Clock Calibration Data

Spacecraft Clock Calibrations - daily parameters for conversion of Spacecraft Clock counts to seconds since spacecraft epoch (spacecraft epoch is the time when the spacecraft was turned on). Received daily from Flight Operations Team (FOT) at GSFC.

The ASC receives clock calibration data from the FOT at GSFC in the form of ASCII-format reports. ASC then sorts, checks and appends the reports to a database in NCSA HDF format. ASC can provide software to users to access the database. The software includes routines to convert spacecraft clock counts to ACEepoch (seconds since Jan 1, 1996).

Important Note

The ACE Clock Calibration process has not included the spacecraft to earth transmission delay. As a consequence, if the spacecraft range is 1.5e6 km, an event with a clock calibrated UTC of 12:34:45z will actually have occurred at 12:34:40z.

The timestamps in most of the ACE Level 2 data suffer from this ~5-second incorrect time-offset. Almost all of the data have cadence >> 5 seconds, so this has not been considered a serious problem.

The timestamps for the MAG 1-second cadence data have been corrected, i.e. for these two data sets, the spacecraft to earth transmission delay has been accounted-for in the timestamps.

The clock calibration can be corrected by subtracting the range delay for that particular day from the calculated UTC.

corrected UTC = UTC - (range / 2.99792e5 km/s)

The range can be calculated using any geocentric coordinate system (e.g. GSE).

Dates                     min & max range            subtract
---------------------   --------------------    -----------------
launch   to 10/22/97,     40483 - 1473600 km    0.135 - 4.915 sec
10/22/97 to 12/31/97,   1473600 - 1338223 km    4.915 - 4.464 sec
01/01/98 to current,    1393766 - 1602432 km    4.649 - 5.345 sec
The range varies with both the 6-month L1 orbit and the annual solar orbit.

Accessing Clock Calibration Data

The Spacecraft Clock Calibrations are available in a HDF format database. The HDF file contains all the attitude/orbit and clock calibration reports received by ASC to date and is updated daily. There are two ways to access the data:

Last Updated: December 2004
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