GPKit

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Different libraries make different choices. This page sets GPKit's beside those of other Swift and Python libraries that run SGP4, as each project describes itself. It's about approach, not quality: each library is good at what it sets out to do.

For test results across many libraries, see gpconf's own pages, where each count is a result against one version on one date, not a verdict on the project.G-25

At a glance

GPKit 1.0.0 SatelliteKit 2.1.2 python-sgp4 2.27 Skyfield 1.55
Language Swift Swift Python, with compiled C++ (and a pure-Python fallback) Python
Licence MIT MIT MIT MIT
Dependencies none none (uses Foundation) none required; NumPy for the array API certifi, jplephem, numpy, sgp4
SGP4 a Swift transcription of Vallado's SGP4.cpp derived from Orekit, following Vallado et al.'s 2006 paper and code compiles Vallado's official C++ uses python-sgp4
TLE / 3LE ✓ ✓ ✓ ✓
OMM CSV ✓ not in its docs (2.1.2, checked 2026-10-31) ✓ through python-sgp4's OMM support
OMM JSON ✓ ✓ ✓ through python-sgp4's OMM support
OMM XML ✓ ✓ ✓ through python-sgp4's OMM support
OMM KVN ✓ not in its docs (2.1.2, checked 2026-10-31) not in its docs (2.27, checked 2026-10-31) not in its docs (1.55, checked 2026-10-31)
Catalog number held as Int, 0 to 999,999,999 UInt (noradIndex) integer satnum, and a 5-character satnum_str model.satnum, from python-sgp4
Alpha-5 in TLEs decoded on the way in; written by the TLE writer decoded supported, up to 339999 through python-sgp4
Gravity constants WGS-72 default; WGS-84, WGS-72-old selectable WGS-72 WGS-72 default; WGS-72-old, WGS-84 selectable python-sgp4's
Pass finding ✓ rise, culmination, set not in its docs (2.1.2, checked 2026-10-31) not in its docs (2.27, checked 2026-10-31); it gives positions and points to Skyfield ✓ rise, culminate, set
Look angles ✓ ✓ azimuth, elevation, distance not in its docs (2.27, checked 2026-10-31) ✓
Doppler ✓ downlink and uplink not in its docs (2.1.2, checked 2026-10-31) not in its docs (2.27, checked 2026-10-31) not in its docs (1.55, checked 2026-10-31)
Sunlit check none: GPKit has no sun model sun and moon positions not in its docs (2.27, checked 2026-10-31) ✓
Platforms macOS 13, iOS 16, tvOS 16, visionOS 1, Linux macOS 11, iOS 12, tvOS 12, watchOS 10, visionOS 2 Python 2.7, 3.10–3.14 Python 2.7, 3.7–3.13
How it says it's checked against the C++ compiled as it is, within 1 micrometre; Skyfield; python-sgp4; gpconf agrees with Orekit's test output and the paper's passes the automated tests of Vallado et al.'s 2010 reference release not in its docs (1.55, checked 2026-10-31)

"Not in its docs" means the project's own documentation, at the version named and on the date checked, doesn't describe it. It doesn't mean the project can't do it.

C-01–C-04 for each column; GPKit's fromK-02K-04K-24K-30K-31K-40K-42K-43K-55K-60K-82.

The choices, in words

Catalog numbers. GPKit holds every catalog number as an Int and keeps Alpha-5 only in a TLE's five columns.K-24 SatelliteKit holds it as a UInt and decodes Alpha-5.C-02 python-sgp4 gives an integer satnum and a 5-character satnum_str, and supports Alpha-5.C-03

Where SGP4 comes from. GPKit transcribes Vallado's SGP4.cpp into Swift, statement for statement, and tests the transcription against the C++ compiled as it is.K-30K-40 python-sgp4 compiles Vallado's C++ itself, and Skyfield builds on python-sgp4.C-03C-04 SatelliteKit is derived from Orekit's implementation of the same published algorithms.C-02

