# Time Synchronization Notes

*The following was posted by Tom Nolan to Slack in November 2015.*

The ExoMars rover doesn\'t use the SCLK terminology. In some other
missions there is an SCLK, which is a counter onboard the spacecraft
that counts up at some frequency. For those missions, you usually use a
toolkit to convert SCLK to some usable time format.

The rover uses Rover Elapsed Time (RET), which is a 6-byte quantity: 4
bytes of seconds, and 2 bytes of subsecond ticks (65536 ticks = 1
second). They have not specified what time corresponds to 0 RET, but at
some point they presumably will.

The FSW uses 3 different tick counts in various places: 100-microsecond
ticks, 10-millisecond ticks, and 1-second ticks. The 100-us ticks start
from power-on, because they\'re kept in a register in the FPGA. The
other two start from FSW boot time, because they\'re kept in the flight
software.

The telemetry packets contain 100-us ticks (what you\'re calling \"FSW
ticks\"). The correspondence between ticks and RET is recorded in the
digital status packet: whenever I get the time command from the rover, I
record the RET contained in the command and the 100-us ticks when I got
the command (a \"time at the tone\" concept). From this information, you
can reconstruct the RET for each packet, but then you still need to do
the RET-to-time conversion.

We\'re currently using a convention embedded in the EGSE for the
RET-to-time correspondence (I think it\'s seconds since 1/1/2000, but I
don\'t know for sure). But we need to anticipate that there will be
other RET-to-time conventions when we integrate with the rover. I
suspect that there is no requirement at the rover level to make RET
unique. So during rover I&T, they will probably start RET=0 at power-on,
and we\'ll potentially get data sets with overlapping times every time
the rover is power-cycled.

There is also the possibility that RET will start over again at 0 on
Mars, which would add to our headaches.