What kinds of radars SPAA have
Since update "locked on" SPAA's and certain planes can use radars in more in-depth ways.
There are 2 general types of radar for SPAA currently in the game:
Rangefinder - Does not search for targets specifically, but can tell that "something is flying", assists you with targeting and shows you how far target is. In some cases sets fuses for self-destroying ammo.
Active radar - Searches surroundings for any aircrafts, when online. Can be locked on target to provide detailed information on it, provides accurate information. May be "sector", or "circular" view based. Easily detectable by counter-radar.
How to use active radars
When you use any vehicle with any radar, you will see these 2 things on your screen:
1) Radar screen
2) Radar assisted compass
Whenever you don't have any locked target, your radar either searches for targets on it's own, or follows your turret, depending on it's type and current search mode. It's current direction is represented by a second line on radar. This line must hit a target to "spot" it. Be aware, that small planes can avoid detection, and obstacles like trees and hills obscure radar's vision.
Even if your active radar fails to spot a target, or is offline, generally SPAA can still select and target enemy, that enters their "field of view" via passive rangefinders. The accuracy of this method is low.
Whenever your active radar detects any target, you can "lock on" to one of them by pressing a key. Default keybinding is alt+F, which is strongly adviced to bind to something else, because ALT key activates mouse cursor and breaks lock-ons. It's suggested to aim at enemy in sniper mode, if there are many vessels in a line, or radar will definitely get confused and target wrong one (or even your ally).
Your turret will receive the feed, adjust ammunition self-destruction fuses and follow the target automatically. You will now see a "true" targeting reticle, which 100% guarantees, that your target will go down, as long as it doesn't change speed or direction.
Your only concern now is to prevent your opponent from dodging your volley. For this reason, you shouldn't waste too much ammo firing in the same direction, if your SPAA's accuracy is adequate. Try firing all around the targeting reticle, to hit enemy, even if they did change their course.
For super long range snipes (~4-6km engagements) you might have to stop your radar from searching randomly and scan certain area specifically. This is done by a key (Not set by default, name in settings is "change radar search mode"). This will stop radar dish and force it to follow your turret's current direction. After that you need to look into direction you think your enemy is going to come from (enemy airfield, for example). Since radar is static in this mode, you might want to move your turret a bit, just in case plane took a bit different route. You also should note, that smaller planes that fly near to ground are harder to detect, and even this trick might not work on them.
How to use rangefinders.
Warning: Example is written with M163 in mind. It may be different for SIDAM 25 and other SPAA.
To use rangefinder substitute to full radar, you have to go to gunner (sniper) sight and point your crosshairs onto plane that interests you, then press lock-on key (default keybinding is alt+F, like with the rest of radars, is recommended to change to single key). As a result, you will get a lock on:
Now you just have to aim at true targeting reticle and destroy enemy plane before it broke visual contact by flying behind trees and buildings.
Drawbacks of using the radar
Certain planes and helicopters have radar's detection system, and as such, whenever your radar "hit" them, they see radar alerts.
Some aircrafts have fully fledged out radars and will see you clearly as soon as you try to track them.
Here is an example of what happens, when you got noticed by a helicopter:
Certain helicopters and planes may have no problems with carpet-bombing your position with rockets, just to make sure you aren't going to fire at them ever again. To avoid being bombed like this, you can turn radar off with a key (default alt+R). This will shut down radar search, and you won't be able to lock on anymore. Certain SPAA's also hide their radar behind them, making their profile much smaller and harder to notice by tanks around them.
Despite losing Lock-on, you usually still retain your "fake" targeting reticle from rangefinder and can fire at enemy semi-precisely, however, in most SPAA's cases you can't hit targets that are too far away from you like this, because ammo will self-destruct at it's default range. (about 2.5 km away)
SPAA's that have radars in some form
Gepard — Radar station for circular viewing and target tracking. Radar is pulled back, when offline.
Chieftain Marksman — Radar station for circular viewing and target tracking.
AMX-30 DCA — Radar station for circular viewing and target tracking.
M247 — Radar station for circular viewing and target tracking.
OTOMATIC — Radar station for circular viewing and target tracking.
Type 87 — Radar station for circular viewing and target tracking. Radar is pulled back, when offline.
ZPRK 2S6 - Radar station for circular viewing and target tracking. Radar is pulled back, when offline.
ZSU-23-4 — Radar station for sector search and target tracking.
M163 — Radio rangefinder and ballistic calculator.
SIDAM 25 - A laser rangefinder, as well as optical tracking device.