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

From: HH (htrinh_at_hotmail.com)
Date: 08/04/04 

Date: Wed, 04 Aug 2004 21:12:29 GMT

Actually, I wonder why latest Garmin Legend C and Vista C only have 12
channels. In Calgary, probably because of the altitude, I can see 12
satellites about 90% of time and that's not counting the two WAAS
satellites. I just wish that I can track 14 satellites like the new
Magellan. Tracking 16 satellites is even better.

"Randolph J. Herber" <herber@dcdrjh.fnal.gov> wrote in message
news:cer3sn$ggf$1@info4.fnal.gov...
>
> 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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