A retroreflective communication system (RCS), is a state-of-the-art system that provides (1) two-way optical communication using only one transmitter and (2) multipath communication among remote locations. RCSs were conceived for use in communication among stations on the ground and aboard spacecraft in flight.

In the basic mode of operation of an RCS, a laser beam is transmitted from a first station to a second (remote) station that is equipped with a precise retroreflector and a light modulator. The second station can thus modulate the beam with new information and return the beam to the first station. The second station is characterized as passive in the limited sense that it does not contain a laser or other transmitter. Because of this passivity, the size, weight, and power consumption of the second station can be much less than in other communication systems; these characteristics make the RCS particularly attractive for spacecraft communications.

Other spacecraft communication systems can include repeaters mounted on towers or launched into orbit to extend their ranges and/or to enable transmission over the horizon. A typical repeater in such a system consists of a receiver and a transmitter connected to one or more antennas (in a radio system) or lens subsystems (in an optical system). Complicating matters, orbiting transmitters and receivers are often powered by batteries that must be recharged by solar photovoltaic arrays. For long-range communication, the amount of power that must be provided to a transmitter is several orders of magnitude greater than that needed to operate the associated receiver. Therefore, electrical subsystems must be sized to satisfy the peak power demands of transmitters. When multiple remote sites receive simultaneous communications from a transmitter, its power must be shared among these sites. Not surprisingly the weight, size, and complexity of a power system, a transmitter, and an antenna launched on a satellite significantly affect the cost of launching. Yet another disadvantage of maintaining a transmitter in orbit is that the satellite that carries the transmitter must be repeatedly repositioned for precise pointing of the transmitted beam(s). Because of the large amounts of energy that must be expended to reposition the mass of the communication equipment and the rest of the satellite, the useful orbital life of the satellite is limited by the consumption of fuel.

By reducing the consumption of power in a communication satellite, the design of a low-bandwidth RCS can reduce the use of fuel and thereby increase the useful orbital life of a satellite. The modulator in a passive RCS unit is a liquid-crystal shutter (LCS) that varies the polarization of a beam of light. An LCS, which functions similarly to the liquid-crystal display in a wristwatch, is powered by a photocell that generates power when exposed to light from a transmitting source. An LCS consumes so little power that a button-sized lithium cell could well support continuous operations for several years. Although such practical limits as bandwidth and temperature extremes might alter this, an RCS should be capable of communicating data from the Moon to the Earth by use of a laser transmitter and an optical telescope, expending little more energy at the Moon site than is used to power an LCD wristwatch.

Another strength of the RCS is its ability to dramatically reduce requirements for precise aiming. In an RCS, any number of locations on Earth could simultaneously receive information from a passive unit on the Moon, using a few precise points. A typical envelope for capturing and returning a beam of light by a single retroreflector is a 22°-half-angle cone about the main optical axis of the retroreflector. One can group together multiple retroreflectors aimed at different angles to increase the effective capture-and-return envelope. The pointing requirements for communication at extreme distances are dictated only by the capture-and-return envelope. Because the divergence angle of a beam returned by a typical precise retroreflector is ≤ 1 arc second, a significant fraction of incident light is returned to its source.

An RCS could be used, for example, to communicate information from an active unit on Earth to a similar unit on the Moon, where such information would be stored and reflected back to a second active unit on Earth. In fact, retroreflectors left on the Moon during lunar landings in the 1970s have already been tracked from Earth with laser apparatuses similar to those of RCSs. RCS remote units could also reflect information from microrover landers to parent spacecraft orbiting a planet.

For spacecraft communications, the RCS concept offers the potential for major improvement over systems now in use. Reduced weight and size, the possibility of minimizing transmitting antennas and pointing requirements, and a passive design (consumption of little electrical power and generation of negligible heat) place the RCS concept ahead of its competitors. These characteristics could be exploited to reduce the fuel-to-weight ratios of support spacecraft, thereby promoting deeper penetration of outer space for exploration. Though utility is largely limited to outer-space applications like those described above, RCSs might be useful in some terrestrial applications because the RCS concept offers a unique combination of passiveness of a remote station, a secure link, and very low power or self-powered operation of the remote station; no other communication system offers this combination of features.

This work was done by Leo G. Monford, Jr., of Johnson Space Center. MSC-22781

Electronics Tech Briefs Magazine

This article first appeared in the February, 2000 issue of Electronics Tech Briefs Magazine.

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