There are two sets of time stamps provided within the WBD LEVEL1 data files.
Formally, they are referred to as UT_OBT (Universal Time_Onboard Time) and
UT_GRT (Universal Time_Ground Receive Time). The following describes the
origin of these time stamps, their accuracies, and their preferred
applications.

UT_OBT

The origin of UT_OBT is the master ultrastable oscillator on each spacecraft,
or more specifically, the counter that is incremented by it.  Every WBD
transfer frame (the fundamental telemetery unit from the spacecraft) contains
the value of this counter at the instant at which the frame is transmitted.
During ground processing, the WBD team at Iowa uses this counter value plus
the tcal (time calibration) files supplied by ESOC (European Space Operations
Centre) to obtain the Universal Time (UT) associated with that counter value.
Next, internal delays within the spacecraft (from the instant of measurement
at WBD, through processing within the Onboard Data Handling System, which
provides the onboard counter value and constructs the transfer frame) are
calculated and applied, thus arriving at the UT time of measurement at WBD.
This is referred to as UT_OBT.  This time stamp will usually be within 2
milliseconds (msec) of "true" UT, as required by the specifications for OBT,
but there are brief periods where UT_OBT may differ from "true" UT by up to 4
msec.  The difference from "true" UT is primarily the result of slow drifts of
the spacecraft oscillators, which result in the tcal files gradually losing
accuracy.  Once this drift results in a difference between calculated UT and
"true" UT of more than 2 msec, ESOC carries out a time calibration at its
ground station, and then generates revised tcal files that reset the
difference back to zero.

The method used to obtain the UT_OBT time stamps for all frames in any given
WBD pass (which can be anywhere in length up to about 6 hours) is to determine
the time of the first full frame, and then apply a "delta time" to this for
each successive frame of the pass, where "delta time" is the known time
interval between frames based on the data transfer rate and the tick rate of
the onboard oscillator.

In order to obtain UT_OBT time stamps with better accuracy, there are WEC
(Wave Experiment Consortium) time correction files that can be obtained from
the Cluster Active Archive (CAA) (http://caa.estec.esa.int/caa/).  These files
have names in the format C#_CP_DWP_TCOR_YYYYMMDD_Vnn.cef where # is the
spacecraft number, YYYY is the year, MM is the two-digit month, DD is the two
digit day, and nn is a version number.  There are three columns in these
files, the first containing a Date/Time string (Epoch), the second an offset
value (Offset), and the third a difference value (Diff).  For WBD data, only
the Epoch and Diff values are needed to calculate a time correction.  The data
in each file are divided into blocks by rows that have the value -1.0e31
entered in both the Offset and Diff columns.  Each block consists of a start
and end Epoch value, with associated Offset and Diff values.  To obtain the
correction at a given instant, find the block that contains that instant
within its bounds, and linearly interpolate between the two Diff values.  The
result (which may be negative) is the number of microseconds (usec) to add to
UT_OBT to obtain a UT time stamp which should be accurate to +/- 10 usec.

UT_OBT is directly comparable to time stamps used by all other instruments on
the Cluster spacecraft, for which the data are stored onboard and transmitted
to ESA ground stations for processing.  Thus, WBD UT_OBT is the most
appropriate time stamp to use when comparing WBD data to that of other
instruments on these spacecraft.

UT_GRT

The origin of UT_OBT is the Earth Received Time (ERT) parameter applied to
every WBD transfer frame that is received on the ground by Deep Space Network
(DSN) ground stations.  ERT is the UT time at which DSN received the transfer
frame on the ground, and is accurate to within 10 usec.  This time is then
adjusted for time of flight and other data path effects to give a UT time
stamp corresponding to the time of measurement at the WBD instrument, also
accurate to within 10 usec.  As mentioned above for UT_OBT, there are slow
drifts within the spacecraft oscillators, and these also lead to time drifts
in UT_GRT.  The ESOC-supplied tcal files are used to obtain a value for the
oscillator tick rate, but the actual rate will drift relative to one obtained
from the tcal.  Since UT_GRT is obtained by fixing the time of the first frame
of a WBD data pass and then applying a "delta time" (which depends on the tick
rate) to it for each successive frame of the pass (anywhere in length up to
about 6 hours), any drift in the tick rate will show up as a small drift in
UT_GRT over the duration of the pass.  The Iowa WBD team is planning to
produce time correction files for UT_GRT that will correct for these small
drifts.

Occasionally, especially early in the Cluster mission (up to mid 2003) there
will be WBD LEVEL1 files that contain only fill data for UT_GRT.  The reason
for this is that the hardware and software being used by DSN during this
period would occasionally introduce time offsets into their ERT time stamps on
the order of a few to tens of msec.  Since there is no way to determine the
exact amount of this offset after the fact, Iowa was forced to insert fill
data for the UT_GRT time stamp.  In these cases, UT_OBT should be used, along
with the WEC time correction files, to get accuracy of +/- 10 usec.

UT_GRT is primarily for use in performing high time-resolution comparisons
between the WBD data from the various Cluster spacecraft.  However, with the
introduction of the WEC time correction files, a comparable level of accuracy
to UT_GRT can be obtained by applying the WEC time correction to UT_OBT.


*** WARNING! ***

In cases where the end-user might be considering calculating UT_OBT or UT_GRT
on their own (instead of using the values provided in the LEVEL1 files),
please be aware that the calculation of either of these time stamps is a
non-trivial task.  Although most of the data necessary for such a calculation
is included in the LEVEL1 files, and although the accompanying documentation
files describe many of the concepts and procedures that are involved, there
are many pitfalls involved in performing the actual calculations.  Please
think twice before attempting to calculate any time stamps directly from the
raw data.