Tire Pressure Monitor System, how it works

[gc2488]

What's the real deal with the Hummer TPMS?
Does it use radio technology and monitor
the pressure of all tires including the spare
even when the spare is not being used?

I asked because a friend said that some pressure monitor
systems really just compare wheel speeds and assume
pressure is low when one wheel speed is higher than
the others on average.


In my H3 manual the original text said this:

Your vehicle is equipped with a Tire Pressure Monitor
System (TPMS). This system uses radio and sensor technology
to check tire pressure levels.


But, the manual came with an addendum page for the TPMS
with no mention of radio.

Just wondering... If some pressure monitor systems uses
radio to communicate pressure, I'd like to know details
like if batteries are involved and how often the pressure
is communicated to the main computer.

Fun. I seem to recall that the HMMWV and H1 have
super cool tire inflation/deflation systems.