CORAVG specifies the correlator averaging time in seconds.
For the DiFX software correlator on the VLBA, there are two options for how the specified average time is treated. If all average times are to contain exactly the same amount of data and the time tags are to match exactly the mean time of the data, the interval must be an integer number of FFTs and of short-term accumulator intervals. The default behavior of the correlator is to adjust the average time to the nearest option that fulfills these criteria. That may well not be a very ``round'' looking number. The adjusted time will usually deviate from the requested time by only a few percent, although in some extremes of narrow bandwidth and many spectral channels, it can be as high as a factor of SQRT(2). An alternative behavior is offered where the correlator takes the exact value specified. It then bins the scan into intervals of that length and averages any short term integrations that fall in a bin. The data are given the time tag of the center of the bin. This allows the user to choose any desired integration time exactly. But the amount of data contributing to each integration will vary, typically by one short-term accumulation. Also the true mean time of a data point will be offset from the time tag by a variable fraction of a short-term accumulation period. To invoke this behavior, put the word EXACT as a second argument to CORAVG (not case sensitive).
For the original Socorro VLBA hardware correlator, valid values are an integer N times the speed up factor times 0.131072 seconds. Round values such as 2, 4, 8 seconds etc may be specified and the nearest possible value will be used. This is what is expected from most users.
The correlator default is 2 seconds but, at most frequencies, more is probably ok most of the time. The longer the on-line averaging, the smaller the output data rates and the smaller the output data set.
Note the correlation parameters CORAVG, CORCHAN, CORPOL, CORTAPE, and CORSHIPn must be specified when the project will be correlated in Socorro and data are being recorded.