[time-nuts] Aircraft ping timing

[nuts]

On Sat, 22 Mar 2014 07:40:27 -0000
"David J Taylor" <david-taylor at blueyonder.co.uk> wrote:

> 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

Note that the ADS-B mentioned is just a fancy version of the
transponder that was turned off. 

I ran into your R Pi page about halfway in the process of doing my
Beaglebone Black RTLSDR page. I have RTLSDR and dump1090 running on
Angstrom Linux. I bought the GPS you suggested and will attempt to get
NTP going with it, so expect a question or two off line. ;-)

I wonder if your problem with the timing is related to the R Pi being
nearly maxed out. The Beaglebone Black is about 4x the processing power
of the R Pi, with dump1090 running at 30% CPU. 

Seems to me that MLAT is a function of the distance between receivers,
At 0.5uS, the distance a radio wave travels is 150 meters. Throw in
some error and worst case it, you are still talking less than a km.
Good enough for MLAT considering the option is no location information.

The flightradar24 decoders are using a Beaglebone Black with the "radar
cape". That unit claims a 100MHz clock, so the timing resolution is
10ns. But my recollection is the GPSDO units only do around 100ns when
the dust settles. The radarcape uses an FPGA, so it doesn't have
the computer delay issue and is probably limited by the GPSDO accuracy.

Personally for the cost, I'd take the 1km error from the DVB-T stick
and call it a day.