[pippy]

To expand on 3: most spy satellites operate in sun synchronous polar orbits. This is so they can cover the entire earth and will have their solar panels lit by the sun. This also has the ramification that they'll always be in the same spot in the sky depending on the time of day. If you know this time, that's when you hide your tanks and don't fly your top secret spyplane

[azernik]

Even if they're not in SSO, satellites are hard to hide. The timetables might be more complicated for a rank and file soldier to use, but you can time your actions to be missed by any specific low-orbiting satellite.

[jameshart]

"will have their solar panels lit by the sun" is probably not the primary reason for sun-synchronous orbits (though I guess it is presumably a benefit) - it's more about having a constant source of illumination to enable photography, as well as the ability to photograph the same site with the same shadow angle over time to facilitate comparison.