Dellingr CubeSat Demonstration Mission
Launched in August 2017, Dellingr (named after the Norse God of the dawn), the first 6U CubeSat, is an award-winning demonstration mission operated by NASA and GSFC (Goddard Space Flight Centre). Dellingr tested a suite of new technologies relating to nanosatellite design that aims to make future satellite development easier, more efficient, and more cost-effective. The instruments being tested focus on measurements of the Earth’s magnetosphere and composition of the Earth’s atmosphere.
|Launch date||August 2017|
Dellingr’s sensor complement consists of the INMS (Ion and Neutral Mass Spectrometer) and the DAGR (Distributed Acquisition for Geomagnetic Research).
The INMS takes in situ measurements of both neutral and ion composition and density in the atmosphere in order to define steady state background atmospheric conditions. This data can be used, for example, in studies of the dynamic ionosphere-thermosphere-mesosphere system.
The DAGR instrument includes three science-grade fluxgate magnetometers. One of these is positioned at the end of an extendable 76 cm boom and takes measurements of the Earth’s magnetic field. The other two are positioned internally and measure electromagnetic interference from the spacecraft. Newly developed algorithms then remove this noise from the measurements taken from the external magnetometer.
The INMS takes measurements of ions and neutral hydrogen, helium, nitrogen, and oxygen at an incoming energy range of 0 - 50 eV. Data handling can be commanded at up to 10 ms/frame, which corresponds to approximately 80 m of spatial separation. The instrument takes up 1.3U in volume, weighing 570 g and consuming 1.6 W of power.
The fluxgate magnetometer sensors of the DAGR have a sensitivity of 12 pT/√Hz at 1 Hz and 24-bit A/D. The resolution of these sensors is less than 0.1 nT at a sampling rate of 3.5 Hz. Each sensor weighs 19 g and consumes 750 mW.
Dellingr follows a near circular orbit with an altitude of approximately 400 km and an inclination of 51.6°.
Space and Hardware Components
Dellingr contains several interacting subsystems. C&DH (Command and Data Handling) is performed by the GomSpace NanoMind A712D, which consists of an Amtel ARM microcontroller that runs at 40 MHz, 2 MB of SRAM, and 8 MB of flash memory. This flight computer sends commands and collects telemetry, sending processed data to the L3 Cadet-U radio as needed. Data transmission is achieved through UHF uplink and downlink, with two modified stainless steel retractable tape measure strips functioning as deployable dipole antennas. The radio doubles as primary storage for satellite telemetry and science data and has a 3 Mbit/s downlink speed.
The EPS (Electrical Power Subsystem) comprises six body mounted solar panels, an MPPT (Maximum Power Point Tracking) electrical power system, and two Clyde Space 40 Whr standalone batteries with temperature limits of 0° C to 45° C. The electronics on board generate heat when in use, and this is controlled through a passive Thermal Control Subsystem (TCS). The TCS comprises flaps attached to a thermally sensitive spring. When heated, the spring expands, opening the flaps, and when cooled, the spring retracts, closing the flaps.
To maintain altitude and attitude, Dellingr’s ACS (Attitude Control System) contains fine and coarse sun sensors, an MEMs gyro, a magnetometer, torque coils, and three reaction wheels. Through these, attitude maintenance and 3-axis stabilisation can be achieved.
Dellingr CubeSat Demonstration Mission
A team at NASA/GSFC has developed the first 6U CubeSat (a nanosatellite) and hopes to send it to the ISS in 2017 for deployment from the space station. To be known as Dellingr, after the Norse god of the dawn, the satellite will carry three ionospheric payloads to study the ionosphere - the outer region of Earth's atmosphere populated by charged particles, ionized by incoming solar radiation and magnetospheric particle precipitation. The magnetosphere is Earth's magnetic environment, and sometimes, charged particles in this region are deposited into the atmosphere below.
The development of this larger CubeSat takes the utilization of the CubeSat technology into a new phase. Previous CubeSats did not always provide scientists with the payload volume they desired. This larger size does that, while it also allows more space for systems that increase the success rate of the CubeSat.
The Dellingr nanosatellite, with double the size of 3U CubeSats. Once the domain of university researchers, CubeSats in recent years have increasingly become more popular among government researchers. Motivated in large by their growing capabilities and relatively inexpensive cost, NASA and other government agencies are increasing their investments in CubeSats. NASA now is funding CubeSat mission opportunities through various programs.
In February 2014, Michael Johnson and Michael Hesse of NASA/GSFC pulled together a small team of Goddard engineers and scientists to develop and implement "lean," end-to-end techniques, processes and systems for a 6U CubeSat capable of carrying out formidable scientific missions. As part of its charter, the Dellingr team was to design, test and deliver a flight-ready, fully integrated 6U CubeSat by February 2015, at a fraction of the cost of more traditional satellite missions. 1) 2) 3) 4)
The primary goals of Dellingr were to: 5)
1) Develop a cost-effective Center model for CubeSat development
2) Develop tailored, lean, and scalable end-to-end systems
3) Determine lessons learned, and apply them to the next generation of satellites.
The focus of the Dellingr project was primarily as a pathfinder effort to determine an appropriate level of GSFC processes that should be applied to these types of platforms without burdening the project with excessive requirements or processes while implementing an acceptable level of reliability. The project was also guided by a few defining principles, including:
- Pairing experienced with junior engineers to facilitate training
- Keeping the core team as small as possible, and reaching out to the Center for focused expertise as needed
- Smartly applying GSFC knowledge and tailored procedures
- Minimizing up front component testing and "Test as we fly" to the fullest extent possible
- Utilizing table-top reviews with subject matter experts, when needed – no confirmation gates.
When a tiger team set out to design, build, integrate, and test a large shoebox-sized satellite capable of executing NASA-class science, the group didn't completely appreciate the challenges it would face. It knows now. And because of the experience, Goddard currently is spearheading initiatives to dramatically improve the reliability of small satellite-based systems and missions. 6) 7)
Although the six-unit, or 6U, Dellingr spacecraft is nearly finished and awaiting launch to the International Space Station in the summer of 2017, it took double the time and more funds than budgeted to build a robust CubeSat mission capable of achieving NASA-quality science.
