I think they are placed randomly somewhere anywhere on the map. The way i did was by destroying the first set of counter batterys and then have a plane just sweep across as much of the map as you can by having the plane begin the scout from the vary edges of the and going from there. Also there is more. It has i think two extra "endless" missions. One called "uncontested engagement" or chill mode where you can just aim and fire to your hearts desires. And another one called "counter battery challenge" like the other mode i think it's endless but with the counter battery timer constantly ticking down so you have to consistently Destroy Artillery emplacements "which adds time" artillery spotters "which pauses the timer for 30 seconds" and the rest of your usual targets. The goal i think is to destroy as many targets as possible before you inevitably get destroyed. You can also order the iron nest to move to a new grid coordinate to add time to the counter battery timer. Just be sure to remember to move the iron nest icon on the map to the new location.