Pt. Defense Targetting?

Hey!  It's time once again to play "STUMP THE PANEL"!

OK, seriously, vehicle combat in MT.  I was looking at the specs. for the
Sword Worlds TL11 Gram grav-tank (inside cover, "101 Vehicles") and I noticed
2 things which puzzle me.

First, the tank has a "TL11 Point Defense".  This is listed in the MT Ref.
Man. as:

        "  Point defense targetting allows the weapon to fire at incoming
         missiles and projectiles.  Hardpoint mounted weapons automatically
         have this capability.
        "  For each direct fire weapon which must also serve in a point
         defense capacity, install one point defense targetting module."

Now, since the Gram grav-tank has the following weapons:
                                Pen/        Max.    Auto  Dngr
                     Ammo Rnds. Attn Dmg    Range   Tgts  Spc  Sig  ROF
        Plasma PA-11  -    -    44/5 20   VDist(5.1) 2     15   H   40
        3 cm AutoCan HE   500    2   6    VLong(3.5) 4     3    M   200
                     HEAP 500    5   4    VLong(3.5) 4     -    M   200
                     KEAP 500    4   4    VLong(3.5) 4     -    M   200

I assume that the PA-11 automatically has PTD (Pt. Def. Targetting) and
the one add-on mentioned is for the autocannon.  Anyway, how does this
get used in vehicle combat?  Does the tank gunner give up a shot at a
target during a combat round, and elect to use PDT to eliminate incoming
rounds instead?  Or does PDT automatically provide some kind of defensive
DM for the tank and no explicit crew action is required?  I can't find
any references in either MT or in Classic Traveller which clear this up.

Second question: where does the 3 cm Auto-Cannon come from?  If I were
to design my own tank, using the tables in the Ref. Man., I wouldn't have
3 cm as an available choice.  On pg. 77 (Table 23), the sizes start at
4 cm and increase from there.  Did the GDW folks design this tank using
the old rules from Striker (which I don't have)?

Inquiring pre-combatants want to know.

        "Look.  All of this talking is getting us nowhere!  I think we
         should try a more direct approach."

        "MY GOD!!  THAT'S A HAND GRENADE!!"

        "Right.  It's faster than imperial arbitration, and it hurts less."

        Mark F. Cook

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