Arms control pacts can be verified - includes related article

The first two drawbacks were overcome in the mid-'70s with a newtechnology called charge-coupled devices (CCDs). Using essentially the same technology as that in home video cameras, they take and transmit pictures electronically. Each CCD consists of an array of tiny sensors, or pixels (for picture elements), numbering in the thousands, arranged in a grid. As light waves fall on the array, each sensor stores a quantity of electrons proportional to the intensity of the light that has hit it. A counter tallies up the number of electrons each sensor has accumulated, and that number, as well as the location of the sensor in the grid, is tranmitted to a receiving center on the ground. There a computer uses the information to construct on a TV- type screen an exact copy of the original image captured by the CCD. Meanwhile, aboard the satellite, as the electrons drain from the individual pixels the array becomes ready for another exposure. All this happens in milliseconds. No film has to be ejected, captured, and developed. Someone sitting at a console in Fort Belvoir, Va., the site of the CIA's satellite imagery center, can see in real time what the satellite is seeing in the Soviet Union. If something tweaks his curiosity, he can zoom the lens and take a more detailed picture on the next pass. And the CCD array can be used again and again, inexhaustibly.

How much the camera will see depends on its resolution, the sizeof the smallest object s it can distinguish. That depends on its distance from the scene, its focal length, and the size of the individual pixel. The smaller the pixel d and the longer the focal length f, the smaller s will be -- that is, the better the resolution of the system. Let's say the satellite is at an altitude h of 100 kilometers, that the individual pixel size is 2 microns, and that the focal length of the camera is 4 meters. You quickly find out that the resolution on the ground of the hypothetical camera is 5 centimeters, which means it can detect objects as small as two inches from an altitude of about 60 miles above the earth.*

Although the capabilities of the satellite cameras are secret, stories circulate that we've been able to follow the pucks during ice hockey games at outdoor rinks in Moscow or see astonishing closeup details of Soviet submarines. Says one intelligence analyst, ''You can tell if the guys on the bridge watch have their parka hoods up.''

High resolution can be a mixed blessing, however. Say the cameratakes a picture of a scene 100 x 100 meters on the ground. If the resolution is one meter, the picture will be composed of 100 x 100, or 10,000 pixels. But if it's 5 centimeters, the number of pixels increases to 2,000 x 2,000, or 4,000,000 pixels. As a result, 400 times as much information will be transmitted

to the receiving station. This means the satellite must have a very large transmitting antenna (to handle the increased flow of data) and a lot of electrical power, which isn't always available from its solar cells. Ground controllers often order photo satellites to conserve their energy by taking pictures at lower resolution.