The Mysterious Effects Of Contra-Gravity

Your ship is sitting on the pad.  You turn on the contra-grav system.

What happens?

GURPS 3e Space; p.120 says:

Contragravity.
     This superscience technology allows the ship to "screen" itself from some or all of the gravitational pull of a planet or other body.  In practice, this means its weight is reduced to zero for takeoff or landing; thus, only a small amount of thrust is needed.
     A ship with sufficient CG lift can hover in midair, or float gently up or down into orbit.  To do this, it needs enough CG lift to neutralize its weight under the local gravity field.  A half-space light contragravity (LCG) system can neutralize up to 1,000 tons.  A one-space heavy contragravity (HCG) system can nuetralize up to 5,000 tons.  In both cases, this is weight under local gravity - a ship on a 2G world will need twice as much CG!
     It the GM wishes to make contragravity less effective, simply reduce the weight it can nuetralize.

GURPS: Traveller p.107 merely mentions that the normally TL12 contragavity is introduced at TL8.

G:T Starships says:

Contragravity Systems [TL8]
     Contragravity systems cancel out all natural gravitational forces acting on an object up to their rated lifting capacity, as discussed on pp. GT 107 and S120.  In GURPS Traveller, where reactionless thrusters are common, such technology is available but only necessary in very specialized applications.
     Each module counteracts 450 sTons of weight at TL8, 1,500 sTons at TL9 and 5,000 sTons at TL10+.  The module provides no lateral thrust.

And that's it!  That's all I got.

I was perfectly happy with when it was activated the ship just sat there until you applied some thrust.

Then Geff (real life fucking mentat) and Ben (doctorate astrophysics) started asking questions with their tasty chess club brains.

Geff emphatically stated that something with a weight of zero effectively has a density of zero so it's going to float on the local atmosphere, so activating the system means you're going to go up.  Quite quickly in fact.

Edit:
The reason we don't float is that our weight exceeds the weight of the volume of air we displace.  Reducing the weight of the ship is increasing its buoyancy.  A Traveller dTon is 500 cubic feet.  The weight of a cubic foot of air is about 0.0807 lb. per cu-ft.  Thus if our 400 dTon ship's weight is brought below 16,140 lbs it's lighter than air and will move upward.  The standard CG module does 5,000 TONS of reduction.

Your vector as applied by the rotation of the body also applies.

These questions also raised other concerns that were never fully addressed.

If I cannot thrust straight out of the atmosphere, but I have a winged ship...  Can I fly out to orbit?  The reactionless engines in Traveller are unaffected by the density of the air and can make a constant thrust for years thanks to GURPS removal of the power plant fuel requirement.  Since the maximum air speed is indicated air speed, in theory I could keep gaining altitude and ground speed until I got past the point where I needed the wings.

Weigh in here if you have an opinion, but you're going to have to show your work not just make unbacked assertions.

I've been running the game on some VERY simplified tables and equations and lately I've been thinking they are a bit lacking.