DSCOVR (Deep Space Climate Observatory)
DSCOVR is the former renamed NASA/NOAA mission Triana, proposed in 1998 by then Vice President Al Gore. The goal of Triana was to observe Earth as a planet (i.e. continuous full disk observation of the sunlit Earth) from L1, the first Lagrangian Point in the Earth-Sun system. The mission was named Triana after Rodrigo de Triana, the lookout, who spotted the New World on Christopher Columbus’s first expedition in 1492. 1)
Some background: The Triana mission development proceeded for 21 months, (a launch of the observatory was planned for early 2002), before it was de-manifested from the Space Shuttle in the spring of 2001 by the new Bush Administration in office. The official reasons stated were: “A constrained Shuttle flight rate of six per year, established in the 2001 NASA budget planning process, required that NASA give priority to the primary ISS (International Space Station) payloads, a Hubble Space Telescope (HST) reboost was needed, and microgravity experiments were planned to be launched.” Consequently, it was not possible to identify a definitive launch date for Triana. 2) 3) 4) 5) 6) 7) 8) 9)
Figure 1: Illustration of the Triana observatory configuration for Shuttle flight (two views of the undeployed S/C)
Legend to Figure 1: The Shuttle configuration contained also GUS (Gyroscopic Upper Stage) next to the Observatory. GUS was intended to boost the Observatory from LEO to the LOI (Lissajous Orbit Insertion) point.
The Triana observatory has been in a state of “Stable Suspension” since November 2001 (stored in a clean room at GSFC. After the mission was placed into suspension, it was renamed to DSCOVR (Deep Space Climate Observatory).
In 2008, the stored spacecraft was removed from storage. Power-on tests were performed to assess the current status of the observatory. This testing was successful, finding DSCOVR in nearly perfect condition after over 7 years of storage. In 2009, NASA provided funding to refurbish and recalibrate the two Earth science instruments, NISTAR and EPIC. In 2011, after a decade of stable suspension, NOAA, NASA, and the USAF are aligning again with plans to fully refurbish and launch DSCOVR possibly in 2014 (Ref. 1). Triana provided a solid foundation for DSCOVR, including advanced work on all observatory subsystems.
While maintaining its original instrument complement, DSCOVR will have a new primary function to monitor space weather. The satellite’s original purpose was to provide a near-continuous view of the entire Earth and make a live image available via the Internet.
The majority of the structure was intact, however the flight transponder, star tracker, IMU (gyros), and a reaction wheel had been deintegrated from the flight observatory and moved to multiple locations at GSFC. Some of the GSE (Ground Support Equipment) was impounded and some of the GSE had been reabsorbed into the GSFC infrastructure.
To distinguish this report from previous versions it was called “The Serotine Report.” Named after the serotinous or “late opening” pine cone which lies dormant for many years until activated by high heat: such as fire. The name of the report also has become attached to that team.
The DSCOVR mission, to be refurbished at NOAA expense and launched by the USAF, gives a unique opportunity to NASA to obtain unprecedented time resolution solar wind measurements for a minimal cost. The DSCOVR spacecraft is already built (Figure 2) and requires only an 18 month refurbishment to be ready for an operational space weather and scientific research mission. 10)
The 1 AU, near-Earth solar wind has been observed by a number of NASA and international spacecraft over the past decades. However, particle instrumentation technology limitations did not allow the direct observation of the varying properties of the thermal solar wind particles in the kinetic regime, which requires measurements with better than 1 Hz cadence. Observations at this kinetic scale are essential to understand how the solar wind is continuously heated as it propagates away from the Sun, how small scale magnetic reconnection operates in the 1 AU solar wind, and how interplanetary shocks can accelerate particles to high energies.
