It depends on what you want to call "detection". I always thought it was under 200' to avoid being a dot on enemy radar and to avoid creating a dar bar but if it's 500', great. So this minimum keeps enemy's from seeing you no matter what, but, friendlies still see your dot (but not a dar bar from you). As mentioned, the base under attack warning sounds no matter what as soon as you cross the local radar circle. Here's the part 2: Say you're on a bomber run to the enemy's HQ. You can avoid being a dot on enemy radar by avoiding the local radar circles. You're still on dar bar but not a dot. All the enemy will know is that you're in the sector you're in and that you're not within a local radar circle (thus being able to reduce their seach area somewhat). While this doesn't seem like much of an advantage it actually can be at times.