[time-nuts] [Fwd: Accurate Thunderbolt position]

[Lux, James P]

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On Behalf Of J. Forster
Sent: Wednesday, July 15, 2009 10:54 AM
To: time-nuts at febo.com
Subject: Re: [time-nuts] [Fwd: Accurate Thunderbolt position]

"Mark Sims" <holrum at hotmail.com> wrote:
>>
>> My Ashtech Z12 system has located my g-spot to within 4mm lat/lon
>> and 27mm altitude.

I forwarded the full post to a good friend who knows a LOT about GPS and
herewith is his reply:

<snip of excellent explanation>
>
>> A GPS guy I know comments that when you start talking down in the
>> sub-meter sorts of accuracies, particularly for absolute
>> measurements...
>
> I wish people would not imagine that GPS measurements are "absolute."
> If you don't determine position with respect to reference points on
> the ground by DGPS, then you are determining position with respect to
> a particular combination of satellite orbital position coordinates and
> clock-offset parameters that you got from real-time broadcasts or
> perhaps later via the Internet from someone.  Those position-
> coordinate and clock-offset parameter values were determined by
> someone who _assumed_ position-coordinate values for certain ground
> stations.  (Real-time broadcasts suffer substantially from
> extrapolation; and short-time orbit-determinations usually also
> involve a significant amount of extrapolation.)  There ain't no such
> thing as absolute position, any more than there is "absolute time."
> All position and all time measurements are _relative_ to some man-made
> and man-maintained "standard."  If you are talking about state-of-the-
> art, research-grade measurements, then it is essential to understand
> the relevant standards.

Yes indeed..

>
>
>> there's a whole raft of factors that are all of the same general
>> magnitude
>> that you need to take into account: tidal deformation, ionosphere,
>> multipath, thermal distortion of your antenna, changes in the cable
>> due to
>> temperature, etc.etc.etc.>
>
> The factors recited in the above-quoted paragraph are _NOT_ all of the
> same magnitude.  Not even close.
>

When I was talking to my GPS guy, we were talking single channel, non-geodetic receivers and antennas, and his point was that there's a lot of stuff that you need to be aware of when you go the next step beyond a simple handheld GPS in your car.  

As you've mentioned, dual frequency, good antennas, etc. are all a given when doing mm scale measurements.  The tidal deformation one was interesting to me, because I hadn't thought that it was that big.  Model-out-able, certainly, but interesting none-the-less.