At the main menu press N+SPACEBAR to enable these codes:

FREE = Collisions off
AMMO = Powerful weapons
KIEV = View FMV sequences
KAMOV = Sound test
SOYUZ = Low gravity
HAPPY = Invincibility
KURSK = Invincibility
BLAST = Unlimited ammo
RUBLE = Weapons upgrade
RUSSI = Full screen pause
EMPTY = Enemy stops firing
LENIN = Disable enemy fire
MINSK = Disable all cheats
KIROV = Infinite ammunition
SNEEG = Sea Hunter mini-game
DNEPR = Display collision boxes
BURAN = Disable collision detect
VOLGA = Random color terrain borders
ROGOV = Random color terrain polygons
BUGGY = Preview unreleased game (Bug Rider)