AAReST (Autonomous Assembly of a Reconfigurable Space Telescope)
AAReST is a collaborative effort between Caltech ( California Institute of Technology) of Pasadena, CA, and SSC (Surrey Space Centre) of the University of Surrey, UK, with the top-level objectives of developing a low-cost, small satellite mission to demonstrate all key technological aspects of autonomous assembly and reconfiguration of a space telescope based on an optical focal plane assembly (camera) and multiple mirror elements synthesizing a single coherent aperture, and to demonstrate the capability of providing high-quality images using a such a telescope. The AAReST project, which started in 2009, has the goal to be ready for launch in 2016. 1) 2) 3) 4) 5)
Background: The assembly of future large space telescopes requires new approaches. The project believes that an in-orbit assembly holds a potential promise of a low-cost and practical solution in the near term, provided much of the assembly can be carried out autonomously. To gain experience, and to provide risk reduction, a combined micro/nanosatellite demonstration mission of proposed to focus on the required optical technology (adaptive mirrors, phase-sensitive detectors) and autonomous rendezvous and docking technology (inter-satellite links, relative position sensing, automated docking mechanisms). The mission will involve two "3U" CubeSat-like nanosatellites ("MirrorSats") each carrying an electrically actuated adaptive mirror, and each capable of autonomous un-docking and re-docking with a small central "15U" class micro/nanosatellite core, which houses two fixed mirrors and a boom-deployed focal plane assembly. All three spacecraft will be launched as a single ~40 kg microsatellite package. The spacecraft busses are based on heritage designs of SSTL missions, while the optics, imaging sensors and shape adjusting adaptive mirrors (with their associated adjustment mechanisms) are provided by Caltech/JPL. The spacecraft busses provide precise orbit and attitude control, with inter-satellite links and optical navigation to mediate the docking process. The docking system itself is based on the electromagnetic docking system being developed at SSC, together with rendezvous sensing technology developed for STRaND-2 (Surrey Training, Research and Nanosatellite Demonstrator-2).
AAReST mission concept: From the top level mission objectives, a series of more detailed system design requirements follow:
• The mission must involve multiple spacecraft elements.
• All spacecraft elements must be self-supporting and "intelligent" and must cooperate to provide systems autonomy – this implies they must be each capable of independent free-flight and have an ISL (Inter-Satellite Link) capability.
• Spacecraft elements must be agile and maneuverable and be able to separate and re-connect in different configurations – this implies an effective AOCS (Attitude and Orbit Control System), and RDV&D (Rendezvous & Docking) capability (sensors and actuators).
• All spacecraft elements must lock together rigidly and precisely and provide a stable platform for imaging – this implies a precision docking adapter and precision ADCS (Attitude Determination and Control System).
• The mirror elements must be capable of independent motion and/or shape adjustment and wavefront sensing in order to synthesize a single coherent aperture.
These requirements have resulted in the composite spacecraft concept shown in Figure 1. The spacecraft bus, ISL, AOCS, precision ADCS, propulsion and RDV&D systems are largely the responsibility of the Surrey team, with support from the University of Stellenbosch (South Africa). The optical systems (mirrors, cameras, wavefront sensors, mirror selection system), aperture synthesis, and deployable boom are largely the responsibility of Caltech, with support from AFRL (Air Force Research Laboratory) and NASA/JPL (Jet Propulsion Laboratory).
Figure 1: AAReST spacecraft elements (image credit: AAReST collaboration)
The AAReST mission will involve two nanosatellite class vehicles based on 3U CubeSat structures (MirrorSats), which each carry a DMP (Deformable Mirror Payload) and a central 15U microsatellite ("CoreSat"), which houses two rigid RMPs (Reference Mirror Payloads) and a Camera Package mounted on a deployable carbon fiber composite boom. All three spacecraft will be launched as a single ~40 kg microsatellite package with a stowed volume of 0.4 m x 0.4 m x 0.6 m (Figure 2).
