Gravity numerical data: provenance and reuse assessment
=======================================================

This notice covers the normalized numerical fields in gravity/earth/EGM96.cof,
JGM2.cof and JGM3.cof. It does not relicense provider papers, figures, software,
or raw observational databases. No numerical values were changed for GMAT Py.

EGM96
-----
All 65,338 Cnm/Snm pairs exactly match the official NGA archive member EGM96
(degree/order 360), compared as Decimal values. Absent zonal Sn0 is zero.
The current GMAT-format file, model GM, and reference radius are preserved.

Official archive: https://earth-info.nga.mil/php/download.php?file=egm-96spherical
Archive SHA256: 1f21ab8151c1b9fe25f483a4f6b78acdbf5306daf923725017b83d87a5f33472
EGM96 member SHA256: 1e5e6c30343989b8e2eda0bb96bde06ef05981eec69934d6e44aace4f0d6a9d5
Shipped EGM96.cof SHA256: 4735c9301e20ee8db09b6d0882a24671b327cd35ec5740ea497c8eeb43a39807

NGA's public-information policy permits copying/distributing website information
unless otherwise specified, and requests appropriate credits:
https://www.nga.mil/resources/Privacy_Policy.html
No contrary terms were found in the official archive README files. Applying
this agency policy to the official earth-info.nga.mil download is an assessment
of its scope, supported separately by the factual-data basis below.

Credit NASA GSFC/NIMA (now NGA), their collaborators, and Lemoine et al.,
NASA TP-1998-206861, The Development of the Joint NASA GSFC and the National
Imagery and Mapping Agency (NIMA) Geopotential Model EGM96.
Official model description: https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84
Scientific history: https://cddis.nasa.gov/926/egm96/doc/S_1.HTML

JGM2 and JGM3: GMAT legacy variants
----------------------------------
These complete degree/order 70 tables retain the coefficients distributed by
GMAT, including inherited C20 variants. They are not exact, unmodified copies
of the ICGEM files. Directly substituting the ICGEM tables changes propagation.

JGM2: all 2,553 model rows match ICGEM except (2,0). Shipped C20 is
-0.000484165390000000; ICGEM C20 is -0.000484165480, a +9e-11 difference.
The dated July 13, 1995 JGM2F70 file in GMAT has the ICGEM C20 value. The
reason/epoch of the active file's inherited change remains undocumented;
its size is not the permanent-tide conversion. ICGEM's tiny nonzero S20 has
no harmonic effect because the order-zero sine factor vanishes.
Credit Nerem et al. (1994), Gravity Model Development for TOPEX/POSEIDON:
Joint Gravity Models 1 and 2, https://doi.org/10.1029/94JC01376
Source: https://icgem.gfz.de/getmodel/gfc/291f7d127f49fe3bcb4a633a06e8b50f461d4b13ff8e7f2046644a8148b57fc6/JGM2.gfc
Source SHA256: 0779cf74f216d5b38c7a4c4a1a25d596b659605deb7731f00874c17e9ed3f5af
Shipped JGM2.cof SHA256: abac32a233e8662885778018d02e41553a5b6e5e1ee7332d2f69c362423025d3

JGM3: shipped C20 is -0.000484165374886470; ICGEM C20 is
-0.000484169548456, a +4.173569530e-9 difference. IERS 2003 section 6.3
identifies original JGM3 as zero-tide. GMAT's loader classifies the shipped
value as tide-free. The difference has the sign and scale of permanent-tide
removal, but the exact historical conversion parameters are unverified.
IERS 2003 equation 12 with k20=0.3 gives +4.17423864e-9, not the exact
shipped difference. Every other C/S agrees within half ICGEM's last published
decimal unit (maximum absolute differences C: 3.70e-18; S: 4.60e-18).
Credit Tapley et al. (1996), The Joint Gravity Model 3,
https://doi.org/10.1029/96JB01645
Source: https://icgem.gfz.de/getmodel/gfc/a3375e01a717ac162962138a5e94f10466b71aa4a130d7f7d5b18ab3d5f90c3d/JGM3.gfc
Source SHA256: 2ec791eec795689291974dd3602b304029da93d3fea30096daa3b368fb231d03
Shipped JGM3.cof SHA256: a622342e7c5421ee0b0f01f1b5cd38e6be87f1118fbd81b8c695e8c969603819
Tide convention: https://iers-conventions.obspm.fr/archive/2003/chapter6/tn32_c6.pdf

Factual-data assessment
-----------------------
The coefficient-only fields are computed scientific results in mechanical,
complete degree/order tables. We assess their reuse under the U.S. factual-data
exclusion, not a newly invented Apache/CC provider license. U.S. Copyright
Office Compendium 300 sections 313.3(C), 313.4(D), and 312.2 distinguish facts
and common-property tables from creative expression/compilation:
https://www.copyright.gov/comp3/chap300/ch300-copyrightable-authorship.pdf
17 USC 102(b)/103(b): https://www.copyright.gov/title17/92chap1.html
Section 105 is additional support for official government contributions,
but alone cannot cover contractor/university contributions to joint models.
ICGEM's imprint defers third-party data rights; it is not a universal CC grant:
https://icgem.gfz.de/imprint

The remaining C20 questions concern scientific lineage. No per-file JGM reuse
restriction was identified. This assessment is not a worldwide legal guarantee.
python/tools/verify_gravity_provenance.py reproduces the numerical comparison
against hash-pinned official source files without modifying any data.
