98 lines
3.9 KiB
Markdown
98 lines
3.9 KiB
Markdown
---
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title: Visibility, access and obstruction
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tags: visibility, access, elevation, horizon, obstruction, availability
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summary: The difference between a satellite being overhead and a user being able to use it, and what the gap between them is called.
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---
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# Visibility, access and obstruction
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A satellite in view is not a satellite in use. The distinction is the
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difference between coverage and availability, and almost every disappointing
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service is explained by it.
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## Definition
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**Visibility** is the geometric condition: the satellite is above the local
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horizon. **Access** is the condition that includes the link: the elevation is
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high enough for the link to close, the pointing can be resolved, and the
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propagation is acceptable. **Availability** is the fraction of the time an
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access is available to a particular user.
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**Obstruction** is anything in the way: terrain, a building, another vehicle,
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and in the worst case the payload of a satellite in a higher orbit passing
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across the line of sight.
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## Mathematics
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### The geometric pass
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A satellite in a circular orbit at semi-major axis `a` passes a latitude
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`phi` with a maximum elevation `e_max` that follows from the geometry,
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```text
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e_max such that the elevation is arctan of the perpendicular
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distance from the observer to the orbit plane, divided
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by the distance along the line of sight
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```
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with two limits worth naming. At the pole a satellite in an inclined orbit
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reaches only `90 - inclination` degrees of elevation at best, so a
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high-inclination constellation has poor coverage at high latitude from any
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single satellite, and relies on many satellites rather than on high ones.
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At the equator a satellite in an equatorial orbit passes directly overhead and
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the elevation approaches 90 degrees, which is the best case in the system.
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### The elevation mask
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The link budget requires a minimum elevation because below it the path
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length grows, the pointing becomes coarse, and the Doppler rate becomes
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large enough that tracking is harder. The mask is a design parameter
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rather than a physical one, and choosing a lower mask increases coverage and
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degrades the link on every pass, which is a trade rather than a free
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choice.
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### Obstruction and its statistics
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Terrestrial obstruction is binary for a ground user, so the relevant figure
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is a **probability of availability** rather than an average. For a satellite
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obstruction, the relevant figure is the probability that an intersection
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occurs during an access, which depends on the relative motion and on the
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angular separation of the two orbits.
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## Unit conventions
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- Elevation and azimuth in degrees when describing a pass, radians in any
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computation.
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- Access time in seconds; availability as a fraction or as a percentage.
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- Angles of separation in degrees.
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## Applicability
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The gap between coverage and availability is large and is usually the whole
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story of a service complaint. A constellation may have 100 percent
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geometric coverage over a country and a service that is available for 96
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percent of the time, and the remaining four percent is what a user
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remembers.
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The obstruction term is what makes the ground segment matter. The same
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constellation with a different set of ground station sites has a different
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availability, and no amount of satellites moves it, because the obstruction
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is local.
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A low elevation mask raises the headline coverage number and lowers the
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per-pass link margin at the same time. Those two effects are usually not
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reported together, which is why a coverage figure without a mask is not
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interpretable.
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## Standards
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- Elevation mask conventions and availability reporting: not standardised;
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the relevant practice is the regulatory service definition in the
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licensing.
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## See also
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- [[constellations]] for the coverage figure this is measured against
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- [[link-budget]] for why the mask is not zero
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- [[handover-and-routing]] for what happens between accesses |