FlightAware Discussions

Elevation question on local SkyView


#1

Hi all… I started reporting ADS-B data a little over a week ago and had a question regarding elevation… I’m using the Orange Pro Stick along with the FA antenna, which is mounted inside my attic… I’m 3.6 miles from KISP… The elevation at my location is 171’, and my antenna is at 190’. I’m using a package installation of PiAware 3.6.3 and using dump1090-fa. One thing I have noticed when looking at flights that land at KISP is that the chart on the side of SkyView shows the altitude of planes that landed at appx minus 200 - minus 225’… I know the elevation at KISP is 99’. Can someone explain to me why this is? I do have both my location and antenna elevation entered in my settings.

Thanks,
Rick


#2

Transponders, especially older models, generally transmit pressure altitude. It is an easy STD to stick to.
ATC have local instruments that help them convert it to actual altitude.
This may account for the differences you are seeing.


#3

Actually all transponders respond with pressure altitude because that is required by the standard.

Some newer transponders will also transmit geometric altitude but not as frequently or only if queried by radar (not quite sure).

That is only for MLAT and not for display purposes.

While the planes transmit pressure altitude via transponder the pilots can adjust the altimeter in their cockpit to the local pressure. (https://en.wikipedia.org/wiki/Altimeter_setting)


#4

Thanks… Would AMSL Altitude be closer to what I would be expecting?


#5

Pressure altitudes are referenced to mean sea level. But not corrected for local difference to standard pressure.

You can put in the airport elevation and Islip altimeter setting into for example this calculator:
(http://imageserver.fltplan.com/PressureAndDensityAltitudeCalc.htm?USER=PILOT)

Altimeter setting / local pressure you can find for example here:
(https://flightaware.com/resources/airport/KISP/weather)

Currently it’s 30.38 inHg
Airport elevation for Islip is 100ft AMSL

That means the current pressure altitude on the ground at Islip is -321 feet.

So currently you can subtract 321 feet from the altitudes in skyview if you want to know the feet AGL.
If the local pressure changes that will also change.

Does that help you? Your question didn’t make too much sense to me.


#6

Thanks… Ultimately, I was just curious about a simple question – when one of the 737s or A320’s fly over the house, and my receiver is showing them at an altitude of 800’, how high are they above the house? Sorry if the question didn’t make sense…


#7

Real one is Geometric altitude.

The altitude used by controllers and pilots is mainly air pressure-based (barometric), for historical reasons (and not only). Imagine the GPS signal is lost for some reason… what they will do then?
The air pressure at your location, ground level, varies a lot depending of temperature, and other weather-related conditions. If you know the air pressure at ground level, you can calculate the real altitude from air-pressure one.


#8

And, I just realized that geometric is displayed on the info screen when you click on an aircraft… Thanks all for the help.