VLBA Sensitivity Upgrade

Jon Romney, Walter Brisken, and R. Craig Walker

The Digital Backend (DBE) sub-band processor module, the first element of the upgraded 4-Gbps data path, passed a major milestone with the satisfactory evaluation of the second prototype ROACH (formerly "iBOB-2") board. Limited production of copies of this board has begun for partners in the development collaboration, including the NRAO, the South African KAT project, and the UC-Berkeley CASPER Laboratory. Development of the FPGA firmware has begun, in collaboration with Haystack Observatory, which is also producing the application-layer control software. Plans for integrating this unit into the VLBA station-control system are under development. One option being explored would be based on the EVLA control system.

The only new hardware required for the Mark 5C wideband recording system, a 10-Gigabit Ethernet input interface mezzanine board, is under test by Conduant Corporation, using prototype boards and prototype FPGA code delivered recently by subcontractors. Development of the control software, as an extension of existing Mark 5A/B control software, is under way at Haystack Observatory. Integration of the recorder into the VLBA station-control system is being done in parallel with the DBE, described above.

Functional testing of the DiFX software correlator, and NRAO's implementation of it within the VLBA operational environment, is approaching completion, with the incidence of bugs becoming vanishingly small. Agreement with results from the original VLBA correlator is now excellent, both in the primary FITS UV table, and in the associated auxiliary tables as well. A substantially enhanced computation of the (u,v,w) interferometer coordinates has been incorporated, including corrections for aberration and other relativistic effects, appropriate for observations of even very near field, moving objects. More formal tests are planned for October, including functional and science-based comparisons to both the original correlator and existing correlators at other institutions. An operator's GUI, and interfaces to the VLBA Operations Management System and the NRAO archive, are under development and well advanced.