GPS

[Al Johnson]

On Monday 07 July 2008, Kai Römer wrote:
> Hi Al,
> If you need testers, please contact me. I have several gta02v5
> available and can do tests in Germany Munich.

First stab uses the example perl functions from ublox for generating the 
aid-ini data, replacing their hardcoded x,y,z with values for my location. 
The copyright notice on the example code says you can't do anything with it 
without permission so I can't give you the script, but I can tell you how to 
reproduce it ;-)

Get the AssistNow online client application note from:
http://people.openmoko.org/matt_hsu/ImplementationAssistNowServerAndClient(GPS.G4-SW-05017-C).pdf

Create a new script aid-ini.pl and start with:

#!/usr/bin/perl
print(clientdata_prepare());

Go to section B - Sample Server implementation and append subroutines 
clientdata_prepare and ubx_checksum to aid-ini.pl

You need to replace the $posx, $posy and $posz values in clientdata_prepare 
with some that match your location. These are ECEF coordinated in m. There's 
an explanation of the calculation method in:
http://www.u-blox.com/customersupport/docs/GPS.G1-X-00006.pdf

Alternatively you can use the attached spreadsheet if it survives the list. 
Just replace the lat and lon with values for your location.

You probably want to change the time accuracy to reflect the accuracy of the 
Freerunner clock, and possibly the accuracy of your location estimate.

Now copy the script to somewhere suitable on the Freerunner and make it 
executable. I'm using /usr/local/bin. You need to install perl if you don't 
have it already:
opkg install perl

Switch on the GPS then run the script:
/usr/local/bin/aid-ini.pl > /dev/ttySAC1

If you cat /dev/ttySAC1 you should be able to see it using the current time 
according to your Freerunner. TangoGPS makes it easier to see what it's 
doing. In the only test I've managed so far it got a fix with a poor view of 
the sky, while my Garmin Geko was still struggling to see 3 sats. It wasn't 
quick, but it was better than the Garmin. It would be interesting if you 
could try 2 units side by side, one with aid-ini and one without, to see if 
it really makes a difference or if I was just lucky. I'll try to get some 
more testing in, but the weather isn't good here, and I don't want to get my 
new toy wet ;-)
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