Plan contacts around real constraints

Earth blocks the direct path when a spacecraft is below a station’s horizon. Above it, usable reception still depends on local obstructions, antenna pointing, radio compatibility and signal quality. Near-equatorial geography can be useful for some trajectories, but it does not provide continuous access to every orbit.

A separate example at the same site makes the local environment tangible. ESA’s navigation office reports that construction and growing trees obstructed the original MALI navigation receiver’s sky view. The replacement MAL 2 monument was installed at the edge of the site in 2008. This was a GNSS receiving installation, not the MAL-X tracking dish; the two should not be treated as interchangeable equipment.

Consider an invented twelve-minute pass. Acquiring the signal takes two minutes, and a required reconfiguration occupies another minute. If these are separate, non-overlapping interruptions, nine minutes remain for useful transfer. At a steady useful rate of 0.5 megabits per second, the budget is 9 times 60 times 0.5, or 270 megabits.

This is a simplified teaching budget, not a Malindi operating specification. Real planning must account for rate changes, margins and losses. The important habit is to define which time and which rate you are multiplying. A quoted interval for visibility is not automatically the interval carrying usable data.

Count usable contact time

Imagine one tracking terminal that can point to only one of two different spacecraft at a time. Mission A requests 10:00–10:08, and mission B requests 10:07–10:15. Their requested windows overlap for one minute. The schedule cannot promise both full contacts unless the system or the commitments change.

A scheduler can consider another compatible terminal, a different pass, shorter accepted contacts or explicit priorities. Each choice must respect the mission requirements. Adding the lengths of two requests without checking their overlap would hide the conflict rather than solve it.