Enabling Spacecraft Formation Flying through Spaceborne GPSand Enhanced Automation Technologies==

Frank H. Bauer*, Kate Hartman*, Jonathan P. How**,John Bristow*, David Weidow*, and Franz Busse**

*NASA Goddard Space Flight CenterGreenbelt, Maryland, 20771

301-286-8496, frank.bauer@gsfc.nasa.gov

**Department of Aeronautics and AstronauticsStanford University, Stanford, CA 94035

jhow@leland.stanford.edu

= Presented at the 1999 ION-GPS Conference, Nashville, TN, September 15, 1999.

Biographies

Frank H. Bauer is the Chief of the Guidance, Navigationand Control (GN&C) Center at NASA’s Goddard SpaceFlight Center. . He received his Bachelor's and Master'sdegree in Aeronautics and Astronautics from PurdueUniversity. Mr. Bauer's technical research interestsinclude spaceborne applications of the Global PositioningSystem (GPS) and space vehicle formation flying

Kate Hartman is the Distributed Spacecraft Thrust AreaManager for NASA’s Cross Enterprise TechnologyDevelopment Program. She received a Bachelor of Artsdegree in Astronomy from the University of Pennsylvania,and a Masters of Science degree in Applied Mathematicsfrom John Hopkins University. Ms. Hartman’s primarytechnical interests are mission design, trajectorydetermination and control, Kalman filtering techniques,onboard navigation systems, and formation control ofmultiple cooperating satellites.

Jonathan P. How is an Assistant Professor in theDepartment of Aeronautics and Astronautics at StanfordUniversity. He received his B.A.Sc (1987) from theUniversity of Toronto, and SM (1990) and Ph.D. (1993)from MIT, both in the Dept. of Aeronautics andAstronautics. Mr. How's technical research interestsinclude sensing and control of formation flying vehiclesand spaceborne applications of GPS.

John Bristow is the Guidance, Navigation and ControlCenter (GNCC) Formation Flying and Virtual PlatformTechnology Development Program manager. He receivedhis Bachelor's degree in Computer Science fromNortheastern University. Mr. Bristow’s technical researchinterests include mission design, autonomous spacecraftcontrol and space vehicle formation flying.

Franz Busse is a Ph.D. candidate in the Dept. ofAeronautics and Astronautics at Stanford University. He

received his SB (1995) from MIT and MS (1998) fromStanford University.

Abstract

Formation Flying is quickly revolutionizing the way thespace community conducts autonomous science missionsaround the Earth and in space. This technologicalrevolution will provide new, innovative ways for thiscommunity to gather scientific information, share thisinformation between space vehicles and the ground, andexpedite the Human exploration of space. Once fullymatured, this technology will result in swarms of spacevehicles flying as a virtual platform and gatheringsignificantly more and better science data than is possibletoday. Formation flying will be enabled through thedevelopment and deployment of spaceborne differentialGlobal Positioning System (GPS) technology and throughinnovative spacecraft autonomy techniques. This paperprovides an overview of the current status ofNASA/DoD/Industry/University partnership to bringFormation Flying technology to the forefront as quickly aspossible, the hurdles that need to be overcome to achievethe formation flying vision, and the team's approach totransfer this technology to space. It will also describesome of the formation flying testbeds, such as Orion, thatare being developed to demonstrate and validate theseinnovative GPS sensing and formation controltechnologies.

Introduction

Earth and Space scientists are just beginning tounderstand the full potential of space vehicle formationflying. In a few short years, this technology has gonefrom something that the space community considered anoddity and a very high risk to one that has been fullyembraced by Earth and space scientists around the world.Just prior to the selection of the New Millennium ProgramEarth Orbiter-1 (EO-1) mission in 1996—the first

Figure 1: Mars Aerobots

autonomous formation flying Earth science mission—therewere only one or two formation flying concepts beingconsidered by NASA. This has changed dramatically.Table 1 depicts the Earth and space mission sets currentlybeing considered by NASA and the Air Force ResearchLaboratory (AFRL). Clearly, the substantial benefitsgleaned from obtaining simultaneous measurements fromnumerous formation flying vehicles has resulted in avirtual explosion of future Earth and space sciencemission sets.

A simple analogy can be used to illustrate the fundamentalchange that occurs in Earth and space science whenformation flying is employed. The analogy used is theobservation of a football game from two differentperspectives. The Earth and space science measurementsperformed today are comparable to viewing a footballgame through the perspective provided by still imagestaken at a very slow rate. Using formation flyingtechnology, the visual perspective and understanding willbe radically changed. In the football example, this wouldbe analogous to watching a video of the game using manyfixed and/or movable cameras. For the scientists, this newperspective should provide a unique “birds eye view” of theEarth and universe.

For example, the current complement of Earth Sciencemissions perform somewhat infrequent measurements oftargeted areas of the Earth using very large, expensivespacecraft platforms (e.g. Landsat-7 which takes 16 daysto retrace its ground swath). In the future, swarms ofinexpensive miniature space vehicles, flying in formation,will replace these expensive spacecraft platforms. Thesespacecraft formations will provide continuousmeasurements of the processes and events effecting theEarth. Space science will also be significantly impactedby formation flying technology. For example, the spacescience community’s ability to understand the events andprocesses that occur between the Sun and the Earth (the socalled Sun-Earth connection) is limited to a just a fewspacecraft in various Earth and Heliocentric orbits. Asignificant improvement in the understanding of thedynamics of the magnetosphere can be accomplished ifthese spacecraft were replaced by an armada of miniaturescience probes flying around the Earth and Sun in a looseformation. Significant improvements in space-basedinterferometery can be accomplished by flying severalspacecraft in formation, increasing the number ofinstruments comprising the system & eliminating therestrictions imposed by the use of physical structures toestablish maintain, and control instrument separation andstability. As shown, the benefits of formation flyingpropagate throughout the entire Space Science Enterprise—Origins, Sun-Earth Connection, Structure and Evolutionof the Universe, and Solar System Exploration.

Formation flying will also change the way NASA and thespace community conducts the Human Exploration andDevelopment of Space. In the future, autonomous SpaceShuttle and Space Station rendezvous and docking usingGPS-based formation flying will become commonplace.Very low cost scientific payloads, such as Spartan, will bedeployed from Space Station, fly in formation, andautonomously return for eventual retrieval. In the future,aerobots, autonomous balloon systems flying in formationusing GPS-like navigation sensing will be conductingscientific investigations around the planet Mars (figure 1).

