Re: birding-aus GPS's

To: "Birding-Aus" <>
Subject: Re: birding-aus GPS's
From: "Paul Taylor" <>
Date: Sat, 21 Aug 1999 11:28:24 +1000
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

   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.

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