All ACE level 2 data records contain timing data in the same format. The timing data indicate the start of the integration period for the data record, in Universal Time (UTC), and are formatted as follows:
year - int32 year
day - int32 day of year (first day of year is day 1)
hr - int32 hour of day
min - int32 minute of hour
sec - float32 seconds (accurate to millisec)
fp_year - float64 floating point (fractional) year
fp_doy - float64 floating point day of year (starting at 1.0)
ACEepoch - float64 ACE epoch time in seconds since Jan 1, 1996 at 0hr UT
The ACE Spacecraft 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, ~ 5 seconds earlier. The time offset is roughly constant since ACE entered the L1 libration point orbit in early 1998.
The timestamps in all of the ACE Level 2 data except the MAG 1-second data currently suffer from this ~5-second incorrect time-offset. For most ACE Level 2 data sets, the offset may perhaps not be a serious problem, since the time-resolution of these data are much greater than 5 seconds. But it depends on how the data are being used, and what they are being compared to...
For the MAG 1-second Level 2data, the offset has been corrected.
For other ACE data sets, the question of reprocessing the data to correct this timing problem is under review.
For other 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.
J. Bartels defined his calendar based on observations of daily geomagnetic activity. Over long periods the geomagnetic recurrence rate is very close to 27 days. (The equatorial rotation rate of the Sun is also close to 27 days.) Rotation 1 Day 1 in this sequence was assigned arbitrarily by J. Bartels to February 8, 1832.
A list of
bartels rotation start-times for the ACE mission (text file).
A list of bartels rotation start-times from 1997 to 2016 (PDF).
A list of bartels rotation start-times from 2004 to 2023 (PDF).
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Last Updated: 13 May, 2016