Calibration Signals
VLBA stations support several different types of calibration measurements.
Two calibration signals are injected near the beginning of the primary data path, detected elsewhere in the VLBA system, and applied subsequently:
The switched-noise system injects well calibrated, broadband noise, switched at 80 Hz in a 50% duty-cycle square wave. This noise signal is synchronously detected in the RDBE, to provide a time-tagged system-temperature table that is delivered with the primary fringe visibility data. Application of these measurements for amplitude calibration is discussed separately.
The pulse-cal system injects a series of pulses at intervals of 1.0 or 0.2 microseconds, to generate monochromatic, phase-stable tones, spaced at multiples of 1 MHz or 5 MHz. The tones are detected currently in the VLBA DiFX correlator, and eventually will be available from the RDBE. Application of these measurements for phase calibration is discussed separately.
There is also a round trip cable calibration scheme that monitors the length of the signal cables, to enable corrections for temperature and pointing induced variations.