The experience underscored a hard truth, said Michael Johnson, the chief technologist of Goddard's Applied Engineering and Technology Directorate, who championed the Dellingr pilot project. "In many cases, our desired level of performance wasn't there," he said, referring to many small satellite-related components and subsystems offered by commercial vendors. "The Dellingr team encountered more than a handful of technical challenges with components right out of the box that affected the project's cost and schedule. This wasn't a theoretical activity. Some of the systems simply did not meet our expectations."
Johnson and a team of other experts initiated the GMSA (Goddard Modular SmallSat Architecture). The effort is aimed at developing overarching system designs and technologies to dramatically reduce SmallSat mission risks without significantly increasing the platforms' cost. As part of this initiative that also addresses mitigating the adverse effects of radiation, Goddard technologists have made progress developing new mission-critical command and data handling and electrical power-system technologies, he said.
MUSTANG (Modular Unified Space Technology Avionics for Next Generation) mission: MUSTANG — through its use of mix-and-match electronics cards — acts as the mission's brain and central nervous system, controlling every function needed to gather scientific data from a Small Explorer (SMEX)-type mission.
The Dellingr nanosatellite (30 x 20 x 10 cm) was designed from a clean sheet and is largely based on COTS (Commercial Off The Shelf) components to keep costs down. However, in the process of developing the satellite platform, it became clear that COTS components oftentimes do not work together as advertised, requiring an additional engineering effort and driving up costs. Nevertheless, Dellingr represents an adaptable platform suitable for a number of applications to achieve compelling science on a limited budget. The satellite platform provides sufficient power margin for a variety of instruments and the craft's attitude determination and control system provides precise pointing and stability needed by Earth-remote-sensing missions as well as astrophysics payloads. 8)
Dellingr is a 3-axis stabilized spacecraft consisting of fine and coarse sun sensors, MEMs gyro, magnetometer, torque coils and three reaction wheels (Figure 1). The communication system is UHF uplink and downlink with a deployable dipole antenna. In addition, the spacecraft includes six body mounted solar panels with a maximum power point tracking (MPPT) electrical power system and lithium polymer batteries. A passive thermal system maintains temperature of components within limits. The on-board computer utilizes the in-house core Flight Software (cFS) and handles the command and data handling in addition to the guidance, navigation and control processing.
The instruments were selected based on their level of maturity and achievable compelling science. The Spacecraft bus and orbit operations were architected based on the instruments and science requirements. Requirements were periodically reevaluated as the system matured in an effort to reduce cost and schedule without compromising or significantly affecting the science products. The pointing requirement is one example of such a trade. Initially the INMS instrument required pointing knowledge of less than 0.5 degree. Such requirement would leverage a more expensive solution while compelling science could still be achieved at a relaxed pointing knowledge enabling a cheaper solution with sun sensor and MEMs gyro instead of star trackers. This is a common theme across the project up until delivery and operations. A combined science-bus team effort is paramount for trades between science requirements and bus capability decisions, compromising at both ends to reach science goals and a feasible technical solution within budget.
A second example of a trade between science and bus is the decision to use only body mounted solar panels. Deployed arrays were not implemented in an effort to minimize technical complexity. Instead, science operations were modified to allow for charging orbits in between science orbits. This adaptation enabled science goals to be met with reduced programmatic and development costs.
An International Space Station (ISS) deployment was selected because that represented the quickest pathway to launch for a 6U satellite. This orbit presents challenges from the power and thermal standpoints because of the beta angle changes. At the same time, the orbit offers a benign radiation environment.
Mechanical structure and mechanisms: Since the mission lacked a manifest throughout the development, the project had to assume interface requirements. As such, the structure was designed to the PSC (Planetary Systems Corporation) standard. Overall volume allocation is 366 mm x 239 mm x 113 mm with 2 tabs along the base edge which serves as the interface with the deployer. The NanoRacks 6U deployer, used to jettison Dellingr from the ISS, is compatible with the PSC standard.
The majority of the bus components are mounted to a baseplate, while the remaining sides of the structure are solar closeout panels joined by structural bars (Figure 2). The baseplate is the primary load path since it contains the deployer mounting edge and it also houses the majority of the mass.
Dellingr contains two deployables, the UHF antenna and a magnetometer boom, each utilizing the same type of release mechanism developed during the mission (Figure 3).
The two UHF antennas are modified stainless steel retractable tape measure strips. Each strip was cut to length, the original coating removed, and a white paint coating added. The UHF antenna strips protrude from an opening in each of the 3U panels and fold into a single point at the adjoining 2U panel while held by the release mechanism (Figure 4). The strips immediately retake their linear shape upon release.
The magnetometer boom consists of an upper and lower arm, with elbow and shoulder hinges (Figure5). The boom is stowed in a pocket on the underside of the spacecraft baseplate, and is restrained for launch near the shoulder hinge, with passive restraint at the elbow hinge. The magnetometer mounts to the end of the 52 cm, 0.2 kg, boom assembly.
The UHF antenna and magnetometer boom utilize the same DANY (Diminutive Assembly for Nanosatellite deploYables) release mechanism developed internally in 2013 (U.S. PTO Number 9,546,008). Each release mechanism consists of two spring-loaded plungers held in place by a plastic retaining bar of ABS Plus material . The plungers will retract under the spring force once the plastic piece is heated by redundant heating elements. Each mechanism also contains redundant separation switches to confirm proper functionality of the release action.
EPS (Electrical Power Subsystem): The final flight power subsystem consists of a Clyde Space 3rd generation EPS, two Clyde Space 40 Whr standalone batteries, and fixed solar panels produced in-house. The original design used a similar EPS and three 30 Whr batteries (90 W total) from Clyde Space. These components were used throughout a large part of Dellingr's development but, ultimately, had to be replaced by the newer battery components for a variety of reasons, including ISS safety compliance. Such compliance also forced a reduction of the battery capacity to 80 W hr maximum and required that inhibits entirely isolate the battery. Clyde Space was able to provide 40 W hr batteries that met the inhibit specifications and underwent the extra testing required. This reduction in battery capacity, along with higher than expected power requirements for other subsystems, required a modification to the mission profile to allow for more charging orbits (sun-pointing) for each science orbit.
A design compromise was reached to account for the number of switched power buses available on the EPS. Even with ten available, additional switches were added to a special services card and some components share a single switched bus (e.g., all three reaction wheels are on the same bus).
