The American observer Ingersoll reported at this time that "as to the accuracy of the bombing of military objectives, here I make no qualifications. The aim is surprisingly, astonishingly, amazingly inaccurate ... The physical damage to civilian London, to sum up, was more general and more extensive than I had imagined. The damage to military targets much less", and stated that he had seen numerous examples of untouched targets surrounded by buildings destroyed by errant bombs. For example, in two months of bombing, Battersea Power Station, perhaps the largest single target in London, had only received one minor hit ("a nick"). No bridge over the Thames had been hit, and the docks were still functioning despite great damage. An airfield was hit 56 times but the runways were never damaged and the field was never out of operation, despite German pilots' familiarity with it from prewar commercial flights. Ingersoll wrote that the difference between the failure of the German campaign against military targets versus its success in continental Europe was the RAF retaining control of the air.:7980,174
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): What did German pilots use to gain knowledge of an airfield?
prewar commercial flights.