# How did SR-71 spy, flying at 80,000 ft and 3500 km/h?

The SR-71 Blackbird is a famous supersonic reconnaissance/spy aircraft, undoubtedly one of the most amazing flying machines ever. Now, with the capabilities it had:

Maximum speed: Mach 3.3 (2,200+ mph, 3,540+ km/h, 1,910+ knots) at 80,000 ft (24,000 m)

How was it used for spying? It must have been difficult to focus on any object on the ground, from such a great height and then many problems are encountered with high speed photography. How were these and many other problems overcome?

• Cameras are faster than the speed of sound :) – egid Jun 26 '15 at 5:17
• Actually that depends @egid. en.wikipedia.org/wiki/Shutter_(photography) – papirtiger Jun 26 '15 at 5:28
• I spy with my hugely complex big expensive and top secret eye ... – PlasmaHH Jun 26 '15 at 10:50
• Consider that satellites "fly" higher and faster and provide quite good images of the earth. – Manu H Jun 26 '15 at 12:38

The SR-71 Blackbird was equipped with specialized cameras. It would overfly the area of interest and take high-resolution pictures of the ground.

Of particular interest is the Technical Objective Camera (TEOC). It could shoot pictures with a 6'' resolution from the operational altitude of the SR-71. This task was not simple as the aircraft would have moved several hundred feet forwards during the exposure time of the picture which was (depending on the settings) around one or two tenths of a second. Without some kind of correction, this would have resulted in a picture that was blurry in the direction of motion of the aircraft.

To solve this, the velocity over height ratio number was fed to the camera, either manually as computed by the pilot or automatically as computed by the sensors and computer of the SR-71 (and it turns out that the machine did a better job at this than the pilot) and while the picture was taken, the high-precision mirror that directed the camera FOV downwards would rotate in the opposite direction towards which the airplane was moving, thus cancelling it out.

• Great answer. Do you happen to have any information on how large the entire area covered by the picture was, and how the pictures were taken (i.e. plan one large picture of a specific area, or fly over an area and automatically shoot smaller pictures at a constant rate)? – Turch Jun 26 '15 at 12:14
• This picture shows an approximate size of the ground aerea that can be photographed using the various cameras on board the SR-71. The yellow areas on the middle are what the TEOC can cover. As you can see, if the camera is targeted towards the ground on a right angle they are 2.4 nautical mile wide squares. – ravingraven Jun 26 '15 at 12:42
• Do you have a source for this? It sounds like a very interesting mechanism! – ROIMaison Jun 26 '15 at 12:47
• This badly written website as well as those two books. – ravingraven Jun 26 '15 at 13:19

This is the section of the original SR-71 operation manual that describes in detail how the camera works. The plane was equipped with an Optical Bar Camera

"The Optical Bar Camera (OBC) is a high resolution panoramic camera with a "folded" ----------- lens system. It provides continuous ------- or --------- coverage along the flight track through an angle of 70 degrees on each side of the aircraft.

Note: the ----- are blacked out sections in the manual.

The manual goes on to talk about the operations of the camera and also notes that it takes 5 inch film with is way bigger (and subsequently higher "resolution") than the 35mm you would use in a home camera.

Here are the actual cameras. The left is the Technical Objective Camera and the right is the Optical Bar Camera.

The resolution of the TEOC was pretty great

Programmable system with a 48 inch focal length. The TEOC resolution was 110 lines per millimeter, which equates to about 6" ground resolution from an operational altitude. The TEOC's were mounted both sides of Chine controlled by a computer

The OBC seems like it may have been more size than resolution

The OBC could photograph 100,000 square miles of the Earth's surface per hour. Film image 72 miles wide, & film length 10,500ft.

And of course you cant have a fancy plane with out lots of buttons and knobs. The control systems for the cameras looked like this

For what its worth it seems that both of these cameras were really high tech panning control systems for cameras. They are very similar to Equatorial Mount cameras which are used to photograph stars (since the earth is moving), it's also called a barn door tracker.

• Thanks for the photos. I didn't know the word "chine", so here it is: en.wikipedia.org/wiki/Chine_(aeronautics) – ChrisW Jun 26 '15 at 17:11
• @ChrisW You can edit those links in directly. – Lilienthal Jun 26 '15 at 18:34
• @Lilienthal Thanks but no, the word "Chine" is in quoted text and the link isn't really part of the quote. – ChrisW Jun 26 '15 at 18:37
• It seems the image source link was edited out ill put it back in. – Dave Jun 26 '15 at 18:45
• Film image 72 miles wide, & film length 10,500ft. What does that mean, surely the actual film wasn't that big?! – Michael Jun 27 '15 at 6:17

Yes, the OBC (by Itek) did have 10,500 feet of 5" wide very fine grain Kodak film. As I recall, in non-overlapping mode it could cover over 2900 miles of terrain, and in stereo (overlapping mode) 1300 or so miles. So, each negative was 4.5" x 72". It also had v/r sensors and servos to tilt the view fore & aft during each exposure for image motion compensation as in the Hycon TEOC. The OBC principle was utilized in other reconnaissance aircraft at lower altitudes as well.