During launch, the MirrorSats, Camera Package and Boom are held rigidly onto the CoreSat via frangibolts and burn-wires. However, once in orbit, the boom will be released to deploy the Camera Package to form a prime focus telescope of 1.2 m focal length, and 0.3o FOV (Field of View), where the primary mirror is divided into a sparse aperture consisting of an arrangement of four 10 cm diameter circular mirrors (2 RMPs and 2 DMPs).
Figure 2: AAReST launch configuration (image credit: AAReST collaboration)
Once initial imaging has been carried out using bright stellar targets, the moon, and possibly terrestrial targets, the reconfiguration aspect of the mission will be tested. The frangibolts holding the MirrorSats will be fired, and the MirrorSats are then just held magnetically onto the CoreSat via small permanent magnets in the docking system. The docking system is electromagnetic with switchable polarities, and thus can overcome the magnetic latching to allow the MirrorSats to separate and re-attach in the different configurations (compact or wide) for more imaging (Figure 3).
Figure 3: AAReST concept demonstrating the launch configuration (left), initial imaging configuration (middle) and secondary imaging configuration (right), image credit: AAReST collaboration)
The 3-point extended EMDSs (Electro-Magnetic Docking Systems) use a Kelvin Clamp arrangement to ensure rigid and repeatable alignment of the MirrorSat and CoreSat spacecraft. This provides the mechanical baseline for the adjustable mirrors.
Using wavefront sensors in the Camera Package, the mirrors can be adjusted and calibrated in order to minimize the size of the mirrors' individual PSFs (Point Spread Functions). The RMPs are rigid in shape, but can be adjusted using tip, tilt and piston motions. The DMPs have these motions, but in addition have the capability of changing shape.
The optics have been designed so that the mirrors can be co-phased to synthesize a common aperture (as would be required for an actual science mission). As well as being challenging optically, this gives significant requirements for the pointing stability of the spacecraft.
Spacecraft busses: The design approach taken to spacecraft is that common to all Surrey's spacecraft, that is:
- Low-cost: Where possible use tried and tested COTS (Commercial-Off-The-Shelf) technology (e.g. leverage CubeSats).
- Heritage: Leverage of Surrey's 35 year SmallSat experience, with particular reference to the SNAP-1 (2000) NanoSat program (butane propulsion and pitch-axis momentum wheel/ magnetic control ADCS) and STRaND-1 (2013) (UK's first CubeSat) missions.
- Incremental: Hardware, software, propulsion and rendezvous/ docking concepts developed through Surrey's STRaND-1, STRaND-2, and QB50/CubeSail and SMESat missions, recently flown or currently under development at SSC.
- Modular approach: Maximize commonality between MirrorSat and CoreSat systems and also with other SSC CubeSat programs.
The MirrorSat requirements can be summarized as:
- Must support the DMP (Deformable Mirror Payload) mechanically and electrically via a 5 V 1 A supply (2 W continuous operational power) and telemetry/telecommand (TTC) via a USB 2.0 interface.
- Must be able to operate independently of other units.
- Must be able to communicate with the CoreSat out to 1 km max distance. (via COTS WiFi based ISL).
- Must be able to undock, rendezvous and re-dock multiple times.
- Must have 3-axis control to ±2o precision and 6 DOF (Degree-of-Freedom) propulsion capability.
- Must provide low/zero power magnetic latch to hold in position on CoreSat in orbit.
- Must be able to safely enter the CoreSat docking system's acceptance cone:
• 20-30 cm distance (mag. capture)
• ±45o full cone angle; < 5 cm offset
• <±10o relative RPY error
• < 1 cm/s closing velocity at 30 cm
• < ±2o relative RPY error at first contact.
Given these requirements, a spacecraft design based on a COTS ISIS (Innovative Solutions In Space BV, Delft, The Netherlands) 3U CubeSat structure was made (Figure 4).
Figure 4: MirrorSat design (image credit: AAReST collaboration)
This comprises the following subsystems:
- DMP Payload: Caltech supplied
- Structure: modified COTS 3U CubeSat – ISIS
- Thermal: passive thermo-optical controlled
- Power: COTS GOMSPACE NanoPower P31u EPS (30 W) power system boosted when docked through the EMDS power sharing system; COTS GOMSPACE NanoPower P110 Series solar panels.
