Real-time VLBI systems using STM-1 and -16 ATM networks

The Communications Research Laboratory has developed two highly precise VLBI systems using high-speed asynchronous transfer mode (ATM) networks with AAL1 corresponding to the constant bit-rate protocol. Instead of being recorded on magnetic tape, the observed data is transmitted through either a 2.488 Gbit/s STM-16 (in synchronous digital hierarchy)/OC-48 (in synchronous optical network) or a 155.52 Mbit/s STM-1/OC-3 ATM network so that cross-correlation processing and data observation are simultaneous. The STM-16 system was specially designed for the Keystone Project measuring crustal deformation in the Tokyo metropolitan area. STM-16 is not a commercial-grade ATM network but only a backbone line, while STM-1 is spread throughout Japan. The STM-1 system was designed considering not only optical fibre links but also satellite links for future international real-time VLBI experiments and it supports burst-mode data transfer by using a 4 Gbit buffer at each station. Both real-time systems and a tape-based system tested simultaneously using the Keystone network, and the differences between the results were within the formal error.