[time-nuts] Aircraft ping timing

[David J Taylor]

From: Hal Murray

There is a newer system getting phased in:  ADS-B
  http://en.wikipedia.org/wiki/Automatic_dependent_surveillance-broadcast

The plane broadcasts it's position and velocity every second.

The SDR folks are having fun with it.  With one of the USB TV receiver 
gizmos
and a Raspberry Pi, you are limited by the height of your antenna vs
curvature of the earth.  A friend with an antenna 20(?) feet up gets planes
out to several hundred miles.  I don't know how far it will work if you have
an antenna on top of a hill.
  http://www.rtl-sdr.com/adsb-aircraft-radar-with-rtl-sdr/
==========================

.. and by sharing this data on a central server you can overcome the 
line-of-sight limitation:

  http://www.satsignal.eu/raspberry-pi/dump1090.html

There is a further development which may be of interest to the more 
mathematically oriented time-nuts, creating positions of aircraft which do 
no send out their position with ADS-B but use earlier responses.  By 
comparing the timing of responses from aircraft which do send position with 
those that don't you can multilaterate a position for the position-less 
aircraft.  This requires timing in the receiver device to a level of about 
100 nanoseconds, most often achieved with a simple crystal oscillator (but 
which needs calibration).  In the DVB-T sticks the usable sampling rate is 
about 2 MHz, and the resulting 500 microsecond resolution appears not to be 
good enough for even a basic multilateration fix.

If anyone is interested further or could help with suggestions on improved 
accuracy with the DVB-T sticks, a suitable place would be the Plane Plotter 
Yahoo group:

  http://groups.yahoo.com/group/planeplotter/

Cheers,
David
-- 
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-taylor at blueyonder.co.uk