[time-nuts] Impact of GPS antenna height measurments

[Michael Wouters]

Tom Van Baak said:

"This note is just a plea not to apply the speed-of-light number or
the "nanosecond a foot" rule-of-thumb out of context."

It works reasonably well as a rule of thumb. It's an upper limit  but
if you wanted to refine it a bit, divide by two. The average value of
sin(x) on [0,pi/2] is 2/pi ie about 1/2.

This agrees quite well with the results of the post-processing I
described earlier.

Cheers
Michael


On Tue, Jun 28, 2016 at 8:49 AM, Tom Van Baak <tvb at leapsecond.com> wrote:
>> So to travel 22 meters is about .000,000,073 Seconds.  Or 73 nanoSecond.
>
> Hi Gary,
>
> I want to echo what Bob just wrote. People get carried away with "a nanosecond is a foot" and think it applies 100% to GPS timing and position, or in this case, elevation errors.
>
> Equating 22 m with 73 ns, or equating 1 foot with 1 ns is only true in the impossibly rare case of one satellite directly above you. In reality, 1) most of the time the SV are further down and so the error is reduced by sin(angle). And, 2) the GPS timing solution is typically based on lots of satellites, not just one, and so the effects of position error is further reduced by the ensemble mean.
>
> An accurate position is desirable. No question about that. This note is just a plea not to apply the speed-of-light number or the "nanosecond a foot" rule-of-thumb out of context.
>
> /tvb
>
>
> ----- Original Message -----
> From: "Gary E. Miller" <gem at rellim.com>
> To: "Mark Barettella via time-nuts" <time-nuts at febo.com>
> Sent: Monday, June 27, 2016 12:31 AM
> Subject: Re: [time-nuts] Impact of GPS antenna height measurments
>
> 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.
>
>
> 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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