Two Antennas (was RE: "Blunkett u-turn on data privacy plans")

[Peter Fairbrother]

David Hansen wrote:

> On 4 Mar 2003 at 17:05, Brian Gladman wrote:
> 
>> However these measurements would require a laboratory environment - as
>> you suggest, it does not seem likely that sub-cm resolution would be
>> possible in differential GPS measurements in a field environment.
> 
> As the two antennas are likely to move by several mm relative to each
> other (due to wind, springy grass, backlash in the mountings and
> whatever) I would be inclined to say that such accuracy is
> impossible.

On top of hills you sometimes see concrete or stone pillars, about
waist-high, with brass cones on top. Antennea are fitted on the cones, and
this gives sub-millimetre accuracy of the antennae's positions.


The antennae they use look like miniature domed flying saucers about the
size of a tea saucer. I have heard of sub-millimetre accuracy over 50km+
ranges (for measuring fault creep), but I don't know how they do it :). I
don't think they use GPS. Next time I see them (probably this summer) I'll
ask. I do know it takes most of a day to make the measurement.


-- 
Peter Fairbrother