It is clear that formation flying will result in a fundamentalchange in the way science measurements will beconducted from space. The mission sets already on the

drawing boards atNASA and the DoDspan the wholespectrum offormation flyingperformancerequirements, from2-3 spacecraft in aloose formation totens or hundreds oftightly controlledspacecraft flying inan armada. Some

missions, such as the Mag Constellation mission, figure 2,will require very loose (1-100 km) formation knowledgeand control. On the other end of the spectrum, spacescience interferometry missions, planet finders (figure 3)and relativity missions will require micro-meter and, insome cases, pico-meter knowledge and control. Thesediffering mission sets require an entire spectrum ofsensing, controlling and actuation capabilities to satisfythese varied requirements challenges.

Fig. 2: Mag Constellation Fig. 3: Planet Finder

Developing the technology to produce virtual platformswill be a long-range challenge. Similar to GPS technology[1], there are several technological “stairsteps” that must beovercome to go from autonomous navigation andconstellation control to one and two-way formation flyingand finally to virtual platforms.

Table 1: Representative List of Satellite Missions Utilizing Formation Flying Techniques

ProjectedLaunch Year

Mission Name Mission Type

99 New Millennium Program (NMP) EO-1 Earth Science01 University Nanosats/Air Force Research Laboratory Technology Demonstrator01 Gravity Recovery and Climate Recovery (GRACE) Earth Science01 MightySat/Orion/Orion Jr. Technology Demonstrator02 Auroral Multiscale Mission (AMM)/APL (MIDEX) Space Science/SEC02 Lightweight Synthetic Aperture Radar (LightSAR) Earth Science03 New Millennium Program (NMP) ST-3 Space Science/ASO03 New Millennium Program (NMP) ST-5 Space Science03 Techsat-21/AFRL Technology Demo05 Magnetospheric Multiscale (MMS) Space Science/SEC05 Space Interferometry Mission (SIM) Space Science/ASO07 Global Precipitation Mission (EOS-9) Earth Science07 Geospace Electrodynamic Connections (GEC) Space Science/SEC08 Constellation-X Space Science/SEU08 Magnetospheric Constellation (MC) Space Science/SEC08 Laser Interferometric Space Antenna (LISA) Space Science/SEU09 DARWIN Space Infrared Interferometer/European Space Agency Space Science11 Terrestrial Planet Finder Space Science/ASO

Astronomical Low Frequency Array (ALFA)/Explorers Space Science05+ Leonardo (GSFC) Earth Science05+ Soil Moisture and Ocean Salinity Observing Mission (EX-4) Earth Science05+ Time-Dependent Gravity Field Mapping Mission (EX-5) Earth Science05+ Vegetation Recovery Mission (EX-6) Earth Science05+ Cold Land Processes Research Mission (EX-7) Earth Science05++ Submillimeter Probe of the Evolution of Cosmic Structure (SPECS) Space Science/SEU15+ MAXIM X-ray Interferometry Mission Space Science/SEU15+ Solar Flotilla, IHC, OHRM, OHRI, ITM, IMC, DSB Con Space Science/SEC15+ NASA Goddard Space Flight Center Earth Sciences Vision Earth Science15+ NASA Institute of Advanced Concepts/Very Large Optics for the Study Space Science15+ NASA Institute of Advanced Concepts /Ultra-high Throughput X-Ray Space Science15+ NASA Institute of Advanced Concepts /Structureless Extremely Large

Yet Very Lightweight Swarm Array Space TelescopeSpace Science

Notes: ASO-Astronomical Search for Origins, SEC-Sun Earth Connections, SSE-Solar System Exploration, SEU- structureand Evolution of the Universe

Figure 4 depicts the planned evolution of formation flyingtechnology from its current state to the achievement ofvirtual platforms. Precisely, a virtual platform is definedas the collective, coordinated operation of multiplespacecraft that are oriented and positioned to achieve pre-defined mission objectives.

A distinction must be made between formation flying andconstellation control. NASA is focusing on the control ofmultiple, cooperating satellites in autonomous formationsthat operate together to accomplish a variety of scienceobjectives. Therefore, formation flying typically involvesactive, real-time, closed-loop control of these satellites inthe formation. Formation flying can also be characterized

as a combination of multiple assets, that is, space vehicles,sub-orbital balloons and surface robots, all operatingautonomously together. Constellation control typicallydoes not require this level of autonomy or real-timecoordination. However, several subsystems, such as thesatellite cross-link communications and data transfer arecritical to both constellations and formations.

Formation Flying: From a Dream to Reality

Converting this formation flying vision into a real productis a formidable task, and both NASA and the Air ForceResearch Laboratory (AFRL) are leading several

Collective Guidance, Navigation & Control

for Multiple

Spacecraft

Multiple SpacecraftNavigate

Collectively

Chase & TargetSpacecraft

NavigateCollectively

Chase Spacecraft Navigates in

Response to Target Spacecraft

On-board delta-V

Execution(Closed Loop)

On-board delta-V

DeterminationOn-Board OrbitDetermination

1998 2000 2002 2006

AutonomousNavigation

AutonomousManeuverPlanning

AutonomousConstellation

Control

One-wayFormation

Flying

Two-wayFormation

Flying

TrueFormation

Flying

VirtualPlatforms

2004

constitutes a significantleap forward in s/c autonomyfrom traditional approaches

increased autonomy toallow multiple s/c to act asa single, cooperative entity

s/c pointingintegrated intocontrol strategy

Table 4: Formation Flying & Virtual Platforms Technology “

government-university-contractor teams to make thishappen. Some of the leading researchers that comprisethis team are from the Goddard Space Flight Center(GSFC), the Jet Propulsion Laboratory (JPL), AFRL, theNaval Research Laboratory (NRL), the Johns HopkinsUniversity Applied Physics Laboratory (APL), StanfordUniversity, Massachusetts Institute of Technology,University of California Los Angeles, and Space Productsand Applications (SPA), Incorporated.

