sentences:  These aircraft were employed for Operation Hallmark, a sensitive intelligence operation in which the Pembrokes were fitted with high-powered reconnaissance cameras to acquire imagery of Soviet and East German military installations and airfields below the tightly controlled air corridors.These were subsequently analyzed by photo intelligence and imagery experts, who recorded any changes in the Warsaw Pact forces facing the West.Alterations in the order of battle, appearance of new equipment and movement of military units were all items of great interest.