On Friday, 15 December 2017 17:36:49 UTC, Terry Casey wrote:
In article 899888a2-39cf-4f8c-871c-134be252a102
@googlegroups.com, says...

The majority of air traffic contols stations DO NOT use GPS.
Not knowing where a plane is and at what height isn't the
best way to 'control air traffic'


GPS systems CAN calculate altitude but, as the GPS receiver
is on the plane, it won't be much use to ATC until a
reliable method of ensuring that this data is also available
to controllers in at least as accurate detail as current
radar systems.

What drivel you spout.
https://en.wikipedia.org/wiki/Air_tr..._beacon_system

Also known as "squawk"

If you're in an aircraft with a transponder/squawk, the operating light comes on every time it's "swept" by ground ATC radar indicating the ID signal has been sent out.