Table 1: History and current status 11)
DSCOVR’s primary mission is space weather observation. The objectives are: 12)
• Solar wind measurements at Earth-Sun Lagrange Point 1 (L1)
• Provision of 1-3 day early warnings of geomagnetic storm intensity. The consequences of solar storms are becoming more significant as society becomes increasingly dependent on technologies, from satellites to the electrical grid, that can be disrupted by a major storm. 13)
• DSCOVR will replace the NASA ACE (Advanced Composition Explorer) spacecraft in orbit since 1997
• Data downlink via the international RTSWnet (Real Time Solar Wind Network).
Mechanical structure: A full environmental test program, including vibration, strength, mass properties, acoustic, pyro shock, and alignment activities, is currently planned for DSCOVR. The Serotine team estimate includes funding to refurbish and/or rebuild portions of the mechanical GSE needed for this testing. During this study, the Serotine team rebuilt and certified the lifting GSE for the DSCOVR spacecraft and the EPIC instrument.
GN&C (Guidance Navigation and Control): The approach for DSCOVR includes using the original ACS dynamic simulator and not upgrading to a more costly advanced system, such as the one used for LRO (Lunar Reconnaissance Orbiter).
The DSCOVR star tracker was tested during this study using a star field simulator that attached to the tracker shade. While this is not a comprehensive performance test, the tracker did perform nominally. The current plan is to remove the star tracker and return it to Ball Aerospace for refurbishment. This will include some minor star tracker shade adjustments. Based on the testing performed by the Serotine team, no issues are expected during this refurbishment.
During the Serotine study, the DSCOVR reaction wheels were powered on and commanded to an assortment of speeds. All wheels performed nominally. The wheels were developed in-house at GSFC, and as part of DSCOVR’s refurbishment effort, will be removed from the spacecraft and sent to the development lab for comprehensive performance testing and evaluation. These wheels can not be disassembled due to their pressed on assembly techniques. However, stored samples of the original grease will be tested to verify the integrity of the wheels’ lubricant, which can not be directly sampled. This grease was also tested a few years ago and showed no degradation. The same grease was also used in an identical wheel that continues to perform flawlessly in its eighth year of powered life testing. Based on this and recent DSCOVR spacecraft testing, the Serotine team does not expect any issues with the DSCOVR wheels.
Figure 3: Illustration of the DSCOVR spacecraft and its components/sensor complement (image credit: NASA)
Launch: A launch of DSCOVR is planned for late 2014 on a vehicle sponsored by the USAF. In December 2012, the USAF awarded a contract to SpaceX to launch DSCOVR aboard a Falcon 9 vehicle. 14)
Orbit: Lissajous orbit about L1, the first Lagrangian Point in the Earth-Sun system (1.5 million km from Earth in the direction of the Sun).
Sensor complement: (EPIC, NISTAR, PlasMag instrument suite)
The Earth viewing instruments on DSCOVR have a continuous view of the entire sunlit face of the Earth.
EPIC (Earth Polychromatic Imaging Camera)
The EPIC instrument was managed by SIO (Scripps Institution of Oceanography) at USCD (University of California at San Diego) and built by LMATC (Lockheed Martin’s Advanced Technology Center) in Palo Alto, CA. The objective is to measure ozone amounts, aerosol amounts, cloud height and phase, hotspot land properties (a view of the land from angles where shadows are a minimum), and UV radiation estimates at the Earth's surface. EPIC is able to view the entire sunlit Earth from sunrise to sunset at an almost constant scattering angle between 165-178º. The EPIC channels were selected to match closely with TOMS in the UV region and with MODIS in the visible range; hence, the data products will be very similar and can be directly compared. These comparisons will validate both the calibration and data reduction algorithms.
The instrument system consists of the EPIC instrument, MEB (Mechanisms Electronics Box) and EC (EPIC Computer) which controls the instrument and interfaces to the spacecraft avionics. EPIC’s telescope, built by SSG Inc., is a reflecting Ritchey-Chrétien design with an aperture diameter of 30.5 cm, f 9.38, a FOV (Field of View) of 0.6º, and an angular sampling resolution of 1.07 arcsec. Once at L1, Earth (plus 100 km) varies from 0.45º to 0.53º full width.
