FA Rules
Note: The rules presented were taken from the Techspecs.zip file from Morrigu's SR FTP Archive Site because basically these are good rules ( I think):
INDIRECT FIRE
Though a number of indirect-fire weapons (grenade and rocket launchers, and even a mortar) are available, no rules for indirect fire have appeared.
With indirect fire, the target is not within line-of-sight of the attacker. If the target is within LOS of the attacker, use the normal combat rules. Now, if the target is not in the attacker's LOS, but within the maximum range of the weapon, the attack has a base Target Number of 8, independant of the range between target and attacker. Naturally, a target outside maximum range cannot be hit. This Target Number is modified only for recoil, attacker movement, and multiple targets (page 89, SRII); if the weapon is equipped with a Smartlink Level II (page 84, Fields of Fire), apply a -1 modifier to the Target Number. No visibility modifiers apply, because the target is invisible to the attacker anyway.
Then, a skill test is rolled for the weapon. A succesful roll does not mean the target is hit directly; instead, a succesful roll reduces the scatter, using the rules for grenades (pages 96 and 97, SRII). Whether the target is inside the blast radius or not depends on the number of successes rolled to reduce the scatter. Only if scatter is reduced to 0 meters does the target take a direct hit: any excess successes are used to stage the damage up in such a case.
Incidentally, these rules would also apply to grenades thrown over walls or around corners at targets that are out of the line of sight of the thrower.
Observers
The chance of scoring a hit are low when using indirect fire, mainly because the attacker cannot observe the fall of his rounds. Observers can change this, but the observer must have some method of communicating with the firer. Each time the observer sees a round fall, he can relay this information to the firer by making an Intelligence test against a Target Number 4. Apply all appropriate modifiers to this Target Number as if the observer were firing a weapon at the target. Making this test costs the observer a Complex Action. If the test is successful, the firer's Target Number goes down by 1, irrelevant of the number of successes scored by the observer.
Designated Rounds
It is possible to "illuminate" the target with a target designator. The rules for these appear in Fields of Fire, on page 85. The only modifications needed are that the person firing the weapon makes a skill test against a Target Number 4; success means the round arrives close enough to the target to be guided by the character using the designator, while failure means that it doesn't. The designating character then rolls a skill test to succesfully designate the target.

Addendums
Observer
  Plainly stated, observers communicate by radio (as any US Army FO what's one of his most important items, and more than likely he'll say "my radio"). But since SR doesn't seem to worry, it's good to have a good comm system (like, Phillips, since he may be giving his position for any Polar missions he may shoot) to have. Another thing, since the invention of Battletac, and the FDDM system, allows an FO to send a fire request digitally (we can do that now, but a little more limited) without having to worry about voice interception. Also realize that FO's are a priority number one target on the battlefield, as we are the eyes of the artillery, and noone likes a hell of alot of HE coming down on thier heads via FO.
  Designated Rounds
  The only problem with this rule is that a) designated rounds hit their target (if that wasn't the case, what's the point of laser guided rounds and LGBs if they didn't "hit the spot". Granted it may be true for radar guided, but most commonly used is the laser, and when I need to drop an APC with a Copperhead, I want it to hit, not a "few feet"), and even then if the target moves in the next phase (if allowed), the acquirer will naturally have to make another Test to reacquire. Also, this rule assumes that the missile will be there the next combat phase. No, if that were the case, I wouldn't need an FDC to tell me when to lase a target, because depending on the range from the firing gun to me (the observer), it would naturally take at least less than thirty seconds, or pretty much 10 combat turns (or less) for you to wait until that round gets in the area. Now, modern SR technology has the ability to judge time of flight (TOF) of that round, as well as current tech (that's where cannon FDC's need to know so they can tell the observers when to start lasing). Depending on the TOF (which I have to figure out a formula), FDC in a cannon battery will call "Designate, now, over" to the observer, which with his designator, illuminates the target for the round to home in on. This is called thirteen (13) seconds before the round impacts. Now one thing that FASA of course missed was the fact that cloud cover can seriously screw a laser guided munition as it won't have time to maneuver to the designated target. To determine time, assume each 1,000 meters from you to the firing gun is 1 second, so naturally, a 155mm howitzer firing 25,000 meters (25 klicks) = 25 seconds TOF. Simple, and divide by three (round down) to determine how many Combat Turns the round actually takes. Now, the 13 seconds is pretty much SOP for us FO's in the Army. However, most like to make it close, like 5 seconds, since it really only takes a second for the round to acquire, provided the shot round was in the right area. Fortunately, we're not going to take into account a missed shot, since if you started lasing earlier (around 3 Combat Turns) at -10 seconds, we can "lasso" the errant round (but this is really GM's discretion really, you might really screw things up for the fired round) and still make the kill.
  The general rule of thumb is that if it's low cloud cover, it won't work (less than 3000m as a general rule). You can fire at a Medium cloud height, just make sure that the target is static. High cloud cover (or better yet, none) and anything is practically game. Another small thing about designated munitions is the firing unit has to know your position. The reason is that a designate mission is basically what is referred to as a Polar mission, where the direction in mils and the distance from the observer is given and the firing battery makes the shot. For Polar to work, FDC (Fire Direction Center) has to know your position within a six digit (100m error) or more grid to make the proper calculations to attack the target. When you call the mission in, the firing gun loads the guided round, and is shot per your direction and distance given. Naturally, the round upon reaching the area will start searching for the laser/radar beam to the target. Battletac and the FDDM will automatically send your grid position data when you requuest air strikes, mortar, FA, or some other form of fire support. One inherent danger in laser designation is not hitting a friend, but what is known as Angle T, or when the Gun to Target (G-T) line is 800 mils or more from the Observer-to Target (O-T) line. What this simply means is that the round will track where it is being emitted from, not to where the spot is actually at (on target). While I may hear hooting about "well, I got this IFF code thing here". Bullshit, there was a CALFEX at Fort Sill, and that's what happened, some observers got killed because of that Angle T, and all Copperhead rounds are coded to the laser designator it's firing from. How to do this is well, somewhat difficult, since FASA doesn't really worry about this thing, and well assumes that you're always in the perfect position. Sooo, the best way to determine this is to make your GM make the call, with an Indirect Fire test made. It's a matter of GM to yay or nay what you're after depending on whether you're good or going to die, per your rolls and decision. However, if you do miss, refer to the rules above.
  The Mighty FA Cannon
  This modifier paragraph is on the use of burst fire 152/155mm cannons. It simply is that for every round, add a +3 modifier per round. Naturally, these massive barrels have huge muzzle brakes to offset this. Each brake (regardless of caliber) offers 3 points in recoil reduction. Also, the vehicles Body offers an additional 2 points due to the sturdiness of the vehicle. The barrel can accept Gunnery Recoil Adjusters, as well as Vehicle Gyro Stabilization, per standard rules for each, and you cannot mount both, it's either or.
  Eric Johnson, 1998.