ACS (Attitude Control System): The ACS is comprised of a complement of sensors and actuators comparable to larger satellites. The main premise of the attitude determination scheme is to combine information from sun sensors, magnetometers, and inertial rate sensors in whatever combination is best at any given position in orbit. Once the attitude is determined, three reaction wheels are commanded to null errors relative to a selected target attitude and rate. System angular momentum tends to increase in the chosen mission attitudes, so magnetic torque coils in the body face solar panels are employed to remove that momentum over time.
The ACS algorithms were developed in a Matlab/Simulink simulation based on a heritage simulation and ACS design from the SDO ( Solar Dynamics Observatory) mission. The simulation includes translational and rotational dynamics, multibody gravitational models, an up-to-date magnetic field model, and a set of representative sensor and actuator models. Selected functionality, such as ACS Failure Detection and Correction, was omitted from the simulation to meet Dellingr's low-cost and fast turnaround requirements. The ACS team used the simulation to develop an algorithm document, upon which the actual C-based ACS Flight Software (FSW) was developed. The ACS includes two main wheel-based Proportional–Integral–Derivative controllers — Sun Pointing and LVLH (Local-Vertical, Local Horizontal) — and simple magnetic momentum management algorithms. An extended Kalman filter, representing essentially a stripped-down version of the heritage filter from SDO, was implemented to achieve desired onboard attitude knowledge.
The ACS mode application was one of several applications that run on the cFS architecture. Important ACS hardware processes such as GPS data ingest, reaction wheel tachometer processing, and gyro readouts, are organized into their own applications or sub-applications for priority processing in advance of the ACS mode execution, or in some cases, more frequent execution than the baseline 1Hz ACS sample rate.
RF communications: Dellingr uses an L3 Cadet-U radio for RF communication and primary storage for satellite telemetry and science data. The Cadet-U radio is a half-duplex UHF transceiver with downlink speed of 3 Mbit/s. The radio interfaces with the NanoMind through a 57600 baud serial port. Its storage area is divided into HIGH-FIFO (smaller size) and LOW-FIFO (larger size). Dellingr used this feature to store the most recent housekeeping data in the HIGH-FIFO memory while saving the rest of telemetry and science data in LOW-FIFO.
C&DH (Command and Data Handling): Dellingr uses a GomSpace NanoMind A712D as the flight computer. The GomSpace NanoMind consists of an Amtel ARM microcontroller that runs at 40 MHz, 2 MB of SRAM, and 8 MB of flash memory. The NanoMind interfaces with each subsystem, sending commands and collecting telemetry that the NanoMind then processes and sends to the radio as needed. The ACS algorithms are also run on the NanoMind.
Several communication buses are provided by the NanoMind, including I2C, SPI and three UARTs. These are further supplemented by the SSC (Special Services Card). The SSC provides additional Analog-to-Digital (A2D) inputs, GPIO (General Purpose Input/Output) pins, and additional UARTS.
The NanoMind was delivered with firmware that included the FreeRTOS real time operating system, a complete set of device drivers for the communications buses, and a diagnostic shell. The GomSpace Diagnostic shell was utilized as a framework for diagnostic tests, allowing hardware checkout and aliveness tests to be run without reloading the software.
TCS (Thermal Control Subsystem): The TCS is a passive design relying on heat conduction from powered components into the common aluminum baseplate which radiates to space. The solar cells are body mounted on all of the sides, which means that the sides of Dellingr that view direct sun can get hot and not make for a good radiator. The interior surfaces were coated with low emissivity material to better protect Dellingr from getting too hot when the solar cells were pointed to the sun. The nadir pointing side was used as the baseplate and radiator since it doesn't get direct sun. The solar cells on the baseplate side are high emissivity, and the exposed metal was coated with high emissivity Teflon impregnated anodize. The baseplate was designed to be higher in mass than required by structural analysis in order to dampen the transient temperature swing during low beta angles when Dellingr is going from full sun into eclipse behind Earth.
The thermal limiting component were the batteries, with flight allowable temperature limits of 0°C and +45°C. The radiator area was adequate to keep them from reaching the 45°C hot limit. The internal battery heaters have sufficient power to overcome the rapid cooling that occurs when going into eclipse.
The L3 Cadet radio produces a hot temperature concern since it dissipates a total of 10 W when it transmits during a 10 minute pass. To mitigate this problem, the radio was mounted on the housing with the use of NuSil thermal interface material to enhance the thermal path and dampen the temperature spike when the radio transmits. In addition, FDC (Failure Detection and Correction) will preclude Lo-FIFO transmissions if the cadet radio temperature exceeds a certain value.
The magnetometer boom was anodized for high emissivity to avoid overheating during sun exposure. The magnetometer at the boom tip is lightweight and changes rapidly in temperature as the environment changes from full sun to eclipse. A tailorable emittance coating was manufactured with a low enough emissivity to slow down the temperature drop during eclipse and an even lower solar absorptance to slow down the temperature spike during sun exposure.
Neither component nor subsystem level TVAC (Thermal Vacuum) testing was done except for release mechanism, boom and solar panel engineering units. A total of 8 thermal cycles were done at the system level. A variety of problems were found, and some components had to be repaired or replaced. A lack of component level testing precluded driving them to the qualification temperature levels since the batteries were limiting the temperature range at the system level.
Four thermal balance points were done. Since each side of Dellingr saw different sink temperatures for a particular orbit and no sides were insulated, the most appropriate way to run a thermal balance would've been to have different GSE test thermal zones looking at each side. Due to cost limitations, it was decided to use a two-zone approach where the baseplate saw one temperature, and the other 5 sides of the spacecraft saw a different one. Settings were tweaked to match both temperature predictions and heat flows. Thermal balance test results demonstrated that the system is safely running cooler than predicted, which is a more desirable and manageable situation than running warmer.
Introduction of a new thermal-control technology for CubeSats: An older technology once proper for preventing spacecraft gadgetry from getting too hot or too cold has been resurrected and repurposed for an emerging class of small satellites now playing an increasingly larger role in space exploration, technology demonstration, and scientific research. NASA PI Allison Evans and her team at the Goddard Space Flight Center have successfully miniaturized the thermal-control technology and now plan to flight test it on the maiden flight of the GSFC-developed Dellingr spacecraft, a new-fangled 6U CubeSat purposely created to easily accommodate NASA-class science investigations at a lower cost. The tiny Dellingr measures 30 cm long and 10 cm high, and could launch later this year, developers said. 9)
Overcoming thermal-control challenges: The higher availability of power has created some challenges. How do mission designers regulate the temperature of the more power-hungry or temperature-sensitive flight instruments inside the satellite bus that can be as small or smaller than a cereal box? Electronic thermal-control devices add mass and consume valuable space, making them less appropriate for small satellites. "One of the things I observed at the time was that no one had begun developing passive thermal-control technology for CubeSats," Evans said.
