GNSS in Space Part 1 Formation Flying Radio Frequency Missions, Techniques, and Technology
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 the apogee) will be used to demonstrate and/or FF. The main demonstrations are objective is to validate the GNC system, formation flying for astronomical and as follows: relative position and pointing determi- scientific missions as well as to observe • GNC maneuvering experiments with nation systems (RF, and coarse and fine the solar corona. a high level of autonomy: autono- optical), and actuators. This mission During the perigee (portion of orbit mous formation flying, homing and will also validate formation acquisition, nearest to the Earth), the spacecraft will rendezvous, proximity operations, maneuvering, and collision avoidance revert to normal, gravitationally deter- and final approach and recede opera- algorithms as well as test different oper- mined orbits to reduce fuel consump- tion. The experiments are run mainly ational scenarios and formation flying tion, with the thrusters then being used by SSC with important contributions architectures. only for collision avoidance. The perigee from DLR. The second objective is to observe the pass will also demonstrate formation • GPS-based navigation system experi- solar corona, a luminous plasma ”atmo- flying configurations required for LEO ment from DLR, which will evaluate sphere” that surrounds the sun. The total earth observation missions. real-time differential GPS as a sensor light output from the solar corona is less for autonomous formation flying than one-millionth of that radiated by PROBA-3 Guidance & Control • evaluation of the VBS as a multirange, the disk of the sun. This enormous dis- The relative positions of the two space- range-tracking and RdV sensor parity in apparent brightness gives rise craft are determined by the S-band RF • demonstration flight test of the FFRF to the need to use an “occulter” to block metrology subsystem, which functions subsystem in open- and closed-loop light coming from the solar disk so as for separations between five meters and configuration. to be able to observe the corona better. eight kilometers, with an accuracy of Further, the mission also has some (See Figure 3.) centimeters. Increased accuracy for secondary goals mainly oriented toward PROBA-3 comprises two indepen- separations up to 500 meters will be
42 InsideGNSS november/december 2008 www.insidegnss.com Future FF Missions Overview A remarkably large number of forma- tion-flying and rendevouz missions are planned over the next 10 years or so by European and other nations or groups of nations. Tables 1–3 summarize these missions as well as the probability of FIGURE 4 The PROBA-3 occulter spacecraft FIGURE 5 The PROBA-3 coronagraph spacecraft their being realized by 2020. Table 4 provides a classification of the obtained using optical laser techniques spacecraft will employ a star tracker for FF missions with respect to the objectives having both coarse and fine sensors to absolute attitude determination. of the mission (space science, earth obser- refine the relative position measure- Concerning the observation of the vation, and technology demonstrations). ments. solar corona, the FF control system allows The combined system is expected to the use of a two-component space system Conclusion achieve a relative positioning accuracy with the external occulter on one space- GNSS constellations are the most practi- of about 100 micrometers over a sepa- craft (Figure 4) and the optical instru- cal way to perform RF formation flying ration range of 25–500 meters. Multiple ment on the other spacecraft Figure( 5) at in LEO, and autonomous two-way trans- thrusters, using either cold gas or ion approximately 100–150 meters apart. The mission of GNSS-like S-band signals is a technology, will be used to maintain the stability of the formation is linked to the better way to