Figure 4: Photo of the EPIC instrument system (image credit: NASA)
Table 2: EPIC performance parameters
Figure 5: Schematic view of the Cassegrain telescope (image credit: NASA, Ref. 11)
Table 3: Expected EPIC data products
• Ozone: total column
• Aerosol properties: aerosol index, aerosol optical thickness, aerosol height
• Cloud & surface properties: cloud fraction, cloud height, surface albedo
• Vegetation properties: vegetation index and LAI (Leaf Area Index)
• RGB: colored image of the Earth’s sunlit face.
EPIC challenges (Ref. 11):
• Geolocation: The spacecraft jitter is expected to be on the order of 1 pixel. This increases EPIC’s effective field of view. The ‘edge’ of the Earth and the outline of the continents will have to be used to exactly geo-locate the images. This is especially important for the algorithms based on ratios of channels.
• Stray light: EPIC’s (spatial) stray light is significant and must be corrected. A very complex stray light correction algorithm is being developed. It is based on laboratory measured point spread functions and calculations of an optical model.
• Instrument stability: The radiometric stability of EPIC will be tracked using the measured reflectivity over ice-covered surfaces and by periodic images of the moon’s sunlit face at a nearly constant phase angle when it is furthest from the Earth as seen from L1.
NISTAR (National Institute of Standards and Technology Advanced Radiometer)
The NISTAR instrument was designed and developed by NIST (National Institute of Standards and Technologies) of Gaithersburg, MD and and Ball Aerospace of Boulder, CO. The objective was to measure the radiance output from the sunlit Earth over a broad portion of the spectrum (UV and VIS as reflected radiation, IR as emitted radiation) in order to detect changes in the Earth's energy balance (climate studies).
The NISTAR instrument package includes four detectors: three active-cavity electrical substitution radiometers and one silicon photodiode channel to measure the “total Earth reflected and emitted radiant power” received in the direction of the spacecraft. 15)
NISTAR measures the absolute irradiance integrated over the entire sunlit face of the Earth in 4 broadband channels minute-by-minute (Ref. 11).
1) A UV to far infrared (0.2 µm to 100 µm) channel to measure the total radiant power UV, VIS and IR wavelengths emerging from the Earth.
2) A solar (0.2 µm to 4 µm) channel to measure reflected solar radiance in UV, VIS and NIR wavelengths.
3) A near infrared (0.7 µm to 4 µm) channel to measure the reflected IR solar radiation
4) Photodiode channel (0.3 µm to 1.1 µm) for monitoring of radiometer filter elements (channel for calibration reference).
The goal of NISTAR is to measure the Earth’s energy balance (solar input and Earth reflection and radiation to space) with sufficient accuracy (0.1%) to improve our understanding of the effects of changes caused by human activities and natural phenomena.
In 2010, the NISTAR instrument was calibrated against a portable version of the NIST SIRCUS (Spectral Irradiance and Radiance Responsivity Calibrations using Uniform Sources) facility. The calibration was performed with the NISTAR space-flight instrument in a thermal vacuum chamber in a clean-room environment at NIST. This calibration included system-level measurements of the relative spectral response of the NISTAR bands using a wavelength-tunable laser, and absolute responsivity measurements of each of the four NISTAR detectors at 532 nm. The standard uncertainty of the absolute responsivity calibration obtained using this technique was 0.12 % (k=1).
Figure 6: Photo of the NISTAR instrument under test (image credit: NASA)
NISTAR has a FOV of 1º, sufficient to see and image the full Earth disk whose FOV is approximately 0.5º as seen from L1. The photodiode channel has been included in order to obtain a faster time series (<1 s) than what can be obtained by the cavity radiometers. The channel is used to provide for the tracking stability of the filters, to verify co-alignment of NISTAR and EPIC, and to continuously observe the solar reflected broadband radiation from Earth with high temporal resolution.
