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Re: [gpsd-dev] gpsd - AT command control on modems

From: Gary E. Miller
Subject: Re: [gpsd-dev] gpsd - AT command control on modems
Date: Mon, 15 Jul 2019 12:00:28 -0700

Yo Hal!

On Mon, 15 Jul 2019 04:45:48 -0700
Hal Murray <address@hidden> wrote:

> > There is no benefit to keeping the almanac when you have a 72
> > channel receiver.  You just listen to all sats all the time.   
> You can't listen to a satellite until you have the almanac and rough
> time and location so you can work out the Doppler and know what
> frequency to listen to.

Sorta true for old GPS receivers, but not needed for the new ones.

If what you said were true, then you would have a chicken and egg
problem.  If you need the almanac to lock onto a sat, and you do a cold
start you can never lock on to any sat to get the almanac.

Note the new ZED-F9P does not even bother to get the Almanac:

ZED-F9P_IntegrationManual_(UBX-18010802).pdf Section 3.9

> Without the almanac you can use trial and error to find a frequency
> slot with a signal in it and then use that to read the almanac --
> cold start.

Or, use a 60 channel receiver that just listens to all slots at the same
time.  Note that GPS and QZSS use only one frequency that is shared by
all the birds (1.57542 GHz).

The sats differ by PRN, not by frequency.

> I don't know how many frequency slots there are in a typical receiver.

Old ones start at 6 channels.  The u-blox 8 has 32 and the u-blox 9 has 60.

You can see how many channels a u-blox has by polling UBX-CFG-GNSS.  

u-blox 8:

 msgVer 0  numTrkChHw 32 numTrkChUse 32 numConfigBlocks 7

u-blox 9:

 msgVer 0  numTrkChHw 60 numTrkChUse 60 numConfigBlocks 6
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        address@hidden  Tel:+1 541 382 8588

            Veritas liberabit vos. -- Quid est veritas?
    "If you can’t measure it, you can’t improve it." - Lord Kelvin

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