NASA’s primary focus for formation flying technology isthrough the “Distributed Spacecraft” thrust area of the CrossEnterprise Technology Development Program (CETDP).The research within this thrust area is focused on thecollaborative behavior of multiple space vehicles thatform a distributed network of individual vehicles acting asa single functional unit while exhibiting a commonsystem-wide capability to accomplish various missiongoals. A combined Government-University-Industry teamhas identified 6 formation flying technology focus areasthat require further research to reach the goals of futuremissions. These technology focus areas, shown in figure5, include:

1) Sensor development2) Actuator development3) Telecommunications—inter-spacecraft communication4) Formation Control Strategies5) Computing and Data Management, and6) Tools & Testbeds

As figure 5 depicts, the sensor development andtelecommunications focus areas rely very heavily on GPS-based systems. The development and deployment ofrobust GPS systems (focus 1) to sense absolute andrelative navigation is critical to enable autonomousformation flying to occur. These receiver systems arebeing modified to also transmit formation control data tothe virtual platform—providing a telecommunicationscapability (focus 3) to the formation. Embedded in thesetransceivers and in the discrete formation space vehiclesare the computers (focus 5) and the autonomous formationcontrol algorithms (focus 4) that process the sensor dataand issue formation control commands to the fleet. Thevehicle actuators (focus 2) then reposition and reorient(attitude & navigation) the vehicles in the formation basedon the above control commands to achieve the missionrequirements.

Validating the performance and capabilities of these newformation flying technologies will be very challenging.Thus a complement of tools in concert with ground andon-orbit testbeds (focus 6) are being developed tominimize risk and reduce research costs. The rest of thispaper describes these six focus areas in more detail,including the current status of each of these and the futuredirections.

Focus Area 1: Sensors

The ability to determine and control the relative positions,orientations, and their respective velocities for a vehicle

1.3Telecommunications

FormationFlying

1.4Formation

Control

1.5Computing & Data

Management

1.6Tools andTestbeds

1.1Sensors

1.2Actuators

Product Line Descriptions Sample Products

emphasis: developing new sensing technologies

relative positions & velocities of vehicles within fleetsrelative orientations among vehicles within fleets

emphasis: insuring Formation Flying techniques can beaccommodated

precision, efficient, high mobility orbit & attitudecontrol systems

Autonomous Formation Flying sensorDifferential GPS techniques

Unique Camera/Trackers/Imagers*GPS receivers*, gyros*

emphasis: adapting communication technologies to new uses

inter-spacecraft communications systems

emphasis: establishing the required infrastructure fortechnology development, verification, & application

mission analysis and design toolsflight software emulation environments

component, subsystem, system level verification

NMP Earth Observer-1FreeFlyer mission design tool

6-DOF Coordinated Platform Testbed*High Fidelity Formation Flying Testbed*

DoD University Nano-sats*

emphasis: developing new control methodsand architectures

fleet control paradigmsvehicle control algorithms

Unified orbit & attitude control techniquesDe-centralized control techniques

Synchronized Formation Rotation andControl Techniques*

emphasis: insuring Formation Flying techniques can beaccommodated

high performance processorshigh capacity data storage

real-time distributed computing

Formation flying derived requirementsactuator performance specifications

reaction wheels*

Formation flying derived requirementsonboard computing & data management

performance specifications

Formation flying derived requirementsRF-Based TransceiversOptical Comm Systems

Product Lines

Figure 5: Formation Flying Focus Areas

or fleet of vehicles is only as effective as the sensors thatare on-board these vehicles. To this end, the formationflying team is emphasizing the development of newrelative and absolute sensing techniques.

Spaceborne GPS—For formation flying, the capstoneposition and timing sensor technology is spaceborne GPS.Several teams, including NASA GSFC, APL, and JPL, areworking with university and industry partners to move thistechnology to the forefront [1]. As described in [1], it willtake three generations of receiver developments to achievethe GPS (transmit/receive) space vehicle crosslink sensorthat is needed for future formation flying missions. Whenthis third generation GPS receiver is coupled withautonomous on-board maneuver planning and orbitcontrol software, and formation control algorithms,formation flying becomes feasible.

In addition to relative and absolute positioning, GPSprovides low cost, spacecraft timing systems, vehicleattitude determination and attitude control, andautonomous data transmissions over ground stations.Miniaturized copies of a GPS receiver can also serve asthe heart of an autonomous formation flying micro-sciencecraft providing attitude and orbit sensing, attitude

commanding, orbit control, command and data handlingservices, formation control transmission and reception andscience instrument timing all in one package.

VISNAV—Many formation flying missions, in particular,interferometry missions, rely on high precision relativeposition and attitude knowledge. Although GPS canprovide this capability near Earth, deep space missionsmust rely on other technologies. One of these alternativetechnologies is the vision-based navigation (VISNAV)system under development by Texas A&M University.VISNAV comprises an optical sensor of a new kindcombined with specific light sources (beacons) in order toachieve a selective or “intelligent” vision. The sensor ismade up of a Position Sensing Diode (PSD) placed in thefocal plane of a wide-angle lens. When the rectangularsilicon area of the PSD is illuminated by the energy from abeacon, it generates electrical currents that are processedto determine position coordinates. The concept ofintelligent vision is that the PSD can be used to see onlyspecific light sources or beacons. The light sources canbe at the end of a deployed boom of a single spacecraft fordetermining alignment, or on other spacecraft in aformation to determine position and attitude.

Attitude Sensing—If formation flying technologists aregoing to achieve the concept of virtual platforms, asshown in figure 4, a set of absolute and relative attitudesensing devices are needed. Very precise, autonomousstar tracking and gyro systems are needed to support therequirements demanded from formation flying missionssuch as the interferometry and relativity missions. Inaddition, developing fleets of very low cost spacecraft willrequire inexpensive, miniaturized attitude sensors such asmicrogyros, and low weight, low power attitude trackingdevices. To this end, the CETDP Distributed Spacecraftthrust area is teaming up with the Nanosat thrust area toensure these sensors are available for formation flyingmissions of the future.

Focus Area 2: Actuators

The ability to redirect specific spacecraft as well as theentire formation in translation and rotation is contingentupon the installation of adequately sized attitude andtrajectory actuators. From an attitude perspective, theseare usually reaction wheels and thrusters. From atrajectory perspective, this is primarily thrusters.Formation control puts high demands on these spacecraftactuators. New technologies are necessary to ensuresufficient resources are available onboard to maintain theformation. These technologies must support higherpointing constraints, provide greater precision thrustcapability and significantly improve use of propellantexpendables since a greater maneuvering frequency isexpected to maintain precision relative positioning.

