| Description: | In the early 1960's, the Jodrell Bank-Malvern 127 km microwave-linked interferometer demonstrated that many quasars and other active galaxies were unresolved. There was a clear need for much longer baselines, and with microwave-linked technology then limited to only a few hundred kilometres, various groups around the world (e.g. Canada, the US, Russia and the UK) began to develop tape recording systems. In the mid-1960's, several groups achieved the first Very Long Baseline Interferometry (VLBI) detections, using independent clocks and frequency standards at each of the telescopes. The data were recorded at each telescope on magnetic tapes. There were several experimental failures in those early days but progress was rapid, and the first transncontinental VLBI "fringes" (detections) were achieved in the spring of 1967 by the Canadian and US (NRAO/Cornell) groups. The first intercontinental fringes soon followed (to Onsala, Sweden) in January 1968 (incidentally Onsala was involved in the first transatlantic e-VLBI fringes in 2004). The original US NRAO MkI digital VLBI recording system (pictured above) had a total bit-rate of 720 kbits/sec - about 1000 times less than the current Mk5 system. The data correlation was performed by an IBM 360-50 mainframe computer (also pictured above).
Perhaps the most exciting early science result was the discovery of apparent super-luminal motion. The application of VLBI techniques to geo-physics, resulted in the measurement of continental plate motion, and VLBI is still the only method that can accurately measure the absolute orientation of the Earth in space. The calibration techniques of self-calibration, hybrid mapping and phase-referencing all have their origins in VLBI. To get an idea of the technology available back in 1967, the US NRAO MkI digital VLBI recording System had a total bit-rate of 720 kbits/sec - about 1000 times less than the current Mk5 system. VLBI has clearly come a long way over the last 40 years!
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