The sub-satellite point is the spot on Earth's surface directly beneath a satellite, where a line from the satellite to Earth's centre meets the ground.
Draw an imaginary line from a satellite straight down to the centre of Earth. Where that line crosses the surface is the sub-satellite point. If you stood exactly there, the satellite would be straight overhead.
When the ISS tracker shows the station's latitude and longitude, it is giving the sub-satellite point. The altitude is the height of the station above that point.
The marker on the map is drawn at the sub-satellite point too. As the satellite moves, the point slides across the map. The line it leaves behind is the ground track.
From its height, a satellite can see a circle of Earth around its sub-satellite point. The higher it is, the bigger the circle.
So a geostationary satellite sees a far bigger slice of Earth at any moment than the ISS, which sees only a patch at a time.
For a geostationary satellite, the orbit is circular and lies in the equator's plane, and the period matches Earth's spin. Its sub-satellite point therefore stays put at one spot on the equator. A dish pointed at it never needs to move.
The map marker sits on the station's sub-satellite point, and the Latitude and Longitude cards give that point's coordinates. Open the ISS tracker
Finding it from a position vector. Orbit propagators such as SGP4 give a satellite's position in an Earth-centred inertial frame (x, y, z) that does not rotate with Earth. To get the sub-satellite point:
1. Geocentric latitude: φ = asin(z / r), with r = √(x² + y² + z²). 2. Right ascension: α = atan2(y, x). 3. Longitude: λ = α − θ_G, where θ_G is the Greenwich sidereal angle — how far Earth has turned since a reference time. Earth turns 360° in one sidereal day (23.9345 h = 86,164 s), or about 0.004178° per second. 4. Wrap λ into −180° … +180°.
The altitude is r − R (on a sphere). Real trackers use an ellipsoid for the last step, which is why φ differs slightly from the latitude a GPS receiver would give.
Coverage circle. A satellite at height h sees the surface out to the horizon. The Earth-centre half-angle λ₀ of that circle satisfies
cos λ₀ = R / (R + h)
In practice a ground station needs the satellite some degrees above the horizon, so useful coverage is smaller than these limits.
It is the point where the straight line joining a satellite to the centre of Earth meets Earth's surface — the point directly below the satellite.
The satellite orbits over the equator in a circle with a period equal to Earth's rotation period, so it stays above the same point.
cos λ₀ = 6,371/6,771 ≈ 0.941, so λ₀ ≈ 19.8°, a ground radius of roughly 2,200 km.