Re: birding-aus GPS's

To: "Paul Taylor" <>
Subject: Re: birding-aus GPS's
From: Tony Russell <>
Date: Sat, 21 Aug 1999 19:08:00 +0900
Excellent Paul. Thank you so much.

At 11:28  21/08/99 +1000, you wrote:
>Tony wrote:
>> [snip]
>> Obviously some of you are having similar start up delays with Magellan and
>> other brands. But two queries remain.
>> 1) Getting a fix with just two satellites shown as locked in? Doesn't it
>> need three as indicated on p6 of the handbook?
>Yes; you need at least three satellites for a 2D fix (i.e. Lat./Long.), and
>at least four satellites for a 3D fix (Lat./Long. + height.)  ("At least",
>because the geometry of the satellites is important.  Occasionally you may
>have three satellites locked in and still not get a position fix; you'd
>have to be *really* unlucky to not get a fix with four satellites locked
>It's possible the unit did have a three satellite lock, and "lost" a
>satellite.  Garmin units have a form of "dead reckoning" to estimate
>the position based on the last known position, speed and direction of
>travel; after 15 seconds, they give up and report "Poor coverage" (and
>no position.)  Other units might display the last known position until
>it gets another position fix, but there should be some warning that
>this is the case.
>> 2) Why does my friend's Garmin (GPS2), which he uses much less frequently
>> than I use mine, start up so fast? ( I know why he laughs).
>Apart from the fact they are different models, it could be a number of
>1) Cold start shortcuts.  Most Garmin units remember the last known position
>   even when the power is turned off; this is used to speed up cold starts
>   since it has some idea where it is (vs. a very cold start where it knows
>   nothing.)  Even if you have moved a considerable distance since the last
>   position fix, they offer a menu of approximate locations (e.g. "Australia
>   - NSW") that also speed up the process.  (If neither of these work, it
>   still goes through the labourious satellite search process - five
>   Other units may offer similar features; check your user manual for tips
>   on speeding up acquisition times.
>2) Antenna performance.  The two main antenna designs for handheld GPS units
>   are "patch" antennas and helical antennas; most units use the "patch"
>   variety but the Garmin GPS II, II+ etc. use helicals.  I haven't seen any
>   data to suggest that one type is better than the other, but there will be
>   some differences in signal-to-noise ratios, sensitivity profiles etc.
>   will affect performance.
>   As with TV antennas, you may need to change the orientation of the unit
>   or antenna to get the best performance; moving a few metres can also make
>   a difference if there are trees etc. around to interfere with the signal
>   (either block it, or causing multipath reflections like "ghosting" on TV
>   pictures.)
>   An external antenna may help if your internal antenna is not getting
>   strong signals; they are usually amplified, so your battery consumption
>   is increased.  These are of no use if there is no signal available e.g.
>   under rainforest canopies.
>3) Different usage patterns.  If your friend's unit has picked up the latest
>   satellite data and your's hasn't, he will get a faster lock even if
>   everything else appears equal.  (Akin to used car salesmen who warm up
>   their cars' engines before the customers arrive... :)
>   Paul Taylor                                  Veni, vidi, tici -
>                           I came, I saw, I ticked.
>To unsubscribe from this list, please send a message to
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Tony Russell,
Adelaide, South Australia
Phone no. deleted, too many rude calls.

   Overbearing austerity is always the companion of solitude.
                                        Plato. ( Epistle to Dion)
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