What's in the box. python-sgp4 returns positions and velocities in TEME and leaves frame conversions to libraries like Skyfield.C-03 Skyfield adds observers, rise and set, and sunlight.C-04 GPKit and SatelliteKit both include look angles. GPKit adds a pass search and Doppler, and 1.0.0 leaves out the sun; GPKit's main, not yet in a release, adds a sun model.K-55K-60C-02K-96

Formats. GPKit reads OMM in CSV, JSON, XML and KVN, and TLE, all into one record type.K-22K-12

One public report, cited with permission

In a public comment on python-sgp4's issue #173, the owner of GPKit and gpconf showed that python-sgp4 2.27's from_alpha5('A+123') and from_alpha5('A1_23') both return 100123, because the digits after the letter are read with int().X-01 GPKit's Alpha-5 decoder refuses a sign or any character but a digit after the letter, and a test on main checks it on A+123 and A1_23.K-23K-103 Read the comment →

Sources

  1. G-25 "a count is a result against that version on that date, not a verdict on the project". Source: gpconf: README.md:242.
  2. C-01 GPKit's column of the table. Source: the K- claims cited under the table.
  3. C-04 Skyfield 1.55: Python, MIT; requires certifi, jplephem, numpy, sgp4; wraps python-sgp4; TLE files (with name line) and from_omm; find_events (rise, culminate, set); altaz; is_sunlit; Python 2.7, 3.7–3.13; no satellite Doppler or KVN described. Source: installed skyfield-1.55.dist-info/METADATA:3-27; skyfield/sgp4lib.py:2-8,141-151,215-216; skyfield/positionlib.py:314,570-571; skyfield/iokit.py:428-436.
  4. K-02 A Swift package with no dependencies: OMM (CSV, JSON, XML, KVN) and TLE readers, a TLE writer, SGP4, passes with look angles and Doppler. Source: GPKit: README.md:17-18; Package.swift:14.
  5. K-04 Platforms: macOS 13, iOS 16, tvOS 16, visionOS 1. Linux is built in CI. Source: GPKit: Package.swift:9; .github/workflows/ci.yml:16.
  6. K-24 Catalog numbers are an Int; OMM NORAD_CAT_ID takes up to nine digits; Alpha-5 is refused in OMM. Source: GPKit: OMM.swift:4-17; ElementSet.swift:9-10; OMMTests.swift:58-70.
  7. K-30 SGP4 is a port of Vallado's SGP4.cpp (CelesTrak/fundamentals-of-astrodynamics 49df0479c950), with names and order of operations kept. Source: GPKit: NOTICE:9-17; SGP4Core.swift:1-15.
  8. K-31 Gravity constants WGS-72 (default), WGS-84 and WGS-72-old. Source: GPKit: SGP4Core.swift:78-108; Propagator.swift:76-94.
  9. K-40 SGP4 vs the unmodified C++: 33 cases (24 deep space), 2,349 steps, equal error codes, agreeing within 1 micrometre (asserted). Source: GPKit: SGP4Tests.swift:74-75,102-126.
  10. K-42 Skyfield 1.55: five public element sets over 40° N, 75° W, two days each; 49 passes; 95 rises and sets within 0.166 s; 47 culminations within 0.349 s; look-angle bounds. Source: GPKit: conformance/skyfield-2026-10-07.txt:1-19; test PassTests.swift:168-248.
  11. K-43 python-sgp4 2.27: 14,411 element sets, 86,466 states (0, 90, 720, 1,440, 4,320 and 10,080 min), largest difference 0.0024 mm, no differing error codes. Not run in CI. Source: GPKit: conformance/python-sgp4-crosscheck-2026-10-07.txt:5-10; CrossCheckTests.swift:5-14.
  12. K-55 GPKit has no sun or visibility model and no azimuth-dependent horizon. Source: GPKit: a grep of Sources for sun/eclipse/sunlit/twilight found nothing; signature passes(over:from:for:above:).
