Al Johnson <openm...@mazikeen.demon.co.uk> writes: > On Saturday 04 July 2009, Paul Fertser wrote: >> Sebastian Krzyszkowiak <seba.d...@gmail.com> writes: >> > On 7/4/09, Hammer Armin <aham...@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 -- Be free, use free (http://www.gnu.org/philosophy/free-sw.html) software! mailto:fercer...@gmail.com _______________________________________________ Openmoko community mailing list community@lists.openmoko.org http://lists.openmoko.org/mailman/listinfo/community