GPS Problems

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NOTE: Much of the information in this page is duplicated at GTA02 GPS Problems; these pages should probably be merged one way or the other

I would suggest keeping this page, as it seems more complete to me - Artificer 11:25, 17 July 2008 (UTC)


See GPS for details on how to evaluate the Freerunner GPS with command line utilities.

Contents

Problem: it takes a long time (10min+) before the Freerunner gets first GPS data

It was found that the OpenMoko FreeRunner GPS has a long Time To First Fix (TTFF) with the SD card in the phone. This thread indicates that there is interference from the SD card slot, as without the SD card inserted it the TTFF are much shorter.

More specific: the GPS signal level drops by -20dBm, i.e. factor 100. Signal strength of a GPS satellite above your head is around -127dBm. Needed strength for a first fix is -145dBm with a minimum of three satellites. Once the GPS chip has a fix, it can operate at signal levels of -157dBm, thus making it possible to operate while there is data transfer from/to the SD card.
(From http://lists.openmoko.org/pipermail/community/2008-July/022202.html )

Solutions

The people of OpenMoko know about the problem and are working on it. A software workaround is under testing. It turns off the SD card clock when the SD card is not being used. One will be able to use the SD card at the same time as the GPS except for the first approximately 30 seconds.

OpenMoko is also examining a hardware solution.

Discussion

The following is based on Alasal's post to the community mailing list.

What is TTFF?
TTFF means Time To First Fix of the GPS. This is the time the GPS needs to get the first clue on where you are on planet earth. So you have to do this only once. After you have a FF (first fix), you can get more fixes without any problems with the SD card in the phone.


So if we have a first fix, the SD card doesn't block the GPS anymore?
Yes, the SD card doesn't block the GPS if we have a first fix. (Some people even claim it's more stable)


Why do we need a first fix?
According to Al Johnson, the Antares4 chip on the GTA02 doesn't have the memory needed to store almanac and ephemeris, last known position or time. This means that every start is a true cold start. It is possible to save location, almanac and ephemeris at GPS shut down, and restore these information at the next startup, but that does not seem implemented yet.


Does the lower signal also affect accuracy?
It is a good deal more jittery. However, if you drive around in a city you will get a lot more error from signal reflections than from the jitter.
(From http://lists.openmoko.org/pipermail/community/2008-July/022209.html )


Do they have a solution?
Yes. There is already a highly alpha software workaround. The software just turns off the SD card clock when the SD card is not being used. They are also examining a hardware solution.
The software fix is still in testing. Expect first results this noon (Thursday, 17th July 2008) (Taipeh time).
Also, the hardware fix needed is rather easy. No need to open the shielding or housing. We are working on a document which gives you detailed instructions.
(From http://lists.openmoko.org/pipermail/community/2008-July/022242.html )


Do we have to return our FreeRunners to OpenMoko?
Probably not, because the software workaround should work too.
But we can't read from the SD card when the GPS is on?
Wrong, you will be able to read the SD card when the GPS is on. The clock for the SD card used to run constantly. Now the kernel turns it off when the SD card is not in use. As long as you are not constantly reading from the SD card, you should see much better first fix times. Obtaining a GPS fix does not affect SD performance.
Did Openmoko test the GPS with a SD card in it?
Yes.
Why didn't they find it while testing the Freerunner?
Because they have to do the tests with a fake GPS signal and with that signal it worked (In a fab you don't have a decent GPS signal). They have already modified the test, so it corresponds to the GPS signal better.

Information about faulty antennas

  • If removing the SD card doesn't help and you suspect your antenna, try unplugging and replugging the internal GPS connector. (See Disassembling_Neo1973 about 2/3 of the way down. (this may differ on freerunner, pictures appreciated)

Also note that there is a trac ticket for this issue: #1542

FreeRunner_GPS_antenna_repair_SOP Indicates that some (rare?) units may have bad soldering, and includes a guide to fix. This will presumably void your warranty.