Several initiatives are underway to develop actuators toenable very low cost formation flying. This activity isbeing sponsored by both the Distributed SpacecraftControl and the Nanosat thrust areas in NASA and byAFRL’s TechSat 21 program. Of particular interest aremicro-reaction wheels and micro-thrusters. These areneeded to support very small micro and nanosatellites aswell the extremely fine pointing needed to achieve therequirements for formation flying missions such as planetfinder, interferometry, synthetic aperture radar, andrelativity. In NASA, micro-thruster research is beingconducted at GSFC in the Guidance, Navigation andControl Center to support future formation flying testbedssuch as ST-5 and Orion. At AFRL as at GSFC, a greatdeal of effort is being spent on Micro-Electro-MechanicalSystems (MEMS), pulse plasma and colloid thrusters.

NASA GSFC is sponsoring the development of a PulsePlasma Thruster (PPT) as an experimental technology forthe New Millennium Program (NMP) Earth Orbiter-1(EO-1). The PPT uses solid Teflon propellant and iscapable of delivering high specific impulse (900-1200sec), very low impulse bits (10-1000uN-s) at low averagepower (<1 to 100W).

Colloid microthrusters (with thrust in the milli-Newtonrange) are a promising new technology in the field ofsmall spacecraft propulsion. Because of their small sizeand low weight these devices are particularly interestingto missions incorporating formation flying andnanosatellites. Colloid thrusters work by acceleratingcharge particles using an electrostatic field. Forpropellant, the thrusters use a conductive liquid that istypically doped with salt to increase charge carryingcapacity. The liquid is channeled through a tiny orifice.At the back opening of the orifice, a high electrostaticfield is applied causing imbalance of surface forces thatcaused the liquid to breakdown to small charged droplets.The same electrostatic field then accelerates thesedroplets, thus creating thrust.

Current research, based at the Stanford Plasma DynamicsLaboratory (PDL), aims to better understand the workingmechanism of colloid thrusters, and to develop anintegrated, micro-electro-mechanically based colloidthruster for space propulsion. To enable small-scaleposition control, these thrusters can supply vectored thruston the order of 0.11 milli-Newtons, and have a specificimpulse of approximately 1000 seconds. The StanfordUniversity thrusters are scheduled to fly on Stanford/SantaClara Emerald formation under the AFRL Universitynanosatellite program.

Focus Area 3: Telecommunications

Formation Flying cannot be accomplished without anadequate inter-spacecraft communications medium. Thespecific medium used for formation flying spacecraft isprimarily a function of the performance and science datagathering requirements for the mission. Missionsrequiring low to medium formation knowledge andcontrol (km to cm-level) and nominal data requirementswill use Radio Frequency (RF)-based telecommunicationscapabilities. Currently, researchers are investigating GPS-like transceivers as the primary RF-based formation flyingcommunications medium. Missions requiring highperformance formation knowledge and control (sub-cm topicometer) and/or very high data rates (>10 Mbps) willrequire optical communications methods to support thevirtual platform. The current research thrusts in this focusarea are described in the following paragraphs.

RF-Based Transceivers

APL CLT—The Applied Physics Laboratory (APL) CrossLink Transceiver (CLT) represents an integrated crosslinkcommunication and relative navigation system formultiple distributed spacecraft flying in formation.[2][3][4][5]. The CLT (figure 6) will support inter-spacecraft communications at a nominal rate of 5-20kbpsfocused primarily upon the distribution of command and

Figure 6: APL CLT

control information. This includes the dissemination ofdirectives for vehicle coordination and well as ancillarydata exchange needed to support intelligent controlstrategies. The nature of the crosslink data is not limited tohigh-level control directives, indeed lower level controlapproaches (e.g., decentralized control methodologies thatrely upon state vector sharing) are also supported. Thedata fields for inter-spacecraft communications aregeneric and protocol dependent, with a nominal,packetized approach that is CCSDS compliant.

A critical aspect of the CLT crosslink communicationsapproach is that it is explicitly designed to supportformation flying missions, which require capabilities suchas dynamic adaptivity, scalability, and robustness. Assuch, the CLT is designed to simultaneously receive datafrom multiple spacecraft and the signal structure is suchthat it will support a variety of logical command andcontrol architectures (e.g., centralized, hierarchical, fullydistributed) by providing a flexible communicationsinfrastructure.

The CLT providesboth an absolute andrelative navigationsolution (positionand velocity) andprovides precisiontime recovery and asteered one pulse-per-second output.Orbit determinationis provided byreception of GPSsignals andprocessing by an

extended Kalman filter. In addition, crosslink signalssupport relative navigation, with the potential for bothdirect solutions as well as relative GPS solutions whichrely upon the computation of double differences of GPSdata in a pairwise manner among spacecraft. Rangingusing crosslink pseudorandom noise codes cansupplement the GPS measurements in the relativenavigation solution to enable more rapid convergence ofthe solution for low Earth orbit missions. The crosslinkranging data could entirely replace the GPS measurementsfor highly elliptical or deep space missions. In this casethe relative clock error and range can be solved on a per-link basis. The ranges establish the relative geometry ofthe constellation, subject to an indeterminate rigid-bodyrotation.

JPL AFF—The NASA New Millennium Program (NMP)Space Technology (ST)-3 mission is expected todemonstrate various elements of the technology requiredfor space interferometry, including the Autonomous

Formation Flying (AFF) sensor [6], and an autonomousreconfigurable formation control system [7,8].

