Calibration Signals
VLBA stations support several different types of calibration measurements.
Two calibration signals are injected near the beginning of the primary data path, and detected elsewhere in the VLBA system, with derived corrections applied in data analysis:
The switched-noise system injects well calibrated, broadband noise, switched on-off in a 50% duty cycle at 80 Hz. 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 in the VLBA DiFX correlator. 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.
