SSC Home Page
Spitzer Science Center
 SSC Home -  Active Mission -  Overview -  IRAC -  IRS -  MIPS -  Data Archive -  Data Analysis & Tools
Knowledgebase: Data processing
What time keywords should I use from the header for the exact start of my AOR?
Posted by Elena Scire on 21 November 2008 11:04 AM

In a nutshell, DATE_OBS is the most accurate, absolute measurement for comparing with astronomical emphemerides. SCLK_OBS (in real sec) is good for relative timing comparisons within an AOR or even a campaign.

More words on this:

  • DATE_OBS value is a real date-time string representing the absolute UTC time at the very start of data aquisition or "array activity" for the DCE, i.e, the start of any boost (for DCENUM=0) or, the start of a reset (for DCENUMs > 0). (It has nothing to do with image timing or readouts.)
  • UTCS_OBS is the number of real seconds past midnight, year 2000 at the _UTC_ time of the DCE start (or array activity as above).

Both DATE_OBS and UTCS_OBS are computed from spacecraft telemetry clock counters (SCLK_OBS coarse and fine bit ticks) and calibrated using an atomic clock in Boulder to correct for on-board drifts.