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| title | tags | summary |
|---|---|---|
| Concept overview | overview, scope, how-to-read | What this documentation covers, how it is organised, and how to read a concept page that describes more than one implementation. |
Concept overview
This documentation describes spacecraft, orbits, and the measurement and communication around them. It is a teaching resource: it explains what the quantities are, what units they carry, how they are computed, and where each method stops being valid. It is not a manual for one library, and a page here usually describes a family of approaches rather than a single one.
What it covers
The subject is a satellite from launch to link closure:
- Foundations. Units, dimensions, and the numerical behaviour of a computed number.
- Time and geometry. Time scales, leap seconds, calendars, reference frames, the Earth rotation angle, and the reference ellipsoid.
- Orbit. Orbital elements and their formats, the perturbation sources that move an orbit, the models that propagate it, and the manoeuvres that change it.
- Radio and propagation. How motion changes the received frequency, what makes a link close, what the atmosphere does, and what interference does.
- Constellation and network. How satellites are arranged, when one can be used, and how a session survives the satellite handing it over.
- Operations. The measurement and control chain, and the planning that allocates fuel, contact, and safety.
- Simulation. What a simulation can and cannot establish.
How it is organised
The sections follow the layers a spacecraft system is naturally built in, from the arithmetic at the bottom to the network at the top. The ordering is a reading order, not a dependency: a page on link budgets assumes the orbit and the atmosphere, not the other way round.
Each page follows the same shape, so a reader knows where to look:
- Definition of the quantity or the model.
- Mathematics, with the symbols defined.
- Unit conventions, including the ones that differ between fields.
- Applicability, where the method stops being valid.
- Standards, naming the public source the definition follows.
- See also, linking the concepts that depend on this one.
A cross reference names another page; for example the page on time-scales is reached from here, and the page on maneuvers refers back to it.
How to read a page
Two things are worth knowing in advance.
A page names more than one implementation wherever more than one is used, and says which is which. Where a specific library is relevant, it is named; where it is not, the page is about the problem rather than about a product.
The unit conventions section is not housekeeping. A number without its unit and its reference is not a physical quantity, and a large part of the practical errors in this field are unit errors rather than concept errors.
Where the implementation is
The pages above describe the field. A separate section describes what one particular library implements of it, including how it is organised and where its algorithms come from. That separation is deliberate: the concepts stand on their own, and the implementation is one answer to them rather than their definition.
The online tools exist for the same reason. A reader who has just read about a propagation model can run it, which is a stronger check than reading.
See also
- units-and-dimensions for the convention that runs through every page
- numerical-methods for what "computed" means on these pages