It's all speculation. we are to assume that Cooper knew exactly when to go down the stairs to see the lights from the dam? he gives no indication to timing anything. how would he have known they would fly close to it anyway? then of course the cloud coverage.
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My contention is that Cooper had no indication of timing anything, he had no idea of when or where he was going to jump, and he didn't even care. I remember discussion on the DZ Forum about Tina helping Cooper get the aft stairs to go down. Tina gave no indication that Cooper was agitated, irritated, or panicked about the situation. In fact, he seemed cool, calm, and collected. Some people thought Cooper's original intention was to jump as soon as possible after leaving Seattle -- like around Tacoma (which is close to McChord AFB) or Olympia. Getting the aft stairs to deploy took longer than expected. No big deal, Cooper wasn't even annoyed. Tina didn't indicate he seemed rushed about anything -- he got agitated in Seattle when the fueling took so long, that's pretty much it. When he was cutting parachute chord and strapping the money bag to his waist, Tina didn't say he seemed rushed or panicked. Either this guy had nerves of steel and ice running through his veins, or he was specially trained to do this sort of thing -- like a special ops, creme de la creme, CIA ops, Navy Seals kind of guy -- or both. For him, it was just another day at the office.
As for using the lights of Vancouver as his guide -- that seems pretty far-fetched. With all that cloud cover (I was there that night, remember -- lots of rain and clouds for sure), the Vancouver lights would have done little to help him pinpoint a drop zone -- he could have landed anywhere -- on I-5, on someone's roof, in the River, on the north end of Vancouver, or the south end -- where?. On that night, he couldn't have known anything with any certainty.
Why wasn't he worried about when and where he would actually jump? He didn't seem to be overly concerned about those matters. Possible explanation: he had lots of help on the ground, several pickup points along V-23 -- it didn't really matter where he jumped out -- help was only a stone's throw away. Was he a hired hand? Did he grow old in the witness protection program? A possibility -- why not?