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Map is about 45-60 seconds behind actual position

Posted: Fri Dec 19, 2014 7:36 pm
by wizardpc
My house is directly below the Base leg for the International Airport nearby. I routinely get jets passing over at about 1600ft AGL (I can read tail numbers in the daylight).

Being in America, not all our planes transmit in Mode-S. But last night, I was outside with the dog and saw a 737 overhead. Pulled up the map and the marker for that aircraft was about 45 seconds behind the actual location. Is that normal?

Re: Map is about 45-60 seconds behind actual position

Posted: Mon Dec 22, 2014 2:49 am
by agw
It depends :) If you see that for all aircraft then something is lagging somewhere, or intentionally introducing a delay (when dedicated ADSB receivers first came out they would add a five minute delay).

However, if it's just the occasional aircraft that's in the wrong place then what you're probably seeing is an aircraft that's taking its position from an inertial guidance system (http://en.wikipedia.org/wiki/Inertial_navigation_system) instead of a GPS. Depending on when it was last told the aircraft's true position, how accurate that position was and how far the aircraft has flown since then they can be a bit out by the time the aircraft comes to land.

The probable error margin for a position is transmitted by the aircraft but at the moment VRS doesn't show that on the map, it just shows the reported position.

Re: Map is about 45-60 seconds behind actual position

Posted: Mon Dec 22, 2014 2:30 pm
by wizardpc
It's all aircraft that I've observed (about a dozen). I'm using an RTL SDR stick with ADSBSharp, not a dedicated receiver.

Re: Map is about 45-60 seconds behind actual position

Posted: Mon Dec 22, 2014 2:55 pm
by jonfear
Hi

It maybe that ADSB# is introducing the delay. I always suggest that people use dump1090 which is available in many flavours including Windows. It has other benefits which may become more important next year.

Cheers

Jon