External GPS antenna question

[Paul Fertser]

Al Johnson <openmoko at mazikeen.demon.co.uk> writes:
> On Saturday 04 July 2009, Paul Fertser wrote:
>> Sebastian Krzyszkowiak <seba.dos1 at gmail.com> writes:
>> > On 7/4/09, Hammer Armin <ahammer at datazug.ch> wrote:
>> >> With the internal gps antenna, the first fix last quite long -
>> >> with the external - within 5min to 10min I get a fix.
>> >
>> > WTF? I'm getting fix easly with *internal* GPS antenna in 30 seconds -
>> > 2 minutes...
>>
>> Because you already have almanac (and sometimes ephemeris) data.
>
> With a good sky view and suitable constellation TTFF from cold is ~40s with 
> the internal antenna, rootfs on SD and an early A05 without the capacitor on 
> SD. For it to take >2min I need heavy urban canyoning, being indoors or in a 
> vehicle, or an unusually poor constellation.

Let me cite the site that looks to be authoritive enough [1]: 

"The navigation message is a continuous 50 bits/second data stream
modulated onto the carrier signal of every satellite. It is a
telemetry message, and the data is transmitted in logical units called
frames. For GPS a frame is 1500 bits long, so takes 30 seconds to be
transmitted. Every satellite begins to transmit a frame precisely on
the minute and half minute, according to its own clock. Each frame is
divided into five subframes, each 300 bits long. Subframes 1, 2 and 3
contain the high accuracy ephemeris and clock offset data. The "data
content" (which I'll define later) of these three subframes is the
same for a given satellite for consecutive frames for periods lasting
as long as two hours. New subframe 1, 2 and 3 data sets usually begin
to be transmitted precisely on the hour (see description of uploads
and cutovers below). Subframe 1 contains second degree polynomial
coefficients used to calculate the satellite clock offset. Subframes 2
and 3 contain orbital parameters.  Subframes 4 and 5 are
"subcommutated, which means that consecutive subframes have different
"data content". This data does repeat, but 25 consecutive frames of
subframe 4 and 5 data must be collected before the receiver has all of
the unique "data content" being transmitted by the satellite. A
satellite transmits the same "data content" in subframes 4 and 5 until
it is next uploaded, or usually for about 24 hours. Subframes 4 and 5
contain the almanac data and some related health and configuration
data."

25 * 0.5 min = 12.5 min . I assume this gives a good estimate of TTFF

If you already have almanac (which is the usual case as almanac is
usually valid for months), you'll get a fix in a little more than 30
seconds (time required to receive ephemeris from visible sattelite).

[1] http://gpsinformation.net/gpssignal.htm 
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