Re: GPS - Can't get a fix
 

You may find this advice extracted from http://fragostech.com/MaemoMapper/ helpful.

Failure to Lock Troubleshooting

The GPS driver writes a almanac /var/lib/gps/nvd_data which it uses in subsequent times when the driver is started. Almanac contains orbit information on all satellites, clock corrections, and atmospheric delay parameters. It is transmitted by a GPS satellite to a GPS receiver, where it facilitates rapid satellite vehicle acquisition within GPS receivers. It appears that if the N810 is powered down during this write the almanac will be corrupted. As a result it is recommended that after terminating Maemo Mapper during a session with a successful lock you save an almanac copy for backup purposes. Open a terminal window and run:
cp /var/lib/gps/nvd_data /var/lib/gps/nvd_data.bak

If there come a time when the GPS lock won't occur even considerably past a half hour, the saved almanac should be restored. First terminate Maemo Mapper and then open a terminal window and run:
cp /var/lib/gps/nvd_data.bak /var/lib/gps/nvd_data

You will have to restart the GPS by opening the N810 Control Panel, tap GPS location, uncheck Enable GPS and tap the OK button. Lastly tap GPS location again, check Enable GPS and tap OK. When restarting your restored nvd_data almanac will be used.

This may not be a cure all but has been reported to restore correct operation in a number of instances. Hopefully this issue will be fixed or accommodated in the GPS driver.

Re: GPS - Can't get a fix
 

This theory would imply many people have a GPS hardware malfunction. You do have 3 years of warranty in EU.

Re: GPS - Can't get a fix
 

I'm beginning to wonder what role an Internet connection plays in getting the initial fix. I've installed and run agps some weeks ago. Just now my N810 was in off line mode and I started Maemo Mapper. It found no satellites so I reset the GPS by disabling and then enabling. Still no satellites. Next I put the tablet in online mode and established a WiFi connection. Again I reset the GPS and now I have satellites in view and acquired a fix within a minute or two. Is this a coincidence or have I discovered a solution? More experimentation is in order. Comments anyone.

Re: GPS - Can't get a fix
 

What you describe is exactly what I would have expected: With A-GPS on it can help only if you have active internet connection. Without internet connection A-GPS can not pull visible satellites data from the internet. Once you opened internet connection and activated GPS, A-GPS downloaded visible satellites data from internet and got fix faster.

However, even without internet connection the tablet should find satellites after some time (minutes...) and get fix after all. If you did have your tablet looking for satellites say ~3 minutes and still didn't have a single satellite, that isn't normal.

Re: GPS - Can't get a fix
 

For some reason I'd thought that agps built a list of sites when it was 1st run and continued to use that list until directly run again. It would appear that I'm mistaken on that point and need to update my Maemo Mapper HowTo which discusses and recommends the use of agps.

Re: GPS - Can't get a fix
 

fragos:

You may find these links helpful:
http://www.nokia.com/betalabs/agps-tablet
http://betalabs.nokia.com/blog/2008/...n810/#comments

EDIT: I saw that you have already posted comments there... So no news for you? :o

Re: GPS - Can't get a fix
 

Hmm, after installing minigpsd, Maemo Mapper can't find the (internal) GPS anymore.. :(

Re: GPS - Can't get a fix
 

Just to avoid confusion about almanacs and ephemeris data.

nvd_data probably contains both the almanac and ephemerides actually. The almanac gives the long term (6 week iirc) rough orbital predictions for all the satellites and the ephemerides give the short term (30min?) corrections to account for a variety of things including atmospheric disturbances (on a per satellite basis, and just for those in view).

See here for example: http://gpsinformation.net/main/almanac.txt

Re: GPS - Can't get a fix
 

Hmm, by turning off GPS and turning it on again, the (wayfinder) map application finds the GPS, and so does Maemo Mapper. But despite using AGPS, maemo mapper only shows 1 satellite in view (strength 33), 0 in use - and if i quit the wayfinder map app, Maemo Mapper looses the GPS, so somehow gpsd/minigpsd gets killed.

Re: GPS - Can't get a fix
 

Yesterday I did some more experiments with the N810 GPS.

For testing I did try to get a fix in my house downstairs. In that location it is normally impossible to get a fix, but I just wanted to see what signal strength MaemoMapper GPS details show. After about 10 minutes of wait I had signal from 3 satellites. The strongest was about "30". I did not get a fix. (All this with A-GPS enabled.)

I moved upstairs where the signal strength is better. My strongest signal improved to about "33-35". After this GPS started to find more satellites, and finally got a fix in about 3 minutes.

Once the GPS has fix the satellite signal strength bars are updated much more frequently, maybe about once per second. I experiment how much holding tablet in hand in different positions affects signal strength. I did find some surpising results:

1) In general holding tablet on hand anywhere else than upper left corner increased signal strength significantly (up to "10"...) This appear to be due to "grounding" the tablet chassis.

2) Holding hand above (close but not touching) the tablet upper left corner significantly increased signal strength on some satellites. Some in excess of "10"! It appear that the most gain was with satellites where hand was exactly placed between satellite and N810 GPS antenna (located at upper left corner?)

3) Combining 1) and 2) doesn't seem to bring any more gain.

These are my basic findings... Especially the 2) surprised me a lot. The only explanation (?) I can think is that the hand will form an antenna element (similar to the "sticks" in front of the yagi/dipole antenna) to increase antenna gain. Now you guys with more experience in RF and antenna theory can chime in... :rolleyes: