[time-nuts] GPS position

[Bob Camp]

Hi

All of the things we yack about in terms of antenna location and timing, also apply
to location and self survey. The gotcha is that they apply to a somewhat greater 
extent. 

A timing receiver can be pretty happy with a 30 degree view to the horizon.
It’s not perfect, but it can work. For self survey, getting down to the horizon matters
a bit more. Four sats almost straight overhead will do fine for timing … not so fine for location. 

You can do timing with a single sat. It’s not ideal, but again, it does work. You are only
getting started doing a survey with four sats locked on. I can sit here with a plug in module
and get it to do some really strange stuff by blocking this or that direction. Swinging things 
ten or twenty meters is pretty easy once you figure out the “right” location to make it all 
happen. 

Lots of variables ….

Bob

> On May 2, 2016, at 7:53 PM, Joseph Gray <jgray at zianet.com> wrote:
> 
> Nick,
> 
> Comparing the coordinates from the different units, I'm seeing about
> an 8 ft spread in latitude and about 3 ft in longitude. The elevation
> is much worse - about a 30 m spread. All units are on the same antenna
> and splitter.
> 
> I used this calculator:
> http://www.csgnetwork.com/degreelenllavcalc.html
> 
> These numbers are from self surveys at different times. I will soon be
> doing a major overhaul of the shelf that holds that equipment. When I
> do, I'll have all units start a survey at the same time to see how
> much of a difference it makes. I have three different model GPSDO's,
> although all of them are HP varients (even the Lucent).
> 
> Joe Gray
> W5JG
> 
> 
> On Mon, May 2, 2016 at 4:21 PM, Nick Sayer via time-nuts
> <time-nuts at febo.com> wrote:
>> I haven’t tried, but if you take two GPS receivers of different manufacturers and feed them the same antenna feed… to what sort of tolerance can you expect their solutions to coincide?
>> 
>> I mean, obviously they’re *supposed* to show the same location, but I can imagine that the math can come up with inexact solutions that may differ systematically (obviously at low resolution) between different manufacturers.
>> 
>> Or are those errors swamped by other factors that are not receiver specific?
>> 
>>> On May 2, 2016, at 2:44 PM, Joseph Gray <jgray at zianet.com> wrote:
>>> 
>>> Sorry, no Thunderbolts. TVB emailed me to ask if I had one, to make
>>> the same suggestion.
>>> 
>>> Joe Gray
>>> W5JG
>>> 
>>> On Mon, May 2, 2016 at 11:38 AM, Mark Sims <holrum at hotmail.com> wrote:
>>>> You could try Lady Heather's precision 48 hour survey.  It basically does a weighted average of median filtered fixes  over 1 hour time intervals.  The median filtering automatically throws outliers out of the data.
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