- ADCS: COTS CubeSense, CubeControl, CubeComputer, with 1 pitch axis MW as per the QB-50 spacecraft giving 3-axis control to at least ± 2o precision.
- GPS Rx.: for long range operations >10 m separation
- OBC1: Raspberry-Pi, WiFi, and DMP support
- ISL Comms/Data: 2-off 2.4 GHz WiFi; USB
- OBC2: Raspberry Pi – RDV/ Docking/ LIDAR control and redundant WiFi support
- Docking Sensor: Softkinetic DS325 LIDAR/ Camera for RDV
- Docking illumination/optical RDV: (IR LEDs; ADCS nadir sensor)
- EM Docking System: SSC designed EMDS
- Launch Retaining Structure: frangibolts + EMDS
- On-Orbit Retaining Structure: permanent magnets + EMDS
- Propulsion: Surrey designed 9 thruster 6 DOF butane ~10 m/s ?V
The structure, power, ADCS and GPS systems all have flight heritage.
Figure 5: MirrorSat system block diagram (image credit: AAReST collaboration)
Figure 6: MirrorSat internal system layout (image credit: AAReST collaboration)
The CoreSat requirements can be summarized as:
- Must be able to point accurately (< 0.1o 3? error all axes)
- Must be stable in attitude (< 0.02o/s for 600 s) during payload operations.
- Must be able to slew at >3o/s for RDV maneuvers.
- Must be able to mechanically support 2 RMPs (Reference Mirror Payloads) and to supply each of them with 2 W power at 5 V.
- Must provide up to 5 W at 5 V power and I2C communications to the "camera" (image data transfer only) and support boom.
- Must provide up to 5 W at 5 V power to both docked MirrorSats
- Must be able to communicate with the MirrorSats via WiFi and to the ground via a VHF U/L (1.2 kbps) & UHF D/L (9.6 kbit/s)
- Must be able to operate with Sun >20o off optical (Z) axis.
- Must be able to independently sense MirrorSats during RDV/docking
- Must provide hold-downs for MirrorSats, camera and boom during launch
- Must provide launcher interface.
Given these requirements, a spacecraft design based on two COTS ISIS 6U structures placed either side of a COTS ISIS 3U CubeSat structure was made (Figure 7).
Figure 7: CoreSat design showing the front and back view (image credit: AAReST collaboration)
This comprises the following subsystems:
- RMP Payload: 2-off, Caltech supplied
- Structure: modified COTS 3U and two 6U CubeSat – ISIS
- Thermal: passive thermo-optical controlled
- Power: 2-off COTS GOMSPACE NanoPower P31u-S EPS (30 W) power systems cross-strapped and boosted when docked through the EMDS power sharing system; 2-off COTS GOMSPACE NanoPower BP-4 battery packs; COTS GOMSPACE NanoPower P110 Series solar panels.
- ADCS: 2-off COTS CubeSense, CubeControl, and CubeComputer – High Precision 3-axis control <0.1o error systems; 2-off: CubeStar Star Cameras; 2-off STIM210 a high-performance multi-axis gyro IMU (Inertial Measurement Unit) modules + 1 off Viscoelastic vibration damped 4 RWA (Reaction Wheel Assembly)
- GPS Rx.: 2-off for long range opoperations >10 m separation
- OBC1: 2-off Raspberry-Pi, WiFi, and DMP support
- ISL Comms/Data: 4-off 2.4 GHz WiFi; USB
- RF communications: COTS ISIS VHF uplink receiver at 1200 bit/s; VHF quarter-wave monopole antenna; COTS ISIS UHF downlink transmitter at 9600 bit/s; UHF quarter-wave monopole antenna.
- OBC2: 2-off Raspberry Pi – RDV/ docking/ LIDAR control and redundant WiFi support
- Docking sensor: 2-off Softkinetic DS325 LIDAR/ Camera for RDV
- Docking illumination/Optical RDV: 2-off (IR LEDs; ADCS Nadir Sensor)
- EM docking system: 2-off SSC designed EMDS + 2-off passive magnetic docking ports
- Launch retaining structure: frangibolts + EMDS
- On-Orbit retaining structure: permanent magnets + EMDS
- Attach fitting: RUAGSpace PAS 175 launcher interface (low shock - < 200 g) or Dassault MicroSat launcher interface.
