Wednesday, August 09, 2017

What we were doing in the 80's (VLBI)

Arthur Niell, Marshall Cohen, David Rogstad, and Martin Ewing, ca 1982

See full photo album.

PDP-11s were hot.  Helical scan video recording was just the thing for recording high-speed radio interferometer (VLBI) data.  In a Caltech-JPL collaboration, we built what was then the largest correlator to process data streams from up to 5 telescopes at once -- providing correlations (fringes) between all 10 pairs.   Getting any fringes at all required delicate synchronization of the telescopes (Loran C timing and Hydrogen maser frequency standards).  Geometric corrections were tricky, too. You had to know the baselines between telescopes accurately, giving the delay and doppler frequency offsets.   To manage all this, we had MSI-TTL logic, core memories, a PDP-11/40, and Forth software.  The hardest part -- keeping those tape machines running!  They were modified IVC and Ampex helical scan recorders, semi-professional.  A later version of the system used standard consumer VHS recorders, which were just becoming available.

One bit sampling was the choice for best signal-to-noise ratio on a broadband "continuum" radio source.

All this came to light again when I started digging into some old files.
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