SPACE COMMUNICATIONS

A grou¥of researchers from the Communications Research Laboratory (CRL; Tokyo, Japan) and NASA`s Jet Propulsion Laboratory (JPL; Pasadena, CA) is conducting laser-based communications tests between the Japanese Engineering Test Satellite (ETS-VI) and a US ground station. The tests, which are expected to last through 1996, are intended to validate performance of systems for space-to-space and ground-to-space communications with lasers.

Jan 1st, 1996

SPACE COMMUNICATIONS

International tests validate satellite links

Paul Mortensen

A grou¥of researchers from the Communications Research Laboratory (CRL; Tokyo, Japan) and NASA`s Jet Propulsion Laboratory (JPL; Pasadena, CA) is conducting laser-based communications tests between the Japanese Engineering Test Satellite (ETS-VI) and a US ground station. The tests, which are expected to last through 1996, are intended to validate performance of systems for space-to-space and ground-to-space communications with lasers.

Recent tests include transmission of a 10-W argon-ion laser beam of divergence 0.001° from a 0.6-m-diameter telescope at JPL`s Table Mountain Facility (TMF) near Wrightwood, CA, to the Japanese satellite. This uplink laser beam was detected and tracked by the satellite and used as a pointing reference to return a second beam of divergence 0.002° from the satellite`s communications package back to a 1.2-m-diameter telescope, also located at TMF. The ground-telescope-to-satellite distance of the test was 40,000 km and the uplink beam diameter at the spacecraft was 800 m, which, according to JPL, is equivalent to transmitting a laser signal from Los Angeles, CA, and hitting a target in Washington, DC, whose diameter is half the height of the Washington Monument (169 m).

Japan`s National Space Development Agency (NASDA) launched the ETS-VI satellite in 1994. The craft, which carries experimental laser communications equipment (LCE), failed in geostationary orbit due to engine trouble and was subsequently transferred into a three-day subrecurrent orbit suitable for communications experiments.

The satellite`s communications equipment includes a 30-mW-peak-power 830-nm diode laser for downlink transmission, an avalanche photodiode, a point-ahead mechanism (PAM), and a quadrant detector for PAM control. In addition the ETS-VI is capable of laser communications reception, incident laser beam tracking, and optical power measurement. The equipment package includes a two-axis gimbal mirror, 7.5-cm-diameter telescope, acquisition sensor (CCD), fine-pointing mechanism, and quadrant detector tracking sensor.

The downlink transmission rate of the LCE is 1.024 Mbit/s. A rate of 100 Mbit/s will, however, be possible when a new satellite is launched by NASDA that upgrades the peak power of the laser diode to 200 mW, increases the telescope diameter from 7.5 to 20 cm, and adds amplifiers.

Compensating for the atmosphere

The LCE was originally intended for use only with CRL`s main ground station in Tokyo, so its acquisition area was limited to 0.2° around its target-pointing direction. With the satellite`s failure to achieve a geostationary orbit, however, the LCE can be accessed if the satellite`s attitude is controlled such that its boresight is pointed at the target ground station.

To overcome the limitations of its optical ground site in Tokyo, the CRL is relying on international cooperation. At 100 m above sea level the air around the Tokyo site is dense, polluted, and turbulent. Such conditions cause severe attenuation and scintillation of the uplink laser light; consequently, the fine-tracking loo¥has not been stabilized yet and works continuously only for a few seconds--20 s is currently the best result. On the other hand, at TMF, which is 2300 m above sea level, conditions for uplink and downlink transmissions are much better.

To compensate for attenuation of uplink transmission by atmospheric turbulence, CRL is planning experiments using adaptive optics (see figure on p. 30). A CRL researcher notes that, with this system, laser light transmitted from satellites can be thought of as an artificial guide star. CRL will use adaptive optics with the optical interorbit communications engineering test satellite that will be launched by NASDA in 1998.

With distances between geostationary and low earth orbit satellites u¥to 45,000 km, onboard equipment that can cope with beam divergence will be critical. Key systems will include "beam acquisition" devices to initially acquire an incoming laser beam with the accuracy required to initiate a communication link, "beam tracking" to receive the incoming laser beam with an accuracy better than 1 µrad to maintain the communication link, and "beam pointing" to accurately transmit a laser beam toward the position where the other satellite will be. CRL researchers expect transmission rates of about 50 Mbit/s using an 847-nm diode laser.

More in Research