Start times in 4 wapps differs in pulsar observing

06jan07


    An archive of the  headers from the wapp pulsar files is made each month. At the end of the month, all of the pulsar files on disc are scanned and the headers are archived. Many pulsar data files are removed from the online disc before the end of each month, so this process only archives a fraction of the data actually taken.

    An observation can use 1 to 4 wapps. When starting an observation, the 4 wapps are told to configure themselves and then start taking data on the next hardware 1 second tick. The start time for each wapp is stored in the header for that wapps datafile.

    The start of datataking is determined by a hardware 1 second tick (locked to the hydrogen maser). The time recorded in the header comes from the local clock on each wapp. This time is maintained by the ntp Daemon on each wapp.
 
    There has been a problem where the time on each wapp was drifting (see ntp problems with wapps). This seemed to clear up when the wapp linux kernels were upgraded.

    I recently went through the wapp pulsar archive and looked at the number of times the start times for the different wapps used in an observation differed.  The data covered jan05 through jan07. The time difference was usually 1 second.
 
    The plots shows when the wapp start_times differed (.ps) (.pdf):

    The question is whether the start time in the header is the real start time (one or more wapps started late) or all of the wapps started together and the time in the header is wrong. When the ntp time was drifting then the time in the header could have been wrong. After the kernels were upgraded my guess is that the times in the header are correct and the wapps are actually starting at different times.


How pulsar observations are started.

    As far as i can tell, the wapp startup  sequence for pulsare observing is:


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