The Lockheed U-2, nicknamed "Dragon Lady", is a single-engine, very high-altitude reconnaissance aircraft operated by the United States Air Force (USAF) and previously flown by the Central Intelligence Agency (CIA). It provides day and night, very high-altitude (70,000 feet / 21,000 meters), all-weather intelligence gathering. The aircraft is also used for electronic sensor research and development, satellite calibration, and satellite data validation.
The first flight occurred at the Groom Lake test site (Area 51) on 1 August 1955. The U-2 remains in frontline service more than 50 years later despite the advent of surveillance satellites. This is primarily due to the ability to direct flights to objectives at short notice, which satellites cannot do. The U-2 has outlasted its Mach 3 SR-71 replacement, which was retired in 1998.
A tactical reconnaissance version, the TR-1A, first flew in August 1981. A distinguishing feature of these aircraft is the addition of a large instrumentation "superpod" under each wing. Designed for standoff tactical reconnaissance in Europe, the TR-1A was structurally identical to the U-2R. The last U-2 and TR-1 aircraft were delivered to the Air Force in October 1989. In 1992 all TR-1s and U-2s (all U-2Rs) were designated U-2Rs. The two-seat trainer variant of the TR-1, the TR-1B, was redesignated as the TU-2R. After upgrading with the F-118-101 engine, the former U-2Rs were designated the U-2S Senior Year.
Source: Wikipedia, "Lockheed U-2", available under the CC-BY-SA License.