Concepts

Concepts explain the models, algorithms, and data conventions used by DiffOrb. Read these pages when you want to understand what DiffOrb computes and what assumptions its APIs rely on.

Recommended Reading Order

1. Read Time Scales And Epoch Storage first. It defines the epoch rules used across the library.
2. Continue with Frames And State Representation. It explains how State combines a Cartesian vector, an epoch, and a reference frame.
3. Read Keplerian Elements if your inputs or outputs use osculating element sets rather than Cartesian states.
4. Read Batch Inputs And Shapes to understand how scalar and batch inputs share the same interface style.
5. Read Earth Rotation And Terrestrial Geometry next if you work with ITRS sites or ITRS -> GCRS conversion.
6. Read Earth Orientation Parameters next if you work with modern UT1, polar motion, or Earth-based observations.
7. Continue with Observer Site Keys And Observer Types to see how fixed ground, roving ground, and space observers map into one Site model.
8. Read Light-Time Model if you work with one-way optical light time, two-way radar light time, or radar Doppler.
9. Read Photocenter Correction if you work with comet center-of-light corrections.
10. Continue with Dynamical Models to see how accelerations are defined.
11. Continue with Numerical Integrators And Dense Trajectories to see how states are propagated and queried.
12. Read Ephemeris Products to see how optical, radar, vector, element, apsides, and close-approach products share one propagated path.
13. Read Orbit Determination Overview before the orbit-fitting pages.
14. Read Initial Orbit Determination, then Differential Correction.
15. Finish with Weighting And Debiasing Models and Outlier Rejection when you need the error model and rejection rules.

Current Concepts

• Time Scales And Epoch Storage: canonical TT, UT1, UTC, TAI, and TDB handling.
• Frames And State Representation: Cartesian state-vector storage and reference-frame labels.
• Keplerian Elements: osculating element storage, units, epoch rules, and Cartesian conversion boundaries.
• Batch Inputs And Shapes: scalar and batch inputs, point-wise broadcasting, Cartesian-product grids, and PyTree dispatch.
• Earth Rotation And Terrestrial Geometry: the ITRS -> GCRS transformation model used for ground sites.
• Earth Orientation Parameters: measured Earth-rotation data used by modern time conversion and ground geometry.
• Observer Site Keys And Observer Types: fixed-ground, roving-ground, and space-based observers, and how their observer keys map into Site.
• Light-Time Model: one-way optical light time, two-way radar light time, and radar Doppler.
• Photocenter Correction: comet center-of-light corrections for optical astrometry.
• Dynamical Models: ephemeris-backed perturbing bodies and built-in acceleration laws.
• Numerical Integrators And Dense Trajectories: integration methods and dense trajectories.
• Ephemeris Products: optical, radar, vector, element, apsides, and close-approach products.
• Orbit Determination Overview: the whole path from an initial orbit to a fitted solution.
• Initial Orbit Determination: the short-arc optical seed used before fitting.
• Differential Correction: residuals, weights, automatic differentiation, and damped least squares.
• Weighting And Debiasing Models: inverse-variance weighting and catalog-based correction of optical astrometry.
• Outlier Rejection: chi-square rejection and recovery around the differential-correction solve.

Where To Go Next

• Use Guides when you know the task and want the shortest reliable path.
• Use Workflows for end-to-end workflows such as ephemeris generation and orbit determination.
• Use the API when you need exact arguments and return fields.

Scope

These pages describe the model layer only. They cover time, state vectors, reference frames, Keplerian elements, batch shapes, Earth orientation data, observer sites, terrestrial geometry, light-time models, photocenter correction, dynamical models, propagation, ephemeris products, observation weighting, debiasing, differential correction, and outlier rejection. They do not replace the API pages or full workflows.