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Observation Time Correction

We have duplicated (to the second!) the observation times listed in early computer printouts produced by Copernicus principal investigators by adding the first SET time to the GMT observation times listed in the raw and coadded scan data files. The feeling is that our listed observation times refer to the time the Copernicus satellite crosses the longitude of the ascending node (i.e., LAN). It is still unclear however whether this reference time refers to the orbit in which the observation was made or to the orbit in which the data was last dumped.

In any event, the actual calculations for deriving the start and end observation times (from the IDL program, which are based on data extracted from the Copernicus raw data files, are as follows:

 ddate = (yr_obs-1900)*1.0D+03              ; decimal date 
       + day_obs + (hr_obs/24.0D0) 
       + (min_obs/(24.0*6.0D01))
       + (sec_obs/(24.0*3.6D03)) 
 fst = (frst_set * 15.72887)/(24.0*3.6D03)  ; first SET time in days
 lst = (last_set * 15.72887)/(24.0*3.6D03)  ; last SET time in days
 startobs = ddate(i)+fst(i)                 ; start time
 endobs = ddate(i)+lst(i)                   ; end time

The SET times mentioned above refer to Spacecraft Equivalent Time (SET) and represent units of 15.72887 seconds.