KPL/FK SPICE Lunar Reference Frame Specification Kernel ===================================================================== Original file name: moon_de440_200625.tf Creation date: 2021 June 25 16:00 Created by: Nat Bachman (NAIF/JPL) Version description: This frame kernel contains lunar frame specifications compatible with the current lunar binary PCK file moon_pa_de440_200625.bpc The above PCK contains lunar orientation data from the DE440 JPL Planetary Ephemeris. Frames Specified by this Kernel ===================================================================== Frame Name Relative to Type Frame ID ------------------------- ----------------- ----- -------- MOON_PA_DE440 ICRF/J2000 PCK 31008 MOON_ME_DE440_ME421 MOON_PA_DE440 FIXED 31009 This kernel does not map the default frame aliases MOON_PA MOON_ME to frames based on the DE440 ephemeris. Introduction ===================================================================== This kernel specifies lunar body-fixed reference frames for use by SPICE-based application software. These reference frames are associated with high-accuracy lunar orientation data provided by the JPL Solar System Dynamics Group's planetary ephemerides (both trajectory and lunar orientation data are stored in these ephemeris files). These ephemerides have names of the form DEnnn (DE stands for "developmental ephemeris"). The frames specified by this kernel are realizations of two different lunar reference systems: Principal Axes (PA) system -------------------------- The axes of this system are defined by the principal axes of the Moon. Due to the nature of the Moon's orbit and rotation, the Z axis of this system does not coincide with the Moon's mean spin axis, nor does the X axis coincide with the mean direction to the center of the Earth (in contrast with the ME system defined below). Lunar principal axes frames realizing the lunar PA system and specified by this kernel are associated with JPL planetary ephemerides. Each new JPL planetary ephemeris can (but does not necessarily) define a new realization of the lunar principal axes system. Coordinates of lunar surface features expressed in lunar PA frames can change slightly from one lunar ephemeris version to the next. Mean Earth/Polar Axis (ME) system --------------------------------- The Lunar mean Earth/polar axis system is a lunar body-fixed reference system used in the IAU/IAG Working Group Report [2] to describe the orientation of the Moon relative to the ICRF frame. The +Z axis of this system is aligned with the north mean lunar rotation axis, while the prime meridian contains the mean Earth direction. This system is also sometimes called the "mean Earth/mean rotation axis" system or "mean Earth" system. The mean directions used to define the axes of a mean Earth/polar axis reference frame realizing the lunar ME system and specified by this kernel are associated with a given JPL planetary ephemeris version. The rotation between the mean Earth frame for a given ephemeris version and the associated principal axes frame is given by a constant matrix (see [1]). For the current JPL planetary ephemeris (DE), this kernel includes specifications of the corresponding principal axes and mean Earth/ polar axis frames. The names of these frames are MOON_PA_DE440 and MOON_ME_DE440_ME421 The rotation from the former to the latter frame is described in [1] as the rotation "from the DE440 PA frame to DE421 MER" (mean Earth/mean rotation)" reference frame. Regarding Use of the ICRF in SPICE ================================== The IERS Celestial Reference Frame (ICRF) is offset from the J2000 reference frame (equivalent to EME 2000) by a small rotation: the J2000 pole offset magnitude is about 18 milliarcseconds (mas) and the equinox offset magnitude is approximately 78 milliarcseconds (see [3]). Certain SPICE data products use the frame label "J2000" for data that actually are referenced to the ICRF. This is the case for SPK files containing JPL version DE4nn planetary ephemerides, for orientation data from generic text PCKs, and for binary PCKs, including binary lunar PCKs used in conjunction with this lunar frame kernel. Consequently, when SPICE computes the rotation between the "J2000" frame and either of the lunar PA or ME frames, what's computed is actually the rotation between the ICRF and the respective lunar frame. Similarly, when SPICE is used to compute the state given by a JPL DE planetary ephemeris SPK file of one ephemeris object relative to another (for example, the state of the Moon with respect to the Earth), expressed relative to the frame "J2000," the state is actually expressed relative to the ICRF. Because SPICE is already using the ICRF, users normally need not use the J2000-to-ICRF transformation to adjust results computed with SPICE. Comparison of DE440 and DE421 Lunar Reference Frames ==================================================== Differences in the orientation of the frames MOON_ME_DE421 MOON_ME_DE440_ME421 were measured as follows: For the time range 1900 JAN 01 00:00:00.000 TDB (-3155716800.0000 TDB seconds) to 2051 JAN 01 00:00:00.000 TDB ( 1609416000.0000 TDB seconds) the maximum rotation angle, taken over ~1M samples, was 5.8204420784597E-07 radians This corresponds to a displacement of slightly over 1 m on a great circle. For the time range 2000 JAN 01 00:00:00.000 TDB (-43200.000000000 TDB seconds) to 2040 JAN 01 00:00:00.000 TDB ( 1262260800.0000 TDB seconds) the maximum rotation angle, taken over ~1M samples, was 3.0709840342771E-07 radians This corresponds to a displacement of approximately 53.4 cm on a great circle. References ===================================================================== [1] Ryan S. Park, et al, "The JPL Planetary and Lunar Ephemerides DE440 and DE441," The Astronomical Journal, Volume 161, Number 3. 2021 AJ 161 105. URL: https://iopscience.iop.org/article/10.3847/1538-3881/abd414/meta [2] Archinal, B.A., A'Hearn, M.F., Bowell, E., Conrad, A., Consolmagno, G.J., Courtin, R., Fukushima, T., Hestroffer, D., Hilton, J.L., Krasinsky, G.A., Neumann, G., Oberst, J., Seidelmann, P.K., Stooke, P., Tholen, D.J., Thomas, P.C., and Williams, I.P. "Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2009." [3] Roncoli, R. (2005). "Lunar Constants and Models Document," JPL D-32296. Frame Specifications ===================================================================== MOON_PA_DE440 is the name of the lunar principal axes reference frame defined by JPL's DE440 planetary ephemeris. \begindata FRAME_MOON_PA_DE440 = 31008 FRAME_31008_NAME = 'MOON_PA_DE440' FRAME_31008_CLASS = 2 FRAME_31008_CLASS_ID = 31008 FRAME_31008_CENTER = 301 \begintext MOON_ME_DE440_ME421 is the name of a reference frame defined by a constant rotational offset from the DE440 principal axes frame, such that the frame is closely aligned with the DE421 lunar mean Earth/polar axis frame. Rotation angles are from [1]. The angles and axis sequence shown below represent the inverse of the cited rotation; hence the angles are negated and the sequence is reversed. \begindata FRAME_MOON_ME_DE440_ME421 = 31009 FRAME_31009_NAME = 'MOON_ME_DE440_ME421' FRAME_31009_CLASS = 4 FRAME_31009_CLASS_ID = 31009 FRAME_31009_CENTER = 301 TKFRAME_31009_SPEC = 'ANGLES' TKFRAME_31009_RELATIVE = 'MOON_PA_DE440' TKFRAME_31009_ANGLES = ( 67.8526 78.6944 0.2785 ) TKFRAME_31009_AXES = ( 3, 2, 1 ) TKFRAME_31009_UNITS = 'ARCSECONDS' \begintext ===================================================================== End of kernel