I feel compelled to add my $0.04 (with
inflation) to this thread. While it is quite true that license plates are not typically mounted on the tops of cars, there are a couple of factors that should be taken into consideration from a photinterp (
Photographic Interpretation) perspective.
One: While I realize that the license plate is the notional root of this thread, it should be kept in mind that only in a few circumstances would you need to know the license plate! If you're looking for the location of a vehicle, then unless the vehicle in question is in a congested area the color, make and approximate location should allow you to garner a pretty good idea as to which car you're looking at.
Two: Many spy satellite shots (I reference here the ones available publicly, for example at the Federation of American Scientists site at http://www.fas.org) are in fact taken from low angles. This is because a low-flying satellite is moving so quickly over the ground that it is only 'overhead' a brief amount of time - but has much more time as it approaches and retreats from the target point to observe. Thus, of the imagery acquired in a pass by the sat, most of the imagery will have been acquired at a low (less than 60 deg.) angle of incidence to the target. Of course, this doesn't mean the license plate is facing the right way, or that in fact the plate is readable at all given the above.
Three: Aircraft, public vehicles (buses and the like) and military vehicles often do have ID info visible from the air. Watch Speed again, or look at the wings of a large commercial aircraft. While not guaranteed, it's quite likely the latter has its aircraft number on the wings, for example.
I know this is sorta party-poopin' and off the original (license plate) topic, but it's relevant to the broader question of the limitations of satellite reconnaissance.
Roninspoon:
heh. Actually, I'm just more of a blabbermouth. Qualified? Ssschy-e-e-a-h.