perform FFRF in HEO or required relative positions. requirement of keeping the optical pupil within the Lagrange points. PRISMA is a Both spacecraft will carry GPS receiv- plane in the shadow of the occulter. unique opportunity in Europe, both tech- ers to provide timing synchronization Lateral positioning accuracy is nically and programmatically, to validate and relative navigation during perigee about ±2.5millimeters and longitudinal under real conditions the basic feature of passage. GPS signals can be used at least is about ±250 millimeters. The absolute any non-LEO future FF mission—the 25,000 kilometers from Earth — and attitude of the satellite hosting the exter- RF-based autonomous metrology, using even more if carrier-to-noise spectral nal occulter is required to be about ±40 GPS-C/A-like signals and techniques. density (C/N0) threshold reduction tech- arcseconds. With the proposed PROBA- By mid-2009, an autonomous RFFF niques are used. So, the signals should be 3 arrangement, accurate measurements sensor shall be flying onboard the useful for at least one-third of the pro- will likely be possible from 1.05–3.2 PRISMA satellites. This sensor will posed orbit and possibly all of it. Each solar radii. use GPS-like signals in S-band. Later,
Mission Orbit Launch Date Number of Spacecraft Baseline Main Field Agency PRISMA LEO 2009 2 FF: 100m–10km RdV: FF/RdV technology SSC/DLR/CNES/CDTI 10km -> 0 demonstration TerraSAR-X and LEO 2007/09 2 150m–10km Earth’s digital model (DEM) DLR/Astrium TanDEM-X and SAR interferometry PROBA-3 HEO (24 h) 2012 2 (1 occulter and 1 observer) Demo: 25m–250m FF/RdV (solar corona) tech- ESA Corona: 100m–150m nology demonstration RdV: 10km -> 0 NanoForm* LEO 2013 5 (1 mother and 4 children) 200m EO (SAR) ASI SABRINA* LEO 2012 2 100s m–100s km EO (SAR) ASI Simbol-X HEO (4 days) 2013 2 (1 mirror and 1 detector) 20m X-ray telescope ASI/CNES Xeus L2 2019 2 (1 mirror and 1 detector) 35m X-ray telescope ESA Darwin L2 2020+ 4 (3 telescopes, 1 beam 10–1300m Space IR interferometer ESA combiner) PEGASE L2 2017 3 (2 telescopes, 1 beam 50m–500m Infrared interferometer CNES combiner) MAX HEO or L2 2015+ 2 (1 mirror and 1 detector) 80 (90m) Gamma-ray telescope CNES GRL L2 2020+ 2 (1 mirror and 1 detector) 500 Gamma-ray telescope ESA Romulus LEO 2020+ 4 in sequence 18m–21m–30m Military. Synth.aperture for CNES/ONERA high image resolution * NanoForm and SABRINA are missions in competition. Only one will be selected after phase A
TABLE 1. Overview of the European FF missions [Probability of mission launch in medium term (<2020) High; Medium; Low] www.insidegnss.com november/december 2008 InsideGNSS 43 working papers
Mission Orbit Launch Date Number of Spacecraft Baseline Main Field Agency SMART-OLEV GEO 2012 2 (OLEV & GEO S/C) 10s km -> 0 Commercial service for GEO life OSS (Sw) extension NextMarx Mars 2015 2 (600 kg target and 50 kg 3000km -> 0 Demonstration of autonomous ESA chaser) rendezvous MSR Mars 2020+ 2 (obiter and sample con- 3000km ->0 Mars exploration. Autonomous ESA tainer) long-range RdV LEO 2015+ 2 (CSTS-ISS) 1000km -> 0 Manned mission. RdV&D with ESA/RRSC/JAXA ISS. ATV/Soyuz type CSTS Moon 2020+ 5 1000km -> 0 Manned mission to the Moon. ESA/Russia/JAXA Assembly and docking in LEO
TABLE 2. Overview of the European rendezvous missions [Probability of mission launch in medium term (<2020) High; Medium; Low]
Mission Orbit Launch Date Number of S/Cs Baseline Main Field Agency MagCON HEO perigee:1RE 2020 30+ Mean inter-spacecraft Science. magnetosphere NASA (USA) apogee: 7–27 RE separation of ~2 RE MMS HEO perigee:1.2 2013 4 in tetrahedron 10 km at apogee Science. magnetosphere NASA RE apogee: 12 RE SPECS ? 2015 3 100m–1 km Sub-mm telescope NASA ALFA Retrograde 106 ? 16 Circular array 100km Imaging interferometry NASA km from the earth max diameter TPF L2 2015 4 or 5 (3 or 4 telescopes, 1 100m–1000m IR interferometry NASA beam combiner) New Millennium Earth-trailing ? 2 (1 combiner and 1 collector) 50m–1000m FF and technology demon- NASA heliocentric stration MAXIM L2? 2020 33 (mirror formation); 1 500km-2000 km X-ray telescope NASA detector CAN-X4/5 LEO 2008 2 (< 5 kg) < 5000 m FF and technology demon- CSA (Canada) stration JC2sat LEO 2 (20 kg) FF and technology demon- CSA/JAXA (Japan) stration
