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 antennas 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, not previously used by a communication satellite, is utilized, 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.) 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 pre-determined 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. 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. We have been forced to await
replacement of two transmitters, the operating system, and several
other HDR components, due to hardware failures. Although the ACTS
personnel and BBN have been extremely cooperative in restoring service
on such occasions, the impact of the reliability issue is that at
least one observer must be sent to Hawaii to use the telescope, in
case of ACTS-related equipment malfunctions.
The remote nature of most high-quality observing sites exacerbates this problem. Even in Hawaii, BBN maintains only a small field office, making HDR maintenance costly and time-consuming. A truly remote site, which would most benefit from remote observing techniques, also requires the highest degree of robustness from the equipment. In our opinion, the ACTS system is insufficiently robust to provide true remote observing with large (i.e., highly competitive) telescopes, due primarily to its limited scope and experimental nature. However, one of the ACTS Project's primary goals is to stimulate commercial high-speed communications satellite development. These systems may eventually play a role in remote astronomical observing systems.
Another difficulty we have encountered is that the transmitters in the ACTS HDR stations are not designed to run continuously, due to the finite lifetime of certain critical components, but rather must be switched on and off as needed. This method of operation demands human intervention at the beginning and end of every satellite session, a procedure that has been non-trivial to organize and would prove difficult in more remote observatory locations. The Hawaii location itself poses a final problem for us due to the large yearly rainfall at the location of the HDR in Honolulu. Because the frequency band at which ACTS operates is highly susceptible to rain fade, we have lost several runs in the past year due to rain in Honolulu. In essence, the use of the ACTS system for remote observing adds a weather constraint such that it must be clear (i.e., not raining) at both of the ground station sites, as well as at the observatory itself! (We note that this is another area in which future commercial high-speed communications satellites may provide improvements over ACTS.)