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From: Craig Haggart, Accelerator Ops (ssrl01.slac.stanford.edu!HAGGART)
Date: Tue Oct 15 1996 - 22:12:38 EDT
Derrick Young (DEYoung-LANMAIL.RMC.COM) said:
> ...US Navy...fast frigate...has the center of their radar
> [antenna] 110 feet in the air. This gives them an effective
> radar horizon of between 10 and 12 miles...
and
> All the extra power that you can generate will not do any
> good, if your effective range is only 2 to 4 miles.
But that effective range/horizon only applies if you're looking
solely at the SEA SURFACE at that distance. Things that stick up
above the surface can be seen at a greater distance -- ships,
land, and rain squalls, for example. Remember, the taller or
higher something is, the farther away you (and your radar) can
see it.
If you were perched halfway up your mast, how far out to sea do
you suppose you could spot a large ship? How about the Golden
Gate Bridge? What about the island of Moorea?
Here's how to figure it out: the visible distance (in miles) is
1.23 times the square root of your height (in feet) plus 1.23
times the square root of the object's height (in feet).
Radar, in fact, can usually see a bit farther -- generally about
15% farther -- due to atmospheric diffraction. To figure out
your "radio horizon," use the formula D = (1.41 x sqrt H1) +
(1.41 x sqrt H2). Again, disance (D) is in miles and heights
(H1 and H2) are in feet.
So if your radar antenna is at the spreaders (say, 35 feet above
the water), you could pick up a container ship's superstructure
(call it 60 feet above the water) at about 20 miles. If your
radar transmitter has enough power, you could see an island
with dramatic peaks from more than 70 miles away.
Getting back to that frigate with a 110-foot antenna height:
Those swabbies can paint the Golden Gate Bridge from Southeast
Farallon island, which is about 30 miles out.
-Craig Haggart, KC6VHO
haggart-slac.stanford.edu
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