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Old September 9th 16, 11:04 AM posted to uk.railway,uk.rec.gps
Peter Crosland
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Posts: 17
Default GPS receivers and possible OHLE interference

On 09/09/2016 10:44, NY wrote:
"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...

It also depends a lot on the software and hardware for the particular
GPS. The better ones have accelerometers built in that keep track during
signal interruptions. Others also use them to assume that you will
continue on the road you are on. I have never seen such weird results as
shown in the map you listed.
Peter Crosland

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