DANDE (Drag and Atmospheric Neutral Density Explorer)
DANDE is a microsatellite, a sphere of 46 cm in diameter and a mass of ~50 kg, under development by students at the University of Colorado at Boulder (UCB), CO, and administered by the Colorado Space Grant Consortium (CoSGC) in partnership with the Aerospace Engineering Science Department (ASEN). The goal of the DANDE mission is to provide an improved understanding of the satellite drag environment in the lower-thermosphere at low-cost. 1) 2)
In addition to being a unique educational forum for teaching design and systems engineering, the mission is a response to government and industry needs for near-real time space-weather and drag prediction models. These products are important to operators of low earth orbiting satellites with precision navigation needs and include both government and industry.
The DANDE project is part of the University Nanosat Program (UNP) jointly administered by AIAA (American Institute of Aeronautics and Astronautics), AFOSR (Air Force Office of Scientific Research), and AFRL (Air Force Research Laboratory). DANDE was selected the winner in January 2009 from a US national competition (NanoSat-5, which started in 2007) among 10 other universities to build and test a working satellite. The first-place award meant that the project is being sponsored by the government. The plans call for the satellite to be launched in 2011-12.
The DANDE government collaborators interact with UCB engineering students and include the AFOSR, AFRL, AFSPC (Air Force Space Command Space Analysis Center), NRL (Naval Research Laboratory), the NOAA (National Oceanographic and Atmospheric Administration) Space Weather Prediction Center, and NASA/GSFC (Goddard Space Flight Center).
DANDE’s mission objective is to perform in-situ measurements of the neutral thermosphere in the general region of 350 (shuttle) to an elliptical 200 km x 1200 km orbit (high high inclination would be ideal). This region is highly variable, and coupled to both solar activity and lower weather patterns in ways which are not clearly understood. The winds, weather, and variability of this region have serious effects on spacecraft maneuvering and lifetimes, and is an important area of study. 3) 4) 5)
To achieve its goals, DANDE will make measurements using spacecraft radar tracking for orbit determination and two on-board instruments. Tracking will be done through a collaborative agreement with AFSPC, providing high-priority precision tracking for drag. The dual instrument measurement approach selected involves a novel accelerometer instrument capable of µg resolution and a neutral mass spectrometer to sense the wind speed and density on orbit.
Although past missions like AE-A (Atmospheric Explorer-A, launch April 3, 1963), DE-2 (Dynamics Explorer-2, launch Aug. 3, 1982), and CHAMP (Challenging Minisatellite Payload, launch July 15, 2000) have measured either wind vectors or mass density, there have been none which have measured the two parameters together while having a well determined coefficient of drag. By measuring both these properties, one can deduce the contribution of the Vw and CD components and thus significantly reduce the uncertainty in the density measurement.
Naturally, the PI (Andrew C. Nicholas) of the two ANDE (Atmospheric Neutral Density Experiment) missions of NRL (shuttle launch of ANDE on Dec. 10, 2006, and deployment of ANDE-2 from shuttle on July 30, 2009) provided good advice to the UCB project team in building the spherical DANDE spacecraft. However, while ANDE is a more perfect sphere than DANDE, it does not make in-situ measurements of acceleration but relies on orbit tracking through SLR. Another difference is that ANDE is designed for shuttle launch while the DANDE design allows for an interface to ESPA [EELV (Evolved Expendable Launch Vehicle) Secondary Payload Adapter] while remaining compatible with shuttle requirements.
For the DANDE drag mission to be successful for science measurements, the shape of the spacecraft must be spherical which imposes challenges to the design of the structure, power, and communication subsystems.
