Self Propelled Guns
possess a special targeting mechanism, which is commonly referred to as artillery sights.
These sights allow an SPG to scan its surroundings by the means of probing it with ultrasound waves, which result in complex feedback that the SPG learns to interpret.
In an experienced SPG's mind, this feedback creates something very similar to images that they would acquire from their optical senses.
It takes a lot of practice to master this technique, and of course their equipment has to be of a certain quality as well - higher tiers usually have easier access to sights that allow them to see farther.
This unique ability comes with a downside: A great amount of computing power is required to process the feedback, which results in SPGs having to focus entirely on it to effectively use it. Other classes don't direly need such a mechanism, therefore are very seldomly equipped with it. It takes away valuable resources from their other abilities such as brawling and sniping, and requires a great amount of time and concentration, which makes it useless for anything other than indirect fire.
Another downside is that the ultrasound waves of multiple SPGs using them at the same time can interfere with each other. When they are close together and use them at the same time, the result of the feedback is useless. It is important for the SPGs of a team to coordinate their use. An ingenious exploit of this problem is to set up a jamming transmitter that constantly disrupts the feedback of the enemy SPG(s), but these are difficult to get a hold of and require a power source. Also, they make the team's own SPGs useless; thus are only used by teams that don't have any artillery, and mostly in stationary camps.
Depending on the waves' reach, it is possible for SPGs to sense the presence of enemy artillery. Locating the exact source of the other SPG's waves is very difficult, but possible under certain circumstances and with enough practice.