  Formation-flying studies to date have required continuous and minute
corrections of the orbital elements and attitudes of the spacecraft.This
increases the complexity, and associated risk, of controlling the formation,
which often makes formation-flying studies infeasible for technological and
economic reasons. Passive formation-flying is a novel space-flight concept,
which offers a remedy to those problems. Spacecraft in a passive formation are
allowed to drift and rotate slowly, but by using advanced metrology and
statistical modelling methods, their relative positions, velocities, and
orientations are determined with very high accuracy. The metrology data is used
directly by the payloads to compensate for spacecraft motions in software. The
normally very stringent spacecraft control requirements are thereby relaxed,
which significantly reduces mission complexity and cost. Space-borne
low-frequency radio astronomy has been identified as a key science application
for a conceptual pathfinder mission using this novel approach. The mission,
called FIRST (Formation-flying sub-Ionospheric Radio astronomy Science and
Technology) Explorer, is currently under study by the European Space Agency
(ESA). Its objective is to demonstrate passive formation-flying and at the same
time perform unique world class science with a very high serendipity factor, by
opening a new frequency window to astronomy.