Table 1: Science requirements for minimum success
Figure 1: Illustration of the DANDE satellite, the solar cells are in blue color (image credit: UCB, CoSGC)
The DANDE project selected a sphere structure of the spacecraft for the measurement of the drag. To produce an accurate drag profile, the spacecraft needed to be as spherical as possible. This requirement created numerous design challenges, including conforming photovoltaic panels and flush-mounted antennas to the spherical structure. 6)
The spacecraft is spin-stabilized around the orbit normal vector, meaning the instruments aligned with the “equator” of the spinning sphere will scan the velocity vector at a predicable rate. The nominal spin rate is 10 rpm. Considering an initial near-polar circular orbit of 350 km in altitude, the spacecraft is expected to spend approximately 3 months in orbit prior reentry.
EPS (Electrical Power Subsystem): The smooth spherical shape of the outer spacecraft structure poses a considerable design effort in mounting and arranging the solar cells. The problem is to mate inherently flat solar panels to a curved surface structure. DANDE's final design concept in solar cell mounting borrowed techniques (traditional epoxy) that were used in the Hermes CubeSat of CoSGC and in the SwissCube projects. With this hybrid design, the solar panels on board conform to the small surface area available and modify the exterior of the sphere to a point that can still be modeled in terms of its effect on the overall drag of the satellite.
Figure 2: The DANDE spacecraft showing spin alignment orientation (image credit: UCB)
Figure 3: Photo of the DANDE microsatellite (image credit: UCB)
Launch: The DANDE spacecraft was launched as a secondary payload on September 29, 2013 on a SpaceX Falcon V.1.1 vehicle from VAFB, CA. The primary payload on this flight was CASSIOPE of Canada. 7) 8) 9)
The secondary payloads on this flight are:
• CUSat-1 and CUSat-2, microsatellites (each of ~41 kg) of Cornell University, Ithaca, N.Y.
• DANDE (Drag and Atmospheric Neutral Density Explorer), a microsatellite (<50 kg) of the University of Colorado at Boulder.
• POPACS (Polar Orbiting Passive Atmospheric Calibration Sphere), a 3U CubeSat of several US universities and entities including: MSU (Morehead State University), Gil Moore (POPACS Project Director), the University of Arkansas, PSC (Planetary Systems Corporation), Silver Spring, MD, MSU (Montana State University), Drexel University (Philadelphia), et al.
Orbit: Elliptical polar orbit, 324 km x 1500 km, inclination =80º, period = 103 minutes (14 orbits/day).
DANDE separation system:
The DANDE spacecraft is required to interface to the launch vehicle via the ESPA ring containing a circular 24 bolt pattern. At the desired time following launch, the ESPA ring will eject (via an independent ESPA separation system) the DANDE system from the launch vehicle allowing the initiation of the mission. To connect the spherical DANDE spacecraft to the ESPA ring, an interface bracket was created (Figure 4). 10)
Upon ESPA separation the DANDE system (containing both the interface bracket and sphere) are ejected. At a later time, the spacecraft will be commanded to independently separate from the interface bracket enabling the start of the science portion of the mission.
Figure 4: Architecture of the baseline separation system (image credit: UCB)
Sensor complement (ACC, WTS)
The primary instruments, the ACC (Accelerometer suite) and WTS (Wind and Temperature Spectrometer), are aligned together to enable velocity vector scanning in the nominal attitude state. The spacecraft structure is also a part of this measurement system as it houses the instruments and interacts directly with the atmospheric gas. Accordingly, requirements on the level of cross-sectional area variation were derived from the science analysis. The allowable center of gravity offset was determined by the maximum allowable drag torques for performing acceleration measurements. The instrument locations and relative sizes are shown in Figure 5 (the accelerometer suite is item and the WTS is item 11). 11)
Figure 5: Engineering diagram of the DANDE spacecraft assembly (image credit: UCB)
Figure 6: Hemisphere of DANDE on lightband adapter bracket with science gathering component (image credit: UCB)
Figure 7: The accelerometer subsystem (left) and the WTS (Wind and Temperature Spectrometer) at right, (image credit: UCB)
ACC (Accelerometer Suite):
The objective is to measure spacecraft acceleration and deceleration due to local variations in density and in-track winds. The accelerometer suite uses six radially-mounted commercial-grade accelerometers, and a unique data-processing scheme which takes advantage of the spacecraft’s spin, to measure nano-g accelerations using relatively inexpensive micro-g components (Ref. 1).
