The main deterrent to the implementation of remote observing has always been the problem of obtaining an affordable and reliable connection with adequate bandwidth. NASA's Advanced Communications Technology Satellite was built as a prototype system to explore new modes of high speed transmission for digital data. It provides this capability at rates reaching up to OC-12 (622 Mbit/sec) via advanced on-board switching and multiple dynamically hopping spot beam antennae for selected areas of the United States, including Pasadena and Hawaii, although the steerable antenna used to reach sites not in the continental U.S. is only capable of OC-3 (155 Mbit/sec) speed. The 20-30 GHz frequency band has been employed for the first time by a communication satellite, with extensive rain fade compensation.
ACTS was launched on September 12, 1993 by Space Shuttle Discovery and now occupies a geostationary orbit at 100 west longitude. It has survived almost twice as long as its planned mission duration of two years, but is now nearing the end of its lifetime, which is limited by the fuel resources required to maintain its stationary position. (Current plans involving steerable ground stations may be implemented to extend the usable lifetime of the satellite even further.) The ACTS program is administered by NASA's Lewis Research Center (LeRC) in Cleveland, Ohio.
Bolt, Beranek, and Newman, Inc. (BBN) designed, built, and maintains the high data rate (HDR) ground stations that provide a gateway between ACTS and ground-based fiber optic networks and supercomputer interfaces. Five of the semi-portable HDR terminals have been built; they are allocated to the various ACTS experiments for predetermined lengths of time, then moved to another location. (For more information on the ACTS satellite and program, see the Gigabit Satellite Network web page .) Each HDR ground station includes a 12-foot dish permanently pointed at the satellite and an equipment trailer containing a real-time Unix control system with SONET I/O boards, burst modem, and high-output transmitter (see Figure 1). Due to the experimental nature of these ground stations, the often harsh environmental conditions, and the inherent complexity of high-speed communications equipment, the HDR stations have proved to be the weakest link in our network. This network has been used to support remote observing, remote diagnosis of problems, remote software development, and other related tasks. In the sections that follow, we will outline the network architecture and topology, characterize the performance of the network, demonstrate remote operation of a specific instrument on the Keck II Telescope, and suggest future directions for remote observing with high-speed networks. We will close with a summary of the benefits and difficulties which we have encountered during the course of this ACTS demonstration project.