The Gemini Agena Target Vehicle (GATV) was an orbital target vehicle used by NASA to test rendezvous and docking maneuvers during the Gemini program.
The ability to rendezvous in space was important for the subsequent Apollo program, enabling the lunar module to be coupled to the command module during the flight to the Moon. The necessary techniques for this were researched and learned in the Gemini program.
The GATV consisted of an Agena upper stage (manufactured by Lockheed) and a docking adapter (manufactured by McDonnell). The GATV was a 7.93 meter long cylinder with a diameter of 1.52 meters and a dry mass of 1834 kg. At launch it carried 6375 kg of propellant and gas. The forward section of the Agena airframe held the guidance, flight control electronics, telemetry, command, tracking, electrical power, and propellant pressurization equipment. The primary and secondary propulsion systems were at the aft end of the target vehicle with the attitude control gas tanks, and the main propellant (fuel and oxidizer) tanks were located in the mid-section. Propulsion was via a bi-propellant system, using unsymmetrical dimethylhydrazine (UDMH) and inhibited red fuming nitric acid (IRFNA). The Agena propulsion systems could be run while the Gemini was docked, allowing the GATV to be used to change the orbit of the docked pair. A minimum of five engine starts was possible. The docking cone was connected to the front end by shock absorbing dampers. Acquisition running lights and target vehicle status display indicators were situated on the front end. A 2.1 meter long retractable L-band boom antenna extended from the side of the cylinder near the front. Tracking and command of the GATV were also aided by a rendezvous beacon, two spiral L-band antennas, two tracking antennas (C-band and S-band), two VHF telemetry antennas, and a UHF command antenna. Micrometeoroid packages and other experiments could also be mounted on the GATV.
To anticipate potential problems during the development of the GATV, NASA decided in 1965 to develop an alternative targeting vehicle that could at least perform the docking maneuver. For this purpose, a docking adapter was fitted with an attitude control section of the Gemini capsule. An Agena upper stage was not required for launch. The resulting solution, consisting of an Atlas rocket plus a docking adapter, was called the Augmented Target Docking Adapter (ATDA) and was ready for the Gemini 8 mission.
The ATDA was a short cylinder, 1.52 m in diameter by 3.33 m long, with a mass of 794 kg, consisting of a target docking adapter cone mounted on front and containing a communications system, a guidance and control system, and a reaction control system. It also had running lights, but unlike the Gemini Agena Target Vehicles, the ATDA was not stabilized and did not have independent propulsion to allow manuevers while docked with the Gemini. After it became clear that the GATV would be ready for this mission, the ATDA was initially stored.
During the Gemini 9 mission, the Atlas launch vehicle, which was supposed to transport the GATV into orbit, failed. An additional Agena upper stage with a docking adapter was not available on short notice, so NASA decided to resort to a backup plan. The stored ATDA was reactivated and launched as a backup target. This meant that the astronauts would not be able to use the Agena upper stage to launch themselves into a higher orbit, but the docking maneuver could still be performed as planned.
However, during the rendezvous maneuver, it turned out that the payload fairing had not detached, leaving the docking adapter unusable.
A total of six Gemini Agena Target Vehicles (GATV) and one Augmented Target Docking Adapter (ATDA) were launched: