The LEAP SECONDS header record of an observation file.
current is the current leap-second count. delta_future, week and day
are nil where the record leaves the field blank; a blank field before a
written one keeps its place when the record is written back.
time_system is the identifier in columns 25 to 27 exactly as written, or
nil where the field is blank. A blank field and an explicit "GPS" are
different statements and are both kept. RINEX 3.03 introduced "BDS" and
"GPS", RINEX 3.05 renamed "BDS" to "BDT", and RINEX 4 permits only
"GPS"; the writers refuse an identifier the target version does not
support rather than rewriting it.