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New implementation! (Jan. 2012)

Pointing and live time history (FT2, LS-005)

This is a fundamental data product used by the science tools. The current definition (version 10 of July 25, 2006 as of this writing) is posted on the Guidelines for Science Tools Design page maintained by Masa Hirayama. This page includes a link to the ft2.tpl template file that is the current working definition of the FT2 format as used by the Science Tools.unmigrated-wiki-markup

Basically, the file is intended to include the necessary information (other than tables of effective area) for calculating exposures. So it has the position and attitude of the LAT for regularly-spaced time intervals (nominally 30 seconds, although shorter intervals are used when the LAT enters and exits the South Atlantic Anomaly). As currently planned, SAA entry and exit will correspond to the stop and start time for the data taking 'runs' in flight. For orbits that miss the SAA, a run stop/start will be issued when the LAT crosses the orbital plane heading north. \[anders\] The reasons for the forced stop/starts can be found [here|https://confluence.slac.stanford.edu/download/attachments/17880/borgland_runLength_SciOps10272006.pdf?version=1]. For these [anders] The reasons for the forced stop/starts can be found here. For these stop/starts, the interval between the entries may also be less than 30 s. For each of the intervals in an FT2 file the accumulated live time is also recorded.

This page is open for comments, and for posting additional issues. For clarity, please add the comments in the section to which they refer, by editing the page directly. When the comments settle down, I will post as JIRA issues any specific changes that we converge on for the definition of the FT2 format.

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And when the LAT is just scanning, can we make do with a longer interval, say 1 minute? How severe is the tradeoff in accuracy? The motivation for making the interval longer would be to make FT2 files smaller and to increase the speed of exposure calculations.

Wiki MarkupThis is kind of a minor point, but may need to be clarified: During the time between the stop (LPASTOP) of a run and the LPASTART of the next (if a stop/start is issued outside an SAA passage - see above), the LAT will be dead as far as data taking is concerned. If a time interval is not explicitly included in an FT2 file, i.e., if the time of a START entry is not the same as the time of the previous STOP entry, we can just assume that the missing interval was dead time, right? \ [anders\] Any time between an LPASTOP and the subsequent LPASTART is deadtime i.e. we did not take any data.

(D. Band 4/6/07) First, is this an ICD/FFD issue, given that every row provides the start and stop time? Second, I think it makes sense to increase the time resolution when necessary--perhaps we should establish an angular change criterion?

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(Ormes, from e-mail 4/16/07) I don't know how the electronics was finally done. I do know these numbers, live and dead were supposed to be measured independently in the electronics. Maybe it was not done in the end.

Wiki Markup(_Borgland_, from e-mail 4/16/07) There is no deadtime counter since by definition the instrument is dead. There is only a livetime counter which increments in 50 ns tickes all the time the instrument is alive i.e. not busy/dead reading out etc. There is also an elapsed counter which is the total time. The livetime
fraction between events N and N+M is just
LiveTimeFraction = \ [Livetime(N+M)-LiveTime(N)\]/\[ElapseTtime(N+M)-ElapsedTime(N)\]

and the deadtime (for whoever is interested in that) is just 1 - LivetimeFraction.

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5/16/07: Removing the DEADTIME column has been submitted as a JIRA request, https://jira.slac.stanford.edu/browse/DATAPROD-1^{Image Removed}

3. McIlwain coordinates

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5/16/07: Adding the geomagnetic latitude has been submitted as a JIRA request, https://jira.slac.stanford.edu/browse/DATAPROD-2^{Image Removed}

4. IN_SAA column

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