JULIAN DATES IN THE DEEP IMPACT AND EPOXI ARCHIVES
AND
THEIR INTERACTION WITH LEAP SECONDS
History
=======
2010-09-13 B.T.Carcich Initial version
Table of Contents
=================
References
Prerequisites
Summary
Julian dates and leap seconds in data sets from the Deep Impact Instruments
Overview
Details
Julian dates in the DI data sets
JD UTC values in the DI data sets
JD TDB values in the DI data sets
Julian dates in the PDS DI FITS headers
Julian dates in the PDS DI data labels
Cognizant persons
References
==========
[1] NAIF/SPICE TIME required reading document, available from
http://naif.jpl.nasa.gov/.
[2] Astronomical Almanac Online. (2010). U.S. Nautical Almanac Office and
Her Majesty's Nautical Almanac Office. http://asa.usno.navy.mil/ and
http://asa.hmnao.com/.
[3] Planetary Data System Standards Reference, Version 3.9, JPL D-7669
Part 2, Jet Propulsion Laboratory, California Institute of Technology,
Pasadena, California, USA, 2009.
[4] Pence, W.; Chiappetti, L.; Page, C.; Shaw, R.; Stobie, E.; (IAUFWG
Technical Panel), Definition of the Flexible Image Transport System
(FITS). FITS Standard version 3.0 Internat. Astronomical Union
Commiss. 5: Documentation and Astronomical Data. FITS Working Group
(2008) http://fits.gsfc.nasa.gov/standard30/fits_standard30.pdf
Prerequisites
=============
=====================================================================
== N.B. it is assumed that the reader is familiar with the time ==
== required reading material from the NAIF/SPICE toolkit ==
== documentation [1], specifically with terms and concepts ==
== such as Julian Date, UTC, TDB, leapseconds, &c. Refer to ==
== the references above for details. ==
=====================================================================
Summary
========
The Deep Impact (DI) and EPOXI data sets to-date span two leap seconds:
2005-DEC-31T23:59:60 and 2008-DEC-31T23:59:60. Therefore, any differences
calculated between DI UTC JDs which span one or both of those leap seconds
must add one, or two seconds to the calculated result.
With two exceptions, all keywords using Julian dates (JDs) in the FITS
headers and PDS data file labels of the Deep Impact (DI) data sets are
UTC JDs. This means that any two JDs from these data sets may not
be arbitrarily associated by simple differencing of the JD values.
The two exceptions are FITS header keywords OBSMIDJT and KPKSSBJT which
are JD TDB values, and are plainly labelled as such in the header keyword
comments.
Julian dates and leap seconds in data sets from the Deep Impact Instruments
===========================================================================
Overview
--------
The purpose of this document is to alert the data set user to pitfalls
when calculating differences between Julian Dates within the data sets
and between Julian Dates from the data sets with other time systems.
Almost all JD time representations in the DI data sets are UTC JDs and
therefore require that leap seconds be taken into account when
calculating time differentials between UTC JDs.
Details
-------
There are several representations of time used in the DI data sets; one
of them is Julian Date. The Julian Date system uses a number which
represents the time since an epoch, specifically
Julian date (JD): the interval of time in days and fractions of a day,
since 4713 B.C. January 1, Greenwich noon, Julian proleptic calendar
[2].
That definition, from the Glossary of reference [2], goes on to prescribe
the following:
In precise work, the timescale, e.g., Terrestrial Time (TT) or
Universal Time (UT), should be specified [2].
This intent of this document is to fulfill that prescription. The issue
is that calculating time differences between Julian dates involves more
than subtracting two floating point numbers.
The specific pitfall is that Julian dates look like real numbers, e.g.
2452545.000000, which appear to represent a time using two parts: an
integral part which corresponds to a number of whole days; a fractional
change part which corresponds to a fractional day. However, in some
contexts, one day in the integral part may not represent a change of
86,400 seconds in time. This is due to leap seconds, an explanation of
which is outside the scope of this document. The reader is referred to
the references above plus available literature, but suffice to say that
since the January 2005 launch of the DI spacecraft, two leap seconds have
occurred: 2005-DEC-31T23:59:60; 2008-DEC-31T23:59:60.
Julian dates in the DI data sets
--------------------------------
There are two kinds of Julian dates used in the FITS headers and in the
PDS data labels of the DI data. The most used kind is JD UTC, aka UT JD
and UTC JD. The second kind is JD TDB.
JD UTC values in the DI data sets
---------------------------------
Due to leap seconds, and like UTC calendar times, JD UTC values are not
continuous with respect to continuous time systems such as TT or TDB, and
should therefore not be treated as real numbers subject to arithmetic
operations such as addition and subtraction. JD UTC values should instead
be thought of as symbols representing specific times; in that sense JD
UTCs are analogous to calendar strings such e.g.
2010-09-13T01:23:45.6789 UTC
N.B. The JD UTC time system is discontinuous across leap seconds and
therefore has no way to represent time during a leap second.
However, there have been no DI data taken during any leap second
that would require such a problem to be resolved.
JD TDB values in the DI data sets
---------------------------------
JD TDB values are continuous and linear with respect to the continuous
TDB time system, and typically differed from JD UTC values by about
0.000075d (~65s) during the DI mission.
For details regarding time conversions between UTC times and other time
systems the reader is referred to the NAIF/SPICE toolkit library and its
attendant documentation, specifically the TIME required reading document
[1].
Julian dates in the PDS DI data labels
--------------------------------------
In the PDS labels, the relevant PDS keywords using Julian Dates are
START_JULIAN_DATE_VALUE
MID_JULIAN_DATE_VALUE
STOP_JULIAN_DATE_VALUE
Although not defined by the PDS standard [3], these keywords'
floating-point values have been written as UT JDs at the direction
of the Science Team.
Julian dates in the PDS DI FITS headers
---------------------------------------
In the FITS headers of the DI data, all JDs, except two, are UT JDs.
The two JDs in the FITS headers are comments defining them as TDB JDs:
OBSMIDJT= ... / [JD TDB] Barycentric time at mid-obs at s/c
KPKSSBJT= ... / [JD TDB] When light reaches Solar Barycenter
The reader is directed to the FITS standard [4] for the format of keyword
comments in FITS headers.
Again, for details regarding time conversions between UTC times and other
time systems the reader is directed to the NAIF/SPICE toolkit library and
its attendant documentation, specifically the TIME required reading
document [1].
Cognizant persons
=================
This document was written by Brian Carcich, Cornell University,
Ithaca, NY, and reviewed by members of the EPOXI Science Team.