13.2 The Pulse Cal System

VLBA observers using more than 1 BBC will want to sum over the BBCs to reduce noise levels. This should not be done with the raw signals delivered by the BBCs: the independent local oscillators in each BBC introduce an unknown phase offset from one BBC to the next, so such a summation of the raw signals would be incoherent. A so-called ``phase cal'' or ``pulse cal'' system (Thompson 1995) is available at VLBA stations to overcome this problem. This system, in conjunction with the LO cable length measuring system, is also used to measure changes in the delays through the cables and electronics which must be removed for accurate geodetic and astrometric observations. The pulse cal system consists of a pulse generator and a sine-wave detector. The interval between the pulses can be either 0.2 or 1 microsecond. They are injected into the signal path at the receivers and serve to define the delay reference point for astrometry. The weak pulses appear in the spectrum as a ``comb'' of very narrow, weak spectral lines at intervals of 1 MHz (or, optionally, 5 MHz). The detector measures the phase of one or more of these lines, and their relative offsets can be used to correct the phases of data from different BBCs. The VLBA pulse cal data are logged as a function of time and delivered in a PC table (Section 11). AIPS (Section 20.1) software can be used to load and apply these data. However, some VLBA observers may still want to use a strong compact source to do a ``manual'' phase cal if necessary (Diamond 1995). For example, spectral line users will not want the pulse cal ``comb'' in their spectra, so they should ensure that their observing schedules both disable the pulse cal generators and include observations suitable for a manual phase cal. Manual phase calibration also is likely to be necessary for any non-VLBA stations included in an observation, because they may have no tone generators or detectors. In addition, it is necessary at 3 mm, where the VLBA antennas have no pulse calibration tones.