[time-nuts] Speaking of Costas loops

[Attila Kinali]

On Tue, 02 Jul 2013 11:21:24 -0700
ed breya <eb at telight.com> wrote:

> For my needs, I'm more of a frequency-nut - I usually don't care 
> about getting time info, but I'd like perfect 10 MHz for reference. 
> Can using only the carriers lead to simple ways to get the same (or 
> better) frequency stability as a conventional GPSDO, but without the 
> time and location info, or is it pointless to worry about it, and 
> just go with full GPS decoding of everything? Or, is carrier-phase 
> just an enhancement only if you already have the full GPS info?

The GPS carrier varies IIRC +/-6kHz (=+/-4ppm) due to doppler shift for a
stationary GPS receiver. Ie you need at least the satelites relative velocity
to correct for that, which in turn means you need to know where you are
relative to the satelite and where the satelite is heading. And to get that
you need the full solution for x/y/z/t and the almanach data.

After you have that, you can use carrier phase tracking to further improve
the accuracy (or is it the precision?) of your solution. I know i have
seen papers discussing that, but i cannot recall which they were.
I guess a couple of google queries should get you enough to read on that.

An interesting idea is to use long integration times (up to 100s) of the
GPS signal to get a better SNR and thus higher accuracy. But this in turn
needs that the refrence oscillator has a stability comparable to the Rb
reference on board of the GPS up to the integration time (if i have
understood the discussion in [1] and [2] correctly).

On the other hand, i know a guy who does sub-cm positioning with unmodified
LEA6-T, by logging their satelite phase data and heavy post processing over
hours of data and comparing it to a neaby basline of two stations with
known coordinates [3]. They are currently aiming at sub-mm resolution.

And just in case: Averaging the doppler shift out does not work out because
the distribution of the satelites and their velocity vectors has a non
zero mean for most locations on earth.

			Attila Kinali


[1] "Effect of oscillator instability on GNSS signal integration time",
by Gaggero, 2008 (Master Thesis)
http://plan.geomatics.ucalgary.ca/papers/msc_thesis_gaggero_feb08.pdf

[2] "Ultra-stable Oscillators: Limits of GNSS Coherent Integration",
by Gaggero, Borio, 2008
http://plan.geomatics.ucalgary.ca/papers/ion08_ultrastable_pascalg_26sep08.pdf

[3] "GPS-Equipped Wireless Sensor Network Node for High-Accuracy Positioning
Applications", by Buchli, Sutton, Beutel, 2012
http://www.tik.ee.ethz.ch/~bbuchli/pubs/BSB2012_published.pdf

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