She embraced the challenge, turning to a technology used initially in the 1960s — a large panel of louvers measuring a up to 1m in diameter. Like venetian blinds, the louvered flaps would open or close depending on whether an instrument needed to shed or conserve heat. The new thermal-control louver technology operates in much the same way as its forebear. It, too, requires no electronics and is completely passive.
The device measures 10 cm on a side and can be linked together to accommodate almost any small-satellite mission, whether it is a 3U CubeSat or something larger than even Dellingr. Each unit includes front and back plates, flaps, and springs. The back plate is painted with a white, highly emissive paint — boron nitride nano mesh (BNNM) — developed by Goddard materials expert Mark Hasegawa. The front plate and flaps are made of aluminum, which aren't as emissive.
"The bimetallic springs do the heavy lifting," Evans said. Measuring just 6 mm in diameter, the springs are made of two different types of metal. Attached to the highly emissive back plate, they uncurl if one of the metals gets too hot, forcing the flaps to open. When the spring cools down, it reverts back to its original shape and the flaps close.
Testing proves effectiveness: Since building the device, Evans and her team have put the unit through the paces to determine performance. In a benchtop lifecycle test, the team ran more than 12,900 cycles exposing the device to temperatures of between 32º to 55º Celsius. The bimetallic springs experienced no failures, she said. The team also carried out eight thermal-vacuum test cycles at temperatures of between minus -20º and 85º Celsius, finding that the technology dissipated an amount of thermal energy significant to a CubeSat. A vibration test also indicated the technology had achieved qualification levels.
The proof of its effectiveness, however, will come during the Dellingr mission, she said. During that technology demonstration involving a smaller experimental version, she is anticipating that the device will prove its effectiveness as a spaceflight technology.
Thermal louver experiment demonstration: For future CubeSat missions, thermal louvers could be a passive means of thermal control to stabilize internal temperatures. The Thermal Louver Experiment is intended to raise the TRL of an in-house GSFC development for this class of thermal louvers. As shown in Figure 12, the Thermal Louver Experiment consists of a single flap and bimetal spring combination thermally isolated from the spacecraft and monitored with both an infrared motion sensor for flap movement and a pair of thermocouples for temperature detection. At discrete times during the mission, ground commands will be sent to power on the heater. Experiment success relies on the spring heating up, resulting in the flap opening and triggering the proximity sensor.
• August 2, 2017: Along for the ride on Dellingr's maiden journey is a suite of miniaturized NASA-developed technologies — one no larger than a fingernail — that in many cases already have proven their mettle in suborbital or space demonstrations, boosting confidence that they will perform as designed once in orbit. Scientists and engineers at NASA/GSFC built all the instruments, primarily with research-and-development program funding. 10)
• Dellingr was delivered to NanoRacks on May 31, 2017 and is being processed for a planned August launch aboard the Falcon-9 Commercial Resupply (CRS) SpX-12 as part of ELaNa-22 Mission.
• August 8, 2016: The construction of the Dellingr nanosatellite is complete, this is followed for environmental testing. 11)
The Dellingr 6U CubeSat was launched as a secondary payload on August 14, 2017 (16:31.37 UTC) from Launch Complex 39A (LC-39A) at NASA's Kennedy Space Center, Florida. The primary mission was the SpaceX CRS-12 Dragon logistics flight on a Falcon-9 v1.2 vehicle to the ISS. 12) 13) 14)
The Falcon 9 rocket's reusable first stage performed a controlled landing on Landing Zone 1 (LZ1) at Cape Canaveral Air Force Station.
SpaceX's twelfth contracted cargo resupply mission with NASA to the International Space Station deliver ed more than 2,910 kg of science and research, crew supplies and vehicle hardware to the orbital laboratory and its crew. This included 1,652 kg of pressurized cargo with packaging bound for the ISS, and 1,258 kg of unpressurized cargo composed of the CREAM (Cosmic-Ray Energetics and Mass) instrument, to be mounted externally to the ISS (mounting on a facility outside the station's Japanese Kibo module). The CREAM payload will spend at least three years sampling particles sent speeding through the universe by cataclysmic supernova explosions, and perhaps other exotic phenomena like dark matter. 15)
Orbit: Near circular orbit, altitude of ~400 km, inclination = 51.6º.
The secondary payloads are
Four small satellites inside the Dragon capsule will be transferred inside the space station for deployment later this year.
• Kestrel Eye-2M is a pathfinder microsatellite (~50 kg) for a potential constellation of Earth-imaging spacecraft for the U.S. military. From the ISS orbit, Kestrel Eye-2M's optical camera will be able to spot objects on Earth's surface about the size of a car. The objective Kestrel Eye imagery data is to downlink directly to provide rapid situational awareness to Army brigade combat teams in theater without the need for continental United States relays.
• ASTERIA (Arcsecond Space Telescope Enabling Research in Astrophysics), a 6U CubeSat (12 kg) of MIT and NASA/JPL. The objective is to test miniature telescope components that could be used in future small satellites to observe stars and search for exoplanets.
• Dellingr, a NASA demonstration mission on a 6U CubeSat.
• OSIRIS (Orbital Satellite for Investigating the Response of the Ionosphere to Stimulation and Space Weather) is a 3U Cubesat of PSU (Penn State University), University Park, PA, USA. Working in coordination with the Arecibo Observatory, a giant radar antenna in Puerto Rico, OSIRIS-3U will fly into a region of the ionosphere heated to simulate the conditions caused by solar storms.
• Spring 2019: A miniaturized fluxgate magnetometer that helped stop the Dellingr spacecraft from a potentially mission-ending spin has secured a flight aboard a NASA CubeSat mission — NASA's first with Brazil — and is now undergoing a significant upgrade that would benefit both space- and ground-based data collection. 16)
- Developed exclusively with support from Goddard's Internal Research and Development Program (IRAD), the miniaturized fluxgate magnetometer proved that scientists could reduce the size of these powerful instruments and gather scientifically useful magnetic-field measurements from small platforms sometimes no larger than a shoebox, said PI (Principal Investigator) Todd Bonalsky.
