Re: [Melb] POTS (Position of Trains System)

[Jackson Harding <jaxsonjo@camtech.net.au>]

Dave Proctor wrote:
> 
> Richard <richard_snook@primus.com.au> wrote in message
> 3830DC58.7F200683@primus.com.au">news:3830DC58.7F200683@primus.com.au...
> > DOes the current system employ a true GPS system...ie train is tracked (no
> pun intended) as per normal GPS to say within
> > 10feet??
> 
> Isn't that DGPS (Differentil Global Positioning System)> GSP is only valid
> to approx 20 metres or so. unless on the military band. There are ways
> around this though (DGPS is one of them).
> 
> --
> DaveProctor
> thadocta AT dingoblue.net.au


GPS on the civilian band is meant to be accurate to within 100m 95% of
the time.  This means that for 5% of the time (roughly one hour per day)
it may be more than that.  Most current model receivers usually manage
better than that (20m is about right).  For critical tracking
applications one needs to use DGPS, which said by some to be as accurate
or even more accurate than the military systems.  The idea is simple, a
hyper accurate survey is made of a location and a GPS unit stationed
there.  It then compares it's known location with the GPS signal and
transmits a correction factor to any nearby DGPS unit.  

As for sources of error the most important one is Selective Availability
or SA.  The US Navy who control the system deliberately degrade the
signal to prevent potential adversaries of the US and her allies using
the system against them.  They detune the signal continuously and in a
pseudo random fashion.  If you sit still and watch a GPS position plot
you can actually see it wander all over the place, at times by quite a
bit.  Over time the error will average out but that can take quite some
time and is not suited to tracking a moving vehicle.  The continuation
of SA is going to be reviewed sometime next year.  The other sources of
error include unfavourable satellite position giving less accurate
solutions due to the trigonometry involved and refraction errors due to
atmospheric conditions.

Jackson


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