This is a real time system and the data arrives in packets at intervals of 16 or 128 seconds depending on the mode. Each packet has a MET time header and this is the time displayed on the screen - the time of the last packet to be received. Thus on a display this time is not that of the first or the last data point on the screen. The last packet received is always plotted at the right hand side of the screen. The plot header gives sufficient information to be able to calculate the time of any particular data bin (0 - 1023).

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Time (UT) = UT of index + [ (data bin - index bin) x bin size ] - 2.4 seconds

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Index bin and binsize are related as follows for a display having 1024 bins:

bin size (sec)   index bin     span (sec)
-------------------------------------------
     0.125           1            128
     0.25          512            256
     0.5           768            512
     1.0           896           1024
     2.0           960           2048
     4.0           982           4096
     8.0          1008           8192
    16.0          1016          16384
    32.0          1020          32768
    64.0          1022          65536

and for a display having 512 bins (0-511):

bin size (sec)   index bin     span (sec)
-------------------------------------------
    16.0           511           8192
    32.0           511.5        16384
    64.0           511.75       32768

The 2.4 second term in the formula above is the result of a combination of errors in the conversion between UT and MET in the displays and the spacecraft clock.

Example:

//XTE PCA Science Monitoring
//Standard Mode 1 Data
//Date and Time created : 12/11/96 18:41:58
///socops/tmp/pcaStd1LightCurve.data
//Data status is :  binsize = 1 sec.  compression factor = 8
//Last/Current  Partition : MET = 92983680 UT = 12/12/96 04:47:59 
//No. of data integrated =  1 
//variables written are :  pcuSUM
//No. Vec.    Length 
2 1024     
0 145
1 152
2 140
3 148
...
1023 143

Suppose we have seen a feature in the light curve at bin 621, and we want to figure out the time at which it occurred.

• The binsize was set to 1 sec, so the index bin (see above) is 896. That means that the counts in bin 896 occurred at 04:47:59 (last/current partition time, above).
• Using the formula above the table,
  Time(UT) = 04:47:59 + [(621 - 896) * 1 sec] = 04:47:59 - 275 sec = 04:43:01 UT.
• Due to the error mentioned in the first paragraph, the actual time is:
  Time(UT) = calculated time(UT) - 2.4 sec