I noticed some of the traffic being tracked on my receiver are flying underground. Intuitively, I knew this could not be. So I Googled and learned. I get why it is happening.
So, is there a chart, a rule of thumb, an algorithm, or some such that I could use to make the adjustment based on the pressure deviation from 29.92?
Blue Sky question. Is there a method where I could input the correction factor so that the data on my dump1090-fa/ page is more accurate?
and convert the altitude in the updateData() function i suppose.
Just need to get the current altimeter somehow, i’ll leave that to your imagination, there are boxes on the webpage to enter minimum altitude, this could be a way.
# h = (1 - (P / 1013.25)**0.190284) * 145366.45
# P = (1 - h / 145366.45)**(1/0.190284) * 1013.25
This is essentially what the reported altitude is, it’s really reporting a pressure P, interpreted as a height h assuming a standard atmosphere. Of course, the atmosphere is never standard…
One rule of thumb I’ve seen is 30ft per 1hPa difference from 1013.25; this is a fairly rough approximation.
If what you want to know is height above MSL (or AGL, if you know the local terrain height) then a simpler approach for ADS-B equipped aircraft is to use the GNSS/GPS altitude, which is a geometric height above the ellipsoid and so is unaffected by pressure. (nb: you do need to adjust for the geoid height to get height AMSL, but that doesn’t vary a lot over smaller distances)
Yep. Just the MSL. I am not clear on if you are saying there is an option I can click that sill provide this info. Can you elaborate on what you mean by the ellipsoid?
Geometric altitude (= GPS height above ellipsoid) is reported by skyview when you select an aircraft:
The geometric altitude that is reported is a height above the WGS84 ellipsoid. This ellipsoid is a standardized approximation of the shape of the Earth, basically a slightly squashed sphere with specific dimensions. Local gravitational variation means that sea level doesn’t exactly follow the ellipsoid; the difference is the “geoid height”. There are fairly complex models (EGM96 et al) that model the shape of the geoid.
Latitude: 52° N = 52° 0' 0" N
Longitude: 0° E = 0° 0' 0" E
GPS ellipsoidal height: 1000 (meters)
Geoid height: 46.300 (meters)
Orthometric height (height above EGM96 geoid which approximates mean sea level): 953.7 (meters)
(Note: orthometric height = GPS ellipsoidal height - geoid height)
i.e. a “geometric altitude” of 1000m near Cambridge is actually 953m above MSL.
The geoid height doesn’t vary much unless you move long distances, so you can probably just find the geoid height for your receiver location once and use that for the aircraft you see.
Thanks for the idea. Instead of doing the conversion, could I replace the Barometric altitude with the Geometric Altitude in the popup hint that pops when one points at an airplane?
The index.html span id references $("#highlighted_altitude").text(in script.js
So you could for example duplicate those two with different name to also display geometric altitude.
That way there is not nothing displayed when the geometric altitude is missing.
Choose your local airport. Simple scripting to get that data. You only need to get your closest airport. It will be close enough for any aircraft you can pickup.
For aircraft at and above 18,000 feet, you would want the ALT to be set to 29.92 in-Hg. So, I would suggest omitting any of those aircraft from referencing your local pressure.
If you have a Pro Stick Plus, try 38 in your location.
Basically you want to turn up the gain as high as you can without losing aircraft when they are flying low and close to your antenna.
Think of the gain as volume and the receiver can’t understand something that is too loud.
But if it is too quiet the receiver can’t understand it either.
Of course you only have one volume control for all the aircraft, which can be very far away and very close.
So it’s almost always a compromise.
(The antenna “hears” better towards the horizon than 30 degrees or up. So if a plane passes at more than 20000 ft it is either far enough away or at a weaker receiving angle of the antenna, so it never overloads the receiver. But if the plane is flying at 5000 ft or lower, it is much easier for it to be too loud to be heard.
Also don’t worry about the receiver, it can “hear” perfectly fine as soon as the strong signal is over. So it can still hear the other airplanes)