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

[Onno Meyer]

> > Neat idea!
> > 
> > How about a waterborn (possibly submersible for additional stealth and 
> > storm protection) version of this to make it easier to give  more 
> > world-wide coverage of water rich planets?
> 
> Yes and no. As described, this one is for a world somewhere between Mars 
> and Luna, or perhaps slightly more atmosphere. Note the life support.

I posted this on a JTAS discussion board:

I have thought a bit about the chance of submarines (or submerged 
System Defense Boats) against orbiting starships. My thoughts are
still incomplete, but perhaps you have something to add ...

* Meson guns can hit a submerged target just as easily as any 
  other target within range.
  X-ray lasers will have a substantially reduced range, but a 
  low orbit is not all that far by space combat standards. 
  Traveller missiles are actually Vehicles, so the rules for 
  missile fire in VE don't count. 

* Radscanners with a halfway decent range can detect subs, but 
  they are not targeting-capable sensors (i.e. they detect the 
  approximate bearing, not exact positions). A submarine with 
  radical emission cloaking might have a signature of -6, and
  LEO is a speed/range of -30. Take a Heavy Radscanner with a 
  scan rating of 42, and a halfway decent operator, and the 
  approximate position becomes clear ...

* As far as targeting goes, ladar seems to be the best bet. 
  Any AESA can function as a ladar, at -2 to scan. Scanning 
  through a body of water gives another -8 (p. VE171) and 
  Starships lists the vacuum ranges (another -6). Since 
  TL10 deceptive jammers function against ladar, the sub 
  might have a signature around -18. On the plus side, the
  radscanner is a prior contact for +4. But combined with 
  the -30 for range, not even the UHv. AESA has a chance. 

  Things look quite different with TL9 sensors vs. TL9 
  submarines since jammers are out of the game. 

* Densitometers are worthless because they scan only one 
  cubic yard per seconds. Odds against successful contact 
  are astronomical. 

Summarized, a starship will know roughly were subs or SDBs
are hiding, and it could hit them if it knew exactly where
they are, but it cannot get this kind of exact data. 

That means a sub or SDB with accurate targeting data from 
passive surface sensors can engage starships without fear
of retaliation ... until the invader drops hunter subs to
take the fight to the oceans. 

At TL10, sonar has a maximum nominal range of 200 miles 
and hence scan 25. Unfortunately, VXi25 has Extreme Sound
Baffling at -24. 

... best hope for an accidental ramming.