The structure, power, GPS, RF and launch adapter systems all have flight heritage. Most of the ADCS system (i.e. that common with the QB-50 precursor missions) has also flown.
Figure 8: CoreSat system block diagram (image credit: AAReST collaboration)
Figure 9: CoreSat internal system layout showing the front and back view (image credit: AAReST collaboration)
The MirrorSats each have an AOCS propulsion subsystem which comprises nine 1 W micro-resistojet thrusters to provide 6 DOF motion. The resistojets have a high degree of reliability, low system complexity and can be operated as a cold gas system in the event of heater failure. This is a new development by Surrey, making use of miniaturized technology to fit so many thrusters into a CubeSat form factor.
Figure 10: System layout of the propulsion subsystem in MirrorSat. Note, a +Z thruster is not flown, due to the presence of the DMP on that facet (image credit: AAReST collaboration)
The thrusters use liquefied butane propellant stored at 2 bar and expelled in gaseous phase at 0.5 to 1 bar via a pressure controlled 7 ml plenum. The 150 ml propellant tank holds 80 g liquefied butane, chosen because it has good density, reasonable specific impulse and has no toxic or carcinogenic qualities. Also, Surrey has much flight experience of using the butane propellant since its use on SNAP-1. The plenum and tank are highly integrated as shown in Figure 11.
The spacecraft design requires a 5 – 10 mN thrust range at ~80s Isp, with 10m/s ?V - 6 m/s for ?V maneuvers, and 4 m/s for attitude control and contingency. The resultant Z-axis torque is ~0.5-1 mNm; and the X/Y–axis torque ~2-4 mNm.
The minimum valve opening time is 2 ms (500 Hz), giving a minimum impulse bit of 10-20 µNs corresponding to a minimum angular speed step change of ~ 0.1o/s. The mass is estimated at 880 gram (800 gram dry mass).
The current status is that the thruster has been built and its performance verified. The integrated plenum/propellant tank is in manufacture for delivery in September 2014. An all-up test firing in vacuum will be carried out Q4 2014.
Figure 11: Integrated plenum and propellant tank (image credit: AAReST collaboration)
RDV&D (Rendezvous & Docking system):
The RDV&D uses the SSC Electro-Magnetic Kelvin Clamp Docking System, which comprises four PWM (Pulse Width Modulated, H-bridge-driven, dual polarity electro-magnets, each of over 900 A-turns passing through the body of the both the MirrorSat and CoreSat spacecraft. These are coupled to three "probe and drogue" (60o cone and 45o cup) type mechanical docking ports, arranged to form and extended the area docking surface (Figure 12). A kinematic constraint is established using the Kelvin Clamp principle (3 spheres slotting into 3 V grooves arranged at 120o).
Figure 12: MirrorSats showing the EM docking systems (image credit: AAReST collaboration)
Once the launch hold-down frangibolts are released, the MirrorSats are just held in place on the CoreSat via the docking ports and small permanent magnet latches. When activated, the electromagnets overcome the fixed magnetic attraction, and allow the satellites to separate. By pulsing the magnets, and changing their polarities, the satellite's relative motion can be controlled within the "capture cone" of the docking port. This has been simulated and experimentally verified on air bearing table tests to extend out to some 30 cm from the port's surface, with a half-cone angle of approximately 45o (Figure 13).
Figure 13: Finite element modeling of the magnetic interaction between the docking ports elements (image credit: AAReST collaboration)
The idea is to use the butane propulsion system to maneuver the MirrorSat into the capture cone at an appropriate relative velocity (< 1cm s-1), and then for the final docking maneuver to be controlled electromagnetically by the ports. The relative pose, distance and velocity of the satellites will be determined by the SoftKinetic LIDAR/camera systems present on both the MirrorSats and the CoreSat. During the maneuver, the satellites communicate with each other via the WiFi ISL, and the maneuver is controlled by the RPi OBCs.