The six COTS accelerometers are integrated to custom filtering hardware and software to reduce the measurement noise. Honeywell QA-2000 accelerometers are being used which employ an actively centered proof mass on a deflection beam to measure the acceleration. These devices include an internal temperature sensor which allows for the computation of a temperature dependant bias and scale factor.
The spinning spacecraft will modulate the drag input from each of the six accelerometers by rotating the instruments (Figure 8) through the velocity vector at a known rate. The filters on the spacecraft reduce the noise in all frequency bands except for a narrow bandpass around the spacecraft spin frequency. Assuming a sinusoidal signal model, a linear least squares fit is performed on the data from each accelerometer signal. The six resulting sinusoidal amplitudes indicate the drag acceleration. Finally, the results from each of the six accelerometers are averaged to produce a data product with the required precision.
Figure 8: Acceleration is obtained by modulating the drag signal to the known spacecraft spin rate (image credit: UCB)
The benefit of using the deflection type accelerometers is that calibration constants which define the bias and scale factors, can be determined on the ground. Furthermore, by modulating the drag signal and looking only for the amplitude of that modulation, the system is insensitive to changes in accelerometer bias. An additional benefit of this approach is that the loss of one or two out of six accelerometers results in only a partial loss of useful data allowing the system to degrade gracefully.
The accelerometer suite has a volume of 10 cm x 11.5 cm x 6.25 cm and has a mass of 1.3 kg.
WTS (Wind and Temperature Spectrometer)
WTS is a miniature instrument to measure the wind, temperature and atomic densities of the local neutral atmosphere. This instrument includes a unique “imaging” capability which can measure the number density across a 16 pixel fan, allowing the determination of not only the composition of the local atmosphere [atomic oxygen (O), and molecular nitrogen (N2)], but also the presence of any horizontal cross-track winds (Ref. 1). 12)
WTS contains SDEA (Small-Deflection Energy Analyzer) for energy selection. It is a coarse analyzer (ΔE/E ≈ 0.1) with the ability to resolve the kinetic energy of neutral species entering the aperture at about 7,800 m/s. This corresponds to kinetic energies of about 5 eV (8.5 x 10-19 J) for atomic oxygen and 9 eV (14.9 x 10-19 J) for molecular nitrogen.
The detector is of the type MCP (Microchannel Plate) which amplifies charged particle impacts by setting
Figure 9 describes the motion of a molecule through the WTS instrument and the corresponding sequence of the measurement process is described below.
1) Neutral particle enters the collimator
2) Neutral particle is ionized inside of a field free of electron bombardment
3) Neutral particle enters the energy selector (SDEA) and undergoes acceleration toward the exit slit
4) Once outside of the selector, the particle is accelerated abruptly by a -3 kV potential toward the MCP detector
5) The impact on the MCP causes a cascade of electrons to travel toward one of the anodes which measures the impact (which anode is triggered depends on the angle at which the neutral particle entered the collimator).
Figure 9: Section views of the WTS instrument showing the path of a molecule in blue (image credit: UCB)
The WTS is designed to observe the lower energy particles in the atmosphere, specifically the energy range from 1-14 eV. Further it is designed to only sample the neutral particles in the atmosphere due to practical limitations of measuring the naturally occurring ions at LEO. The limiting factor is that naturally ionized particles whether they be O+, N2 +, H+, etc., their charge with respect to the DANDE satellite cannot be easily measured nor controlled. As DANDE moves through the ionized atmosphere, it is likely to build charge over time and there is no means built into the spacecraft to dissipate this charge. All of this charging takes place in reference to the Earth system, what is nice about the neutral particles is the WTS can charge them internally and then sample them in reference to the DANDE system effectively controlling their energy. This then leads to how the WTS rejects the ions, since they cannot be effectively screened before entering the instrument aperture, ionizes the desired specimens and selects them for observation.
