RC-135 Command Posts  
      In 1960, the Strategic Air Command modified C-135 aircraft to house its Airborne Command Posts, known as "Looking Glass."  They were assigned to the 55th Strategic Reconnaissance Wing, which operated out of Offutt AFB.  The idea was good that the idea of flying command posts has since been expanded into other areas of intelligence and operations.  Today there are several RC-135 command aircraft.  All are assigned to Air Combat Command and are operated by the 55th Wing out of Offutt Air Force Base.   

RC-135U 'Combat Sent'
      Two aircraft are designated RC-135U "Combat Sent" and are characterized by cheek fairings and additional fairings in the chin, boomer, wingtip, tail cone and fin-top positions. Until 1991 they were fitted with "towel rail" antennas above the cheek fairings, but these have been removed. The RC-135Us are believed to have special purposes within the SIGINT fleet, and may also be used to trial new equipment.

RC-135V/W 'Rivet Joint'
       Eight aircraft are designated RC-135V "Rivet Joint,"  Six are the essentially similar to the RC-135W variant. These are the workhorses of the SIGINT fleet and are distinguished by having extended "thimble" noses and large plate aerials under the center-section. External differences between the two variants are restricted to a lengthened cheek fairing on the W-model, which also lacks auxiliary air intakes on its engine pods.
The aircraft is an extensively modified C-135. The Rivet Joint's modifications are primarily related to its on-board sensor suite, which allows the mission crew to detect, identify and locate signals throughout the electromagnetic spectrum. The mission crew can then forward gathered information in a variety of formats to a wide range of consumers via Rivet Joint's extensive communications suite.  The interior seats 32 people, including the cockpit crew, electronic warfare officers, intelligence operators and in-flight maintenance technicians.

RC-135S 'Cobra Ball'
An altogether more specialized role is undertaken by three RC-135S "Cobra Ball" aircraft which normally operate from Shemya Island, Alaska. In addition to "thimble" noses, electronic receivers mounted in cheek fairings and a teardrop-shaped fairing on the aft fuselage, these also have large circular windows in the fuselage for the photography of foreign ballistic-missile tests at long range. The intelligence equipment includes multiple infrared telescopes and is known as the Real Time Optical System (RTOS). These aircraft allow the U.S. to monitor every reentry vehicle flown from Russian test ranges, to determine the capabilities of each Russian missile, new or old.