struct

Kozai::Observer

A ground station: somewhere on the Earth that wants to know where to point.

Positions are WGS-84 geodetic, the same thing a GPS receiver or a map will give you. The earth-fixed position is computed once at construction, because a pass search evaluates look angles thousands of times against a station that has not moved.

station = Kozai::Observer.new(52.3676, 4.9041, 0.005, name: "Amsterdam")
angles = station.look_angles(satellite, Time.utc)
puts "az #{angles.azimuth.round(1)} el #{angles.elevation.round(1)}"

Constructors

new(latitude : Float64, longitude : Float64, altitude_km : Float64 = 0.0, name : String | Nil = nil, minimum_elevation : Float64 = 0.0, refraction : Bool = false)

Creates a station.

latitude is positive north, longitude positive east, altitude_km is height above the WGS-84 ellipsoid.

Raises ArgumentError if the coordinates are not on the Earth.

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Instance methods

altitude_km

Height above the ellipsoid in kilometres.

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ecef

Station position in the earth-fixed frame, kilometres.

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latitude

Latitude in degrees, positive north.

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longitude

Longitude in degrees, positive east.

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look_angles(position_teme : Vector3, velocity_teme : Vector3, gmst : Float64) : Frames::LookAngles

Look angles to a TEME position and velocity at a given sidereal time.

This is the form the pass search uses: it already has the sidereal time in hand and does not want it recomputed per call.

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look_angles(state : SGP4::State, time : Time) : Frames::LookAngles

Look angles to a propagated state at time.

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look_angles(satellite : SGP4::Satellite, time : Time) : Frames::LookAngles | Nil

Look angles to satellite at time.

Returns nil if the propagator could not produce a position — a decayed satellite, or elements too degenerate to use.

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minimum_elevation

Elevation in degrees below which a pass does not count.

Zero is the geometric horizon. Real stations rarely see anything useful below a few degrees, because of terrain, buildings and the extra atmosphere a low signal has to cross.

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name

Station name, for display.

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position

Geodetic position of the station.

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refraction?

Whether reported elevations are apparent rather than geometric.

NOTE: off by default, and that is a decision rather than an omission. Refraction lifts an object near the horizon by 0.57 degrees, which is the largest single effect this software does not apply — larger than everything in the README's simplification list put together. Three reasons it is nonetheless not the default:

It is unpredictable. The bend depends on the pressure, temperature and humidity along the path, none of which a prediction made three days ahead can know. The 0.57 figure is a standard atmosphere, and a real one departs from it by a useful fraction.
It is largest exactly where a station cannot use it. At the horizon terrain, buildings and trees decide what you hear; by ten degrees, where a station starts working, refraction is 0.09 degrees, and an antenna beamwidth is tens.
Geometric elevation is the unambiguous quantity. Two tools that both apply refraction still disagree unless they assume the same atmosphere; two that report geometry agree exactly.

Turn it on to compare against tools that apply it — predict, Gpredict and Heavens-Above all do — or to squeeze the last of a horizon pass.

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to_s(io : IO) : Nil

Same as #inspect(io).

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visible?(satellite : SGP4::Satellite, time : Time) : Bool

Whether satellite is above #minimum_elevation at time.

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