[time-nuts] Sub mm measurements with gps timing antennas?
magnus at rubidium.dyndns.org
Thu Apr 26 22:03:13 UTC 2012
On 04/25/2012 07:05 PM, Tom Van Baak wrote:
> Hi Attila,
> Are you sure the customer said sub-mm and not sub-meter? I know
> post-processing is really helpful, but the LEA-6 is a single frequency
> receiver so all the advantage of L2 is lost for this customer. The
> bullet antenna's don't even have an arrow for North ;-)
Actually, if you do both code and carrier phase you can make ionspheric
measures on L1 alone. This is well known and documented, but I wonder
how many actually attempts it. The thing is, the phase shift due to
ionspheric dispersion has the same size, but different sign as you do
code and carrier phase. What you can do is to resolve the bias between
them and then make ionspheric observations and compensate for it. I've
reported on this before, and Oncore VP receivers where used between a
pair of caesium clocks.
It would be a step up from carrier phase smoothed code measures.
> One thought -- seeing how this is a research project. It might be
> possible to cross-correlate the post-processed data against the
> Az-El of each SV along with ambient temperature over days or
> weeks and thus actually measure the phase center profile as well
> as tempco of the system. This would be no small effort, depending
> on the math and programming skills of the researcher(s), but the
> advantage for them is that is costs time instead of money. Then
> armed with this "calibration" data (possibly unique to each unit),
> it would be possible to reduce these effects, improving precision.
> I have no idea how much. Still, an interesting project.
> A simple test that could be done locally (refrigerator, sauna, etc.)
> would be to measure the tempco of the entire system (antenna,
> cables, LEA-6T) before they deploy it to a mountain. It may also
> be the case that the system has both a temperature coefficient
> and a temperature change coefficient so it's not a simple 2-point
> test. You can probably ignore humidity and barometric pressure.
> Another test would be to rotate the antenna at 1 RPH (revolution
> per hour) and then look for modulation in the post-processed
> solution. This would give a hint of the quality of the antenna. As
> a baseline, try the same test using a precision gps antenna. I
> have spare pin-wheel, choke-ring, and ground-plane antennas
> that I could loan, but surely these are available where you are,
> and probably cheaper than postage from here.
> It seems that everyone else that does sub-ns precision timing or
> mm positioning uses a large combination of tricks: dual-frequency
> antenna and receiver, geodetic-quality antenna, passive or
> active temperature control, phase-stabilized cables, GPS and
> Glonass, external frequency reference, and post-processing.
> Your customer is only using one from this long, expensive list.
> So there may be a lesson there.
External frequency reference in terms of a rubidium should get you quite
far in that regard, without much funds being used.
> Can you share any data they have collected already? I would be
> interested to know how far one could push a LEA-6T.
That would be interesting. Getting one to play with would be fun.
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