Against the background of a growing demand on controlled satellite formations, a demonstration mission is proposed to gain experience for upcoming ESA projects in the next decade.
Gemini (GPS-based Orbit Estimation and Laser Metrology for Intersatellite Navigation) is a technology mission, proposed by DLR’s German Space Operations Center (GSOC), Astrium GmbH, and Vectronic Aerospace GmbH. aiming The major Gemini mission objective is the controlled establishment of a satellite formation in a low-Earth orbit. To that end, advanced in-orbit technologies will be demonstrated based on laser metrology, as well as innovative GPS-based approaches to relative navigation. As part of its technological objectives, the Gemini formation control will entirely be based on an autonomous orbit control approach. Secondary mission objectives concern the separation concept from the launcher, the drift stop and the development of a controlled formation acquisition strategy. As a technology demonstration mission, emphasis is given to an independent verification of the relative distance by means of a laser radar sensor. To allow a formation flying demonstration for a wide range of applications, the technologies for the control of the relative distance cover both the regime of close and wide formations ranging from several hundreds of meters up to 100 km. In contrast to the relaxed orbit control requirements of nowadays formations, Gemini aims at a relative position keeping of several cm to several meters, that is expected to be of significance for many of the upcoming formation flying missions and, in addition, paves the way for even more advanced requirements, such as for SMART 2. To achieve that level of control accuracy, the Gemini sensors have to provide relative position measurements in the range of millimeters or better, that may not be achievable solely using a spaceborne GPS receiver.
Formation Flying Concept
Following the separation of the spacecraft from the launcher, the state vectors of both satellites will first be determined using conventional radiometric angle tracking data in a ground-based orbit determination scenario. After the GPS receivers have been switched on, the GPS position fixes determined on-board are transmitted through telemetry to the ground and an orbit determination of the two spacecraft can be achieved at the meter level. Within that phase, the checkout and verification of the inter-satellite communication link and the onboard navigation algorithm will be conducted in a monitoring mode, to ensure proper operations of the formation flying control system. Depending on the launcher separation mechanism, the formation acquisition phase will stop the resulting drift of the satellites by the on-board thruster system of both satellites based on ground control commands. Once the formation acquisition phase has been completed, the initial formation configuration will have been achieved with a typical separation distance of 100 km.
Once the formation has been established, the autonomous formation control concept handles both narrow formation configurations of 1 km or less as well as wide configurations of up to 100 km. In the latter regime, the on-board orbit determination is based on single raw GPS pseudo-range measurements or derived GPS position fixes. Here, a relative position measurement accuracy at the meter-level is expected with an associated relative orbit control accuracy of better than 10 m. At distances of down to one kilometer, the data exchanged through the inter-satellite link will comprise GPS carrier-phase measurements. This allows to determine the relative spacecraft positions at the centimeter to decimeter level and significantly benefits from the cancellation of common error sources in the GPS carrier phase measurements. At even smaller separations, the orbit control will essentially rely on a laser interferometer, augmented with GPS differential carrier phase measurements. The laser interferometer then provides highly precise range rates, corresponding to a ranging accuracy of about 10 micro-meter and allows for a formation orbit control at the cm-level.
To allow an independent verification of the GPS and laser-metrology measurements, an onboard pulsed-laser radar is part of the Gemini payload. Within the mission operations phase, the different formation flying scenarios are demonstrated, that depend on relative spacecraft distance, the controlling requirements, and the adopted relative navigation algorithm. Within each scenario, an autonomous keeping of the formation within the specified control bounds is performed to demonstrate the high level of automation achievable in formation flying.
Based on the Miniflex satellite concept, a design of the Gemini spacecraft has been developed, that assumes identical spacecraft for redundancy purposes as well as for a cost-efficient implementation. The spacecraft structure is of cubic geometry with a characteristic axis length of 60 cm. Electrical power is provided by three solar panels attached to the structure during launch, two of them being released and unfolded after separation from the launcher. The spacecraft bus itself is built up of three different compartments: the payload segment, the electronics segment and the service segment.
Gill E., Steckling M, Butz P.;Gemini: A Milestone towards Autonomous Formation Flying;ESA Workshop on On-Board Autonomy, October 17-19, ESTEC, Noordwijk (2001).
Gotsmann M, Steckling M., Gill E.;Miniflex satellite concept for precursor and commercial Missions;IAA-B3-1501; 3rd IAA Symposium on Small Satellites for Earth Observation,April 2-6 2001, Berlin; ISBN 3-98685-566-2, 423-427 (2001).