[gurps] [VEHICLE] of the week 731 ? Planetary Defense Battery

[Anthony Jackson]

Evyn MacDude wrote:

> Anthony, it all depends on your wave length,  There are sensors that can
> detect subs from orbit. Power and gross coverage are the limiting factors.

They don't do it by seeing through seawater (to the extent it's actually 
possible, which is unclear and probably classified, it likely involves 
detection of wakes or thermal plumes, neither of which is a targeting 
sensor). Pure water has its greatest transparency in the 400-500 nm 
range, at which point it absorbs about 2% of light per meter, and it 
probably takes about 30% absorption to get a -1 to perception rolls, so 
we're talking -1 per 15 meters. Seawater is rarely perfectly clear, 
actual values are probably -1/10 for clear ocean water, on the order of 
-1/meter for average coastal water.

See http://en.wikipedia.org/wiki/Image:Water_absorption_spectrum.png for 
the absorption spectrum of pure water. 
http://www.seafriends.org.nz/phgraph/water.htm has a table of 'light 
absorption by wavelength' which shows the effects of dirty water.