The IMB-FITS files provide 2 antenna streams and associated tables: the slow (1 Hz) and the fast (128 Hz) traces. For each of those traces, some antenna position parameters are available, plus the associated time as MJD values.
As of today (IMB-FITS version 2), the 2 traces are in the same FITS
extension. Both have the same number of rows, but the fast trace has
128 values per column and per row, while the slow trace has 1 value
per column and per row. However, even if they have the same number of
rows, they are INDEPENDENT: for example, their MJDs are misaligned by
a 2 seconds shift (2 rows). They should not be correlated! In
the IMB-FITS version 3, the 2 traces will be clearly separated in 2
FITS extensions, which will avoid any misinterpretation.
Note that the caveat exposed above explains why MRTCAL selects more
dumps than MIRA in on-the-fly maps: MIRA looks at the antenna slow
column TRACEFLAG to flag out MJD values in the antenna
fast. Because of the 2 seconds shift between the 2 traces, more
time dumps are discarded.