The make up of the ion optics can be broken down into four fundamental parts. First the collimator and ion deflector, the deflector applies an electric field inside the instrument entrance aperture perpendicular to the sample velocity vector. Ionized particles are perturbed from their direction of travel by this field while the neutrals do no respond since they have yet to get a charge. The collimator part of this is a series of knife edges to trim the entering sample to a field of view that is 30º along the spacecraft cross track and 2º along the satellite radial axes. Then neutral species move into an ionizing chamber, which has a beam of electrons to singly ionize the neutrals with respect to the overall DANDE structural ground. Once charged neutrals continue on their initial flight path and enter an energy selector that acts as a parallel plate capacitor where one side is held at ground and the other can be ramped from 0-4 V perturbing the path of the now ionized neutrals as a function of their energy. Once the energy is selected ,neutrals exit and strike a charged MCP (Micro Channel Plate) which produces an signal that can be processed as an impact and factored into an overall number density calculation.
1) Marcin D. Pilinski, Scott E. Palo, “An Innovative Method for Measuring Drag on Small Satellites,” Proceedings of the 23nd Annual AIAA/USU Conference on Small Satellites, Logan, UT, USA, Aug. 10-13, 2009, SSC09-VIII-3
2) Lee Jasper, Kyle Kemble, “Drag and Atmospheric Neutral Density Explorer (DANDE) - Spherical Spacecraft Design Challenges,” March 31, 2009, CoSGC Symposium, URL: http://spacegrant.colorado.edu/COSGC_Projects/symposium_archive/2009/papers/CUes.pdf
3) Marcin Pilinski, “Multi-Instrument Data Analysis of Thermospheric Density and Winds,” 37th COSPAR Scientific Assembly, July 13-20, 2008, Montréal, Canada., p.2439
4) “The Drag and Atmospheric Neutral Density Explorer (DANDE) Measurement Concepts,” URL: http://dande.colorado.edu/files/UN5-SYS901.0_DANDE_Measurement_Methods.pdf
5) Marcin D. Pilinski, Scott E. Palo, “An Innovative Method for Measuring Drag on Small Satellites,” Proceedings of the Symposium on Small Satellite Systems and Services (4S), Funchal, Madeira, Portugal, May 31-June 4, 2010
7) “DANDE has launched and successfully separated from the launch vehicle!,” http://spacegrant.colorado.edu/boulderstudents/boulderprojects/dande
8) Patrick Blau, “SpaceX successfully Launches Upgraded Falcon 9 on 1st Demonstration Flight,” Spaceflight 101, Sept. 29, 2013, URL: http://www.spaceflight101.com/falcon-9-v11-cassiope-launch-updates.html
9) Information provided by Kyle D. Kemble, DANDE Satellite Program Manager, University of Colorado Boulder
10) Bruce L. Davis, Scott E. Palo, “The Design and Development of a Separation System for a Low-Cost Spherical Nanosatellite,” Proceedings of the 23nd Annual AIAA/USU Conference on Small Satellites, Logan, UT, USA, Aug. 10-13, 2009, SSC09-VIII-5, URL: http://www.spacedavis.com/files/portfolio/DANDE_SEP_System_Web.pdf
11) Kyle D. Kemble, “The Challenges of Characterizing a Wind & Temperature Spectrometer for Space Weather Measurements,” Colorado Space Grant Consortium Undergraduate Research Symposium, April 21, 2012, URL: http://www.google.de/url?sa=t&rct=j&q=the%20wind%20and%20temperature%20spectrometer%20(wts)%20andourcUa-CNMKDtAbtxIDoCQ&usg=AFQjCNHlF51rKF35I8GI3uFVysma18MjCQ&bvm=bv.4870560d=rja
12) S. Palo, “The Wind and Temperature Spectrometer (WTS) and accelerometer suite on the DANDE satellite,” 5th European CubeSat Symposium, Royal Military Academy, VKI (Von Karman Institute), Brussels, Belgium, June 3-5, 2013
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.