somewhere near the temporal coordinates of 12/8/01 6:58 PM, the entity known
as Marco Mailand transmitted the following from Newton2k1_at_mac.com:
>>> Differential GPS (dGPS) is one of the more accurate ways to get your
>>> coordinates as you are using more satellites
>> Nope. dGPS doesn't use more satellites. What it does is compare your
>> receiver's information to that of a nearby (usually fixed at a well-known
>> datum) GPS receiver. You are able to get a far more accurate relative
>> position, as the much of the errors in the positions of the two receivers
>> are common -- things like the error introduced by the signals passing thru
>> the ionosphere, since the paths are nearly identical, the errors are too.
>> By differencing the positions of the two receivers, most of the errors are
>> subtracted away.
> Very nice description. However, the US DoD deactivated the artificially
> introduced satellite signal error some time ago, so that any ordinary GPS
> can reach the technically possible accuracy of about 1+/- meter. Of course,
> the DoD has the possibilty to activate the error at any time and I don't
> know the status now, but I could check it if anyone wants to know it :-)
>
dGPS does handle when selective availability is turned on, but the benefits
of dGPS happen even when that's not active. Because the naturally occurring
(and residual technological) errors are largely differenced away, dGPS is
able to break the roughly 1 meter error (achieved by a fixed GPS receiver,
running for a long time) and take us down to the order of a few to 10 cm.
This is enough that it can be used by airplanes operating on taxiways, etc.
>>> IMO a true GPS handheld is about $6,000 US,
>> Good Lord!!! What sort of GPS receiver are you talking about?? I got my
>> Magellan for under $100 US on eBay over a year ago.
> There's also a so called RTK GPS (real time kinematic) which increases the
> accuracy to about 1mm (!) and is used in large construction sites as e.g.
> For tunnel drilling, actually south of Zurich. But I doubt that a RTK GPS is
> of any special use in Safari hunting :-)
I've not heard of the RTK GPS, but based on the accuracy, this sounds like
an interferometric GPS unit. I wasn't aware that these were able to be
built as handheld units, but I've been out of this field for a few years.
- Eric.
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