Over the last two years, much practical simulation work has been carried out on air-bearing tables, using rapid prototyped (3D printed) versions of the docking ports incorporated into EM spacecraft structures floating on air cushions and maneuvered by ducted fans or compressed air gas jets. In these tests, the team initially used the Microsoft KINECT® hardware to form the RDV sensor; however, its size meant that it could not be mounted into the free-floating spacecraft.
The project has now switched to using the Softkinetic DS325 LIDAR/ Camera, which does fit across the CubeSat form factor. Further air-bearing table tests are in progress , and the team is evaluating the performance of the hardware in a simulated space environment. - Through extensive software development, the team has enhanced the performance of the LIDAR system, and proved its utility as a docking sensor (Figure 14).
Figure 14: Measured response of the Softkinetic DS325 LIDAR/ Camera using an advanced algorithm (image credit: AAReST collaboration)
The docking port design has now been refined and full metal prototypes have been produced ready for further testing (Figure 15). Once the complete system has been tested on the 2D air-bearing table, we plan to test under more degrees of freedom by placing the satellite on a freely moving carrier system.
Figure 15: Electromagnetic docking systems (image credit: AAReST collaboration)
The ADCS system for the MirrorSats is identical to that produced for the two QB-50 precursor missions launched in 2014. It is made up from three PC104 (standard CubeSat) boards, housing CubeComputer, the CubeSense processing board, and Cube Control. The ADCS implements an EKF (Extended Kalman Filter) and also contains "B-dot" control software for magnetic control of the attitude.
The boards interface to the following sensors and actuators:
- CMOS camera digital sun sensor
- CMOS camera digital Earth sensor
- 3-axis magnetoresistive Magnetometer
- 3-axis magnetorquer (2 rods + 1 coil)
- Pitch-axis small momentum wheel
- GPS receiver.
Figure 16: Photo of the MirrorSat ADCS (image credit: AAReST collaboration)
The system is adequate for the relatively relaxed ADCS requirements of the MirrorSat (pointing accuracy ~ ±2o in all axes); however, the ADCS requirements for the CoreSat/MirrorSat combination, when imaging, are much more challenging:
- pointing (< 0.1o error (3?) in all axes)
- stability (< 0.02o/s for 600 s during imaging)
- slew-rate (>3o/s about Z (telescope) axis for RDV maneuvers)
- ultra-low vibration at the resonant frequencies of the telescope.
This requires the development of a new, high performance precision ADCS system suitable for the CubeSat form factor. The core of this advanced system is based on the MirrorSat ADCS, but without the pitch-axis momentum wheel. Instead, a new four wheel, reaction-wheel assembly (RWA), utilizing precision balanced wheels with viscoelastic damped mountings, is being developed. Each wheel has the following specification:
- 30 mNms @ 5600 rpm
- 2 mNm nominal torque
- 50 mm x 50 mm x 40 mm volume, mass of 185 g
- 3.4 V - 6.0 V operation (maximum 8 V)
- 1.5 W power consumption at maximum torque
- 0.4 W – 0.1 W in normal operation
The wheels are mounted in a pyramid configuration, give full 3-axis control, with redundancy.
To achieve the pointing/stability requirements, precision sensors are needed. To this end two star cameras (CubeStar) and two precision IMUs (Inertial Measurement Units) are fitted. A third star camera may be necessary to achieve the desire precision. The CubeStar (Figure 17) is a low cost, ultra miniature (<100 g, 46 x 33 x 70mm) star camera developed by Stellenbosch University (South Africa). It has an accuracy (1?) of 0.01o cross bore and 0.03o in roll. It can track 20 stars over a 52o x 27o FOV, and has a maximum tracking rate of 0.3o/s, and an update rate of 1 Hz.
Figure 17: Photos of the CubeStar camera and the STIM210 multi-axis MEMS gyro (image credit: AAReST collaboration)
The IMU is an SSC development under the SMESat (Small and Medium Enterprise Satellite) project (3U CubeSat), based on Sensonor AS (Norway) MEMS gyros. The STIM210 (Figure 17, right) is a high-performance multi-axis gyro module including up to 3 gyroscopes (1, 2 or 3 axes). It has:
- Low bias instability (0.5o/h)
- Low bias drift over temperature gradients (10o/h)
- Low noise (0.15o/?h)
- Short start-up time (1s)
- Integrated electronics.
