[time-nuts] OT - GPS and North

[bg at lysator.liu.se]

Hi Neville

> When you think of time specifications from GPS, the GPS system is a
> poor way to find north.

I do not understand. Please elaborate. A baseline of 10 meter will often
give better than 1mrad accuracy.

> Even with a base line of 1000 metres you only have a fraction of a
> degree.

This is just not true.

> The GPS system may be useful to get accurate time to simplify a star
> observation, from a known (GPS)
> position on this planet, but finding north is still a problem because
> of the accuracy of a small
> number of observations from a star fix.


> Gyrocompasses take some time to get a measurement
> ( one hour) but even their estimate of North  cannot match the
> precision that the GPS system can get us with time.

While legacy mechanical gyrocompasses on a ship, might take time to
converge - it does not take a modern strapdown INS more than about 4
minutes to find north while stationary. But having 4 minutes at hand with
some decent GPS-receivers its easy to get a north-measurement that will be
much better than the best available intertial systems.

> cheers, Neville Michie

--

   Björn