American RC-135 Rivet Joint ELINT (electronic intelligence) aircraft have apparently returned to Iraq after a five year absence. At least one was spotted taking off in Qatar and heading north to Iraq (likely) or Syria (less likely). These aircraft can collect a wide variety of electronic signals in an area, and analyze them quickly and act (as in using onboard jammers). The analysis effort is looking for patterns. The enemy below leaves signs electronically (cell phones, walkie-talkies) or visually (images captured on surveillance cameras). Using the right math and analytical tools (software and computers) and you can quickly discover where the bad guys are coming from, and have the ground troops promptly shell, bomb or raid the location. It’s also possible to find out where people are going as well as the composition and status of a group with these analytical tools. But it all depends on collecting a lot of electronic data for long enough to detect the patterns.
This kind of work was popular with the RC-135 crews (about thirty aircrew and techies) in Iraq, Afghanistan (and elsewhere), because they were getting a chance to do, in a combat zone, what they have long trained for. Moreover, it's relatively risk free, as the aircraft fly beyond the range of machine-gun or shoulder fired anti-aircraft missiles. In addition, the most productive work is done during night missions, when the bad guys can't even see the RC-135's high above. Syria is potentially different because the Syrian government and their Russian allies have jet fighters that could threaten an EC-135.
Other nations, both allied and hostile, have noted the success of the RC-135 and either (like Britain) bought three of the American originals or, like most other nations, built their own. Russia used the Tu-204 airliner for its new RC-135 type electronic warfare aircraft. This two engine jet can carry up to 210 passengers, or 21 tons of cargo. The military version is called the Tu-214 or, for the RC-135 clone, the Tu-214R. Russia has built two of these and although still in development one of them was seen over Syria in early 2016 and was apparently useful enough to keep around.
Russia and China both studied the use of RC-135s in Iraq and Afghanistan and while the Russian response was two Tu-214Rs the Chinese have adapted an aircraft more similar to the C-130 and has been seen using these along contested borders (India and the South China Sea). Many nations, including the United States, has been equipping smaller (twin-engine turboprop) commercial aircraft and larger UAVs to carry out some of the functions of the RC-135.
The U.S. Air Force has been operating RC-135s for over half a century and in 2008 one of them set a record, spending over 50,000 hours in the air since it entered service in 1962. Built on the same airframe as the KC-135 tanker and Boeing 707 airliner. Periodically the air force has to remove KC-135s or RC-135s from service because of metal fatigue problems (usually in the wings, or with the pylons that attach each of the four engines to the wings.) All 732 KC/RC-135s were built between 1956 and 1965. The Boeing 707 commercial transport is actually a civilian version of the original KC-135 (which itself evolved from the World War II B-29 heavy bomber.) Since the 1980s the KC-135 fleet has undergone constant repair and reconstruction. New engines, and new structural components have been added, as older items wore out, or showed signs of wearing out faster than anticipated.
The RC-135s, P-3s and other electronic surveillance and reconnaissance aircraft have been operating out an an American base in Qatar since the 1990s and there are currently 9,000 U.S. troops in Qatar to support air and naval operations through the Persian Gulf, Indian Ocean and northwest Africa.