Phil W Lee wrote:
[snip]
Surely the cost of filling in the GSM blackspots would have been far
less than the cost of creating an entire additional network.
It would have also had the benefit of providing a massive PR boost as
ordinary users would get the benefit of any spare bandwidth.

Err, not really. One of the important things about Tetra is that, in
the event of being out of coverage (indoors or out) you can still do
handset to handset calls. This is particularly useful on a fireground
and also in remote locations. In the latter case you can use handsets
to relay back to the national network so can provide coverage 'on the
fly' which can also involve an aerial platform.

GSM and Tetra had very different design goals and does mean that they
are best at different things. Tetra is designed to provide PTT to large
"talk groups" (possibly hundreds of people) in a small location. GSM
can't do that, for example. On the other hand, Tetra isn't very good at
providing micro, pico and femto cells when you want to have loads of
people in coverage in a dense urban area. And so it goes.