VLBI S2 TIME

The S2 keeps time using the high-rate input clock signal on the C1 cable, refclk(usually 32 MHz). The 1 Hz input pulse is used to identify which 32 MHz pulse corresponds to the 1-sec tick, but as is common in VLBI the user's 1 Hz is only actually recognized at a specific epoch, specifically when setting the "station delay" on the S2. The station delay must be set to 0 before setting the clock, and normally remains at 0 in record applications. After setting the station delay to 0, the S2 internal concept of 1 Hz is maintained simply by a free-running counter: every 32 million refclk cycles.The user's 1 Hz input continues to be used for the station delay measurement displayed on the console screen (and PCFS), but this is for display only and does not affect data recorded on tape (e.g. the user's 1 Hz could go away and things would keep working, although this is not recommended and error messages will start to come out).

If a difference arises between the user's 1 Hz and the S2 internal 1 Hz it will be reflected in the (highlighted) displayed station delay measurement. Any such differences may either be ignored, may be fixed manually by setting the station delay to 0 again, or may be fixed automatically every time they occur by setting the 'scpllrefclkfix' defaults file parameter.

The S2-RT records time on tape by encoding clock information in the auxiliary data channel. The S2-PT uses this information to reconstruct the original input clock signals precisely, so you don't need to understand exactly how it works.

1.) Is setting UT time at the PC enought?

Yes, setting the UT time using the PCFS fmset program is enough. There is no direct hardware interaction necessary such as pressing a button like in MarkIII etc. Note that fmset will automatically set the S2 station delay to 0 if needed. You can also set or check the S2 time using the S2's own console display, which in some sense may be more reliable since there is no "middle man" (PCFS).

2.) How do we know if we are not at the wrong sec. or tick?

The way to do this when using PCFS is simply to look at a nearby UTC wall clock and compare. If no wall clock is visible from where your PCFS screen is located you'll have to come up with something else, for example carry around a good digital watch that you recently synchronized to UTC, or use a portable radio tuned to a time broadcast.

Note that time setting could theoretically be done fully automatically if PCFS was able to access accurate UTC time, but it cannot. The PCFS internal computer time is not guaranteed to be within 1 second of UTC, although it should normally be within a small number of seconds of UTC.

For the full gory details on S2 clock and time management see:
ftp://s2.sgl.ists.ca/pub/s2/doc/timing/timing.ps.Z
This is written mainly for correlation applications, where timing gets
more complicated due to delay and delay rate corrections.