- The instrument made its debut as one of two magnetometers aboard the Dellingr mission, created at Goddard to improve the reliability and resiliency of CubeSat platforms. Shortly after its launch in 2017, the Dellingr spacecraft began to spin, crippling communication and preventing one of the mission's miniaturized mass spectrometers from collecting usable data. To slow down the tumbling, mission controllers wrote and uploaded new software and used Bonalsky's miniaturized fluxgate magnetometer as an attitude sensor to provide the data needed to activate Dellingr's torquers and help stabilize the spinning. Dellingr is now collecting useful data.
- The same instrument has also flown on a couple sounding rocket missions and will gather data on the SPORT (Scintillation Prediction Observations Research Task) mission expected to launch in 2020. The objective of this joint NASA partnership with the Brazilian National Institute for Space Research is understanding the conditions in Earth's ionosphere that lead to scintillation, which can compromise GPS and other transmissions from low-Earth orbit.
- Perhaps more exciting, though, are efforts to develop a self-calibrating, miniaturized magnetometer. It could fly on CubeSats and sounding rockets, but also be used as a ground-based instrument for NASA's first-ever effort to use high-voltage power lines as a super-scale antenna for gathering measurements about geomagnetically induced currents.
- With funding from Goddard's FY19 IRAD program, Bonalsky is continuing an effort he began two years ago to combine the flight-proven miniaturized fluxgate magnetometer that can measure both a magnetic field's magnitude and direction, with an optically pumped atomic magnetometer. With the marriage of the two types, Bonalsky said he would create a self-calibrating instrument.
- "Our miniaturized fluxgate system, which has been so successful on Dellingr and other flight programs, is prone to drift over long periods of time due to wide and repeated temperature variations," Bonalsky said. However, atomic magnetometers, which can only measure the magnetic field's magnitude, not its direction, could maintain the fluxgate's calibration because they aren't prone to drift.
- "A self-calibrating fluxgate magnetometer would be very valuable to us," said Antti Pulkkinen, the Goddard scientist spearheading the power-grid study. "We could put them in the ground and literally walk away without worrying about whether they are properly calibrated. They would do it themselves," Bonalsky added. "I want to put them in the field!"
• October 23, 2018: The road to launch wasn't always smooth, but the scientists and engineers who built the Dellingr spacecraft proved they could develop a more capable and resilient CubeSat platform, and in the process, advance the state-of-the-art in this increasingly more important mission class. 17)
- For the group's efforts, the OCT (Office of the Chief Technologist) at the Goddard Space Flight Center in Greenbelt, Maryland, awarded its IRAD (Internal Research and Development) Team Award to the engineers and scientists who created and now operate Dellingr and its payloads, including an advanced time-of-flight INMS (Ion-Neutral Mass Spectrometer), two miniaturized magnetometer systems, a release mechanism called DANY(Diminutive Assembly for Nanosatellite Deployables), and a miniaturized thermal-control technology.
- The OCT bestows the award annually on those who demonstrate the best in leading-edge research and development.
- Since Dellingr's deployment from the International Space Station in November 2017, the shoebox-sized spacecraft and its miniaturized science and engineering payloads have demonstrated new capabilities and gathered high-quality data about the densities of ionized atom species in Earth's upper atmosphere.
- Just as important, lessons learned from the Dellingr effort retired considerable cost and schedule risks for future SmallSat missions. It matured the skills of Goddard personnel and contributed to the win of three additional CubeSat missions, including petitSat, GTOSat, and BurstCube, said Michael Johnson, the chief technologist of Goddard's Engineering and Technology Directorate and one of the Dellingr project founders. Efforts are now underway to create a follow-on SmallSat architecture — DellingrX — which will extend capabilities beyond CubeSats and be capable of operating outside the relatively benign radiation environment found in LEO (Low Earth Orbit).
- "When our team began developing Dellingr about four years ago, the goal was to build a relatively inexpensive and more reliable CubeSat platform. The goal was to advance the state-of-the-art in small-satellite capabilities," said Goddard Chief Technologist Peter Hughes, in announcing the award [Goddard press release]. "Despite a tight budget and aggressive schedule, this team accomplished this and more. The team inaugurated a new era for scientists wanting to use small, highly resilient satellites to carry out important, and in some cases, never-before-tried science."
Driven by Need
- The quest to build a less-expensive, more resilient CubeSat platform was driven in response to growing interest among NASA and other government scientists. They sought a low-cost platform on which to fly instruments, as well as the ability to fly constellations of these tiny craft to gather simultaneous, multi-point observations — a technique not financially possible with more traditional spacecraft.
- Unfortunately, many low-cost commercially available CubeSat platforms, subsystems, and components used by universities mainly for classroom instruction could not support NASA-class science.
- Developing a new platform, however, proved challenging. The team had planned to use commercial components to reduce costs but getting the parts to play well together required additional engineering and technical effort. The team learned that to minimize costs, it also had to change the way it managed CubeSat missions, eliminating or modifying some standard processes and tests.
- "Resilience is just as important as reliability," Johnson said. With larger missions, designers incorporate redundant systems in the event of a system failure. Small satellites, however, typically don't have the resources to support backup systems. "It's easier to design in redundancy," Johnson added. "It's more difficult to build single-string systems that will work even if something goes wrong. You really have to look at the challenge from a system perspective. The CubeSat has to work as a resilient unit."
- As a result, the team had to think differently, leveraging its knowledge of ground and flight systems and the power of software to correct problems should they occur on orbit.
- That philosophy proved vital shortly after Dellingr was deployed. The platform's command-and-data-handling subsystem experienced an upset and began rebooting every 64 seconds for 10 days. One of the team's engineers, Behnam Azimi, however, had incorporated a hardware capability that allowed Dellingr to recover from the terminal loop, which the team ultimately traced to a one-line bug in a vendor-supplied software driver, Johnson said.
- Innovative thinking was also on display shortly after ground controllers recovered Dellingr from the reset loop. Due to a bug in the attitude-control system, which includes torquers and reaction wheels that rotate a satellite in small amounts, the spacecraft began to spin, preventing the INMS from collecting data.
- Unable to slow down the spacecraft with the existing attitude-control system, the team wrote a new software application, uploaded it, and used one of Dellingr's two miniaturized onboard magnetometers as an attitude sensor, which provided data needed to activate Dellingr's torquers to help stabilize the spinning.
