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Relay Mirror Experiment (RME)

The Relay Mirror Experiment (RME) successfully demonstrated long-range, low-jitter tracking and pointing capabilities appropriate for ground-based laser (GBL) propagation. Relay mirrors have been proposed as a method of increasing the range of laser weapons that is less costly than deploying a larger number of laser weapons. There are many different proposed designs, but in general a relay mirror is an optical system that is stationed at a high altitude and redirects light from a high energy laser (HEL) source to a target.Relay mirrors will only be effective if the beam spreading and beam quality degradation induced by atmospheric aberrations and thermal blooming can be mitigated.

Ball Aerospace & Technologies Corp. led a team of seven government agencies and private firms in the design, fabrication and operation of the Relay Mirror Experiment (RME), a successful element of the Strategic Defense Initiative Organization (SDIO). The space segment was launched in February 1990, aboard a Delta II/6920 and in a stacked configuration with another satellite - the Low-power Atmospheric Compensation Experiment [LACE]. The $107.9-million program demonstrated beam stabilization and pointing and tracking technologies required for a ballistic missile defense system.

The RME was originally conceived to be a shuttle deployed experiment. Shortly after program start, the Challenger disaster occurred, with the promise of extensive delays. A completely new space segment was to be designed incorporating a free-flying spacecraft. During the midphase of the program, a variety of launch vehicles were envisioned to replace the shuttle, requiring the BASD team to design accommodations for Delta, Atlas, and Titan, with a Delta launch being the final solution.

The experiment consisted of a spacecraft bus with optical acquisition aids, which carried the Payload Experiment Package (PEP) and the Wideband Angular Vibration Experiment (WAVE). The spacecraft maneuvered the PEP to track two visible laser beacons and to relay the infrared laser beam between two ground-based sites: the Laser Source Site (LSS) and the Target Scoring Site (TSS), located on the island of Maui, Hawaii. The LSS was located at the Air Force Maui Optical Station on the top of 10,000-foot Mt. Haleakala. The TSS was located approximately 12 miles away at the Experiment Scoring and Control Center near the town of Kihei at sea level.

The Relay Mirror Experiment directed a 1.064-micron laser beam emitted from one ground site to a mirror orbiting at 450km altitude, and then to a ground-based target. The beam traveled over 1200 km. The RME demonstrated that a laser beam can be accurately relayed from the earth to an orbiting satellite 450 kilometers away and then back to a 3-meter target on the ground. Contractors repeatedly failed to bounce a beam off the satellite's two-foot mirror from a laser atop Mount Haleakala on the Hawaiian island of Maui. Finally, on June 25, more than four months after launch, they succeeded in hitting the mirror but were unable to repeat it. After months of frustration, they again succeeded.

Just prior to an engagement, the spacecraft was pointed toward the midpoint of the ground sites. As the spacecraft came over the horizon, the ground stations began their track of the spacecraft, while it began a pitch maneuver to maintain its orientation toward the ground stations. As the spacecraft passed within range, both the LSS and TSS acquired and tracked the laser diode beacon emitting from the spacecraft. The ground systems tracked the spacecraft and illuminated it with green and blue beacon lasers. The Payload Experiment Package (PEP) housed the bisection tracker, a key innovation central to the experiment. The bisection tracker acquired both beacons and controlled steerable mirrors to accomplish fine tracking of the two cooperative beacons. In the process, the relay mirror was precisely positioned to enable a successful relay of a third infrared laser between the two ground sites via the orbiting spacecraft.

Kaman's KD-5100 differential sensors were used to precisely position the 60-centimeter relay mirror on the spacecraft payload to reflect the beams back to the target site. The KD-5100 was instrumental in achieving relay laser beam pointing accuracy that was 16 times better than the experimental goal and line-of-sight stabilization that was 2.3 times better. In more than 60 beam-relay engagements, the whole system, once calibrated, pointed the reflected laser beam 10 times more accurately than the U.S. Air Force had originally requested, with less jitter than expected. Stable and continuous beam relays to the target board were accomplished for as long as 80 seconds.

Once a stable ground track was achieved, and the spacecraft was at least 34 degrees elevation with respect to the horizon, both the LSS and the TSS propagated a visible laser tracking beacon. An intensity peaking servo system was used at both sites to finely peak and center the respective laser tracking beacons on the four retro-reflectors that composed the optical acquisition aids located on the spacecraft. Once the peaking operation was accomplished, the sensors on the PEP locked onto each laser tracking beacon and positioned the 24-inch diameter relay mirror to bisect the angle between the two laser tracking beacons. The infrared relay laser beam, which was coaligned with the LSS laser tracking beacon, was then propagated outward to the spacecraft and relayed back to the target board at the TSS.

A sparse array of 37 telescopes senses the position of the relayed beam at the second ground station for purposes of scoring the pointing capability of the relay mirror. Data from these telescopes is processed to determine the position of the beam as a function of time. These signals contain the effects of atmospheric turbulence on both the uplink and downlink of the IR beam. Spectral and correlation analysis is used on the telescope data to minimize the effects of atmospheric turbulence, as well as other environmental effects. The relay beam position information was sent back to the PEP, which used this information to point the relayed beam to the center of the target board. This configuration was maintained until the spacecraft passed below approximately 34 degrees elevation, when the laser beams ceased propagation.

The spacecraft soon resumed normal on-orbit pointing, signaling the end of the engagement pass. The spacecraft and its payload exceeded mission specifications of a six-month mission life, performing the primary mission for a year with successful contacts 15 times or more a month. The simultaneous beacon tracking jitter at the PEP, and the relay mirror pointing boresight error and jitter performance were sub-microradian and better than performance specifications. By comparison, the jitter was less noticeable than the angle formed by a dime when viewed from 10 miles away. Stable and continuous beam relays to the target board were accomplished for as long as 80 seconds. After the primary mission, RME was used by NASA in laser altimeter research and for training of flight operations personnel. After the primary mission, the Naval Research Laboratory used RME for several laser experiments, and the U.S. Air Force continued to use the system for training flight operations personnel until it reentered the atmosphere in May 1992.

Ball was the prime contractor, responsible for design and fabrication of the PEP, the spacecraft, all three groundbased lasers, the target mount assembly with scoring board at the TSS, the mission operations control computer systems and mission operations. Many of the key technologies employed in the PEP were originally developed for Ball laser communications research and development programs and other laser pointing efforts. Applied Technology Associates, Inc. (ATA) provided the WAVE, the science database and extensive analytical capabilities. Field operations support was provided by Ball, U.S. Air Force Phillips Laboratory, ATA, AVCO/ Textron, Bendix Field Engineering Corporation, Rome Air Development Center and the Consolidated Space Test Center.