WORKING PAPERS GNSS in Space Part 1 Formation Flying Radio Frequency Missions, Techniques, and Technology FIGURE 1 PRISMA satellites (SSC image) Using two or more small satellites can sometimes be better than one, especially when trying to create a large spaceborne instrument for scientific research or experiments.B ut coordinating the alignment of the components of such instruments on separate space vehicles requires highly accurate orientation and positioning. Carrier phase GNSS can provide such precision for spacecraft operating below the altitude of GPS satellites, and GNSS-like techniques can be employed for spacecraft operating in higher orbits. THOMAS GRELIER ormation flying (FF) creates large easiest way because most LEO satellites CENTRE NAtional d’ETUDES SPATIALES (CNES) spaceborne instruments by using are already provided with a GPS or ALBERTO GARCIA several smaller satellites in close GLONASS receiver for orbit and time EUROPEAN SPACE AGENCY (ESA) Fformation. The concept requires determination. very accurate relative positioning and Thanks to GNSS constellations, for- ERIC PÉRAGIN, LAURENT LEstARQUIT, JON HARR, orientation of the spacecrafts, the com- mation flying can be made at higher alti- DOMINIQUE SEGUELA , JEAN-LuC ISSLER plexity of which is largely outweighed tudes with a perigee of up to 25,000 kilo- CNES by the enormous benefit of the extended meters, if GNSS receivers equipped with instrument size compared to traditional low acquisition and tracking thresholds JEAN-BAPTIstE THEVENET, NICOLAS PERRIAULT, one-satellite configurations. are selected. Such receivers have tight CHRIstIAN MEHLEN, CHRIstOPHE ENSENAT, The easiest way to perform forma- coupling between the signal processing NICOLAS WILHELM, ANA-MARIA BADIOLA tion flying with relative attitude and and the onboard orbital Kalman filter MARTINEZ positioning in space is to use signals delivering pseudovelocity and pseudo- THALES ALENIA SPACE broadcast by GNSS satellites. Yet this range aiding to the open or closed delay PABLO COLMENAREJO ideal configuration, which could enable locked loops and phase locked loops. GMV a relative positioning of better than one Even in this case, however, the relative centimeter in certain cases, is limited to positioning accuracies can be degraded formation flying mission in low earth by up to a few meters, or even more orbit (LEO). sometimes, at a high-altitude orbital Such low-altitude operations can apogee. exploit the full visibility of the GNSS As a result of these GNSS accuracy constellations, which is needed to per- limitations, a dedicated formation fly- form the integer carrier phase ambiguity ing radio frequency (RF) technique is resolution required to achieve centime- needed that is accurate and equipped ter-level accuracies. Use of GNSS is the with omnidirectional features emerged 40 InsideGNSS NOVEMBER/DECEMBER 2008 www.insidegnss.com WORKING PAPERS for high and very high altitude orbital mission, and around 2012 it will be used Space Corporation (SSC) via the Kiruna missions. But the use of GNSS-like for PROBA-3, an ESA mission demon- ground station. signals and techniques appears to still strating FF capabilities and making The mission will consist of two be advantageous, because they allow scientific observations. In 2013, it will spacecraft, shown inFigure 1: the Main cost effective accurate measurements, be onboard Simbol-X, a multilateral spacecraft of 140 kilograms with full thanks to the widespread use of GNSS CNES mission implementing an X-ray three-axis reaction wheel–based atti- techniques, even for spacecraft naviga- telescope of two satellites. tude control and three-axis delta-V tion and synchronization. capability, and a second simplified The first part of this two-part column The PRISMA Mission Target spacecraft of 40 kilograms with will describe the PRISMA, PROBA-3, To validate FFRF in flight, CNES will coarse three-axis attitude control, and Simbol-X missions and provide an participate in the experimental PRISMA based on magnetometers, sun sensors, overview of other future FF missions. mission, a technology test bed for forma- and magnetic torquers. Various sen- Part 2 will discuss those missions’ posi- tion flying and rendezvous (RdV) of two sors, depending on the experiment and tioning, orientation, and metrological LEO microsatellites undertaken by the the satellite distance, will measure the requirements, focusing on the forma- Swedish National Space Board (SNSB). relative positions of the space vehicles, tion flying RF (FFRF) techniques and As the first European mission to dem- using the following: instrumentation. onstrate FF, PRISMA is an invaluable • Differential GPS. The DGPS system opportunity for CNES to acquire valu- provided by DLR (German Aero- Institutional Roles