- "Thanks to the heroic efforts of the Dellingr team, the spacecraft recovered from a fast spin, tumbling once every five seconds, to once every 50 minutes," said INMS Principal Investigator Nick Paschalidis. The instrument team turned on the instrument's ion mode and collected mass spectra for hydrogen, helium, and oxygen. "We have collected excellent data," Paschalidis said.
- The now patented and licensed DANY, which released Dellingr's magnetometer boom and UHF antenna, also "worked perfectly and is a huge success," said Dellingr Project Manager Chuck Clagett. And Principal Investigator Allison Evans also reported success with her CubeSat Form Factor Thermal Louver Experiment, a miniaturized thermal-control system involving louvers that open or close depending on whether heat needs to be conserved or shed. The louvers opened when they sensed heat and closed when they didn't.
- Meanwhile, Eftyhia Zesta, the principal investigator on the two magnetometer systems, is still analyzing her data, but considers the flight of the two miniaturized magnetometer systems a success onto themselves.
- "The team has tackled issues head on, is collecting and analyzing data, and has moved CubeSat capabilities from rudimentary, unreliable spacecraft to highly capable science platforms, all while enhancing workforce skills," Hughes said. "I think we'll be enjoying the fruits of the team's labor for years to come."
• October 5, 2018: With a large software upload, which took several weeks because of limitations with the CubeSat radio, the team restored full control of the reaction wheels, allowing Dellingr to maintain its orientation with respect to the Sun. The solar panels can now charge for maximum power generation as Dellingr slowly spins about that axis, collecting data. Almost a year after deployment, and after overcoming a rash of unexpected problems, the team had restored much of Dellingr's functionality. Work on the neutral mode of the INMS instrument continues. 18)
- Dellingr already serves as a testament to the unique challenges associated with packing big science in a small box. Keeping it alive and running for this long was an important goal — a standard mission lifetime for CubeSats has not been validated, and Dellingr's mission may help establish a benchmark. The team successfully achieved a resilient spacecraft mission, while still maintaining the low-cost aspect that is the hallmark of CubeSats. The value of the mission extends to others: papers are already being published outlining best practices learned from the mission, and these lessons learned have contributed to the successful proposals of three new Goddard CubeSat missions: petitSat, GTOSat and BurstCube.
• August 2018: Despite multiple hardware anomalies and failures Dellingr continues to operate in the summer of 2018. We are commissioning the INMS neutral mode, which involves activation of an ionization filament. The key to Dellingr's continued operation is that the most critical subsystems – communication, power and C&DH – still function. With a limited budget and tight schedule, we made the strategic decision to focus most of the extensive ground testing on ensuring the reliability of those three subsystems, and it has paid off for the mission. The ACS system was the last one to be fully integrated and tested; it was also the most difficult and costly subsystem to perform end-to-end tests on to uncover hardware or software issues. The team strategy for reducing ACS risk and controlling test cost was to focus on sun-pointing mode reliability. If Dellingr could sun-point and stay power positive, then we could use our ability to upload software patches to correct pointing or other issues in the control system software. — However, we had not expected that so many hardware issues would affect Dellingr. Still, this on-orbit flexibility was the lifesaving feature that allowed us to continue operations and work around these discrete ACS component issues as well. The bottom line is that this "architecture of resiliency" is always a good thing to build into your CubeSat mission, and the low costs of subsystem components such as sun sensors makes it easy to add in extra hardware. 19)
- Dellingr was primarily an engineering demonstration mission, but also contained two science instruments: a boom-mounted fluxgate magnetometer, and a novel gated time-of-flight ion-neutral mass spectrometer (INMS). The INMS is a key instrument for future ionospheric Heliophysics missions and validating the instrument and characterizing its performance was a key post-launch mission objective. Dellingr was designed to keep the INMS entrance aperture, located on the +Y 2U face, in the ram-pointed direction – i.e. pointed in the direction of spacecraft motion – so that ionospheric particles could enter the aperture to be measured. This orientation required an active attitude-control subsystems with several key components in order to achieve the desired control. The instrument required several weeks of outgassing before we could turn on the high-voltage, a delay that impacted our ability to turn on the instrument given spacecraft anomalies that appeared soon after launch.
- A primary feature of the FDC (Failure Detection and Correction) scheme for Dellingr consisted of a 25-hour reset that power cycled the spacecraft. The spacecraft is reset as part of daily operations, so that it does not interfere with planned activities. There is also a 25-hour reset that will power-cycle the spacecraft if the daily reset fails to reset the spacecraft or the spacecraft exits the daily operations cycle. The 25-hour counter restarts every time the spacecraft reboots. This power cycle was intended to automatically clear any potential issues and single event upsets, while also reducing the FDC development effort. While the 25-hour reset rescued the satellite on more than 1 occasion, it sometimes made troubleshooting and scheduling more difficult in other situations.
- An important aspect of lessons learned from Dellingr operations is the difference between making a spacecraft mission reliable versus making it resilient. From the standpoint of achieving mission success with an appropriate and tolerable level of risk and resource expenditure, it is potentially more cost-effective to focus on resiliency rather than exclusively on reliability.
Spacecraft turned on inside the NanoRacks deployer
20 Nov. 2017
Deployment from ISS
20 Nov. 2017
Contact on first pass
30 Nov. 2017
Anomalous gyro and sun pointing accuracy
19 Dec. 2017
21 Dec. 2017
Determined FSS (Fine Sun Sensors) unusually noisy
25 January 2018
Spacecraft data deleted – unknown cause
26 Jan. - 6 Feb. 2018
Unable to talk to flight computer - continuous reset state
06 Feb. 2918
06 Feb. 2018
INMS turned on; ion data collected
14 - 21 Feb. 2018
Startup RTS001 V1 uploaded
22 Feb. 2018
INMS filament burn in
28 Feb. 2018
Startup RTS001 V2 uploaded
26 March 2018
Spacecraft data deleted – unknown cause
29 March - 5 April 2018
B-dot V1 upload
12 - 17 April 2018
B-dot V2 upload
01 - 07 May 2018
B-dot V3 upload (attempt 1)
08 - 15 May 2018
B-dot V3 upload
01 June 2018 to present
INMS neutral commissioning
The reader is referred to reference 19) to follow the discussion of the various events.
