Issue: While patrolling defended facilities, two or more aircraft hovered at the altitude of about one and a half or two kilometres. Planes flew at a so-called "economical" speed, when the engine consumed a minimum of fuel. At a sudden appearance of the enemy, the aircraft had to gain speed or altitude to repel the attack, and often there was no time to do so.
Patrolling at the altitude of three or four kilometers could provide excellent conditions for repelling an attack due to the altitude margin. However, approaching enemy aircraft were hard to see against the ground background.
Question: How should patrolling be performed to provide effective repelling of enemy attacks?