What Spy Satellites Can See
Satellite photography is a hot topic these days, even in circles far removed from space exploration themes.

Despite this, the same old stories keep circulating, the ever-persistent Cold War myths, such as making out the registration number on a license plate, identifying a military person’s rank ontheir uniform’s shoulder boards, and even reading a newspaper from orbit.

In this article I’m going to tell you whatspace optics are really capable of and we’ll try to figure out if one canindeed see even the tiniest detail from orbit.

First, let me surprise you. Satellite image resolutions better than 50 cm per pixel are not available in Google Maps. Commercial use of more detailed space imagery was prohibited in the US until recently. Thus, all photos of cities showing people going about their business and other such minute details are actually aerial photography, for which there are no publication restrictions.

Such incongruity was always unacceptable for private space companies, so they went to great lengths to lobby a partial lifting of this ban, as a result of which one is now permitted to sell photographs with the resolution ofup to 25 cm per pixel. It is currently the resolution limit for commerical space-based photography.

But snapping a pic within this, not very detailed, resolution range requires very complex technology. For example, DigitalGlobe’s WorldView-3 satellite has the following characteristics:

Image resolution – 31 cm;Telescope mirror diameter – 1,1 m; Cost - $650 mln

That company has recently published a few test photos with the resolution of 40 cm per pixel. They are the most detailed satellite images to have been legally and openly published.

As an example of its effort, DigitalGlobe has posted several photos of the city of Madrid.

As we can see, there are tons of minute details to make out here: You can easily tell cars from trucks, and even spot people swimming in their pools as tiny dots. Madrid was selected for a reason. The closer an object is to the equator, the less cloudy the sky over it is. Dubai is another popular target for orbital photography testing. There are lots of colourful objects to be found there, and the local desert climate facilitates observations.

The huge cost of a commercial satellite capable of such resolutions begs a logical question: How do these satellites pay off? Well, no secret in that. More than 50% of all DigitalGlobe orders come from the Pentagon, the rest being placed by Google and individual clients.

We’ve so far talked about commercial satellites, but what can military and CIA satellites pull off?

It’s more complicated, but overall quite predictable. The most powerful American spy satellite belongs to the legendary Keyhole-11 series. Very little is known about it for certain, even its appearance is far from clear, though amateur astronomers are known to capture images of it from time to time.

It is common knowledge, however, that the Hubble space telescope was constructed on the same production line where spy satellites had been previously manufactured. A few years ago the National Reconnaissance Office donated to NASA two telescopes, 2.4 meters in diameter each, which had been kept under wraps in one of their storage facilities for quite some time.

So, it’s pretty likely that KH-11 is also equipped with a 2.4 m mirror, as is the famous Hubble space telescope.

By way of a simple comparison with the WorldView-3 satellite, which has a 1.1 m mirror, we can work out that the quality of spy satellite images must be approximately 2.3 times better. But there is a catch: WorldView-3 is orbiting the Earth at the altitude of 617 km,whereas the orbital altitude of USA-245, the latest in the KH-11 series, varies between 270 and 970 km.

The Hubble space telescope is theoretically capable of photographing Earth with the resolution of 10-15 cm from the altitude of 700 km. Correspondingly, KH-11 should be able to take photos with the resolution of up to 5 cm in its perigee,the lowest point of its orbit. Ideally, of course, given there is no cloud cover, or smog, or fog, or dust suspended above the object of interest. In addition, the higher the resolution and the closer the satellite to the surface of the Earth, the narrower its field of view and the less its ability to take a wider look. Put simply, photography with such parameters makes sense only incase of pre-investigated objects and photographing through clear skies, at the time when the satellite’s orbit allows it.

Thus, the reason the American military willingly commission  services of American business is that it merely lacks its own technical means, which is why they find it easier to purchase whatever imagery they need, than construct and launch a swarm of satellites, each costing about as much as an average aircraft carrier.

Now I'm going to compare images with different resolutions, based on aerial photography.
Large image size

To summarise, it is actually true that in ideal conditions a single spy satellite could theoretically make out a license plate on a car as a few white pixels. Despite this, it is still technically impossible to read the number on the plate, let alone see stars on shoulder boards or peruse a paper from space.