Figure 7: Photo of the NISTAR instrument (image credit: NASA)
A PTC (Positive Temperature Coefficient) thermistor and a wire-wound low-temperature coefficient heater are bonded to the outside of the 30º conical receiver cavity. The absolute cavity radiometers are designed for optimum power measurements (in the tens of μW range). The optical signal incident on the receiver is only 1 μWcm-2, however, the emission from the receiver cavity to space is estimated to be 30 μWcm¿2 when the shutter is open. There are four digital control loops, three receiver cavity control loops and one for the heat sink. The PTC temperature sensor resistance measurements are performed with AC¿bridge circuits operating between 35 and 155 Hz. The NISTAR absolute cavity radiometers are designed for a noise floor of less than 10 nW (defined as the level at which the SNR is equal to one for a single one second measurement). The NISTAR electronics have a measurement resolution of 10 mΩ and internal equivalent noise of less than that. The NISTAR total instrument mass is 23.5 kg.
DSCOVR’s location at the L1 observing position, rather than in Earth orbit, will permit long integration times, since no scanning will be required. A radiometric accuracy of 0.1-0.2% is expected, a 10-fold improvement in accuracy over current Earth-orbiting satellite data. These will be the only measurements of the entire Earth’s reflected and emitted radiation at the retro reflection angle. As such, NISTAR will provide important missing data not obtainable by any Earth-orbiting satellite.
PlasMag instrument suite:
The PlasMag instrument suite of NASA/GSFC is a comprehensive science and space-weather package that includes a fluxgate vector magnetometer, not present on SOHO, a Faraday Cup solar wind positive ion detector and a top-hat electron electrostatic analyzer. This instrument cluster provides high time resolution measurements in real time and represents the next generation of upstream solar wind monitors intended to provide continuity of measurements started by IMP-8, WIND, SOHO and ACE.
The PlasMag Faraday Cup will provide very high time resolution (0.5 second) solar wind bulk properties in three dimensions, which coupled with magnetic field data (20 vectors/second), will allow the investigation of solar wind waves and turbulence at unprecedented time resolution. This, in turn, will allow new insights into the basic plasma properties: the process of turbulent cascade and the rate of reconnection. Both topics are critical in understanding the nature of coronal heating.
The electron electrostatic analyzer will allow the continual observation of the 3D electron distribution function for various solar wind conditions. Special attention will be given to the supra-thermal component or "strahl" that follows the interplanetary field lines very closely and provides the closest link to the formation of the solar wind in the upper corona. It provides a way of identifying large magnetic loops that are still connected at both ends to the solar corona.
It is important that DSCOVR be at L1 before the ACE mission ends, to allow for cross calibration of the solar instruments, to augment solar wind early warning, and to eventually replace ACE. The DSCOVR PlasMag fluxgate magnetometer will provide crucial continuity of observation of this important interplanetary solar wind parameter. The magnetic field measurements will allow, among other things, the connection of the photospheric magnetic sector structure to 1 AU heliospheric current sheet observations.
Figure 8: Photo of the PlasMag electron electrostatic analyzer (left) and the PlasMag Faraday Cup (right), image credit: NASA
The objective of the top-hat electron spectrometer is to provide high time resolution (<1 s) solar wind electron, full 3D distribution function observations. While electron measurements have an inherently higher uncertainty, the electron spectrometer may extend space weather monitoring to unusually high speed events. The electron spectrometer sits at the tip of mag boom to gain a nearly 4π FOV (Ref. 6).
