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