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    Re: GPS selective availability
    From: David Pike
    Date: 2018 Sep 23, 04:23 -0700

    Hello Geoffrey

    You talked of passive GNSS stations in the UK.  I’ve seen these studs, often on canal banks or sea walls.  They’re like a little stainless steel disc about 35mm dia.  I’m sure I saw at least one in the last couple of weeks taking our river cruiser from Newark to Rippon, the most northerly point on the English canal system, and back.  I’d always thought they were used to check if the location was moving because of continental drift, mining subsidence, collapsing banks or similar.  I suppose they could be used for that too.  They could also be used by an old style (theodolite) surveyor to centre his plumb bob on. 

    Unless you used two surveyors taking instantaneous readings, one at the stud and one at the place to be measured, wouldn’t inaccuracies creep in, because pseudo range inaccuracies change with time and the satellites used?  I use my Garmins in ddd.ddddd mode with kids in a slightly fudged Eratosthenes Earth circumference experiment, and I often note the 1/100,000 degree figure wandering around (60x6080/100,000 = about 4 feet), particularly in longitude where 1/100,000 degree is only about 2 feet in the UK.

    Are they still needed for GNSS correction by modern surveyors using ‘on the fly’ receivers with foolproof aerials (like a baby ‘flying saucer’ on a stick), carrier wave resolution, and using a dual frequency receiver to mathematically remove ionospheric and tropospheric errors? 

    Did you think of changing the datum set on your receiver to ETRS89 (if on, my receivers go no higher than European 79)?  That said, I do believe I read somewhere that as receivers are built to work with WGS84, using inbuilt ‘firmware’ to convert to any other datum will in itself add an additional slight error.  DaveP



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