77 lines
3.7 KiB
Markdown
77 lines
3.7 KiB
Markdown
---
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title: Standards and provenance
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summary: Which public standard or paper each implemented algorithm follows, and how that attribution is recorded.
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tags: standards, references, provenance
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---
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# Standards and provenance
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An implementation is only as trustworthy as the source it follows. Every
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algorithm in the workspace names its source in the module documentation, and
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this document collects those citations in one place.
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## Why the source is part of the interface
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Two implementations of the same named model can disagree by kilometers. The
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reasons are not errors but choices:
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- **The gravity model.** SGP4 was fitted against WGS 72. An element set
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distributed by Space-Track is intended to be used with WGS 72. Using WGS
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84 instead shifts the position by kilometers, and neither result is
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wrong; they answer to different fits.
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- **The definition of an angle.** Latitude may be geodetic or geocentric.
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The two differ by up to about 0.19 degrees at mid latitudes, which is
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about 21 kilometers on the ground.
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- **The time scale.** A position at an instant is only meaningful with the
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scale that instant is on.
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Naming the source makes these choices checkable.
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## Sources by topic
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| Topic | Source | Used by |
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|-------|--------|---------|
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| SGP4 and SDP4 | Vallado, Crawford, Hujsak, Kelso, *Revisiting Spacetrack Report #3*, AIAA 2006-6753, 2006 | `apsis-sgp4` |
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| Two-line element format | CelesTrak, *FAQs: Two-Line Element Set Format*, documenting the original NORAD columns and the checksum rule | `apsis-tle` |
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| Gravity model constants | WGS 72 values as used by the SGP4 reference implementation; WGS 84 from NIMA TR8350.2 | `apsis-sgp4` |
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| Earth Rotation Angle | *IERS Conventions (2010)*, equation 8.32 | `apsis-frames` |
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| Geodetic conversions | NIMA TR8350.2, *Department of Defense World Geodetic System 1984*, third edition, 2000 | `apsis-geodesy` |
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| Julian and Modified Julian Date | IAU conventions as described in the *Explanatory Supplement to the Astronomical Almanac* | `apsis-time` |
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| Terrestrial Time offset | IAU 2000 Resolution B1.9, defining $\text{TT} - \text{TAI} = 32.184\,\text{s}$ | `apsis-time` |
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| Leap seconds | IERS Bulletin C, embedded as a table by `apsis-leap` | `apsis-leap` |
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| Calendar algorithms | Howard Hinnant, *chrono-Compatible Low-Level Date Algorithms*, 2015 | `apsis-time` |
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| Discrete event simulation | Zeigler, Praehofer, Kim, *Theory of Modeling and Simulation*, second edition, 2000 | `apsis-sim-engine` |
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## The difference between a standard and a paper
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A *standard* is maintained and has an authority. WGS 84, the IERS
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conventions, and IERS Bulletin C are standards. A *paper* describes an
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algorithm and its validation. The SGP4 paper is a paper: the algorithm is
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fixed by the paper, and the reference implementation distributed with it is
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the operative description.
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When the two disagree, the implementations here follow the paper and record
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the disagreement, because the element sets were fitted to the paper's model.
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## Reproducing a published value
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A published position for a catalogued satellite can be reproduced when four
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things agree:
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1. The element set, including its epoch and its checksum.
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2. The gravity model. Use WGS 72 unless the source says otherwise.
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3. The frame. SGP4 returns the True Equator Mean Equinox frame.
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4. The time. The propagation time is measured from the element epoch.
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Failing on the fourth is common: the epoch in the element set is a UTC
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instant, and a propagation by "one day" means one day of uniform elapsed
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time, not one UTC calendar day. When the interval crosses a leap second
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insertion the two differ by a second. See [[time-scales]] and
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[[leap-seconds]].
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## See also
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- [[sgp4]] for the propagation model
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- [[geodesy-wgs84]] for the ellipsoid
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- [[reference-frames]] for the frame
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