Corona was replaced by all electronic photo satellites. The first, KH-11 was launched in 1976. By the mid 1980s, all U.S. recon satellites were using, in effect, digital cameras. Resolution continued to get better through the 1980s and 90s. Today it's about 100mm (some say less, but the exact figure is classified, so anyone who knows it and blabs it gets arrested). Better software and hardware made it possible to get photos more quickly and without misleading information. The latest KH-11's (sometimes called KH-12) also have an infrared sensor, enabling them to see through clouds. The latest KH-11s also carry more fuel, allowing an useful life of up to eight years. Earlier model KH birds only lasted a year or two before their fuel ran out (or something broke down.) The current KH satellites can also be serviced by the Space Shuttle.
In the early 1980s, the first radar satellite (Indigo) went up. This was soon renamed Lacrosse and today they are called Vega. With a resolution of about 1.5 meters, they can operate day or night and in any weather.
There are many other types of military satellites up there. Most well known are the 24 GPS birds that provide navigation information. These first went up in the late 1980s. Since the early 1970s, there have been DSP (Defense Support System) infrared satellites in high orbit, looking for ballistic missile launches. There are currently five DSP birds up there. During the 1991 Gulf War, the DSP satellites proved sensitive enough to detect SCUD missile launches in Iraq. There are also over a dozen military communications and weather satellites in orbit.
And then there are the secret satellite programs. Most of these apparently have to do with electronic reconnaissance and electronic warfare. Astronomy buffs have spotted some of these, in some cases clusters of unidentified satellites. America has the largest fleet of military satellites, both in terms of quantity and quality. Its a formidable military asset.
For some forty years, space satellites have been the key American strategic reconnaissance tool. America began designing and building recon satellites in the 1950s. This was the Corona (called Discover for public consumption) series, using film capsules ejected for pick up on earth. The first launch (Discover 1) was in January, 1959. It failed. Discover 13 and 14, in August, 1959, finally succeeded. The cameras in these satellites had a 7.5 meters (25 feet) resolution (items 25 feet or larger could be made out.) This was very expensive, as once the satellite ran out of film, it became space junk. The Corona satellites kept going up, with rapidly improving versions of the Key Hole camera. By 1972, when the Corona program ended, it was getting a resolution of 1.8 meters. Satellites dropping film canisters were used by the United States until the early 1980s. During the Corona program, over a million photos were taken from space. Many of these pictures were of large areas, so accurate maps could be made of otherwise inaccessible places like China and the Soviet Union.