KPL/LSK
Generic LEAPSECONDS Kernel File
===========================================================================
This LEAPSECONDS kernel file contains algorithm for converting
between UTC and ET time systems. It must be loaded into a user's
program via a call to the routine LDPOOL to enable this conversion.
Refer to TIME Required Reading document for more information.
Modifications:
--------------
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 )