Quill was an experimental United States National Reconnaissance Office (NRO) program of the 1960s, which provided the first images of Earth from space using a synthetic aperture radar (SAR). Radar-imaging spacecraft of this design were not intended to be deployed operationally, since it was known that this system's resolution, inferior to that of concurrent experimental airborne systems, would not serve that purpose. Instead, the program's predominant goal was to show whether the propagation of radar waves through a large volume of the atmosphere and ionosphere would dangerously degrade the performance of the synthetic aperture feature.

A detailed description of the program has been made available on-line by NRO.

Initially, the primary benefit seen to be offered by radar imaging was its capability for operating at night and also for imaging through clouds or other atmospheric obstructions which absorbed or scattered waves not only in the visible spectrum but also in the nearby infra-red and ultraviolet. But radar also offered the benefit of a received signal that was already an electrical time-function one, ready for immediate radio re-transmission. So that project became a means for a trial of real-time image-data transmission as well as a trial of orbiting SAR. Since the theory and the state of the art for such transmission were well understood, it was realized that existing means for this part of Quill's mission would be inadequate for showing the level of detail needed to evaluate military threats even if the best imagery proved to be as good as expected. Still, not only were their lessons to be learned from trying, but also any success in such transmission was a hedge against failure to recover the on-board film, a problem that had plagued many of the early photo-intelligence satellites.

Quill satellites were based around the Lockheed RM-81 Agena-D, which also served as the upper stage for orbital insertion. The prime contractor for the orbiting vehicle and its radar payload was Lockheed. To expedite the test, a synthetic-aperture radar designed for airborne use was adapted, by subcontractor Goodyear, to space operation and the long ranges involved, based on criteria developed by the research team at another participating organization whose relationship to Quill has not yet been declassified.

Although only one satellite was needed, a backup model and an engineering model were also produced. Because the first one, OPS 3762, accomplished all of the project's test objectives, only that one was launched. According to an official NRO history, "In the first 20 years of reconnaissance satellite program activity in the United States, Quill was … the only satellite of any nature to proceed from start to finish with a perfect record in launch, orbital operations, readout, and recovery".

To limit the need for strong data-transmission signals from the satellite, ground stations having a very large signal-capture area (antenna area) and subsequent narrow-beam receiving directivity were desired. Available facilities with large-dish antennas capable of rapidly slewing to follow a satellite across the sky existed at New Boston, NH, and at Vandenberg AFB on the California coast. Those locations were also used for up-communication of radar turn-on and turn-off and other payload control commands.

The side-looking antenna required for SAR operation was mounted nearly flush along one side of the Agena's cylindrical body. In orbit, the body was rolled so that the beam was directed at a vertical angle of 55° from horizontal. From orbital altitudes varying around 130 nautical miles (nm), the beam illuminated the Earth's surface along a 10 nm-wide swath generally 160 nm distant and centered about 93 nm to the left of the vehicle's ground track. The 0.006-radian-wide along-track beam illuminated 0.56 along-track nautical miles at a time, continuing to collect returns from each scene point while traveling that far along the Earth's surface, that much data per scene element to be collapsed ("focused"), during later signal processing, into a single measure ("image") of the strength of return from that scene element.

After launch on 21 December 1964, data collection was commanded intermittently during both day and night for four days by ground controller's commands via the tracking stations. The Quill SAR's operation was therefore restricted to vehicle locations within the about 900-statute-mile (1490 km) maximum line-of-sight distances from those two ground stations. Such regions were almost totally within the U.S., but could cover some of Canada from the eastern station and some of Mexico from the western one. Operating times were further restricted, by the controllers, to areas within NORAD (North American Air Defence) territory, avoiding illuminating or imaging any Mexican territory, but that was not done in regard to Canada, the U.S.'s partner in NORAD.