Initial simulations show that this combination of sensors and actuators should meet the demandng ADCS requirements of the mission. The next steps are to:
- assess the RW jitter (Kistler table tests)
- test the new damping system on the RW (Kistler table tests)
- develop a jitter disturbance model and to determine the type of resonance modes we have at spacecraft level, camera level and RW level (Finite Element Analysis)
- to determine the predominant micro-vibration sources from the reaction wheel (Kistler test).
• The CDR (Critical Design Review) is scheduled for September 2015.
• The AAReST project passed its DDD (Detailed Design Review) in September 2014. 6)
Launch: Launch readyness of AAReST is planned for 2016. The telescope is to launch as a small secondary payload (technology demonstration flight) in a stowed state as shown in Figure 3.
Orbit: Current reference orbits: 650 km SSO and ISS.
Telescope Architecture and DMP (Deformable Mirror Payload)
The DMP is supplied by the SSL (Space Structures Laboratory ) of Caltech. The stowed volume of the telescope is 0.5 m x 0.5 m x 0.6 m. After separation from the primary payload, the telescope will deploy its sensor package to the focus of the mirror array using a hinged mast. 7)
Figure 18: AAReST spacecraft components (Caltech/SSL)
Figure 18 depicts the individual components of AAReST. It is comprised of a number of subsystems: the central "Coresat", 2 separate "MirrorCraft", a camera package and a deployable boom. The CoreSat is the central hub of the telescope containing the bulk of the spacecraft's control hardware. The camera package, located at the focus of the telescope contains various elements such as corrective optics, wavefront sensors and an imaging detector. A foldable composite boom separates the camera package from the primary aperture of the telescope.
The MirrorCrafts are small independent spacecraft used to house the deformable mirrors. The deformable mirrors sit atop these spacecraft with the control electronics directly underneath. Each MirrorCraft is equipped with its own propulsion system in order to perform the autonomous reconfiguration and docking maneuvers. The docking procedure is aided through the use of electromagnets and a computer vision system.
Figure 19: .The AAReST spacecraft in the (a) "Launch" configuration, (b) "Compact" imaging mode, and (c) "Wide imaging mode (image credit: Caltech/SSL)
Figure 19 shows the 3 configurations of the AAReST spacecraft. The first is the "Launch" configuration where the deployable boom is folded tightly around the CoreSat. This is done in order to minimize the launch volume. Once in orbit, the boom will deploy and position the camera package at the focus of the telescope. The telescope is now in the "Compact" imaging configuration. At this point in the mission, the initial mirror calibration will take place and the first set of images will be captured. Once all objectives have been completed in this configuration, two of the MirrorCraft will perform an autonomous reconfiguration maneuver and position themselves at the outer edges of the spacecraft. The telescope is now in the "Wide" configuration where the mirror calibration and imaging procedure will be repeated, thus demonstrating the unique capabilities of the deformable mirrors.
Deformable Mirrors: The deformable mirrors are made using thin glass wafers and a layer of piezoelectric polymer. A custom electrode pattern is incorporated onto the backside of the piezoelectric polymer allowing for shape control to be performed. The mirrors have a total stroke of ~40 µm, allowing for the correction of large shape errors. 8) 9)
In the proof-of-concept assembly, 10 cm diameter mirrors with an areal density of 0.6??kg/m2 have been designed, built, and tested to measure their shape-correction performance and verify the finite-element models used for design. The low-cost manufacturing scheme involves low-temperature processing steps (below 140°C) to minimize residual stresses, does not require precision photolithography, and is therefore scalable to larger diameters depending on application requirements.
