Crowds always gather to watch Air Force One touching down. What they don't realize is that the plane is watching them just as much as they're watching it. There are cameras inside the wells which the wheels fit into.
Putting cameras on the underside of the plane was the idea of President Eisenhower back in 1959, and they've been fitted to every model of the plane since. They can identify vehicle registration plates, and check them against security lists to ensure they're not a threat.