On Mar 18, 10:57*pm, wrote:
wrote in message

...
On 18 Mar, 13:59, EE507 wrote: On Mar 18, 1:20 pm, John B wrote:

Indeed, 90s sounds like a fault to me, especially at open-air stations

south of London Bridge where the southern 377s' GPS should already
know where it is (from use on the Brighton - Watford Junction and
Charing Cross - Caterham etc. services)

Would not wheel revolutions help?

No, because the wheels are not a fixed diameter, but get gradually
smaller as they are turned on lathes to remove flats and
imperfections. A small difference in diameter will lead to a large
cummulative error, if no corrections are made. There are other systems
which measure distance travelled (for example radar), but I don't know
how they are affected by different track formations etc.