able FF experience as early as 2009. space Center) is the primary relative Mastery of formation flying is an area The French contribution to PRIS- positioning measurement system on of priority for the French Space Agency MA—Formation Flying In-Orbit Rang- PRISMA (made possible by the LEO (Centre National d’Etudes Spatiales or ing Demonstration (FFIORD) —consists nature of this mission). Experiments CNES) and the European Space Agency of implementing the FFRF subsystem on will mainly be carried out for inter- (ESA) and is driving the agencies’ long- the two PRISMA satellites as a passenger satellite distances greater than 30 term investment in the development of experiment. The FFRF procurement is a meters. The system is based on a fully associated technologies and skills in partnership between CNES and Spain’s redundant set of 12-channel, single- cooperation with other European part- Centre for the Development of Industrial frequency Phoenix GPS receivers ners. The goal of this continuous effort Technology (CDTI). developed by DLR and antennas on is to prepare for future FF missions of In addition to in-orbit validation of each spacecraft. which the CNES Simbol-X project and the sensor, FFIORD will include closed- • Visual-based sensor (VBS). Based on ESA’s PROBA-3, Darwin, and Xeus proj- loop operations with FFRF, thus allowing a star tracker used in many microsat- ects are representative examples. genuine autonomous formation flying ellite missions, this sensor provided Mainly within this European frame- scenarios and associated guidance, navi- by the Technical University of Den- work, CNES and ESA have collaborated gation and control (GNC) algorithms to mark (DTU) will be used to identify and contributed through R&D activities be tested in real-life conditions. the Target spacecraft as a nonstellar to the preliminary design and imple- The PRISMA satellites have arrived object at distances up to 500 kilo- mentation of a new sensor for coarse from Sweden to the test facilities in Tou- meters and to track Target down to metrology—the (FFRF) metrology sub- louse, France. Testing in the vaccuum- very close range, typically 10 meters system. This RF-based sensor, for which solar chamber took breadboard development was initiated place in late Novem- by ESA in 2001, is based partly on exist- ber 2008. Launch of ing spaceborne GPS technology. PRISMA is expected Proposed by CNES in an early ver- during the summer sion in 1991 for the Hermes space plane 2009 as a secondary project, the FFRF equipment was initial- payload into a sun- ly designed and developed by a private synchronous LEO manufacturer. ESA and CNES officially orbit of about 700 selected the FFRF equipment as a coarse kilometers. metrology sensor, FFRF being manda- Expected dura- tory for first-stage formation acquisition tion of the mission on all future European non-LEO forma- is approximately tion flying missions. eight months. All This FFRF generic technology devel- f light operations oped by ESA, CNES and CDTI will be will be controlled FIGURE 2 Main motion around target validated onboard the PRISMA Swedish by t he Swedish www.insidegnss.com NOVEMBER/DECEMBER 2008 InsideGNSS 41 WORKING PAPERS FIGURE 3 Illustration of PROBA-3 formation flying during a sequence of autonomously testing of new technologies appropriate dent, three-axis stabilized spacecrafts scheduled approach maneuvers. for future small-satellite missions. These flying close to each other with the abil- • The RF metrology subsystem. include 1) in-flight testing of high per- ity to accurately control the attitude Designed to handle first stage omni- formance green propellant (HPGP) and and separation of the two spacecraft in directional RF metrology, the FFRF cold gas microthruster propulsion sys- a closed loop. The spacecrafts will fly in a provided by CNES will mainly func- tems, 2) onboard software development high earth orbit (HEO) divided between tion at intersatellite distances from 3 with MATLAB/Simulink and autocode periods of accurate formation flying, meters to 30 kilometers. generation, and 3) validation of new when payload observations will be pos- ground support equipment with multi- sible, and periods of free flight. PRISMA Goals satellite support capability. The length of the formation control The primary goals for PRISMA are tech- period will result from a trade-off analy- nology demonstrations and maneuver PROBA-3: The Next Step sis involving the amount of fuel needed experiments (see Figure 2) examining The ESA PROBA-3 mission is foremost to maintain the orbits when in forma- GNC and sensor technology for a fam- an FF demonstration mission and sec- tion. The formation control part (around ily of future missions that will use RdV ondly am FF scientific mission. The first