[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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

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 3.9

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


Michael J Tubby B.Sc. (Hons) MIET / Technical Director
Email: address@hidden
Direct: +44 (0)1905 752892
Mobile: +44 (0)7973 225144

Thorcom Systems Limited
Phone: +44 (0)1905 756 700
Address: Unit 4, 96B Blackpole Trading Estate West, Worcester, WR3 8TJ, England, UK
Company registered in England & Wales No. 02704696 / VAT Number GB487925681 / EORI GB487925681000

This email and any attachments to it may be confidential and are intended solely for the use of the individual to whom it is addressed.
Any views or opinions expressed are solely those of the author and do not necessarily represent those of Thorcom Systems Limited.
If you are not the intended recipient of this email, you must not take any action based upon its contents or disclose it to any third-party.
Please contact the sender if you believe you have received this email in error.

reply via email to

[Prev in Thread] Current Thread [Next in Thread]