astronomy

Orbital pass

Orbital pass is a astronomy topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Orbital pass rather than just read about it. In short: An orbital pass (or simply pass) is the period in which a spacecraft is above the local horizon, and thus available for line-of-sight communication with a given ground station, receiver, or relay satellite, or for visual sighting. The beginning of a pass is termed acquisition of signal (AOS); the end of a pass is termed loss of signal (LOS).

Key takeaways

Orbital pass belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Orbital pass to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Orbital pass from memory before moving on to harder problems.

Reference excerpt

An orbital pass (or simply pass) is the period in which a spacecraft is above the local horizon, and thus available for line-of-sight communication with a given ground station, receiver, or relay satellite, or for visual sighting. The beginning of a pass is termed acquisition of signal (AOS); the end of a pass is termed loss of signal (LOS). The point at which a spacecraft comes closest to a ground observer is the time of closest approach (TCA).

Timing and duration The timing and duration of passes depends on the characteristics of the orbit a satellite occupies, as well as the ground topography and any occulting objects on the ground (such as buildings), or in space (for planetary probes, or for spacecraft using relay satellites). The longest duration ground pass will be experienced by an observer directly on the ground track of the satellite. Path loss is greatest toward the start and end of a ground pass, as is Doppler shifting for Earth-orbiting satellites. Satellites in geosynchronous orbit may be continuously visible from a single ground station, whereas satellites in low Earth orbit only offer short-duration ground passes (although longer contacts may be made via relay satellite networks such as TDRSS). Satellite constellations, such as those of satellite navigation systems, may be designed so that a minimum subset of the constellation is always visible from any point on the Earth, thereby providing continuous coverage.

Prediction and visibility

A number of web-based and mobile applications produce predictions of passes for known satellites. In order to be observed with the naked eye, a spacecraft must reflect sunlight towards the observer; thus, naked-eye observations are generally restricted to twilight hours, during which the spacecraft is in sunlight but the observer is not. A satellite flare occurs when sunlight is reflected by flat surfaces on the spacecraft. The International Space Station, the largest artificial satellite of Earth, has a maximum apparent magnitude of –5.9, brighter than the planet Venus.

See also Ground track, the path on the surface of the Earth directly below a satellite Satellite revisit period, the time elapsed between observations of the same point on Earth by a satellite Satellite watching, as a hobby

References

Illustrations

Worked examples

Example 1 — a first encounter with Orbital pass

Start with the simplest possible case. Write down what Orbital pass claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In astronomy, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Orbital pass before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Orbital pass ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Orbital pass

In research: Orbital pass appears in astronomy research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Orbital pass in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Orbital pass is common in secondary-school and first-year university syllabi. It links to neighbouring topics Astrodynamics, Spacecraft communication, so understanding it makes those chapters shorter.
In everyday life: Look for Orbital pass outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Orbital pass in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Orbital pass means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Orbital pass out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Orbital pass in simple terms?

Why does Orbital pass matter?

Because it connects several astronomy ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Orbital pass?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Orbital pass.