gpsd and AGPS

[Ian Stirling]

Ken Yale wrote:
> Hello,
<snip my largely incorrect comments thanks for the response!>
> AGPS Status:
<snip>
> Beyond that, the only AGPS capability in the GTA01 is LTO.
 > This is long-term orbit data files downloaded from the network.
> The delivery of the files can be done via wget or the lto_get facility included with OMGUI.  
 > FIC has purchased LTO delivery for the GTA01 for the lifetime of the
> product with the price of the Hammerhead chip.
 > LTO is there now and it is free for FIC customers.
> 
Great! Is there a url for wget, or is it more complex than this?

> Normally, ephemeris satellite data is downloaded at 50 bps from the satellites, but only when the signal strength is above about -142 dBm.
 > Live ephemeris data is good for about 2 hours.
 > With a data connection (I use a USB TCP/IP bridge to a PC, and then 
to the network),
 > we can download 7 days of ephemeris in 3 or 4 seconds, independent of 
GPS signal conditions.

Ephemeris data is broadcast for 12 of every 30s, by each satellite about 
its own orbit.
So, you can get a good position if you've had 30s clear view of the sky 
within the last two hours, in order to download those 500 or so bits per 
satellite.
Or alternatively, at some time in the past weeks, 12.5 minutes of signal 
to download the almanac (12 - which is good for a fair while. (some 3K 
of data)
http://www.colorado.edu/geography/gcraft/notes/gps/gif/databits.gif

I assume you mean LTO in the above.
Are there any nice graphs (or even NMEA logs) of comparisons between 
positions of the LTO files, and downloaded?
(or see the above URL/wget question)
I'm assuming that LTO files are not simply copies of the almanac - but 
more precise orbital data - the almanac has data errors of >1m.

<snip>
> So you can see that direct access to LTO via TCP/IP bypasses lots of complicated infrastructure, and can be accelerated to suit your style of use of the GTA01.  On WindowsCE devices, the LTO mechanism is enhanced by these features, all of which can be added to GTA01 by the OpenMoko community:
> -  cache LTO on a PC.  Upon PC<==>GTA01 sync; squirt the LTO to the GTA01
> -  on GTA01:  detect data connection creation and retrieve LTO if needed
> -  on GTA01:  add a cron job.
> -  for specialized GTA01 environments:  store & forward the LTO file as needed.

The hardware supports in principle more than this - 
http://wiki.openmoko.org/wiki/Server:A-GPS details some possibilities.
But broadly, any assistance model is possible - SMOP -, from downloading 
long-term-orbit files, (who knows, GL may go down under lawsuits, and 
the servers fall over, be eaten by giant mice, ...) to providing only a 
snapshot of the partial data obtained during a short fix, and asking the 
server to provide a position.


> I hope this helps,
> Ken Yale

I notice you mention GTA01 only - is there any significance in this?
Thanks for the info!