[maemo-users] N810 gps troubles

From: Andrew Daviel advax at triumf.ca
Date: Mon Mar 31 20:45:00 EEST 2008
On Mon, 31 Mar 2008, Frederic Crozat wrote:

> Once you get the almanac downloaded, fix time can be shorter (5
> minutes to 10 minutes in bad conditions). And if you got a fix in the

Any idea where the almanac is stored ? In the GPS chip itself ? Does it 
survive power-down or dead battery ? I gather it's a Texas device 
(GPS5300) with not a lot of info on the Web (contact TI if you're a 
cellphone maker wanting millions of the things, hobbyists need not 
apply). The 2-page teaser suggests that it communicates over a serial 
bus, maybe native NMEA0183.

It also says "..enables a rapid time to first fix from weak satellite 
signals exceeding the A-GPS requirements for 3GPP operation"
It's possible that there are some layout issues affecting the performance 
in the N810 - the chip requires an antenna and 11 passive 
components (capacitors, resistors..), so it's possible the design is 
sub-optimal. e.g. compromised to fit it in the space available.

A-GPS being apparently "assisted GPS" used with a signal from cell towers
which I had not previosuly heard of. Perhaps in a true cellphone 
application the cell system sends some approximate location to
speed up the time to fix (for 911 calling in the US)

I had noticed that, compared to my Garmin GPSmap76 (now several years 
old), that the N810 takes longer to acquire a fix but having got one, is 
able to keep it with less signal (indoors in my wood-framed house), at 
least for a while. For the Garmin, initial setup required priming it with 
an approximate position but I see no means to do that on the N810. 
Perhaps it is now considered unnecessary.

> Also remember GPS antenna is located under the zoom in/out button, so
> try to not hold your n810 with your hand over them (at least for
> initial GPS fix).

I had assumed it was the little plastic square just above the camera, so 
that the best orientation was screen side upwards with a good view of the 
sky. Once I had a fix, then I could put it in my pocket riding a bike.

There seems an awful lot of metal in the N810 case, and radio waves 
traditionally don't penetrate. At least, that's what I learned at school 
but the modern devices seem awfully sensitive and the signal can diffract 
through small holes...

I'm not sure Maemo Mapper really has more information available.
As far as I can figure, gpsd translates NMEA from the device to a
generic format and makes it available to multiple clients on 
The NMEA includes $GPGSV GPS Satellites in View. You can see this if you 
connect to the socket with netcat and give an "r" (raw) command, or
"q" or "y".
In Nokia Map the "gps view" shows signal levels and satellite position 
graphically, I presume derived from this information.

  - I'm curious. What time-to-first-fix are people experiencing ?

(I guess I should quantify my own experience..)

Andrew Daviel, TRIUMF, Canada
Tel. +1 (604) 222-7376  (Pacific Time)
Network Security Manager

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