RC-135 is a flying vacuum cleaner of electronic signals. Built on the same airframe as the KC-135 tanker and Boeing 707 airliner, it carries two dozen people to man all the electronic gear. Exactly what kind of electronic signals the RC-135 can pick up is classified, but apparently includes any electronic device the enemy in Afghanistan is using. Not as expensive to operate (about $4,000 an hour) as the E-8, there are only 15 of them in service.
Used together, the two aircraft could monitor over a hundred kilometers of border through the night (when the enemy hit squads like to move around.) Troops on the ground can see what the aircraft system operators see, and quickly act if they spot several vehicles making their way through a rough track in a remote mountain valley. Even though the E-8 has trouble keeping track of stuff in mountainous terrain (the radar is often blocked by the undulating terrain), this is not a new problem and some E-8 crews have experience getting around this.
American commanders in Afghanistan are asking for more electronic surveillance capability, in the form of E-8 and RC-135 aircraft. The E-8 has a ground radar that can spot moving vehicles in a 20x25 kilometer box. The radar can quickly switch from one 20x25 kilometer box to another, and store what it sees, so that when it checks back, it can compare and see what has showed up. The 22 people in the crew monitor whats on the ground and transmit the images back to users on the ground, or bombers looking for targets. The air force has 14 E-8s. These birds cost about $8,000 an hour to operate, but it is thought that by having one or two on call, they can be effective in tracking Taliban and al Qaeda movement in remote areas. Light trucks and SUVs are the favorite transport of the bad guys, and these vehicles show up clearly to E-8 high above (and invisible to people on the ground, at least at night.)