A screening volume is the imaginary box or ellipsoid drawn around a protected satellite for conjunction screening: any catalogued object predicted to enter it triggers a closer look and, if warranted, a warning. Volumes are deliberately generous — kilometres across — so nothing risky slips through unexamined.
Understanding Screening Volume
Two-stage screening: cheap filter, expensive maths
Screening the whole catalogue against itself is a combinatorial monster — tens of thousands of objects yield hundreds of millions of pairs, and each pair must be checked across a multi-day window. The screening volume makes the problem tractable by spending almost nothing per pair: fast filters first discard pairs whose orbits can never intersect (their altitude bands don't overlap), then geometric checks find candidate volume entries, and only the survivors — a tiny fraction — receive full probabilistic treatment with covariances at the predicted TCA. The architecture is a funnel: each stage is more expensive and more accurate than the last, and the screening volume defines the mouth of it.
Who screens, and against what
The US military screens the public catalogue continuously and warns operators worldwide; civil space-traffic pilots and commercial SSA services run parallel screens; and large constellation operators screen their own fleets against both the catalogue and each other's shared ephemerides. Protected volumes are also a human-spaceflight staple: crewed stations fly with special, larger screening perimeters, and any predicted intrusion triggers conservative manoeuvre planning well before the risk is confirmed — with crewed vehicles, the volume errs firmly on the side of dodging.