[time-nuts] Slightly OT - GPS-Based Accurate Direction Finding

Attila Kinali attila at kinali.ch
Thu Aug 26 15:19:39 UTC 2010

On Fri, 27 Aug 2010 00:09:31 +1000
David Smith <david at smithfamily.net.au> wrote:

> Some background - I'm needing an accuracy of 1 degree or better.  The 
> experiments are using digital communication modes and sometimes aircraft 
> scatter so signals are regularly inaudible and often non-existent, so 
> peaking "by ear" is not usually an option.

1 Degree isnt that hard. Even a standard magnetic compas reaches that.
I've quite an old one at home with prismas for aiming with which it
is possible to get better readings than 0.5°. Of course, you'd have
to take the deviations from true north into account. But magnetic maps
should be readily available (at least they were when i was a scout).
> As Atilla says, the software is probably not that fundamentally 
> complicated.  However, the devil is possibly in the detail of aligning 
> sample timing, positioning ...

You don't have to allign sample timing. It is helpfull though, to get
better precision/accuracy. It is enough that you use the relative
phase differences of the SV on each GPS module. This way you can get
rid of the unknown sample phase difference of the GPS modules.
(the principle is the same like normal GPS measurement using a 4th
satelite to get rid of the unknown "time"). 

I just had a look at the LEA6-T protocol specs. The RXM-RAW message
provides you with a phase difference (in L1 cycles) and a frequency
offset (doppler, in Hz) and pseudorange (in meter) for each SV that
is being tracked. With this you should be able to eliminate the
phase and frequency difference/drift between the two modules.

Using a long enough measurement time, you should be able to get
to well below a degree of heading accuracy. I know that the ETH
in Zürich uses LEA6-T modules to get positioning resolutions
better than 2mm with single GPS modules, though they have to collect
more than an hour of phase data for this.

			Attila Kinali

If you want to walk fast, walk alone.
If you want to walk far, walk together.
		-- African proverb

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