SPTC—The Stanford Pseudolite Transceiver Crosslink(SPTC) is being developed at Stanford University as arelative navigation and communication crosslink systemfor formation flying spacecraft. The SPTC was designedusing COTS devices (modems, L1 pseudolites, and anattitude-capable GPS receiver). Carrier phase DifferentialGPS (CDGPS) measurements are used to achieve veryprecise relative positioning. Using GPS measurements, theSPTC is expected to provide relative position accuracieson the order of 2-5 cm, depending on GPS satellitegeometry. Attitude determination to within 0.25 degreesand time transfer between separated vehicles to within 1microsecond is also expected. The pseudolites within theSPTC are used to enhance the operating range (orbitalaltitude) of the CDGPS approach and to provideadditional GPS measurements (improved geometry) atlower altitudes. For inter-vehicle relative positioning, thepseudolite within the SPTC can be used to obtain relativevehicle range measurements to within 0.5 cm. Theonboard pseudolites will also enable the use of CDGPSrelative navigation at Geostationary Earth Orbit using only1-3 GPS satellites. The SPTC system has been testedextensively on the Formation Flying Testbed at StanfordUniversity [9][10]

Stanford Telecom LPT—NASA/GSFC and StanfordTelecommunications, Inc. are jointly sponsoring thedevelopment of a Low Power Transceiver (LPT). TheLPT integrates TDRSS S-band 2-way communicationsand GPS navigation in a compact flexible package. Oncecompleted, the LPT will reflect substantial reductions inpower, size, weight, and cost relative to currenttransceiver options, and is envisioned to serve as thecommunications/navigation subsystem for a wide range ofspace-based science missions. This technology is idealfor formation flying which requires crosslinkcommunications and relative navigation often on smallspacecraft with limited power.

Optical Communications & Metrology Systems

JPL Optical Communications—At NASA, the JetPropulsion Laboratory is conducting research in opticalcommunications systems. These systems will provide asignificant crosslink and downlink conduit for scientificand formation control data. Moreover, these laser-basedsystems can also provide very precise information on therelative positioning and orientation of multiple spacecraftin a formation (micrometers and arc-seconds). Usingthese optical and metrology systems, a “virtual aperture” canbe developed using several spacecraft in a tight formation.This technology is crucial for Earth and space sciencemissions that have very precise formation control

requirements. These mission sets will most likely useboth the RF and optical systems together. The RF systemwill support coarse formation control. Once the formationis in place, the optical-based fine navigation/pointingsystem would take over to tighten the formation until itmet the mission objectives.

Focus Area 4: Formation Control Strategies

Implementation of distributed coordinating satelliteconcepts will require tight maintenance and control of therelative distances and orientations of the participatingsatellites. Thus formation control poses very stringentchallenges in the areas of:

1. Onboard sensing of relative and absolute vehiclepositions/attitudes,

2. Maneuvering, retargeting, collision avoidance, andaperture optimization including resource/taskallocation within the fleet,

3. Modeling the orbital mechanics and the impact ofdifferential drag and solar disturbances,

4. Fleet and vehicle autonomy, including high-level faultdetection and recovery to enhance the missionrobustness,

5. Decentralized control & computation for a fleet ofmany (e.g. from 16 to hundreds) vehicles,

6. Testbeds and simulations to validate the varioussensing and control concepts.

A ground-based command and control system for relativespacecraft positioning would be very complex, heavilyover-burdened, and may not provide sufficiently rapidcorrective control commands. Thus, the overall focus ofthis work is to significantly increase the onboardautonomy of the future spacecraft, thereby reducing theground support required.

Conceptually, autonomous formation flying is a process inwhich an array of spacecraft makes continuousmeasurements of the “array configuration” and uses thesemeasurements to maintain an existing configuration or tosmoothly transition to a new one, all without externalmeasurement or control. Generally speaking, the arrayconfiguration includes both the distances between all pairsof spacecraft in the array and the orientations of thespacecraft in a coordinate frame defined by the array’sinternal geometry. From initialization to targeting, then tomaneuvering, the formation will experience significantchanges in the control requirements. A completelyautonomous, configurable control system must beimplemented to switch between the various system modelsand controllers.

Cooperative formation control can take several forms.Typically, for a small number of spacecraft (e.g. less than

16) operating as a formation, a master/slave or ahierarchical control structure could be used. In themaster/slave scenario, a single spacecraft acts as a leaderand issues commands to the fleet or the fleet reacts to theactions of the leader according to a planned behavior. Thehierarchical control divides the formation into subsets.Each subset has its own leader and a small number of“followers.” However, for large formations, these types ofcontrol carry too much overhead and are impractical. Adistributed or decentralized approach is necessary.

Decentralized architectures are non-hierarchical andcoordination by a central supervisor is not required.Detected failures would then tend to degrade the systemperformance gracefully. Each node in the decentralizednetwork processes only it’s own measurement data inparallel with all of the others.

One example of the decentralized approach being pursuedby NASA/GSFC uses a stand-alone, standard GPS pointsolution to maintain the spacecraft formation [21]. In thisapproach, if each of the vehicles transmits and receivesdata to and from the other vehicles in the formation,relative states can be computed, without the need fordirect measurements of the inter-satellite states. Thiswould enhance the accuracy and allow coordination of theformation maneuvers. If sufficient onboard processorcapacity is available, the GPS measurement data (e.g.pseudo-range, carrier) could be processed for improvedaccuracy. Ideally, if one or more of the vehicles had thecapability to make relative measurements to the otherformation members, all available data could be used tomaximize the relative navigation accuracy.

The team at Stanford University has experimentallyanalyzed and demonstrated several estimationarchitectures (centralized and decentralized) that could beused to perform the differential carrier-phase GPS relativesensing for a large fleet of vehicles [32,33,34]. They havealso demonstrated a multi-level fleet control system thatincludes a coordinator, a planner, and distributedregulators using a fully nonlinear orbital simulation. Alinear programming approach is used to rapidly solve forthe optimal formation maneuvers using the linearizedgroup dynamics (both LEO and deep space) [30]. Thissame control architecture has been experimentallydemonstrated on the Formation Flying Testbed at Stanford[31]. The team is also designing autonomous formationflying algorithms that will be demonstrated on Orion [29].

Several others groups are working on resolving thevarious technical issues listed above. The MIT SpaceSystems Lab is currently working on the aperturesynthesis of a formation flying cluster of satellites [11],with a special emphasis on the optimal placement withinthe aperture for both distributed interferometry and SAR

systems [12,13]. They have also analyzed the effects ofthe primary orbital perturbations on passive formationsand analyzed the ∆V required to overcome theseperturbations [14].

A group at the University of Texas A&M has developedan optimal relative orbit design for minimizing the effectsof the dominant orbital perturbations (using the nonlinearorbital dynamics) [15,16]. They have also developedtechniques to perform fuel optimal control of a twospacecraft rendezvous [17]. Honeywell is analyzing thedistributed resource allocation and collective managementof a large satellite cluster [18].

