GPS soft baseband processing

[quatrox at gmail.com]

On 2008-09-07, Al Johnson <openmoko at mazikeen.demon.co.uk> wrote:
> On Sunday 07 September 2008, Andy Green wrote:
>> Somebody in the thread at some point said:
>> | On 9/7/08, Flemming Richter Mikkelsen <quatrox at gmail.com> wrote:
>> |>  "Our automatic geotagging technology solves these
>> |>  problems. A fragment of RAW GPS RF data is captured
>> |>  in 0.2 seconds and then stored alongside the digital image
>> |>  and the processing is performed later on a PC. Because
>> |>  we separate the GPS capture from the GPS processing,
>> |>  our technology maintains the traditional "instamatic" user
>> |>  experience we are all used to and then pulls consumers
>> |>  to experience a far more interesting way to search and
>> |>  share photographs later."
>> |
>> | Interesting.
>> | Around 1995 I  used about the same technique to sync spectrometer
>> | data to an external authoritative timesource ( in that case the DCF77
>> | signal encoded into 10ms dataframes ) time recovery could achieve
>> | better than 100us
>>
>> It sounds like a fine technique, but it pushes the whole issue into
>> license of the host CPU software then.  They don't seem to mention if
>> they provide it or it is liberally licensed, etc.
>
> There appear to be several open source GPS projects around, including a few
> that do correlation in software.
>
> http://sourceforge.net/project/showfiles.php?group_id=91538&package_id=271229
> http://gnuradio.org/trac/wiki/GlobalPositioningSystem
> http://gnuradio.org/trac/wiki/GlobalNavigationSatelliteSystem

The article I read mentioned all the details of what the chip does and
what is left for software. IIRC, it is basically an FFT and a reverse
transfer function which is needed. There is free implementations of
FFT. It looks like the only problem is that calculating everything each
second might take much CPU, but then again, we can choose to only
calculate every n'th  second or so.