Re: Why are some GPSs so much better than others?

From: Randolph J. Herber (herber_at_dcdrjh.fnal.gov)
Date: 08/04/04 

Date: 4 Aug 2004 16:46:15 GMT

In article <7ub1h01e3e473vvresa6b3oqqe69hvh68p@4ax.com>,
Peter <peter@no-spam-please-6644.co.uk> wrote:

        [SNIP of material to which I am not responding]

>I also wonder what is the point of a 12-channel GPS when most of the
>time there are 8 or fewer satellites in the sky.

>Peter.

>Return address is invalid to help stop junk mail.
>E-mail replies to peter4400@peter2000XY.co.uk but remove the X and the Y.
>Please do NOT copy usenet posts to email - it is NOT necessary.

        Rhetorical question: why have more than one channel when
        the receiver can switch between the satellites rapidly
        (e.g., the early Garmin receivers)?

        1) There can be fourteen GPS satellites above the mask
           horizon at one time at the present time. In my
           experience, with a number of Garmin receivers made over
           the last decade, under field conditions without external
           antennas, eleven satellites is actually quite common.

        2) The WAAS satellites share channels with the GPS satellites.
           There generally are two visible in many regions of the
           world and in the future there may be even more.

        3) If Galileo is successful (and I assume it will, if
           for no other reason than European pride) as many
           as thirty satellites may be above the mask horizon
           at one time.

        4) Spare channels can be used for signal acquisition of
           GPS, WAAS, Galileo, EGNOS (the Galileo equivalent to
           WAAS) satellites about to come above the mask horizon, to
           anticipate Doppler frequency shift effect, etc. They
           also can be used during the initialization of the
           receiver at power-up to massively search for single
           satellites until enough have been found to acquire
           ephemeris, almanac and rough time, location, velocity
           and acceleration data to do a full solution acquisition.

        5) Since the channels share the receiver radio frequency
           front-end, adding channels is relatively cheap as only
           computing resources need to be added to the signal
           demodulation section of the receiver. With massively
           parallel correlators (channels), thousands and even
           hundreds of thousands of channel are practical
           (even at cell phone prices). Such massively parallel
           correlators (channels) permit signal acquisition and
           maintenance even in weak and radio-noisy environments
           (e.g., inside indoor shopping malls and on battle
           fields with jammers operating).

           http://www.clickerado.com/g/gps/positioning.htm

        6) Locations requiring extremely reliable and precise
           solutions may provide ground transmitters of weak
           GPS/Galileo-like signals from several locations
           near the location requiring such signals to provide
           additional navigation signals, differential corrections
           and declarations about the quality of all GPS/Galileo-like
           signals usable at that location (LAAS ). These additional
           signals also require channels.

        7) Consumer grade 16 channel receivers exist. E.g.:

           http://www.highpointscientific.com/meade.html
           http://www.celestron.com/prod_pgs/tel/nx_gps_table.htm
           http://www.aircotec.net/xctrainer.htm

Randolph J. Herber, herber@dcdrjh.fnal.gov, +1 630 840 2966, CD/CDFTF PK-149F,
Mail Stop 318, Fermilab, Kirk & Pine Rds., PO Box 500, Batavia, IL 60510-0500,
USA. (Speaking for myself and not for US, US DOE, FNAL nor URA.) (Product,
trade, or service marks herein belong to their respective owners.)

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