 25, 2009
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- 191
Dennis,
A little history first. The altitude reported to ATC by an encoder has always been pressure altitude (meaning the value referenced to 29.92) not indicated altitude (meaning referenced to the local baro setting). This had to be true, because blind encoders were first and had no local baro info. Encoding altimeters, which do have local baro setting capability had to work the same, or the system would fall apart. The ATC equipment automatically applies the local baro setting to the received data from the A/C.
Now move to today with ADSB-Out. They better report things the same or again, the system would fall apart.
Your flight example shows that your baro setting of 30.38 for 5,500 ft indicated would mean a pressure altutude of about 5,000 ft and that is what Flightaware showed and so did Skyview's pressure altitude.
On a recent flight of mine, I was IFR at 6,000 ft indicated with a baro setting of about 29.86. Flightaware and Skyview pressure altitude were both at 6,100 feet. Both examples make sense to me.
Furthermore, my equipment is all Skyview with the SV-26X (Trig) transponder for ADS-B out. What is your equipment?
A little history first. The altitude reported to ATC by an encoder has always been pressure altitude (meaning the value referenced to 29.92) not indicated altitude (meaning referenced to the local baro setting). This had to be true, because blind encoders were first and had no local baro info. Encoding altimeters, which do have local baro setting capability had to work the same, or the system would fall apart. The ATC equipment automatically applies the local baro setting to the received data from the A/C.
Now move to today with ADSB-Out. They better report things the same or again, the system would fall apart.
Your flight example shows that your baro setting of 30.38 for 5,500 ft indicated would mean a pressure altutude of about 5,000 ft and that is what Flightaware showed and so did Skyview's pressure altitude.
On a recent flight of mine, I was IFR at 6,000 ft indicated with a baro setting of about 29.86. Flightaware and Skyview pressure altitude were both at 6,100 feet. Both examples make sense to me.
Furthermore, my equipment is all Skyview with the SV-26X (Trig) transponder for ADS-B out. What is your equipment?
Tom,
I have the same setup as you for ADSB out. In the past, I recollect (correctly??) seeing an altitude on FlightAware within 100 ft of indicated. On this flight, it was ~500' difference. In your example, I would not be concerned about 100'. 500' makes me wonder. The baro of 30.38 potentially accounts for about 460' so what you say makes sense.
I checked several random flights in the same area at about the same time (similar conditions) but can't sort out what altitude they were actually flying vs what FlightAware reported.
I am hoping the question is more about FlightAware - are they reporting pressure altitude or correcting? Perhaps what ATC sees and FlightAware reports are different.
I'll ask FlightAware.
Dennis
I have the same setup as you for ADSB out. In the past, I recollect (correctly??) seeing an altitude on FlightAware within 100 ft of indicated. On this flight, it was ~500' difference. In your example, I would not be concerned about 100'. 500' makes me wonder. The baro of 30.38 potentially accounts for about 460' so what you say makes sense.
I checked several random flights in the same area at about the same time (similar conditions) but can't sort out what altitude they were actually flying vs what FlightAware reported.
I am hoping the question is more about FlightAware - are they reporting pressure altitude or correcting? Perhaps what ATC sees and FlightAware reports are different.
I'll ask FlightAware.
Dennis
To add a little more, to make the NextGen work better for traffic resolution, ADS-B reports both your pressure altitude and your GPS altitude.
This gets real tricky in areas where there is substantial difference in ground elevation in a short distance. Altimeter setting for Phoenix Deer Valley (DVT) and Prescott, Sedona or Flagstaff will be substantially different even though they are worked by the same approach control sector. I don't know how they handle that at the ATC console level.
The difference is typically around 0.5" of mercury.