"Blunkett u-turn on data privacy plans"

[Ian G Batten]

On Mon, 03 Mar 2003, Brian Beesley wrote:
> Using repeaters fixes the lock-on delay. General accuracy of 10m is fine for 
> tracking movements of targetted individuals - but again use of repeaters 
> allows this to be improved markedly. 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 think you're confused about differential GPS.  The ``repeaters'' you
refer to are simply transmitting the delta between the expected timing
and the perceived timing for each satellite, and although the
differential base station needs to know its location to a high degree of
precision, a user of the differential service merely needs to know that
the source of the deltas is ``close enough'' to its own location to be
valid.  

I'm not sure what you mean by ``repeater station'', but if it transmits
GPS data then it's essentially acting as a (non-moving) satellite.  In
order to use that you'd need to know its location to the same degree of
precision as you want to know your position to.

> The fact that _raw_ GPS positioning is already used in e.g. car
> sat-nav equipment suggests that tracking targets around a city is very
> much feasible,

Such systems do sometimes lose sync, and they need decent antenna.  I
can't recall where it is on my brother's S80, but it was a bootlid
aerial on a Lincoln Town Car I once rented in the US.

> It will be very interesting to see how badly affected these
> applications will be affected when the US "detunes" GPS for military
> reasons during the forthcoming attempt to singe Saddam's moustache.

They've already agreed that selective availability won't be put back on,
as DGPS will pretty much strip it off anyway.  And there's no reason to:
25m precision is good enough for most military purposes.  The reason for
SA was to make GPS not good enough for ballistic missile boost phase
guidance, and it still isn't: DPGS isn't fast enough.  Remember too that
the military have access to the L2 frequency (Precise Positioning
Service) as well as the generally available L1 (Coarse/Acquisition).
Even if both are working at the same basic precision, the use of two
frequencies allows them to strip out ionospheric effects that you'll
need DPGS to do.  Late.

ian