• November 28, 2017: NASA ground controllers have begun checking out and commissioning a shoebox-sized spacecraft that the agency purposely built to show that CubeSat platforms could be cost-effective, reliable, and capable of gathering highly robust science. 20)
- "We're ready to start demonstrating Dellingr's capabilities," said Michael Johnson, chief technologist of the Applied Engineering and Technology Directorate at NASA's Goddard Space Flight Center in Greenbelt, Maryland. He was instrumental in pulling together a small team of scientists and engineers charged with developing the low-cost platform within a relatively short period of time, especially compared with larger, more traditional spacecraft. - "We believe Dellingr will inaugurate a new era for scientists wanting to use small, highly reliable satellites to carry out important, and in some cases, never-before-tried science," Johnson added.
- "We are always looking for potential partners and licensees for our technologies," said Goddard Strategic Partnerships Office Senior Technology Manager Enidia Santiago-Arce, "It's very important to NASA to facilitate licensing of our technologies when possible for the benefit of the broader community."
- The two technologies now available for licensing through Goddard's Strategic Partnerships Office (SPO) are described below:
1) GSC-16900-1: Diminutive Assembly for Nanosatellite deploYables (DANY)
2) GSC-17034-1: CubeSat Form Factor Thermal Control Louvers
• November 21, 2017: Early this morning, NanoRacks successfully completed the Company's 13th CubeSat deployment mission from the International Space Station. As these five CubeSats enter low-Earth orbit, this brings NanoRacks to 176 total CubeSats deployed into space via the NRCSD (NanoRacks CubeSat Deployer). In total, the Company has deployed 193 satellites into space. 23)
- Additionally, NanoRacks is pleased to share that this mission marks the first deployment of the industry standard 6U CubeSats in the 2U x 3U form factor from the NanoRacks ‘Doublewide' Deployers. The 6U satellites deployed were EcAMSat, Dellingr, and ASTERIA.
- "It's critical for us at NanoRacks to grow our CubeSat services with the changing small satellite landscape," says NanoRacks External Payloads Manager Conor Brown. "CubeSats are following a similar path as cellphones – first they were as small as possible, and now they are starting to grow in both size and capability. Now, beyond just accommodating the industry standard 6U form factors, we can deploy CubeSats up to 12U in size from the Space Station, providing opportunities for more advanced payload concepts."
- The NRCSD-13 Mission included satellites launched on the most recent SpaceX and Orbital ATK commercial resupply services missions to station for NASA, which launched Aug. 14 and Nov. 12, 2017, respectively.
- The five CubeSats deployed on the NRCSD-13 mission were:
1) ASTERIA (Arcsecond Space Telescope Enabling Research in Astrophysics), a 6U CubeSat of MIT and JPL.
2) EcAMSat (E. coli AntiMicrobial Satellite), a 6U CubeSat of NASA/ARC.
3) Dellingr, the Radiation Belt Loss Experiment, a 6U CubeSat of NASA/GSFC.
4) TechEdSat-6, a 4U CubeSat developed by San Jose State University and the University of Idaho as a collaborative engineering project with oversight from NASA Ames.
5) OSIRIS-3U (Orbital Satellite for Investigating the Response of the Ionosphere to Stimulation and Space Weather), a 3U CubeSat developed by Pennsylvania State University.
• August 16, 2017: Two days after departing from a launch pad on Florida's Space Coast, the SpaceX Dragon cargo capsule arrived at the International Space Station on August 16 with more than 2,910 kg of experiments and supplies after concluding an automated laser-guided approach. 24)
- Astronaut Jack Fischer aboard the space station used the lab's Canadian-built robotic arm (Canadarm2) to snare the robotic cargo craft at 10:52 GMT on Aug. 16 as they sailed about 400 km over the Pacific Ocean north of New Zealand.
- Around two hours later, ground controllers finished the installation of Dragon on the station's Harmony module, commanding 16 bolts to close and create a firm seal between the two vehicles.
- The station crew opened hatches between the Harmony module and Dragon's pressurized compartment later, a day earlier than planned.
- Flying under contract to NASA, the SpaceX supply ship ferried mostly research hardware, but also carried computer equipment, clothing, fresh food, ice cream and other treats for the crew.
- The cargo mission marked SpaceX's 11th successful operational supply delivery in 12 tries. - NASA inked a $1.6 billion contract with SpaceX in 2008 for 12 logistics flights to the station. This mission wraps up work under the original resupply contract, but NASA extended the agreement for eight additional cargo launches through 2019. SpaceX also has a separate, follow-on contract with NASA for at least flights of upgraded Dragon cargo capsules to the station from 2019 through 2024.
- The station's six-person crew will unload the payloads inside, overseeing a multitude of biological experiments before the ship's departure and return to Earth next month.
Science overview: Earth's upper atmosphere changes in response to "space weather", which is created by the sun's activity. Much of space weather's impacts are observed at high latitudes. It is at these regions, such as across Canada, Iceland, and Scandinavia, where space weather has the biggest visible impact in the form of the aurora. Space weather can also affect radio communication, damage sensitive electronics in our satellites, and damage power transmission infrastructure. Space weather causes changes in Earth's upper atmosphere, and these changes can be measured by scientific satellites in order to better understand these phenomena.
Dellingr carries an advanced gated time-of-flight ion/neutral mass spectrometer (INMS) and three fluxgate magnetometers. Two of these magnetometers are internal to the spacecraft, and will be used to test and validate a new software algorithm that compensates for and removes spacecraft interference; the third magnetometer sits at the end of a 52 cm boom. Together, these instruments will measure the space weather effects of solar wind-magnetosphere coupling on Earth's ion and neutral upper atmosphere.
INMS (Ion and Neutral Mass Spectrometer)
There exists a strong need for in situ measurements of atmospheric neutral and ion composition and density, not only for studies of the dynamic ionosphere-thermosphere-mesosphere system but simply to define the steady state background atmospheric conditions.
The Heliophysics Division of GSFC has developed a compact INMS for in situ measurements of ions and neutrals H, He, N, O, N2, O2 with M/dM of approximately 12 at an incoming energy range of 0-50 eV. The INMS instrument is based on front end optics, post acceleration, gated time of flight, ESA (Electrostatic Analyzer) and CEM (Channel Electron Multiplier) or MCP (Micro Channel Plate) detectors.
