The RRI receivers have three elements that need to change state at the start of an observation, depending on the contents of the schedule database for that observation. These elements are:
The receivers don't start changing state until the instant of the given observation ID (a time in GPS seconds). After that instant, if all is going well, pointing the beamformers takes 0.2 seconds, changing the coarse channels takes 1.5 seconds, and changing the analogue attenuation takes 2.2 seconds (all three changes happen in parallel). All hardware (pointing, AGFO and ASC attenuation) is always sent change commands for every observation.
Sometimes, there are additional delays in some or all receivers, typically due to high load on the main M&C PostgreSQL server, or the PostgreSQL servers inside the individual receivers. That happened often in the past, less so recently.
For visibility files, the amount of data that needs to be discarded will depend on:
Starting in mid 2017, I added two fields to the metafits files:
At the _end_ of an observation, a few seconds of data are missing (any files in the queue are discard), but you shouldn't have to discard any additional data that's been recorded. It's possible that for very old observations, the data might have kept on being recorded into the start of the next observation, but it certainly isn't doing that now.
2012 to 2014(ish): The Dark Ages - chaos and uncertainty, before the civilised era. Code changed day-to-day, there were no dipole tests or flags, and the QUACKTIME and GOODTIME cards were a distant dream. Leap second offsets were hard-coded in dozens of places. Don't trust any timestamps to better than a few seconds. The data in a visibility file starts anywhere from -1 to 4 seconds after the obsid, and the first 0-4 seconds should be discarded, depending on correlator averaging time. This era is why Cotter discards the first four seconds of data, by default. Here be dragons in the data.
2014(ish) to mid 2017: The Renaissance - the code was stabilising, dipole tests and flags exist. From here, visibility files should always start 2 seconds after the start of an observation. There is no QUACKTIM or GOODTIME card in the original metafits files, but newly created metafits files will have those cards.
mid 2017 to now: The Modern Era - stable correlator code, no more leap seconds. QUACKTIM and GOODTIME headers, and data files that _always_ start exactly 2 seconds after the obsid. Because of that 2 seconds without data, only attenuation settings should ever contaminate recorded data, only if the initial and final attenuations are different, and only for one correlator dump time.
2021 onwards: The Future - new (already tested) receiver code drops attenuation change times to < 0.5 seconds. The new MWAX correlator solves all of the problems of the old correlator, and has no bugs of its own. The promise of new receivers heralds the dawn of a new age.