Radar Station

The functions are:

Reduces enemy air units' combat efficiency by 2.5% for each level of radar you have in the territory.
The effectiveness of this factor can be increased by research. A 30% radar efficiency would mean 1.3*2.5.
Coastal radar stations have a combat air battle factor in adjacent seazones. On land, however, each province's radar is used only for that province without any adjacency effects.
Radar stations do not detect naval units .
Radar is absolutely vital for a good air-defence in HOI2 and HOI2 DD versions 1.0 and 1.1:
- Without it the airunits on air-superiority missions will just move towards random provinces within the area.
- With it, the airunit will know about any planes nearby that is heading to/through the province with the radar.
Radar does not provide detection after HOI2DD 1.2 through ARMA 1.2. Two comments by Johan on pg 2 of a long thread: Johan mentions but does not explain the change
In ARMA, the radar naval brigade improves naval engagement range
Since radar is intelligence related, it's always us vs them comparisons for spotting purposes. Slider (open vs closed society), relative tech levels for encryption and decryption, and difficulty levels influence how well radar detects the enemy.