Utility of increased channels on GPS receivers

[Templar42]

I've searched around a bit but can't find a detailed answer to this, although it's been touched on in various places. If its already been discussed to death I'd be grateful for a link to the relevant topic.

There are GPS receivers on the market trumpeting their 32, 44, and 51 hannels. As I understand it a channel is used to receive data from only one satellite, and there are 31 active GPS satellites, 12 at maximum being visible from the ground. I appreciate at 18km altitude (commonly quoted as the maximum operating altitude on units) this number would increase, but not nearly as far as 31, since wherever you are the Earth will be hiding several satellites at least.

Some say that more channels increase performance in urban environments where signals are bouncing off buildings, but I don't understand why this is. Can anyone say whether it is of significant use to have over 20 channels, let alone 32 or more, and explain the benefits of spare channels for bouncing signals in terms an interested layman can understand?

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