Personal tools
NOTE: Much of the information in this page is duplicated at GTA02 GPS Problems; these pages should probably be merged one way or the other

I would suggest keeping this page, as it seems more complete to me - Artificer 11:25, 17 July 2008 (UTC)


See GPS for details on how to evaluate the Freerunner GPS with command line utilities.

Problem: it takes a long time (10min+) before the Freerunner gets first GPS data

It was found that the OpenMoko FreeRunner GPS has a long Time To First Fix (TTFF) with the SD card in the phone. This thread indicates that there is interference from the SD card slot, as without the SD card inserted it the TTFF are much shorter.

More specific: the GPS signal level drops by -20dBm, i.e. factor 100. Signal strength of a GPS satellite above your head is around -127dBm. Needed strength for a first fix is -145dBm with a minimum of three satellites. Once the GPS chip has a fix, it can operate at signal levels of -157dBm, thus making it possible to operate while there is data transfer from/to the SD card.
(From http://lists.openmoko.org/pipermail/community/2008-July/022202.html )

Solutions

The people of OpenMoko know about the problem and are working on it. A software workaround is under testing. It turns off the SD card clock when the SD card is not being used. One will be able to use the SD card at the same time as the GPS except for the first approximately 30 seconds.

OpenMoko is also examining a hardware solution.

Discussion

The following is based on Alasal's post to the community mailing list.

What is TTFF?
TTFF means Time To First Fix of the GPS. This is the time the GPS needs to get the first clue on where you are on planet earth. So you have to do this only once. After you have a FF (first fix), you can get more fixes without any problems with the SD card in the phone.


So if we have a first fix, the SD card doesn't block the GPS anymore?
Yes, the SD card doesn't block the GPS if we have a first fix. (Some people even claim it's more stable)


Why do we need a first fix?
According to Al Johnson, the Antares4 chip on the GTA02 doesn't have the memory needed to store almanac and ephemeris, last known position or time. This means that every start is a true cold start. It is possible to save location, almanac and ephemeris at GPS shut down, and restore these information at the next startup, but that does not seem implemented yet.


Does the lower signal also affect accuracy?
It is a good deal more jittery. However, if you drive around in a city you will get a lot more error from signal reflections than from the jitter.
(From http://lists.openmoko.org/pipermail/community/2008-July/022209.html )


Do they have a solution?
Yes. There is already a highly alpha software workaround. The software just turns off the SD card clock when the SD card is not being used. They are also examining a hardware solution.
The software fix is still in testing. Expect first results this noon (Thursday, 17th July 2008) (Taipeh time).
Also, the hardware fix needed is rather easy. No need to open the shielding or housing. We are working on a document which gives you detailed instructions.
(From http://lists.openmoko.org/pipermail/community/2008-July/022242.html )


Do we have to return our FreeRunners to OpenMoko?
Probably not, because the software workaround should work too.
But we can't read from the SD card when the GPS is on?
Wrong, you will be able to read the SD card when the GPS is on. The clock for the SD card used to run constantly. Now the kernel turns it off when the SD card is not in use. As long as you are not constantly reading from the SD card, you should see much better first fix times. Obtaining a GPS fix does not affect SD performance.
Did Openmoko test the GPS with a SD card in it?
Yes.
Why didn't they find it while testing the Freerunner?
Because they have to do the tests with a fake GPS signal and with that signal it worked (In a fab you don't have a decent GPS signal). They have already modified the test, so it corresponds to the GPS signal better.

Information about faulty antennas

  • If removing the SD card doesn't help and you suspect your antenna, try unplugging and replugging the internal GPS connector. (See Disassembling_Neo1973 about 2/3 of the way down. (this may differ on freerunner, pictures appreciated)

Also note that there is a trac ticket for this issue: #1542

FreeRunner_GPS_antenna_repair_SOP Indicates that some (rare?) units may have bad soldering, and includes a guide to fix. This will presumably void your warranty.