TABLE 3. Overview of the non-European FF missions [Probability of mission launch in medium term (<2020) High; Medium; Low] in 2012, the ESA PROBA-3 and CNES GP4020 chip from Zarlink Semicon- opment of the FFRF terminal signal Simbol-X spacecrafts will demonstrate ductors, Ottawa, Ontario, Canada. processing software the technology in scientific missions in The FFRF subsystem development HEO orbit. is currently in phase C/D, with Thales Additional Resources Alenia Space-F as the prime contrac- [1] Bourga, C., et al., “Autonomous Formation Fly- Manufacturers tor on both the subsystem and FFRF ing RF Ranging Subsystem,” Proceedings of ION PRISMA is a Swedish National Space terminal level. FFRF terminals incor- GNSS 2003, Portland, Oregon, USA, September Board (SNSB) mission, undertaken as porate components and software of the 2003 a multilateral project with additional TAS-F TOPSTAR 3000 spaceborne GPS [2] Cledassou, R., Ferrando, P., “Simbol-X: An Hard X-Ray Formation Flying Mission,” Focusing contributions from CNES, the Ger- receiver. Telescope in Nuclear Astrophysics Gamma Wave man DLR, and the Danish DTU. The In turn, TAS-F is relying on the fol- Workshop, Bonifacio, France, September 2005 prime contractor is the Swedish Space lowing subcontractors: [3] Garcia-Rodríguez, A., Formation Flight (FF) Corporation (SSC), responsible for • Thales Alenia Space España (TAS- Radio-Frequency (RF) Metrology. ESA/ESTEC design, integration, and operation of E, Spain) for development of the RF Technology Dossier, Issue 1.2, Noordwijk, the the space and ground segments, as well modules of the FFRF terminal (RF Netherlands July 2008 as implementation of in-orbit experi- front end, RF transmitter section, RF [4] Godet, J. et al., “Improving Attitude Determi- ments involving autonomous forma- receiver section) nation of Satellites,” Internationnal Workshop on tion flying, homing and rendezvous, • GMV (Spain) for development of Aerospace Applications of the GPS, Breckenridge, and three dimensional proximity opera- the navigation software, including Colorado, USA, February 2000 tions. It employs Phoenix GPS receivers implementation of PVT algorithms [5] Harr, J., et al., “The FFIORD Experiment: CNES’ developed by DLR that incorporates the • Thales Avionics (France) for devel- RF Metrology Validation and Formation Flying
44 InsideGNSS november/december 2008 www.insidegnss.com Demonstration on PRISMA,” 3rd International Formation Flying Symposium on Formation Flying, Missions and Technologies, Noordwijk, Netherlands, April 2008 Space Science Earth Observation Technology Demonstration [6] Issler, J., et al., “Lessons Learned from the Physics Astronomy Aperture Distributed Universities Agencies Use of GPS in Space; Application to the Orbital Synthesis Instruments Blimp TestBed, New use of GALILEO,” Proceedings of ION GNSS 2008, CAN-X4/5 Millenium, PROBA-3, September 2008 PRISMA, [7] Lestarquit, L., et al., “Autonomous Formation TS/TD-X Flying RF Sensor Development for the PRISMA Earth Optical Optical Splitted Mission,” Proceedings of ION GNSS 2006, Fort- Cluster, Crosscale interferometry interferometry Payloads Worth, Texas, USA, September 2006 MMS, MagCon Darwin, TPF TBD EO-1, [8] Persson, S. et al., PRISMA: An Autonomous Pegase, Labeyrie Cloudset Formation Flying Mission, Small Satellite Systems Solar Radio Distributed and Service Symposium, Chia Laguna, Sardinia, ASPICS/PROBA-3 Interferemetry Microwave Radars/ Italy, September 2006 SPECS, ALFA Sounders Romulus, [9] PROBA-3
46 InsideGNSS november/december 2008 www.insidegnss.com