GPS receivers and possible OHLE interference
"Recliner" wrote in message
Even if you have a fix with six it more satellites before getting on the
train, you'll periodically lose it during the journey when the train is in
tunnels, deep cuttings and covered stations. The device will then have to
attempt to get a new fix, which will fail if the windows are shielded or
you don't hold it near a window.
Good point. Maybe I get better results by getting a lock-on before getting
on the train because of the ability to read the "almanac" of currently
available satellites and their locations while there's still good reception.
I've got a little app called GPS Status which can download A-GPS info over
the internet which speeds up time to first lock considerably. Because of the
power consumption of leaving my phone's GPS receiver turned on all the time,
I tend to turn it on only when I need it, so it always needs to get an
up-to-date almanac. If I left it going all the time I'd probably get better
results, but my battery would only last a couple of hours. Making it last
from 8 AM to 6 PM (ie times when I can't be tethered to the charger), even
with GPS, wifi and mobile data normally turned off, is a problem out here in
the country because the phone uses so much power winding up the gain on the
mobile phone receiver to listen for incoming calls - oh to be in an area
that has good mobile reception.
GPS comes into the category of PFM (pure f***ing magic): the maths involved
in calculating your position based on time delays from satellites which
themselves are moving round the earth (*) makes my brain hurt! The fact that
it works at all, even with little quirks like going off course when you get
reflections or can only see a few satellites, is a miracle.
(*) For a long time, I thought that GPS satellites were geostationary, like
Sky satellites. Then I found out that they're not...