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    Re: GPS ground stations
    From: Lu Abel
    Date: 2014 May 15, 13:13 -0700

    As I understand it, there really are two distinct set of ground stations.

    The first set are run by the folks from the US Air Force that operate the satellites.  These ground stations track the satellites and at least a few of them can transmit orders to them.   These satellites are located in the US, South America, Australia, New Zealand, and US-friendy nations such as Britain and South Korea.   See GPS.gov: Control Segment
    The second set are locations only within North America that, by knowing their location to a precision of a few cm, look at GPS signals and say "GPS gives a location that's off by ....."    This data is transmitted to geostationary satellites and rebroadcast down to WAAS-enabled GPS units (most of them sold today).    The idea is to provide location data so precise aircraft can use it to land under so-called zero-zero conditions.   The US Coast Guard also had a WAAS-like system called Differential GPS.   It transmitted low frequency signals (in the 100kHz to 500kHz range, if I recall correctly) with correction data of shoreside locations such as lighthouses.   DGPS was abandoned in favor of WAAS which provides broader coverage and does not require a special low-frequency receiver to receive the corrections.

    Neither of these systems claim receivers in Russia.   But I do seem to have read something about them.   Perhaps the US was using a thaw in relations to site some experimental tracking stations to supplement those in Britain and South Korea??

    From: Noell Wilson <NoReply_Wilson@fer3.com>
    To: luabel@ymail.com
    Sent: Thursday, May 15, 2014 4:31 AM
    Subject: [NavList] Re: GPS ground stations

    There are more technical explanations but the ground stations know where they are, pick up the GPS signal, figure a correction, and broadcast it to local GPS receivers so they can do the same relative correction. It improves the accuracy by a factor of about 5.
    I thought this was an early feature when there was a, rumored, intentional offset and this was a workaround for boats that needed more accuracy close to land. I think today that GPS accuracy just depends on how much money you are willing to spend.
    Maybe it's still needed.
    Forgive me Garmin, but from that site:
    "You've heard the term WAAS, seen it on packaging and ads for Garmin products, and maybe even know it stands for Wide Area Augmentation System. So what is it?
    Basically, it's a system of satellites and ground stations that provide GPS signal corrections, giving you even better position accuracy."
    Regards, Noell
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