Figure 9: Illustration of the boom-mounted electron spectrometer (image credit: NASA)
1) Joe Burt, Bob Smith, “The DSCOVR Mission,” Proceedings of the 2012 IEEE Aerospace Conference, Big Sky, Montana, USA, March 3-10, 2012
2) Francisco P. J. Valero, Jay Herman, Patrick Minnis, William D. Collins, Robert Sadourny, Warren Wiscombe, Ddan Lubin, Keith Ogilie, “Triana - a Deep Space Earth and Solar Observatory,” Report prepared for the National Academy of Sciences by the Triana Science Team (SIO, NASA/GSFC, NIST, LaRC, ARC, NCAR, LMD, LM, LANL, VT), URL: http://www-pm.larc.nasa.gov/triana/NAS.Triana.report.12.99.pdf
3) J. G. Watzin, “The Triana Mission - A Pathfinder Mission to Explore the Utility of using Deep Space in Conducting Earth Observation,” Proceedings of the 51st IAF Congress, Rio de Janeiro, Brazil, Oct. 2-6, 2000
4) S. A. W. Gerstl, F. P. J. Valero, “The Triana Satellite Mission from L1 for Global Vegetation Monitoring,” Proceedings of IEEE/IGARSS'99, Vol. I, Hamburg, June 28-July 2, 1999
6) Quang-Viet Nguyen, “Deep Space Climate Observatory (DSCOVR) Mission Briefing,” Heliophysics Subcommittee Meeting, NASA HQ, Washington, DC, Feb. 27-28, 2012, URL: http://science.nasa.gov/media/medialibrary/2012/03/01/Nguyen-DSCOVR-Mission-Briefing-HPS-v01b.pdf
7) “Deep Space Climate Observatory,” Wikipedia, URL: http://en.wikipedia.org/wiki/Deep_Space_Climate_Observatory
9) Jeff Foust, “People re-DSCOVR an existing program,” April 12, 2013, URL: http://www.spacepolitics.com/2013/04/12/people-re-dscovr-an-existing-program/
10) A. Szabo, K. W. Ogilvie, A. F. Vinas, E. J. Summerlin, “Solar Wind Kinetic Physics - High Time Resolution Solar Wind Measurements from the DSCOVR Mission,” DSCOVR White Paper, URL: http://www8.nationalacademies.org/SSBSurvey/DetailFileDisplay.aspx?id=750&parm_type=HDS
11) Alexander Cede, Jay Herman, Alexander Marshak, “Overview of the Earth Science Instruments on the NASA DSCOVR mission,” ICAP (International Cooperative for Aerosol Prediction), ESA/ESRIN, Frascati, Italy, May 14 – 17, 2012, URL: http://bobcat.aero.und.edu/jzhang/ICAP/AERP/MeetingPDFs/RemoteSensing/Cede-DSCOVR%20%28NXPowerLite%29.pdf
12) Francisco P. J. Valero, Brett C. Bush, Shelly K. Pope, V. Ramanathan, Jay Herman, Keith Ogilvie, Warren J. Wiscombe, Steven Lorentz, Joseph Rice, Patrick Minnis, Peter Pilewskie, William D. Collins, Claude Basdevant, Bernard Legras, Hector Teitelbaum, G. Louis Smith, Alan Strahler, Claudio Tomasi, Ankie J. M. Piters, John Burrows, Irina Melnikova, “NRC Earth Science Decadal Survey-Mission Concept Earth Sciences from the Astronomer’s Perspective, a Deep Space Climate Observatory (DSCOVR) ,”NRC (National Research Council), URL: http://geo-cape.larc.nasa.gov/docs/DSCOVR_RFI_030.pdf
14) “SpaceX Awarded Two EELV Class Missions From The USAF,” Space Travel, Dec. 07, 2012, URL: http://www.space-travel.com/reports/SPACEX_Awarded_Two_EELV_Class_Missions_From_The_USAF_999.html
15) J. P. Rice, S. R. Lorentz, K. Lykke, R. C. Smith,F. P. Valero, “ NISTAR: The NIST Advanced Radiometer,” American Geophysical Union (AGU), Fall Meeting 2011, Dec. 5-9, 2011, San Francisco, CA
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.