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title	tags	summary
Visibility, access and obstruction	visibility, access, elevation, horizon, obstruction, availability	The difference between a satellite being overhead and a user being able to use it, and what the gap between them is called.

Visibility, access and obstruction

A satellite in view is not a satellite in use. The distinction is the difference between coverage and availability, and almost every disappointing service is explained by it.

Definition

Visibility is the geometric condition: the satellite is above the local horizon. Access is the condition that includes the link: the elevation is high enough for the link to close, the pointing can be resolved, and the propagation is acceptable. Availability is the fraction of the time an access is available to a particular user.

Obstruction is anything in the way: terrain, a building, another vehicle, and in the worst case the payload of a satellite in a higher orbit passing across the line of sight.

Mathematics

The geometric pass

A satellite in a circular orbit at semi-major axis a passes a latitude phi with a maximum elevation e_max that follows from the geometry,

e_max such that the elevation is arctan of the perpendicular
        distance from the observer to the orbit plane, divided
        by the distance along the line of sight

with two limits worth naming. At the pole a satellite in an inclined orbit reaches only 90 - inclination degrees of elevation at best, so a high-inclination constellation has poor coverage at high latitude from any single satellite, and relies on many satellites rather than on high ones.

At the equator a satellite in an equatorial orbit passes directly overhead and the elevation approaches 90 degrees, which is the best case in the system.

The elevation mask

The link budget requires a minimum elevation because below it the path length grows, the pointing becomes coarse, and the Doppler rate becomes large enough that tracking is harder. The mask is a design parameter rather than a physical one, and choosing a lower mask increases coverage and degrades the link on every pass, which is a trade rather than a free choice.

Obstruction and its statistics

Terrestrial obstruction is binary for a ground user, so the relevant figure is a probability of availability rather than an average. For a satellite obstruction, the relevant figure is the probability that an intersection occurs during an access, which depends on the relative motion and on the angular separation of the two orbits.

Unit conventions

Elevation and azimuth in degrees when describing a pass, radians in any computation.
Access time in seconds; availability as a fraction or as a percentage.
Angles of separation in degrees.

Applicability

The gap between coverage and availability is large and is usually the whole story of a service complaint. A constellation may have 100 percent geometric coverage over a country and a service that is available for 96 percent of the time, and the remaining four percent is what a user remembers.

The obstruction term is what makes the ground segment matter. The same constellation with a different set of ground station sites has a different availability, and no amount of satellites moves it, because the obstruction is local.

A low elevation mask raises the headline coverage number and lowers the per-pass link margin at the same time. Those two effects are usually not reported together, which is why a coverage figure without a mask is not interpretable.

Standards

Elevation mask conventions and availability reporting: not standardised; the relevant practice is the regulatory service definition in the licensing.

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