Figure 20: Left: Exploded view of the various layers within the deformable mirror. Right: CAD model of the deformable mirror package used for AAReST (image credit: Caltech/SSL)
Deployable boom: The focal length of the telescope is provided by a thin-walled composite boom. The boom, fabricated in collaboration with our partners at AFRL, is constructed using a combination of unidirectional carbon fiber and plain-weave fiber glass. Material is removed at select locations along the length of the boom creating self-deploying tape-spring hinges. Four hinges are implemented in the design of AAReST, allowing the boom to be tightly packaged around the spacecraft for launch. Deployment is performed using a two-stage process as shown in Figure 21. 10)
Figure 21: Overview of the two-stage deployment process for the AAReST spacecraft (image credit: Caltech/SSL)
Camera Package: The camera package houses the optical and electronic hardware required for imaging. Figure 22 is a ray-trace of the optical path within the camera. Light originating from the prime focus of the telescope enters the camera and is passed through a series of collimating lenses. A pair of beam splitters redirect a portion of this light onto two Shack Hartmann wavefront sensors providing knowledge of the shape of the deformable mirrors. The remaining portion travels through a pupil mask and through another set of lenses to re-image the light onto the imaging detector. Images are captured using this detector once the telescope has gone through its calibration process.
Figure 22: Geometric ray-trace of the optical path within the Camera Package (image credit: Caltech/SSL)
1) Craig Underwood, Sergio Pellegrino, Vaios J. Lappas, Chris Bridges, John Baker, "Using CubeSat/Micro-Satellite Technology to Demonstrate the Autonomous Assembly of a Reconfigurable Space Telescope (AAReST)," Proceedings of the 65th International Astronautical Congress (IAC 2014), Toronto, Canada, Sept. 29-Oct. 3, 2014, paper: IAC-14.B4.2.4
2) Craig Underwood, Vaios J. Lappas, Chris Bridges, "AAReST Spacecraft DDR (Detailed Design Review): Spacecraft Bus, Propulsion, RDV/Docking and Precision ADCS," 2014, URL: http://pellegrino.caltech.edu/AAReST_Docs/AAReST_Surrey_DDR_2014.pdf
3) Craig Underwood, Sergio Pellegrino, "Autonomous Assembly of a Reconfigurable Space Tele-scope (AAReST) for Astronomy and Earth Observation," 2011, URL: http://pellegrino.caltech.edu/PUBLICATIONS/Autonomous%20Assembly%20of%20-a%20Reconfigurable%202011.pdf
4) Craig Underwood, Sergio Pellegrino, Vaios Lappas, Chris Bridges, Ben Taylor, Savan Chhaniyara, Theodoros Theodorou, Peter Shaw, Manan Arya, James Breckinridge, Kristina Hogstrom, Keith D. Patterson, John Steeves, Lee Wilson, Nadjim Horr, "Autonomous Assembly of a Reconfiguarble Space Telescope (AAReST) – A CubeSat/Microsatellite Based Technology Demonstrator," Proceedings of the 27th AIAA/USU Conference, Small Satellite Constellations, Logan, Utah, USA, Aug. 10-15, 2013, paper: SSC13-VI-5, URL: http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=2952&context=smallsat
5) Craig Underwood, Sergio Pellegrino, Ben Taylor, Savan Chhaniyara, Nadjim Horri"Autonomous Assembly of a Reconfigurable Space Telescope (AAReST) Rendezvous and Docking on a 2D Test-bed," 9th IAA Symposium on Small Satellites for Earth Observation Berlin, Germany, April 8 -12, 2013, IAA-B9-0508, URL: http://media.dlr.de:8080/erez4/erez?cmd=get&src=os/IAA/archive9/Presentations/IAA-B9-0508.pdf
6) John Baker, "AAReST Detailed Design Review -Mission Overview," Caltech, Sept. 8, 2014, URL: http://pellegrino.caltech.edu/AAReST_Docs/AAReST_Caltech_DDR_2014.pdf
8) Keith Patterson, "Lightweight Deformable Mirrors for future Space Telescopes," Thesis In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy," California Institute of Technology, Pasadena, CA, December 2013, URL: http://thesis.library.caltech.edu/8043/1/patterson_keith_2013_thesis.pdf
9) Keith Patterson, Sergio Pellegrino, "Ultralightweight deformable mirrors," Applied Optics, Vol. 52, Issue 22, 2013, pp: 5327-5341,
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).