  13. K-60 Doppler: received = f(1 − ṙ/c), transmit = f/(1 − ṙ/c), c = 299,792.458 km/s. Source: GPKit: Observer.swift:75-88.
  14. K-82 CI runs gpconf's 5 offline cases; 13 need provider files that gpconf fetches and does not ship. Source: GPKit: README.md:153-155; ci.yml:33-56; tools/check_gpconf.py.
  15. C-02 SatelliteKit 2.1.2: Swift, MIT, no dependencies; TLE/3LE, JSON and XML GP data; noradIndex: UInt; Alpha-5 decoded; derived from Orekit, after Vallado et al. 2006; WGS-72 constants; topPosition gives azimuth, elevation, distance; sun and moon positions; macOS 11, iOS 12, tvOS 12, watchOS 10, visionOS 2; checked against Orekit's and the paper's test output. CSV, KVN, pass finding and Doppler not described. Source: local clone ~/Developer/satkit-pr17-check/repo at tag 2.1.2 (9a87cdb, 2026-06-02): README.md:4,10-13,91,103,144,340,345; Package.swift:1-9,18; LICENSE:1-3; Sources/SatelliteKit/TLE/Propagator.swift:28-40; Astro/Geography.swift:187; Astro/Astronomy.swift:82,118; greps for csv, kvn, doppler, passes: none.
  16. C-03 python-sgp4 2.27: compiles Vallado's official C++ (2023 May 09 release), pure-Python fallback; MIT; TLE and OMM (CSV, JSON, XML; KVN not described); integer satnum and 5-character satnum_str; Alpha-5 supported to 339999; WGS-72 default, WGS-72-old and WGS-84 selectable; TEME positions only, pointing to Skyfield for conversions; passes Vallado et al.'s 2010 reference tests; Python 2.7, 3.10–3.14. Source: installed sgp4-2.27.dist-info/METADATA:3-18,32-37,56-58,66-76,182-185,200-230,502-503,645-646; sgp4/alpha5.py:1-8; sgp4/omm.py:1-29.
  17. K-96 Sun (the sun's position and height at a place, after the NOAA calculators' formulas), Sunlight (the sky at a place: daylight, twilight or dark; whether the satellite is sunlit; phase angle; a brightness estimate) and PassVisibility (when in a pass the satellite is sunlit and when it can be seen with the eye). Source: GPKit main 11b7ab4 (PR #25, 2026-11-07): CHANGELOG.md:24-29; docs/public-api.swift:288, :301, :317, :331.
  18. K-22 OMM in CSV, JSON, XML (505.0-B-3) and KVN (502.0-B-3 §7). Quirks handled as listed. DOCTYPE refused. Source: GPKit: ElementSets.swift:1-13; XML.swift:1-7; README.md:105-119.
  19. K-12 One record type whatever the format; "the TLE is a way in, not the model". Source: GPKit: ElementSet.swift:3-4; README.md:64-65.
  20. X-01 In a public comment on python-sgp4 #173, from_alpha5('A+123') and from_alpha5('A1_23') both return 100123, because the digits after the letter are read with int(). Source: https://github.com/brandon-rhodes/python-sgp4/issues/173#issuecomment-6460454893 (hneogy, 2026-10-28T04:11:27Z), read through GitHub's API on 2026-10-31.
  21. K-23 Alpha-5 as Space-Track defines it: A–Z without I and O = 10–33, ceiling 339999. Test vectors. Source: GPKit: Alpha5.swift:1-46; ValueTests.swift:85-101.
  22. K-103 GPKit's Alpha-5 decoder refuses a sign or any character but a digit after the letter (the same guard in 1.0.0 and on main); a test on main, not yet in a release, checks that A+123 and A1_23 are refused, by the decoder and by the TLE reader. Source: GPKit main 11b7ab4 (PR #25, 2026-11-07): CHANGELOG.md:76 (under "Unreleased"); Tests/GPKitTests/TLETests.swift:38-47 (aSignOrAnUnderscoreAfterTheLetterIsRefused); Sources/GPKit/Alpha5.swift:26, identical at v1.0.0.