NASA GSFC have designed various control systems forthe EO-1/Landsat-7 formation flying experiment (e.g.linear output feedback control, decentralized LQG)[19,20,21,22]. A team of researchers from the VirginiaPolytechnic University & AFRL has developed LQRcontrol of a two-satellite formation [23]. They have alsodeveloped linear output feedback based adaptivenonlinear control for a two-satellite formation (with adetailed nonlinear stability analysis) [24].

The team from UCLA & JPL has investigated thenavigation and control for multiple microspacecraft information (integrating the rotational dynamics into thecontrol and coordination) [7,25]. They have alsodeveloped fuel equalization and adaptive control schemes[26,8], analyzed a formation flying control architecturebased on a leader-follower framework, and presented astability and convergence analysis of the controllermodeled as a hybrid system [27,28].

Focus Area 5: Computing and DataManagement

Computing and data management presents a number ofchallenges both in the hardware and algorithmic areas.Groups at GSFC, JPL, AMES, AFRL, university andindustry are addressing many of these.

Very high performance, low-power, computing devicesare critical for spaceborne GPS, formation flying andnanosatellites. The sophistication of the formation controlalgorithms and the need for sufficient floating pointcompute power for formation flying and GPS applicationsdemand high performance microprocessors. Thesedevices need to be radiation and single event effecttolerant since they are mission critical. Leveraging off thecommercial sector, NASA, AFRL and the commercialspace industry are working feverishly to bring thesedevices to flight-worthy status. Devices such as theStrongARM, Mongoose V, 603e and 720 Power PC andthe RAD6000 represent the current generation ofspaceflight processors.

A great deal of effort is being spent on spacecraftautomation and autonomy. First, some definitions arenecessary. Automation is focused on ground controlwhile autonomy is on-board processing. Automationusually involves scripting product generation and creatinga “lights-out” environment or situation where flightengineers are paged when necessary. The real focus ofthis work is to reduce mission cost. The goals ofautonomy, on the other hand, are to create “thinking”spacecraft that can operate independent of ground support.Autonomy is truly critical in the realm of formation flying.

The introduction of autonomous systems, however,creates it own problems. The nondeterministic nature ofautonomy greatly complicates testing. New techniquesand tools are necessary to determine and fully understandsystem behavior. It is no longer possible to use structuredtesting and try to cover all the “expected” paths oroutcomes.

Another area of study is fault detection and correction.This can be particularly critical in a distributed controlenvironment. To ensure graceful degradation of theformation, failures must be detected and managed. Forexample, some of the distributed spacecraft controltechniques at GSFC are looking at voting schemes todetermine and remove failing spacecraft.

With large numbers of spacecraft collecting potentiallyhuge amounts of data, serious research into datamanagement and reduction is necessary. In someformations, the data must be shared between spacecraft,and allowances must be made for these largecommunication bandwidths. Also, the spacecraft willneed to determine what data should be sent to the ground,what can be thrown away, and what needs to be kept onboard in case the ground requests it. Aging of the datamust also be managed. These are just a few of theproblems that must be solved.

Focus Area 6: Tools & Testbeds

The challenge of deploying distributed spacecraft systemslies not only in controlling the vehicles to achieve andmaintain a specified formation, but also in distributinginformation between the vehicles so that they act as acoordinated system. This requires the development ofadvanced distributed spacecraft control architectures andalgorithms, absolute and relative navigation and attitudesensors, inter-spacecraft communication systems, andinformation management systems. To minimize missionrisk associated when these new technologies are infusedinto formation flying missions, testbeds that will enablecomprehensive simulation and experimentation, arerequired.

Figure 7: GPSTest Facility

Thus, the NASA GSFC Guidance, Navigation and ControlCenter (GNCC) is developing a multi-pronged approachto testbed development. The first prong of this effort is aseries of small testbeds based on specific technologies.For example, the team at GSFC has developed a GPStestbed that includes GPS constellation RF simulators andflight ready receivers. See figure 7. A second prongincludes incorporating these smaller testbeds into a

ground-based FormationFlying Testbed (FFTB). Thistestbed is built around aCOTS product calledVirtualSat Pro. VirtualSat,developed by the HammersCo., is a real-time spacecraftdynamic simulator capable ofsimulating a formation ofspacecraft. It was developedto facilitate flight softwaredevelopment and testing inparallel with hardwaredevelopment. SinceVirtualSat has a plug and playcomponent structure,

hardware can be plugged in place of software simulationmodules as it is developed. These smaller hardware-basedtestbeds can, therefore, be plugged into the FFTB tocreate an extensive flight simulation environment.

The third prong in the NASA testbed strategy is anextensive on-orbit campaign of demonstration missions.These on-orbit demonstrations will validate numerousformation flying technologies, including those mentionedin this paper, in various flight experiments.

NASA/GSFC is not the only group developing formationflying testbeds. The following paragraphs provide moredetails on some of the ground-based and on-orbit testbeds.

Ground-Based Testbeds

Stanford University Formation Flying Testbed (FFTB)—Aformation flying testbed has been created at StanfordUniversity to investigate the guidance, navigation, andcontrol issues associated with precise formation flying[33,34]. See figure 8. The testbed consists of 3 activefree-flying vehicles that move on a 12 ft x 9 ft granitetable. Compressed air thrusters propel the vehicles. Eachvehicle has onboard computing and batteries, andcommunicates with the other vehicles via a wirelessethernet (or modem), making them self-contained andautonomous. These air cushion vehicles are used tosimulate the zero-g dynamics of a spacecraft formation ina horizontal plane. Pseudolites are used in this room toenable the use of CDGPS relative navigation for thisformation. Recent results have demonstrated the

feasibility of using a CDGPS based sensing system (withpseudolites) to achieve relative navigation accuracies onthe order of 2 cm (relative position) and 0.50 (relativeaccuracy) on this formation of prototype space vehicles.

Figure 8: Stanford University Formation FlyingTestbed

3D Formation Flying—A second testbed has beendeveloped to demonstrate formation flight in threedimensions with larger separations between the vehicles[38]. See figure 9. This testbed consists of two blimpsthat fly inside a large highbay (200 ft x 65 ft x 95 ft).These vehicles also have onboard GPS sensors, 5 driveelectronics/motors and 2 communication systems. Theblimp testbed has been used to demonstrate that variousGPS errors, such as the circular polarization effect, can bemodeled and eliminated from the measurement equations.A new, low-cost GPS receiver was developed for thesevehicles, and this will be used for the upcoming Orionspace mission. Results on the blimps have shown veryrobust CDGPS sensing for the formation. The blimpformation has also been tested outside using theNAVSTAR constellation. Basic formation controlstrategies have also been investigated on this blimptestbed.

