|From:||Michael J. Tubby B.Sc. MIET|
|Subject:||Re: [gpsd-dev] gpsd - AT command control on modems|
|Date:||Tue, 16 Jul 2019 08:53:49 +0100|
|User-agent:||Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0|
Gary, et al|
While it is true that several GPS receivers work without needing the system Almanac, you have to be careful as strange things can happen since the Almanac contains the SV Operational Health for each of the SV/RPNs.
Remember that SVN23/PRN23 had a failure of its onboard atomic clock back in Feb 2016 and that any GPS receiver using it in its navigation solution was off by around 13.7uS ... this threw out GPS receivers using it in the position solution by a mile or two. We had hundreds of emergency services vehicles in the south-west of England in the wrong location for several hours until SVN23 was marked bad.
While starting up without the system almanac is common place these days it is optimistic since it assumes that every SVN that you acquire lock to is behaving and there is a chance that one or more is not - there's also increased chance of trusting a spoofed signal... I know, I tried ... its amazing what you can do with a HackRF-One, Rubidium frequency standard borrowed from a friend and Linux GPSSIM software.
On 15/07/2019 20:00, Gary E. Miller wrote:
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 3.9Without 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. https://en.wikipedia.org/wiki/Satellite_navigation#Comparison_of_systemsI 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: UBX-CFG-GNSS: msgVer 0 numTrkChHw 32 numTrkChUse 32 numConfigBlocks 7 ^^^^^^^^^^^^^^ u-blox 9: UBX-CFG-GNSS: msgVer 0 numTrkChHw 60 numTrkChUse 60 numConfigBlocks 6 ^^^^^^^^^^^^^^ RGDS GARY --------------------------------------------------------------------------- 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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