High accuracy formation flight beyond GNSS support

We present a novel use of spacecraft navigation sensors to enable accurate formation flight, which in turn enables heretofore unachievable space mission performances.

Accurate spacecraft formations enable missions with accuracies, operations and robustness unachievable by even the largest space platforms. Moreover, since each spacecraft segment of the formation may be of moderate size and complexity, cost and flexibility advantages relative to use of conventional large spacecraft result.

With few exceptions, space mission performance may benefit from a formation flight configuration. However, to release this potential, the individual segments of the formation must be capable of or forming, maintaining, and if needed to reconfigure formations. The actual acquisition of a loose constellation formation may be achieved with GNSS support, after which the accurate formation may be formed using dedicated instrumentation on each space segment. 

GNSS support may not be available for many applications, e.g. those outside low Earth orbit, or when independence from GNSS is desired for other reasons. A robust, resilient and accurate formation flight sensor-suite must encompass ways to achieve detection and tracking of all spacecraft of the constellation during formation acquisition, maintenance, reconfiguration and dilution.

The key to realization of these objectives is thus to empower each spacecraft with the knowledge of the position and time of the other segments of the constellation. This task requires two novel uses of space navigation sensors, if to be realistic with respect to volume, mass, power and operations.  Where the GNSS-enabled constellations use huge resources on positioning and timing, a constellation of smaller satellites must obtain this information within the constellation. 

We present an instrument concept enabling the required local, autonomous, high accuracy position, attitude and timing of a group of satellites, developed for the ESA PROBA3 formation flight coronagraph. This instrument suite is based on optical, navigation type, cameras and multiple access inter-satellite timing units.

The operations and functionality of this instrument suite has been demonstrated and verified by the ASC star trackers onboard the NASA Juno spacecraft, where literally hundreds of thousands of objects were acquired and tracked, and later by similar instruments on other NASA and ESA missions. Close, accurate, formation flight was first realized by the same instruments on the Swedish PRISMA mission. Full performance of the sensor suite was first demonstrated by the instruments on the ESA PROBA3 mission.

We present the achievable range and accuracies of this novel use of space navigation sensors.