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Low-power Atmospheric Compensation Experiment (LACE)

The Naval Research Laboratory's Low-power Atmospheric Compensation Experiment (LACE) Program was sponsored by the Strategic Defense Initiative Organization (SDIO). In February 1985, SDIO representatives came to NRL to discuss the possibility of an instrumented target satellite for measuring the propagation of laser beams from the ground to space. The concept required the development of one simple sensor to be carried aboard a host satellite.

The spaceborne target was needed to determine the viability of atmospheric compensation techniques being developed at the Massachusetts Institute of Technology Lincoln Laboratory (MIT/LL) under SDIO sponsorship. The concept required the development of one simple sensor to be carried aboard a host satellite. Subsequently, in July 1985, the NRL LACE Program began as the Laser Communication Experiment (LACE). This experiment called for a single sensor to detect infrared (IR) laser emission and flown on NASA's shuttle-launched Long Duration Exposure Facility (LDEF) satellite. The LDEF was then in orbit and scheduled to be retrieved by the shuttle. Since LDEF was a completely passive satellite, supporting subsystems (such as electrical power and communications) would have to be added. As a result of the Challenger accident and the subsequent interruption in shuttle launches, it became clear that the LDEF was not likely to be available as a structure for LACE. Therefore, by June 1986, LACE had become a full satellite Itself instead of a sensor complement to be flown on LDEF. The name was changed to Low-power Atmospheric Compensation Experiment (LACE). Launch was scheduled for August 1988. By June 1986, the sensor requirements had been expanded to include multiple sensors to detect IR laser emission and additional sensors to detect emission from continuous wave (CW) and pulsed lasers emitting in the viqble and ultraviolet (UV) bands. Ultimately, 210 sensors in three arrays distributed over a 4 m by 4 m target board were launched as the sensor array subsystem (SAS).

To support the atmospheric compensation tests, a requirement was added for a retroreflector array. It was to have 252 cornercubes mounted on the end of a boom that could be extended up to 150 ft from the satellite body. Variable length booms had never been flown for more than a few days in space. The LACE satellite has three variable length booms, which can each be extended up to 150 ft. One boom keeps the sensors pointed toward the Earth, one has the retroreflector array mounted on its end, and the third is used to balance the retroreflector array. The retroflector and balance booms have to be moved at the same time (in opposite directions) to keep the satellite's moment-of-inertia constant. These booms were designed to be moved 125 times during the 30-month mission of the satellite. In April 1987, SDIO decided that the Ultraviolet Plume Instrument (UVPI) would be added to the LACE satellite. This forced a major redesign of mechanical and electrical subsystems. As an example, the addition of the UVPI. which is a precision pointing Instrument, invalidated key decisions relating to the spacecraft's attitude measurement.

In early 1988, the Delta launch vehicle assigned to the NRL LACE Program was assigned to a higher priority SDIO mission, Delta Star. From then until January 1989, a number of changes were made by SDIO in the assignment of a launch vehicle. A Titan, Atlas, and Delta again were real contenders. They each had sufficient credibility that the LACE satellite's electrical and mechanical mounting interfaces were redesigned to accommodate each designated launch vehicle. A second satellite, the Relay Mirror Experiment (RME), was added to the launch manifest, and the NRL LACE Program was given launch vehicle integration responsibility for both LACE and RME satellites.

The box shaped 3175-lb LACE satellite was 4.5 ft. by 4.5 ft. by 8 ft. high. The 150 ft. long boom extending from the top of the satellite had a 200 lb. tip mass and magnetic damper to provide gravity-gradient stabilization. Energy storage was accomplished by two nickel-cadmium batteries. The leading boom with the retroreflector array was held in the satellite's velocity vector by a momentum wheel inside the spacecraft. The trailing boom allowed the satellite's moment of inertia about its pitch axis to be held constant as the length of the leading boom was changed to support test requirements. The LACE spacecraft had no onboard propulsion and used a very-high-frequency ground-to-satellite command link operating at 1 kbps.

The objective of the NRL LACE Program was to provide and operate a satellite to carry and support:

• a sensor array subsystem (SAS) to measure at the satellite, as a function of time and spatial distribution, the absolute intensity of low-energy ultraviolet, visible, and Infrared laser beams transmitted from ground laser sites;
• the Ultraviolet Plume Instrument (UVPI) to obtain ultraviolet images of plumes from rockets and to obtain ultraviolet background and clutter data over a wide variety of spatial, temporal, and lighting conditions;
• the U. S. Army Strategic Defense Command's Army Background Experiment (ABE) to measure, as a function of geomagnetic latitude, the neutron background spectrum and the angular distribution of terrestrial neutrons; and
• the U. S. Air Force's (USAF) Radiation Detection Experiment (RDE) to measure electromagnetic radiation.

LACE, also known as Losat-L, was part of a dual payload with RME (Relay Mirror Experiment) carrying laser defense experiments. LACE was built by the Naval Research Laboratory. A DELTA II Model 6925 launch vehicle was used to boost two SDI payloads into orbit from Pad 17B on 14 February 1990. The second RME payload, also known as LOSAT R, validated ground-based laser relay technology in the areas of beam stabilization, pointing and beacon tracking. Following lift-off at 1615:00Z on February 14th, the DELTA II rolled into a flight azimuth of 75 degrees and accomplished the "dogleg" maneuvers required to inject the payloads into their proper orbits. The LOSAT L was injected into a 546-kilometer circular orbit, and it was turned over to the U.S. Naval Research Laboratory to support various SDI studies. After the LOSAT L payload was released, the DELTA II's second stage completed a retrograde burn before releasing the LOSAT R payload into a 470-kilometer circular orbit. The U.S. Air Force Weapons Laboratory in Albuquerque, New Mexico was responsible for the LOSAT R, but a ground station on Maui, Hawaii controlled the spacecraft. The mission was successful.

After launch, many SAS operations were conducted in support of the Massachusetts Institute of Technology Lincoln Laboratory's Short Wavelength Adaptive Techniques (SWAT) Program for compensation of atmospheric-Induced distortions of ground-based laser beams. A completely successful cooperative atmospheric compensation demonstration was first achieved on 30 November 1990. Many more successful demonstrations and tests were conducted until tihe SWAT Program was completed in April 1991. The ULVPI collected high-quality ultraviolet emission images from four rocket launches in four attempts, as well as large quantities of Earth background data. The ABE made highly successful measurements of the neutron background in space almost continuously for 36 months following launch. Tie RDE made aneasurements over 14 months.

In response to SDIO's request, operations with the LACE satellite were terminated on 14 February 1993. The satellite had successfully accomplisheid its mission. The LACE satellite was left in a stable configuration so its retroreflector array can be used as a target. The length of the leading boom was left at 15 ft. Therefore, until tile momentum wheel fails, the retroreflector array will lead the spacecraft's main body. After the momentum wheel falls, the retrorefleclor array will be 15 ft from the main body in an unknown direction.