Achieving Future SVLBI Gbps Data Rates
Space VLBI missions struggle to achieve maximum possible observing sensitivity within the design constraints imposed by the launch vehicle, mass, power, mission cost, and risk. On the other hand, advanced Space VLBI missions are expected to push the mission design in terms of large observing apertures, low-noise front ends, longer integration times, and observing bandwidth - all aimed at better system sensitivity.
To realistically increase sensitivity requires a number of developments. Large deployable reflector antennas to 15-meters and 86 GHz, along with cryogenically cooled low-noise amplifiers are envisioned. Achieving long integration time requires phase referencing, along with cm orbit determination, but promises to deliver integration times of hours. Alternatively, or additionally, water vapor radiometers can be installed at selected ground telescopes to calibrate the troposphere at the observing bands.
Radio telescope receiving bandwidth (for continuum sources) is limited by, and generally less than, the available observing bandwidth (which is not a constraint in the bands of 22 GHz, 43 GHz, and 86 GHz of greatest interest here). The difficulty is in getting the received signals to ground in real-time, along with providing new methods for recording, forwarding, and correlating data at high data rates (1 to 8 Gbps).
Real-time communications transform observing bandwidths (typically 250 MHz to 2000 MHz) into 1 Gbps to 8 Gbps data rates. (Techniques that permit the transmission of the wideband signals in analog form to ground have also been examined.) Additionally, use of the 37 to 38 GHz Space Research frequency allocation for data transmission is expected.
"Achieving Future SVLBI Gbps Data Rates,"[pdf:296KB} by Jim Springett of NeoComm, focuses on real-time communication of 1 to 4 Gbps data rates in digital form over the 37-38 GHz band from Advanced Space VLBI spacecraft to suitable ground stations. Jim begins by addressing a basic difference between the atmospheric loss experienced by ground and space radio telescopes. He then formulates a statistical model for link design aimed a minimizing required spacecraft EIRP. Jim goes on to address the link signal structure, including constraints imposed on out-of-band spectral emissions. Finally, several advanced modulation techniques are reviewed (along with their practical limitations) for transmitting up to 4 Gbps on the 37-38 GHz band.
(Joel Smith, 30-jan-2002)