[time-nuts] Gipsy-Oasis can track continental drift

[Joseph Gwinn]

On Mon, 30 Nov 2015 12:00:01 -0500, time-nuts-request at febo.com wrote:

> Message: 2
> Date: Sun, 29 Nov 2015 08:31:18 -0800
> From: "Tom Van Baak" <tvb at LeapSecond.com>
> To: "Discussion of precise time and frequency measurement"
> 	<time-nuts at febo.com>
> Subject: Re: [time-nuts] Einstein Special on PBS
> Message-ID: <F12DE74B7F824FF8BEB815D51C1C3965 at pc52>
> Content-Type: text/plain;	charset="utf-8"
> 
> Hal,
> 
> Right. The orbits are nominally circular -- but not exactly. The set 
> of orbital parameters cover these details. A quick google search 
> suggests the eccentricity for GPS is around 0.01. Still, that's 
> enough to cause +/- 23 ns of accumulated phase error per orbit. I'm 
> pretty sure the receivers take care of this math, since eccentricity 
> is a key part of any orbit model. I wish we could see the source code 
> to a GPS timing receiver.
> 
> I'm not sure I understand your elevation question. Are you talking 
> about elevation as in mountain vs. sea level altitude? Or elevation 
> as in satellite Az/El?
> 
> GPS satellites in view are about 20,000 km (overhead) to about 25,000 
> km (horizon) away, so the signal gets to you within about 65 to 85 
> ms. Whether you apply the full 4.5e-10 relativistic correction or no 
> correction to the SV clock at all, it makes only a 1 cm 
> time-of-arrival difference. That's why I said for trilateral 
> navigation purposes, the relativistic effects are in the noise. For 
> UTC time-transfer, however, an uncorrected 4.5e-10 frequency error 
> would continuously accumulate, giving 38 us/day phase error, the 
> number you often hear.
> 
> About survey grade -- I suspect the post-processing takes into 
> account anything you can think of, from the shape of the antennas to 
> space weather to the phase of the moon (literally).

Yes, literlly:


.<https://gipsy-oasis.jpl.nasa.gov/>

Ten or twenty years ago, I dug deep into this stuff.  Every module was 
somebody's PhD thesis.

Joe Gwinn


> 
> /tvb
> 
> ----- Original Message ----- 
> From: "Hal Murray" <hmurray at megapathdsl.net>
> To: "Discussion of precise time and frequency measurement" 
> <time-nuts at febo.com>
> Cc: <hmurray at megapathdsl.net>
> Sent: Saturday, November 28, 2015 2:37 PM
> Subject: Re: [time-nuts] Einstein Special on PBS
> 
> 
>> 
>> holrum at hotmail.com said:
>>> The GPS spec implies the satellites have a fixed frequency offset to
>>> compensate for relativistic effects.  But do they actually 
>>> dynamically and/
>>> or individually adjust the frequency to adjust for orbit variations and
>>> eccentricities?         
>> 
>> I think the orbits are circular so the frequency won't depend on the 
>> orbital 
>> position.
>> 
>> The next question is does the math in the receiver have to correct for 
>> changes due to elevation?  Does it become relevant if you are trying for 
>> survey grade results?
>> 
>> -- 
>> These are my opinions.  I hate spam.
>> > 
> End of time-nuts Digest, Vol 136, Issue 32
> ******************************************