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converting IMAGE_TIME UTC to UT1
ncc1701d
post Jun 22 2016, 12:43 AM
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When images are taken the PDS header for a .IMG image file alway gives the "IMAGE_TIME" date in UTC time
like
2006-05-18T01:30:33.015Z which is zulu UTC

If i wanted to to get the matching UT1 that NASA might use for that particular "IMAGE_TIME" I would need to know how they handle leap seconds.

From my reading here
http://tf.nist.gov/pubs/bulletin/leapsecond.htm

It looks like there are two resolutions

It says
"The resolution of the DUT1 correction is 0.1 s, and represents an average value for an extended range of dates.
Therefore, it will not agree exactly with the weekly UT1-UTC(NIST) values shown in the earlier table, which have 1 ms resolution and are updated weekly"


So my question would be . What would NASA use to get the UT1 for that "IMAGE_TIME" if it was making a simulator or something? the 0.1 resolution or the weekly 1 ms resolution?
I might guess the higher resolution but maybe thats over kill or to inconvenient or may doesnt matter which resolution.
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JohnVV
post Jun 22 2016, 12:56 AM
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isis3 and any software that uses the spice data , uses the "naif0011.tls "
a short text file that is documented
ftp://naif.jpl.nasa.gov/pub/naif/generic_kernels/lsk/
CODE
KPL/LSK


LEAPSECONDS KERNEL FILE
===========================================================================

Modifications:
--------------

2015, Jan. 5    NJB  Modified file to account for the leapsecond that
                     will occur on June 30, 2015.

2012, Jan. 5    NJB  Modified file to account for the leapsecond that
                     will occur on June 30, 2012.
                    
2008, Jul. 7    NJB  Modified file to account for the leapsecond that
                     will occur on December 31, 2008.
                    
2005, Aug. 3    NJB  Modified file to account for the leapsecond that
                     will occur on December 31, 2005.
                    
1998, Jul  17   WLT  Modified file to account for the leapsecond that
                     will occur on December 31, 1998.
                    
1997, Feb  22   WLT  Modified file to account for the leapsecond that
                     will occur on June 30, 1997.
                    
1995, Dec  14   KSZ  Corrected date of last leapsecond from 1-1-95
                     to 1-1-96.

1995, Oct  25   WLT  Modified file to account for the leapsecond that
                     will occur on Dec 31, 1995.

1994, Jun  16   WLT  Modified file to account for the leapsecond on
                     June 30, 1994.

1993, Feb. 22  CHA   Modified file to account for the leapsecond on
                     June 30, 1993.

1992, Mar. 6   HAN   Modified file to account for the leapsecond on
                     June 30, 1992.

1990, Oct. 8   HAN   Modified file to account for the leapsecond on
                     Dec. 31, 1990.  


Explanation:
------------

The contents of this file are used by the routine DELTET to compute the
time difference

[1]       DELTA_ET  =  ET - UTC                                        
          
the increment to be applied to UTC to give ET.

The difference between UTC and TAI,

[2]       DELTA_AT  =  TAI - UTC

is always an integral number of seconds. The value of DELTA_AT was 10
seconds in January 1972, and increases by one each time a leap second
is declared. Combining [1] and [2] gives

[3]       DELTA_ET  =  ET - (TAI - DELTA_AT)

                    =  (ET - TAI) + DELTA_AT

The difference (ET - TAI) is periodic, and is given by

[4]       ET - TAI  =  DELTA_T_A  + K sin E

where DELTA_T_A and K are constant, and E is the eccentric anomaly of the
heliocentric orbit of the Earth-Moon barycenter. Equation [4], which ignores
small-period fluctuations, is accurate to about 0.000030 seconds.

The eccentric anomaly E is given by

[5]       E = M + EB sin M

where M is the mean anomaly, which in turn is given by

[6]       M = M  +  M t
               0     1

where t is the number of ephemeris seconds past J2000.

Thus, in order to compute DELTA_ET, the following items are necessary.

          DELTA_TA
          K
          EB
          M0
          M1
          DELTA_AT      after each leap second.

The numbers, and the formulation, are taken from the following sources.

     1) Moyer, T.D., Transformation from Proper Time on Earth to
        Coordinate Time in Solar System Barycentric Space-Time Frame
        of Reference, Parts 1 and 2, Celestial Mechanics 23 (1981),
        33-56 and 57-68.

     2) Moyer, T.D., Effects of Conversion to the J2000 Astronomical
        Reference System on Algorithms for Computing Time Differences
        and Clock Rates, JPL IOM 314.5--942, 1 October 1985.

The variable names used above are consistent with those used in the
Astronomical Almanac.

\begindata

DELTET/DELTA_T_A       =   32.184
DELTET/K               =    1.657D-3
DELTET/EB              =    1.671D-2
DELTET/M               = (  6.239996D0   1.99096871D-7 )

DELTET/DELTA_AT        = ( 10,   @1972-JAN-1
                           11,   @1972-JUL-1    
                           12,   @1973-JAN-1    
                           13,   @1974-JAN-1    
                           14,   @1975-JAN-1          
                           15,   @1976-JAN-1          
                           16,   @1977-JAN-1          
                           17,   @1978-JAN-1          
                           18,   @1979-JAN-1          
                           19,   @1980-JAN-1          
                           20,   @1981-JUL-1          
                           21,   @1982-JUL-1          
                           22,   @1983-JUL-1          
                           23,   @1985-JUL-1          
                           24,   @1988-JAN-1
                           25,   @1990-JAN-1
                           26,   @1991-JAN-1
                           27,   @1992-JUL-1
                           28,   @1993-JUL-1
                           29,   @1994-JUL-1
                           30,   @1996-JAN-1
                           31,   @1997-JUL-1
                           32,   @1999-JAN-1
                           33,   @2006-JAN-1
                           34,   @2009-JAN-1
                           35,   @2012-JUL-1
                           36,   @2015-JUL-1 )

\begintext
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mcaplinger
post Jun 22 2016, 06:15 AM
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One nearly always uses ET, not UT1. I just call NAIF utc2et and let it worry about the details.


--------------------
Disclaimer: This post is based on public information only. Any opinions are my own.
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ncc1701d
post Jun 23 2016, 07:17 PM
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Thank you
Do you by chance know what the K constant and the EB stand for?
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