[time-nuts] Impact of GPS antenna height measurments

[Bob Camp]

Hi

If your time is off because your antenna location is off and you are running a GPSDO …
The error does matter for frequency.

1) Start with a group of sats in the sky all in the direction of the  error vector. Your
GPSDO will get a time solution that is 72 ns early. 

2) Move the group of sats so they are at right angles to the the error vector. Your
GPSDO will get a time solution that is correct (the error does not impact the solution).

3) Move the sats so they are in the exact opposite direction of the error vector. Your
GPSDO now gets a time solution that is 72 ns late. 

As the sats move in the sky, the solution “swings” back and forth between +72 and -72 ns.
It gets a bunch of zeros in there as well. How often you get a “all bad minus” versus “all bad
plus” depends a *lot* on your antenna location. Even if it is simply a 2/3 of the sat’s over there
and 2/3 over here …. there is still a significant swing.

A swing in time will give you a delta frequency. If you otherwise would get 1x10^-11 delta F out 
of your GPSDO, That will double if the +72 to -72 ns swing occurs in 4 hours. 

Bob

> On Jun 27, 2016, at 3:31 AM, Gary E. Miller <gem at rellim.com> wrote:
> 
> Yo Mark!
> 
> On Sun, 26 Jun 2016 09:02:55 -0400
> Mark Barettella via time-nuts <time-nuts at febo.com> wrote:
> 
>> I
>> estimate my antenna’s actual height at about +5 m high and the gps
>> indicates -17 m.
> 
> Others have covered some obvious details.  Different ellipsoids,
> long term surveying, etc.
> 
>> My question is will this adversely influence the
>> accuracy of the gpsdo output?
> 
> Depends on how accurate you need.  I'll assume your estimate is perfect,
> which is that your GPS is reading off by 22 meters.
> 
> The speed of light is  299,792 kilometers/second.
> 
> So to travel 22 meters is about .000,000,073 Seconds.  Or 73 nanoSecond.
> 
> All else being equal, does a constant 73 nanoSec matter to you?
> 
> For comparision, a Trimble RES SMT 360 only promises 15 nanoSec (1 sigma).
> 
> If all you want is a stable frequency from your gpsdo then the offset
> is not relevant.
> 
> RGDS
> GARY
> ---------------------------------------------------------------------------
> Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
> 	gem at rellim.com  Tel:+1 541 382 8588
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