"Blunkett u-turn on data privacy plans"

[Fearghas McKay]

At 21:27 +0000 3/3/03, Roland Perry wrote:
>In message <1165408936-6078119@espace.net>, Fearghas McKay
><fm-lists@st-kilda.org> writes
>>> >Don't forget specialised GPS receiving
>>> >stations are used to track minute movements (down to the order of 100
>>> >microns) in e.g. earthquake research.
>>>
>>>I have never heard of such a thing. Most claim even differential GPS is
>>>only good to a metre.
>>
>>it is differential movement that is being measured not absolute
>>positioning. You have two antenna and you measure where thwy are in
>>relation to each other. Similar tricks have been used on ships for years as
>>it allows the replacement of gyroscopes for measuring roll and yaw.
>
>So what you are saying is that you put two normal GPS receivers a short
>distance apart and measure the difference between their positions, that
>difference increasing slightly if a fissure develops in the ground
>between them?

no not normal GPS receivers - two antenna and a specialised box that
compares the difference.

>And you can detect a difference of 100 microns? Surely that would
>require each receiver to have a readout significant to 100 microns, even
>assuming they drifted and wobbled absolutely in time with one another.

From the conversations I have had with the marine engineers on this I think
that it is not impossible but I don't have source material to verify it.
There was some articles on the use of GPS in New Scientist a couple of
years ago - but the marine conversations were perhaps 8 years ago.

	f