NASA Gets into the ACTS

The whole Ka-band craze can be traced to NASA. When it launched its Advanced Communication Technology Satellite (ACTS) in September 1993, it began a research-and-testing project to determine what it needed to do to make Ka-band satellite communication work. "All the current Ka-band filings [to the ITU] are a direct tribute to ACTS technology," says Erwin Edelman, demonstrations coordinator at NASA's Lewis Research Center.

ACTS proved that it was possible to create an all-digital Ka-band system that could overcome rain fade, a signal-degradation problem resulting from short wavelengths passing through rain. ACTS is a TDMA-based system that uses many of the things you'll find in commercial Ka-band satellite systems, including spot-beam (or multibeam) technology, on-board storage and processing, and all-digital transmission.

Spot-beam. This technology enables an antenna system to subdivide a single large footprint (area of coverage) into many subfootprints. It can then focus these subfootprints (or spot beams) on particular areas. Subdivision enables a high degree of frequency reuse. Rather than spreading the entire frequency over the entire footprint, it spreads subsets of the frequency over smaller footprints. And, most important, it reuses these subsets in nonadjacent footprints.

On-board storage and processing. Most satellites are "bent pipes" -- a signal goes up and then goes back down immediately. On-board storage and processing enables the caching of information until a spot beam is aimed; it also enables intersatellite switching.

All-digital transmission. To overcome rain fade, signals need to be digital so that they can incorporate error codes. According to Edelman, ACTS uses the same TDMA system that you'll find in terrestrial cellular systems.

Together, these technologies enable nearly unheard-of data rates. "The ACTS is theoretically capable of communicating over three 622-Mbps channels," says Edelman. In case you're wondering, that's about 400 T1 lines.