Math problem, projectile accuracy changed with movement of the player

Hi,

I'm having a little problem here.

I'm doing a first person view space shooter. The player's ship is on an orbit around the ennemies (only on the XZ plan (rotate around the Y axis), so on a circular orbit not a spherical) and it can pan up and down from his position on the orbit.

The ship keep firing as you move along with your finger (it's a one finger control system). For the rotation and the panning I've used the Fingergesture scripts.

What I'm trying to achieve now is a sort of inaccuracy factor when moving while shooting. Basically the aim gets less accurate with your speed.
My idea toward that is to do a circle on a perpendicular plan of the actual shooting axis and randomise the vector of the shot from the base of the ship gun to a point in that circle please see attached the shooting.jpg it will make more sense.


At the moment I'm using this code to shoot:

Code (csharp):

1.  
2. void Fire()
3.     {
4.         if(Input.GetButton("Fire1")  Time.time > nextFire)
5.         {
6.             nextFire = Time.time + fireRate;
7.             ForceFire();   
8.         }
9.     }
10.     void ForceFire()
11.     {
12.         GameObject thebullet = Instantiate(Bullet, Shooter.transform.position,this.transform.rotation) as GameObject;
13.         thebullet.rigidbody.AddForce(this.transform.forward*bulletImpulse,ForceMode.Impulse);                                    
14.     }
15.  

Basically I need to create a vector or quaternion of inaccuracy to add to the this.transfrom.rotation.

I really don't get how to manage the two vector (intitial rotation and inaccuracy) and how to link the movement to the inaccuracy. If possible, I would like to make the circle of inaccuracy grow or shrink depending of the speed.

My idea would be to use a random range for the rotation in Z (up/down) and Y(left/right) the speed of the movement would be linked (idk how) with the random range max number (0 being no movement 10° being highest speed).

Questions:

- How can I determine how fast the ship is moving (using the script TBOrbit and TBPan from FingerGesture)?

- Once I've got my random ranged rotation: vector3(x, random.range(0,SpeedCoef),random.range(0,SpeedCoef)) how can I apply it to my quaternion rotation?

- Is calculating that constanly won't kill the performances? is there a better way to do that?


Thanks a lot for your help.