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title	summary	tags
Standards and provenance	Which public standard or paper each implemented algorithm follows, and how that attribution is recorded.	standards, references, provenance

Standards and provenance

An implementation is only as trustworthy as the source it follows. Every algorithm in the workspace names its source in the module documentation, and this document collects those citations in one place.

Why the source is part of the interface

Two implementations of the same named model can disagree by kilometers. The reasons are not errors but choices:

The gravity model. SGP4 was fitted against WGS 72. An element set distributed by Space-Track is intended to be used with WGS 72. Using WGS 84 instead shifts the position by kilometers, and neither result is wrong; they answer to different fits.
The definition of an angle. Latitude may be geodetic or geocentric. The two differ by up to about 0.19 degrees at mid latitudes, which is about 21 kilometers on the ground.
The time scale. A position at an instant is only meaningful with the scale that instant is on.

Naming the source makes these choices checkable.

Sources by topic

Topic	Source	Used by
SGP4 and SDP4	Vallado, Crawford, Hujsak, Kelso, Revisiting Spacetrack Report #3, AIAA 2006-6753, 2006	`apsis-sgp4`
Two-line element format	CelesTrak, FAQs: Two-Line Element Set Format, documenting the original NORAD columns and the checksum rule	`apsis-tle`
Gravity model constants	WGS 72 values as used by the SGP4 reference implementation; WGS 84 from NIMA TR8350.2	`apsis-sgp4`
Earth Rotation Angle	IERS Conventions (2010), equation 8.32	`apsis-frames`
Geodetic conversions	NIMA TR8350.2, Department of Defense World Geodetic System 1984, third edition, 2000	`apsis-geodesy`
Julian and Modified Julian Date	IAU conventions as described in the Explanatory Supplement to the Astronomical Almanac	`apsis-time`
Terrestrial Time offset	IAU 2000 Resolution B1.9, defining `TT - TAI = 32.184 s`	`apsis-time`
Leap seconds	IERS Bulletin C, embedded as a table by `apsis-leap`	`apsis-leap`
Calendar algorithms	Howard Hinnant, chrono-Compatible Low-Level Date Algorithms, 2015	`apsis-time`
Discrete event simulation	Zeigler, Praehofer, Kim, Theory of Modeling and Simulation, second edition, 2000	`apsis-sim-engine`

The difference between a standard and a paper

A standard is maintained and has an authority. WGS 84, the IERS conventions, and IERS Bulletin C are standards. A paper describes an algorithm and its validation. The SGP4 paper is a paper: the algorithm is fixed by the paper, and the reference implementation distributed with it is the operative description.

When the two disagree, the implementations here follow the paper and record the disagreement, because the element sets were fitted to the paper's model.

Reproducing a published value

A published position for a catalogued satellite can be reproduced when four things agree:

The element set, including its epoch and its checksum.
The gravity model. Use WGS 72 unless the source says otherwise.
The frame. SGP4 returns the True Equator Mean Equinox frame.
The time. The propagation time is measured from the element epoch.

Failing on the fourth is common: the epoch in the element set is a UTC instant, and a propagation by "one day" means one day of uniform elapsed time, not one UTC calendar day. When the interval crosses a leap second insertion the two differ by a second. See time-scales and leap-seconds.

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