On Sun, 24 Sep 2006 19:30:51 GMT, "Roger Long"
wrote:

Just one of those questions that you suddenly wonder why you never
wondered about befo

How do all those radar's out there avoid interfering with each other?
Are they all on slightly different frequencies (that seems like a lot
of frequencies) or do they just know that pulses that come back within
a narrow time window are the ones they sent out?

Larry?


Radars that use magnetrons pulse them for a microsecond or less, every
millisecond or two.
The signal travels at a high enough speed that an echo from one
nautical mile takes 12.5 microsecods to get there and back.
An echo from thirty NM away takes 375 microseconds there and back.

That's about a third to a sixth of the interval at most between
successive magnetron pulses.
The indicator, the plan position indicator starts its trace from the
center as soon as the magnetron pulse finishes and brights up for any
echo it sees while the trace travels to the edge in 375 microseconds
(on a thirty NM range). It paints again at a different angle as the
scanner rotates.
If another radar is running close by
and
it points at your rotor
and
its frequency is close enough (they vary during a pulse and they vary
the center frequency - its a dirty device)
and
its pulse occurs while your indicator is painting returns

then
you see a snow storm for a little while.


Brian Whatcott

ps
1 microsecond = 1E-6 sec = 1/1,000,000 sec
1 millisecond = 1E-3 sec = 1/1000 sec
Magnetrons often put out at around 10 gigahertz
= 10,000 MHz
=10,000,000,000 cycles per second
But that's only about 10,000 pulses at a time.