Objective 1: Turn Off The Power Supply For The Radar Facility
At the start swim to the left until you reach the hill with barbed wires. If you crouch/crawl along the barbed wire you'll see that it ends on the top of the hill and you can continue crawling until you're near the power supply compound. Crawl down, approach the guard hut and break his neck, get his dragunov if you like. Check the movement of the 2 patrolling guards here then move to the nearby shack. Hide first then jump on the cement brick and over the wired fence to your left. Go get some nice position where you have a view of the three guards inside the power supply area (check the map below). Shoot them with your silenced SMG and when all are dead move around to the entrance. Still check the movement of the nearby guards and activate the panel to the gate. You have to turn off the two power supplies to complete the objective.
Objective 2: Disable Radar Terminal
To avoid the bad guys on the next area just crawl on the right side of the cliff. You can jump over the wires and then crawl, move close to the wires but not close that you'll hurt yourself... just enough to avoid falling/sliding down. You won't be noticed if you crawl on the right time. If the guy near the satellite dish is approaching just stop or shoot him with your SMG. Continue crawling until you reach the area past objective 2 with lots of trees in it, you can easily jump the wires from here. Head to the radar terminal camp, crouch while inside to avoid enemies' attention and when you reach the end shoot the guy with your SMG while he's looking at the window. Disable radar terminal and exit the camp.
Objective 3: Disable Communications Terminal
After you finished objective 2 a chopper will land near objective 3 unloading enemies to make the mission a little bit harder for you of course. Just leave it to the crawling expert:) Once the chopper's finished unloading, get close and climb the nearby slope. Crawl your way up and avoid being seen by the guards. Check your map for enemy movement, then continue crawling around the hill. There's still one guard standing here, but you can shoot him with your SMG. when you reach the last compound, crouch inside and break the neck of the first guard. Before you snap the next guy's neck check the guard to your left, he'll see you if you crouch so crawl instead and finish the 2nd guy. Open the last door but hide coz there's still one guard left. Peek to see if he's not facing the door anymore then kill him. Disable the communications terminal.