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    Re: GPS selective availability
    From: Geoffrey Kolbe
    Date: 2018 Sep 23, 09:17 -0700

    Hello David

    I think today's differential GNSS systems are clever enough that the monitor cited over the passive GNSS station will radio the correction automatically to the unit being used for the actual survey. You will not need two surveyors.

    As for switching to the ETRS89 system, my receiver (vintage 1998) is similar to yours in that it goes no higher than European 79. It has a remarkable list of geodetic systems in its memory - including such well-known systems as Tambalai 1948 and Pitcairn Astronomical 1967 - but it does not have ETRS89 which seems something of an oversight.

    ETRS89 differs from WGS84 in that it is a system predominantly for Europe, whereas WGS84 is for the whole planet. ETRS89 assumes that the continental plate on which Europe sits is fixed, whereas WGS84 allows this plate to move against its own reference coordinate system. The two systems will then drift slowly apart at a rate of about an inch a year or so


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