Re: Why no iPhone on this list?

George Graves <gmgrav...@xxxxxxxxxxx> wrote:
nospamatall wrote

They don't, as far as I know. Obstacles do. The satellites can't be
directly overhead everywhere, can they

There are so many of them that it doesn't matter. If one is behind a
building, there should be at least three others that aren't.

Much of this comes down to the paradigm of what one considers to be an
"urban" environment. Personally, I hardly call "A" building to be an
urban setting. Think more of the narrow canyons of New York City,
Chicago, etc, instead of suburban sprawl.

Often, with my
GPS receiver, I've counted up to 5 satellites at one time.

Pragmatically, one needs a minimum of 3 to get any fix, and 4 to get a
3D fix, so having only 5 is not really that much of a performance
safety margin.

There's only 27 GPS satellites operational right now. In an idealized
homogensous and two dimensional world, that would be one every 13
degrees. However, in 3D space, the homogeneous arrangement will need
to be more akin to the corner locations on a icosahedral (20 sided
dice), which has a dihedral angle of around 140 degrees.

Continuing to assume homogeneous distribution and applying spatial
geometry, from a tangent plane on a surface (instead of through the
Earth's core), it is impossible for even half of all of the satellites
to ever be visible from a point on the surface. Without doing the
details of the surface area projections, I'd swag the number possibly
visible at ~1/3rd of the total. This would be only 9 satellites.

FWIW, I rarely see that many. I did have a very unusual and
noteworthy event when last fall, I actually had 12 visible at once.
From roughly a decade of GPS use, that's the first and only time I've
seen that happen.

Moving on, if we assume up to 9 in the sky, the perceived spatial area
when split above/below a 45 degree angle from the horizon is roughly
another 1/3rd - 2/3rds statistical split. Thus, you'll have 3 high in
the sky and 6 low.

In urban canyons, all of the low are going to be obstructed.

So...how many satellites are needed to get a fix? 3 for 2D and 4 for
3D. How lucky are you feeling?

FWIW, its my very, very general impression that much of the popularity
of the iPod is due to its utility of being able to provide music/
entertainment while commuting on a subway train. FM reception doesn't
work below ground, which is essentially why the FM tuner 'feature' has
never been an item of meaningful product differentiation either way.

-hh

.

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