Figure 9: Stanford University Blimp Formation FlyingTestbed

Figure 10: EO-1 & Landsat-7 Co-ObservingProgram

On-Orbit Testbeds

EO-1 Enhanced Formation Flying Experiment—Theprimary objective of the enhanced formation flyingexperiment on the EO-1 mission is to demonstrateonboard autonomous navigation and formation flyingcontrol between the EO-1 and Landsat-7 spacecraft. Seefigure 10. An automated mission design and automatedmaneuver planning tool, AutoCon [19], which wasdeveloped by AI Solutions under direction of the GoddardGN&C team, has been used for operational missiondesign. AutoCon is being modified to operate onboardthe spacecraft to support autonomous formation flying.This will be accomplished by having the flight controlsystem plan a maneuver that places EO-1 within 1 minuteof separation from Landsat-7 and then maintain thatseparation to a tight tolerance of 6 seconds for anextended period of time. Flight validation is scheduledfor December 1999.

The GSFC-developed algorithms and software tools forthis demonstration use a modular approach so they caneasily be incorporated onboard future Earth orbitingmissions. These algorithms [19,20,35] will beimplemented using fuzzy logic engines for constraintchecking and control of the formation flying algorithms.JPL and the Air Force Research Laboratory are supplyingadditional formation flying algorithms.

The key benefits of this enhanced formation flyingtechnology are to eliminate routine ground maneuverplanning and commanding requirements, reduce costs,enhance future science investigations, and to advance thetechnology for complete lights-out application for theNew Millennium Program. The system will provide real-time low-cost formation flying control with the flexibilityto meet a broad range of mission requirements including

ground track, inclination, and altitude control asindividual or multiple spacecraft requirements.

Orion—The Orion mission was developed to demonstratetrue formation flying in low earth orbit using very lowcost microsatellites designed and built at StanfordUniversity. See figure 11. This mission will validateseveral key sensing and control issues associated withformation flying, and it represents an important steptowards the virtual platforms envisioned for future EarthSciences Enterprise missions.

Stage A: Stage B: Stage C:

RelativeRanging

Figure 11: Orion—True Formation Flying

The microsatellite design for this mission is based on amodified version of the low-cost low-weight spacecraftbus developed at Stanford University called SQUIRT.Apart from being an excellent testbed for demonstratingthe overall design of an active microsatellite, there arefour important technical objectives of this program:

• On-orbit demonstration of formation control for acluster of micro-satellites using real-time autonomouscontrol software.

• Demonstration of a low-cost, low-power GPS receiverfor real-time attitude & relative navigation and control.The particular emphasis of the program is on usingcarrier-differential GPS (CDGPS) for very preciserelative navigation.

• Demonstration of a real-time inter-vehiclecommunication link to support the CDGPS and controldata. Perform closed-loop control of the formationusing various control architectures (e.g. centralized orleader-follower).

• Investigate several configurations of the fleet todemonstrate the formation flying in fuel-optimizedrelative orientations. Both coarse and precise formationflying will be demonstrated.

The current Orion vehicle hardware design is as follows:

1. Cold-gas thrusters for attitude and station-keepingmaneuvers. There are 4 clusters of 3 nozzles to provide6 DOF control. Four Nitroduck tanks carrying a total of2.5 kg of nitrogen should deliver about 25 m/s of ∆V.

2. Torquer coils and magnetometers will be used as anauxiliary ACS for de-tumbling the satellite during thestart-up mode, thereby allowing the GPS system toreliably obtain its initial signal lock.

3. The communications subsystem provides reliable inter-satellite and ground links. The current design includesthe Hamtronics Transmitter (437.475 MHz carrier, 2.5W output power), Hamtronics R451 Receiver (437.475MHz), SpaceQuest MOD-96 Modem

4. The SpaceQuest space-rated CPU and BekTek OS willbe used for the command and data handling.

5. The basic structure is a modular construction with1/2”aluminum honeycomb trays and panels. The C-channeltray frames have been designed to help with the heatconduction. There are 2 strings of 12 Sanyo CADNICANiCd batteries and 11000 cm2 of solar panels.

6. A Stanford modified Mitel chipset design for theattitude-capable GPS receiver. The receiver supports 6antennas and requires 8 W peak power and 2 W in restmode. The receiver uses a StrongArm CPU forprocessing of the relative navigation data and controlalgorithms. Current simulations of a two vehicle fleet inLEO indicate that, with this receiver, the relativeposition can be estimated at 1 Hz to 2-5 cm (dependingon GPS geometry), and the CDGPS biases can bereliably resolved in under 15 minutes.

The original mission plan had 3 Orion vehicles that wouldbe launched together, initialized, and then perform asequence of coarse and precise formation flying. This hasrecently been changed so that a single Orion vehiclewould fly in formation with two Emerald vehicles that arebeing developed at Stanford and Santa Clara Universitiesas part of the AFRL Nanosat program. While the Emeraldvehicles are less capable than the current Orion spacecraftdesign (they have no significant thrust actuation and havesignificantly reduced onboard computing) this fleet ofthree vehicles should be able to meet all of the originalOrion formation flying sensing and control objectives.

The Orion team is also exploring the possibility of flyingan Orion vehicle in formation with MightySatII.2 as acombined NASA/AFRL program. This configuration willallow substantially more complex station-keepingoperations to be validated on-orbit. A further optionwould be to perform joint scientific experiments using theTRAM radar system under development for theTechSat21 program. In either mission scenario, Orionwill demonstrate and validate many key technologies forfuture formation flying missions.

ST-3—Space Technology-3 (ST-3, previously known as DS-3) is being developed by JPL and Ball Aerospace todemonstrate spaceborne optical interferometry with verylarge baselines (100m-10km). This can be accomplishedusing multiple spacecraft flying in precise formation.While the optical pathlengths over these distances must becontrolled to nanometers, the vehicle accuracies are 1 cmand 1 arcmin in relative range and attitude, respectively.The mission will also be used to test the new relativeranging technology (Autonomous Formation Flyer) andvarious formation flying control algorithms.

