Excellent Paul. Thank you so much.
At 11:28 21/08/99 +1000, you wrote:
>> 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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Adelaide, South Australia
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