To maximize the amount of data returned by space missions such as the Cassini-Huygens mission to Saturn's moon Titan, compression techniques are used to reduce the transmission size. But a minor miscalculation regarding the effect of the Doppler shift on the radio signal used by the Cassini orbiter and Huygens probe nearly prevented the acquisition of a significant fraction of the mission data. Engineers at NASA and the European Space Agency have devised a solution, but the problem might not have arisen if the probe had had the bandwidth to transmit large amounts of data without the need for aggressive compression. Until researchers found a solution, the effects of the Doppler shift on the radio link of the Cassini-Huygens mission threatened to prevent the probe from transmitting information during its descent through Titan's atmosphere. Optical communications systems under development promise to increase bandwidth and solve the problems of radio communications in space. Courtesy of the Jet Propulsion Laboratory. Researchers at the Jet Propulsion Laboratory are investigating an approach that could make such a problem a thing of the past: deep-space laser communications. The high frequency and small divergence of laser beams make it possible to reduce receiver and transmitter apertures and to lower electrical power requirements while increasing data rates by an order of magnitude. Laser communications systems consist of telescopes, lenses, filters, focal planes, fine-steering mirrors, and associated drive and signal-processing electronics. These components have flown in space, some several times, said Hamid Hemmati, manager of optical communications at the Jet Propulsion Laboratory. "What is unproven," he added, "is the demonstration of acquisition, tracking and pointing over the long ranges of deep space." In 1994, with cooperation from the laboratory, Japan's Engineering Test Satellite-6 carried an experimental laser communications payload that successfully demonstrated optical communications, but not from deep-space distances. Because the laser divergence is so small, particularly when compared with the divergence of radio beams, the pointing requirements are much more stringent. For deep-space communications, spacecraft attitude control is insufficient to support laser communications; jitter, therefore, must be eliminated with internal pointing mirrors. Locking to a beacon, such as another laser from Earth, the sun-illuminated Earth itself or the stars, could be used to stabilize the transmission direction. Hemmati believes that precision star-tracking and inertial sensors will offer the most economical guidance solutions for a pointing system. Light: The only solution The Jet Propulsion Laboratory has developed an optical communications demonstrator module capable of communicating at ranges from as near as those associated with unmanned aerial vehicles to those as distant as deep space. A CCD focal plane array provides input to the two-axis fine-pointing mirror, which controls the direction of a Q-switched diode-pumped laser that carries the downlink. A low-noise silicon avalanche photodiode detects the signal. But proving the promise of optical communications may be difficult. Hemmati suggested that successful demonstrations at both near-Earth and deep-space distances would be necessary before mission managers would adopt the technique as their sole means of communications. Because technology demonstrations cost $10 million or $20 million and add mass and power consumption to a spacecraft, they are difficult to come by. The advantages of laser communications, however, are compelling. Data volumes are increasing with such applications as HDTV, real-time video links and human travel to deep space, and optical communications can address these challenges. "For certain near-Earth and deep-space missions," Hemmati said, "optical communications now appears as the only solution."