Hi

If:

1. You are doing a survey of a number of points in an area

2. You want to hit the “1/5 mm accuracy” that your system is rated to :)

3. You really do care

Then:

You point the arrow north to the best of your ability to make all the antenna errors show up in the same direction. It becomes an offset to the entire grid of points rather than an error randomly added to each point. You do a correlation process at your stationary survey (DGPS) locations to remove the error.

If you are not doing that sort of survey work … don’t worry about it.

(Apologies to those who do this for a living, you do indeed care, it’s just the time side that does not care). I’m sure  that “best of your ability” is indeed quite good.

Bob

On 12/15/14, 5:46 PM, Dave M wrote:

If you're using a standard antenna, they've characterized them for the
change in phase center with respect to the direction the signals are
coming from. It's assumed you'll install it level, so elevation is taken
care of.  The remaining uncertainty is the azimuth, hence the north arrow.

Now we can find out how much of a nut you really are.  On choke ring
antennas, I think the maximum shift in phase center with look direction
is on the order of single digit millimeters, or a few ps.

And how accurately do you know what direction is "north".  That could be
a whole project in itself, ranging from moss on trees, to shadows of
sticks and rocks, to observations of Polaris through a theodolite, and
so forth.

To answer which "North", it is True North, not Magnetic.  Orbits,
including GPS, are specified relative to the geographic pole. Magnetic
North moves noticeably over time and place.  True North moves somewhat
over time but only very slightly.

David

On 12/15/14 9:05 PM, Jim Lux wrote:

Dave,

I agree with the others. If you're using dual-frequency GPS with post-processing it might make a few mm or ps difference. This only matters for absolute measurements, or multiple measurements where you plan to compute differentials.

It also depends on the antenna. You can find the phase vs. angle profiles at http://facility.unavco.org

For one example of an antenna calibration see:
www.unavco.org/projects/project-support/development-testing/publications/trimchoke/trimchoke.pdf

The GPS pros take their mm very seriously. If all you're using is a fixed location and commodity GPSDO then it makes no difference at all, not even close.

But to prove us wrong, put the antenna on a 17 hour turn-table, collect data for 6 months, and then see if you see any 17h peaks in the FFT!

/tvb

Jim Lux wrote:

Thanks for the explanations.  I'm not terribly concerned about time, other
than knowing when it's time to eat and sleep... I'm more of a frequency nut
than a time nut.  I have a USGS map and recent survey of my property, so I
know where North is, to a pretty good certainty.

Thanks!!
Dave M

Hi

Since the only way you get frequency is by processing time, you do indeed care about time if you are looking for frequency :)

That said, the error is indeed only a fixed offset and it would not matter in any time solution. There are much larger issues in a time solution. For a precision time application, you would locate the antenna first. Next you run a survey receiver through the same antenna. Once you process the results of the survey, it would take out any phase center error from the antenna.

All that said, yes, it’s not worth worrying about in your case.

Bob

On 12/15/14, 8:10 PM, Tom Van Baak wrote:

Clever idea, but..

Most rotary joints have more phase and amplitude variability than the
antenna.

So you're stuck with rotating back and forth with a cable that's flexing
and now you get to measure the phase variability of the coax.

You really need to have your entire GPS system antenna and receiver on
the rotating table (which will need to have temperature controls, etc.)

Oh, what a pit one can descend into with the goal of reducing everything
to a minimum error.

I was thinking of some sort of non-contact RF bridge that would allow either side to rotate independently. Converting to optical would make this easy, but there must be a way to do it at 1.5 GHz too.

In this case the goal is actually not minimizing error. The goal is to vary each possible error source with its own prime modulation period. Collect lots of data and the FFT tells you how much each error contributes to the pie.

For example, instead of temperature control you modulate temperature by 5C over a 13 hour period. Instead of voltage control you modulate the 5V antenna power by 10% every 1.7 hours, etc.

/tvb

On 12/16/14, 5:59 AM, Tom Van Baak wrote:

two nested coils forming an air core transformer or slip rings are the
typical approach.
(Waveguide at 1.5 GHz is somewhat unwieldy in size).

The trick is in holding mechanical tolerances tight enough.I guess, 1mm
mechanical tolerance (easy, easy) would be comparable to the phase
center displacement.

Yes, but how do you know whether it's coax flex or phase center
displacement that's causing your 17 hour periodicity.

I was thinking not so much reducing error in the overall measurement,
but in reducing the uncertainty in the estimate of the size of each
contributor to the overall system.