The compact sensor has a dual symmetric configuration with the ion and neutral sensor heads on opposite sides and with full electronics in the middle. The neutral front end optics includes thermionic emission ionization and ion blocking grids, and the ion front end optics includes spacecraft potential compensation grids. The electronics include front end, fast gating, HVPS (High Voltage Power Supply), ionizer, TOF (Time of Flight) binning and full bi-directional C&DH digital electronics. The data package includes 400 mass bins each for ions and neutrals and key housekeeping data for instrument health and calibration. The data sampling can be commanded as fast as 10 ms/frame (corresponding to ~80 m spatial separation) in burst mode, and has significant onboard storage capability and data compression scheme. Experimental data from instrument testing with both ions and neutrals will be presented. 25)
A demonstrator INMS-1 was recently launched on the ExoCube 3U CubeSat mission (SMAP launch) on January 31, 2015. A second upgraded INMS is scheduled to be delivered August 2015 to Dellingr 6U CubeSat mission to be launched in 2017.
The Dellingr INMS-2 miniaturized instrument fills a 1.3U volume with a mass of only 570 g and requires nominal power of 1.6 W.
Three heliophysics-related payloads will make the maiden journey. One, the miniaturized INMS, actually was tested for the first time aboard the National Science Foundation's ExoCube mission, which measured the densities of all significant neutral and ionized atom species in the ionosphere, the outer region of the atmosphere where incoming solar radiation ionizes a large fraction of atoms. ExoCube was launched on January 31, 2015.
Measurements of atmospheric neutral and ion composition and density are needed not only for studies of the dynamic ionosphere-thermosphere-mesosphere system but simply to define the steady state background atmospheric conditions. Remote sensing measurements of atomic oxygen density at altitudes between 80-95 km have shown that the density can vary by over an order of magnitude. This causes deviations from the densities estimated by MSIS (a well known empirical model of Earth's atmosphere) by up to a factor of four. CubeSats provide an ideal platform for an ion/neutral mass spectrometer capable of obtaining the in situ measurements that are critical to understanding this complicated system.
DAGR (Distributed Acquisition for Geomagnetic Research)
The DAGR instrument includes three science-grade fluxgate magnetometers. The two internal magnetometers are designed to test new software ‘scrubbing' algorithms that remove interference created by the electronics of the spacecraft. The traditional approach for magnetic field measurements is to place the magnetometer at the end of a long boom, away from magnetic contamination. But through software, it is possible to remove spacecraft interference from magnetometers embedded within the spacecraft, thereby reducing cost and complexity by eliminating the need for a magnetometer boom. By flying both a boom-mounted magnetometer and two internal magnetometers, we will be able to test the capability of the software scrubbing algorithms against ‘pristine' magnetic field data collected by the boom magnetometer (Ref. 5).
Two of Dellingr magnetometers are miniature fluxgate magnetometer developed by GSFC over the last several years (Figure 16). The sensor weights 19 grams, has 24-bit A/D, consumes 750 mW, and has 12 pT/√Hz sensitivity at 1Hz. One of the magnetometer sits on a 52 cm extendable double-hinged boom, and sample at a minimum of least 10 Hz. Measurements in the GSFC coil facility show it has a noise level of <0.1 nT.
The traditional approach used for virtually all science magnetometers to date was to place the sensors on long booms to place them as far away from the spacecraft's magnetic field as possible. Through advances in sensor technology and software, it is now possible to remove spacecraft interference from magnetometers embedded within the spacecraft body, thereby reducing complexity and cost by removing the magnetometer boom which typically represents a deployable structure that comes with inherent complexity and testing needs. Flying a boom-mounted and two internal magnetometers will provide an opportunity for testing the capabilities of the software scrubbing approach by comparing the software-generated solution with what is considered a pristine measurement from the boom magnetometer.
The science magnetometers are miniaturized fluxgate sensors with a resolution better than 0.1 nT at a sampling rate of 3.5 Hz. The magnetometer boom is held against one of the long spacecraft sides for launch and deploys via a shoulder and elbow hinge to position the magnetometer 76 cm from the spacecraft body. In addition to the technical demonstration of the software algorithm, the magnetometer is also used to provide complementary science data to put the particle measurements from INMS into magnetic context.
Three technology demonstration payloads are also part of Dellingr, a Fine Sun Sensor developed at NASA Goddard for CubeSat missions, thermal louvers that could be a solution for future CubeSat thermal control systems, and new deployment actuators for CubeSat appendages like booms and antennas (Ref. 10).
The FSS (Fine Sun Sensor) was developed at Goddard Spaceflight Center as a digital sun sensor for CubeSat missions, enabling precise measurements of the solar vector to orient the satellite and provide attitude information for the instruments.
Thermal control louvers are simple thermal regulating devices that require no complex electronics and operate by opening up when the satellite needs to dissipate heat from its interior and close when the craft needs to warm up. - The device comprises front and back plates, flaps and springs with the back plate painted with a highly emissive coating while the flaps are made of much less emissive aluminum. The flaps automatically open when the satellite reaches a certain temperature by bi-metallic springs that cause the flaps to open as they warm and revert back to their original shape when cooling down, closing the louver. For the Dellingr mission, only one spring and flap is part of the system, outfitted with sensors to validate the system's behavior in an operational environment.
DANDY (Diminutive Assembly for NanoSatellite Deployables) is a new device for unfurling elements on small satellites. It is designed to hold antennas, booms and sunshades in place during a satellite's launch and release into orbit and, upon command, applies current to a heating element which weakens a plastic device holding the retaining pins of the deployable structure. On Dellingr, DANDY is employed for the UHF antenna and the magnetometer boom which will be commanded to open at a pre-set time after deployment.
Dellingr uses the Wallops Flight Facility UHF ground station and Goddard Space Flight Center MOC (Mission Operation Center) for the entire mission (Figure 19). The ITOS (Integrated Test and Operation System), originally developed for the SMEX (SMall EXplorer) program, is used for mission operation and ground control. All commands are initiated in ITOS from the MOC and then delivered to the Space Dynamics Laboratory TITAN ground system at Wallops to modulate and transfer to the satellite. The satellite return signals are demodulated and, after a FEC (Forward Error Correction) decoding process, data is transferred to ITOS in a packet format. ITOS displays the latest housekeeping telemetry and stored science data for later processing and analysis by the science team.
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23) "NanoRacks Completes 13th CubeSat Deployment Mission from Space Station, First "Doublewide" Satellites," NanoRacks, 21 Nov. 2017, URL: http://nanoracks.com/13-cubesat-deployment-doublewide/
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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).