getting coordinates at startup Nuvi 760

I always understood that a gps needed 4 satelites to be able to present
ytou with your position. On my nuvi 760 I check at the satelites screen
when starting up and see that 4 satelites have a solid bar but still no
coordinates are available. Only when a 5th stelites has a solid bar the
coordinates are presented.
Is that normal knowing that 4 stalites give already a unique position.
Thibaud

I have seen that with my 76CSx. If the satellite geometry is poor (as in a
straight line), the unit cannot calculate a position.

Correct. Technically you can get a 2D fix (deducing heigh by assuming
you're on the surface of the earth) from three satellites, and a 3D fix
from four satellites.

The process of 'finding the satellites' involves finding any one
satellite, then downloading almanac data from that satellite. This
tells the unit what time that satellite thinks it is now, and where it
thinks it is now and in the near future. The unit then looks up the
current time in ephemeris data it holds internally and that tells it
which other satellites might be worth looking for. So that takes time.

If the unit has been significantly moved while off, or has been off for
some time, the unit will have to download and store more recent
ephemeris data -- a lot more bits -- from one of the satellites to tell
where each satellite will be and when. I don't know how much data it
has to download or how long that takes, but let's assume it could be a
few seconds.

And even once 4 satellites are found, it still takes a lot of
calculations, so it takes a few seconds to work out and display the
coordinates.

If more than four satellites are visible, the unit tries to do more
than that. For instance it likes to give you a precision ('you are at
X,Y within 17 feet') and the more satellites it can see the better it
can get its precision. It does this because the data from one or more
of the satellites might be being bounced off of a nearby building or
mountain. So finding a fifth or sixth satellite takes time.

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