ST-3 will consist of two spacecraft, each having a degreeof autonomy, but both comprising a single instrument andconstrained to move together in a relative distance of 50-1000 meters. In order to meet the mission goals, thecontrol system must maintain the distances betweenspacecraft to within 1-2 cm, and the relative orientationsof the spacecraft within 1 arcminute per axis. The sensingfor ST-3 will be performed using the AFF describedpreviously. The AFF borrows technology from GPS,using measurements of both radio frequency (RF) carrierphase and a ranging code. Each spacecraft will have atleast one transmitting antenna and two receiving antennas,operating at 30 GHz with a code rate of 100 Mchips persecond. Both spacecraft will be collecting elements for theinterferometer. The vehicles are roughly cubic in shapeand have masses of about 150 kg. Each will use a mirrorwith a diameter of 12 cm to reflect the collected light(wavelengths 500-900 nanometers) to one of thespacecraft that will also serve as the combiner. Becauseof the modest collecting area, the faintest measurablesources will have visual magnitudes in the range 10 to 12.The interferometric baselines will vary in length fromperhaps 50-1000 m. During a planned lifetime of sixmonths, the instrument will demonstrate its ability to pointat specified targets, change baseline length, and maintainthe formation at the required accuracy, as well as to findand track the interferometric fringes and report itsmeasurements back to Earth. The need to maintain thearray configuration without continual thruster firingsmandates that the array operate in a solar orbit similar tothe Earth’s (trailing Earth by approximately 0.1astronomical unit). Clearly, monitoring and controllingthe array configuration will be a crucial element in theoperation of the ST-3 interferometer.

ST-5—Various partners are developing ST-5, called theNanosat Constellation Trailblazer mission [36]. Thismission will use three very small satellites (approx 10kg).The mission objectives are to validate methods ofoperating several spacecraft as a system, and to testvarious technologies (e.g. GPS for relative ranging, low-power communication, Lithium Ion Power System) in theharsh space environment near the boundary of Earth's

Figure 12:TechSat 21

protective magnetic field, or magnetosphere. The launchis planned for in 2003.

TechSat-21—Air Force Research Laboratory (AFRL) isleading the development of microsatellites (10--100kg) toreplace several complex, expensive Air Force satellites,such as MilStar, Defense Support Program, and DefenseMeteorological Satellite Program. The key focus of this

work is to develop newtechnologies, such as Micro-Electro-Mechanical Systems(MEMS), that will lead tolightweight, low-cost, and highlycapable microsatellites. The AFRLis exploring this new paradigm forperforming space missions in aneffort called TechSat-21(Technology Satellite of the 21stCentury) [37]. See figure 12. A

space-based radar mission for Ground Moving TargetIndication was chosen as a stressing case and is the focusof the initial investigation. The program is focused onMEMS development, sparse aperture design, andformation control strategies.

University Nanosats—The University Nanosat programserves as a technology host to test algorithms, softwareand hardware in the space environment. Since there arethree formations of multiple Nanosats, varioustechnologies can be tested and validated in parallel. Theparallel development and deployment of these spacecraft,as well as Orion, provides opportunities for the formationflying community to “climb the technology stair steps” whileminimizing mission risks through multiple on-orbit tests.

DoD, NASA, and Industry are also jointly sponsoring thedevelopment and launch of 10 university nanosatellites(approx 10 kg) to demonstrate miniature bus technologies,formation flying, and distributed satellite capabilities.The satellites are planned to launch from the Shuttle inlate 2001. Three of the teams are focusing ontechnologies that are directly relevant to formation flying.

A team from Arizona State University, University ofColorado Boulder, and New Mexico State University isdeveloping the Three Corner Sat Constellation. Theprimary science objective of the Three Corner Satconstellation is to perform stereo image small (< 100meter), highly dynamic (< 1 minute) scenes includingdeep convective towers, atmospheric waves, and sand/duststorms. To accomplish the science objectives, the 3satellites will form a “virtual formation” in which thesatellites cooperate to perform targeting, data acquisition,and data downlinking. For the mission to beaccomplished, the locations of the satellites will need tobe “in range” and mutually known in order for each to

support its portion of the mission, but physical proximityis not a requirement for the formation network. Stereoimaging only requires a nominal spacing of tens ofkilometers, so these vehicles will not use a propulsionsystem.

A team from Utah State University, University ofWashington, and Virginia Polytechnic Institute & StateUniversity are developing the Ionospheric ObservationNanosatellite Formation (ION-F) to demonstrate satellitecoordination and management technologies. The primaryobjective of the mission is to investigate the ionosphere(Density Structure Sizes, Drifts, Decay Rates) usingmeasurements from the distributed satellites. Variouspropulsion and formation control algorithms are currentlybeing investigated for this mission.

A team from Stanford Space Systems DevelopmentLaboratory, Santa Clara Remote Extreme EnvironmentMechanisms Laboratory, and the Stanford FormationFlying Laboratory is developing Emerald. Emerald isbeing designed as a low-cost demonstration of the basiccomponents of NASA's “virtual spacecraft bus” concept. Inparticular, it will demonstrate the use of Carrier-PhaseDifferential GPS (CDGPS) techniques to autonomouslytrack the relative position and attitude between severalspacecraft. This sensing technology, together with coarseposition control devices (differential drag) and inter-satellite communication links will be used to develop avirtual spacecraft bus comprised of a distributed array ofsimple, low-cost, highly-coordinated vehicles such as aformation of small satellites.

Conclusions

Formation flying technology will make fundamentalchanges in the way the Civil and DoD space communityconducts missions in space. These changes willrevolutionize all space missions of the future: EarthScience, Space Science, Human Exploration and DoD andCommercial ventures. The NASA/AFRL FormationFlying team is on the forefront of the Formation Flyingtechnology effort, providing hardware and softwaresolutions to overcome the current technology hurdles. Aseries of collaborative on-orbit experiments and ground-based tools and testbeds will provide a low cost validationof the Formation Flying hardware and softwarealgorithms. Future missions will rely heavily onspaceborne GPS technology as well as advanced spacevehicle autonomy techniques to enable the construction ofVirtual Platforms in space.

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