MILNET:

MILNET Brief
U.S. Military Satellites

U.S. intelligence satellites fall into several basic categories (See also: collection )

SIGINT (electrical sensing, including COMINT, TELINT, RADINT, ELINT, LASINT, etc.)
PHOTOINT (photo sensing...i.e. film)
Electro-Optical (television, infrared sensors, imaging radar, etc.)
Aggressive (anti-satellite satellite).

In the early years of military satellites, the most common sensors were of the PHOTOINT variety, with infrared and SIGINT satellites following very quickly. Today the watchword for overhead recon is "real time". This literally means the gathering of data from the satellite and transmittal to the ground station and the "observers" on the ground, all as it happens. This being a much desired upgrade over the early versions of recon satellites which spit out film packages or recording packages on a parachute each of which had to be recovered in order to get the data you desired. Also of concern today are the launch vehicles for satellites especially since the Challenger explosion.

U.S. satellites today serve one of three combinations of basic mission in terms of intelligence gathering:

Dedicated : Primary mission is to gather overhead reconaissance
Collateral: Military Satellite with a primary mission is other than space surveillance but either there is a second set of sensors on board or the primary sensor can be used for overhead reconaissance.
Contributing: Non-military satellites with a means for providing overhead surveillance as a second or lower priority. Example: SPOT and its uses by Tactical Air Command (TAC)

Satellite Programs

There have been a number of distinct satellite programs in the public knowlege:

Chalet renamed to Vortex (7000 series, SIGINT)
DSP Defense Support Program (Ballistic missile launch and nuclear detonation detection)
DSCS III Defense Satellite Communications System ( secure SHF voice and data)
DMSP Defense Meteorological Support Program (weather reconnaissance)
GPS Global Positioning System
Jumpseat (Radar Surveillance)
Keyhole CORONA KH-4, GAMBIT KH-7/8, HEXAGON KH-9, KENNAN/CRYSTAL, KH-11,"KH-12")
LACROSSE (Radar Imaging)
Magnum (TELINT, proported to be carried aloft by the Discovery Space Shuttle)
NUDET Nuclear Detonation Detection System (possibly carried aboard GPS, DSP and DMSP)
Rhyolite (7600 series, re-named to Aquacade, UHF and Microwave TELINT)
SAMOS-2 Satellite and Missile Observation System
VELA HOTEL (High orbiting (60,000 miles) x-ray, gamma ray, and neutron sensing nuclear detection)
White Cloud U.S. Navy Ocean Surveillance satellite, intercepts ships to shore, ship to ship and shore to ship tactical and/or strategic communications.

For further information on this and other intelligence topics, see the MILNET Bibliography for:

19. Deep Black , by William E. Burrows.

30. The U.S. Intelligence Community , by Jeffrey T. Richelson