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).
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 all of the ACE Level 2 data suffer from this ~5-second incorrect time-offset. Since the highest-resolution ACE Level 2 data are the MAG 16-second averages, this has not been considered a serious problem. If higher time-resolution data are released in the future (such as MAG 1-second averages), the problem will be fixed for those data sets.
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 secThe 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:
- Access the clock calibration data via our web-based HDF browser. You can use this interface to download in ASCII format whatever subset of the data you want. If you like, you can also use this interface to download the complete HDF file and browse it with your own HDF browser.
(C source code) to read and extract data from the HDF database.
The software package contains documentation of the HDF data structures
used in the database.
This link also has a copy of the current ancillary HDF data file, and
examples of the raw AOR and CCR reports received from GSFC.
It can also be accessed
via anonymous ftp at mussel.srl.caltech.edu, in /pub/ace/ancil.
You'll need HDF libraries for your computer architecture to use this package.
Last Updated: December 2004 Return to ASC Home Page