Characters that have been knocked down, or choose to go prone count as being in the prone position. (You count as going prone by default while setting up a weapon with a bipod on open ground).
Weapon skill tests against prone targets count as ordinary (+ 10).
Weapon skill tests from a prone position count as difficult (- 10).
Ballistic skill tests for characters in prone suffer no penalty. UNLESS
Ballistic skill tests engaged in hand to hand combat count as difficult (- 10).
Dodge tests in prone against ranged attacks suffer no penalty. UNLESS
Dodge tests when engaged in hand to hand combat count as hard(- 20).
Additionally, against ranged attacks you count as one size category smaller if you are prone.
Going prone is a half action.