SORCE (Solar Radiation and Climate Experiment)
SORCE is a NASA-sponsored minisatellite mission. The overall objectives are to continue the precise, long-term measurements of TSI (Total Solar Irradiance) at UV and VNIR wavebands. The science objectives include: 1) 2) 3) 4)
• Daily measurements of the solar UV (120-300 nm) with a spectral resolution of 1 nm. To achieve an absolute accuracy of better than ±5%, and a precision and long-term relative accuracy of ±0.5%.
• To make the first precise measurements of the visible solar irradiance suitable for future climate studies - to obtain a daily measurement of the solar irradiance, between 0.3 and 2 µm, with a spectral resolution (lambda/delta lambda) of at least 1/30, an absolute accuracy of ±0.3% (3 σ), and a precision and relative accuracy of better than ±0.01%/year.
• Improve our understanding of how and why the variability occurs at the Sun and how the variable irradiance affects our atmosphere and climate. To use this knowledge to estimate past and future solar behavior and climate response.
• To make precise and accurate measurements of TSI, connect them to previous TSI measurements to form the long-term climate record. To provide absolute accuracy of 0.03 % (300 parts per million) based on SI units and a relative accuracy of 0.001%/year.
Figure 1: The SORCE spacecraft illustration (image credit: NASA)
SORCE extends the broad data set gathered by the ACRIM instrument series while exploring a new capability to measure solar irradiance in two discreet spectral bands (ten-fold measurement accuracies are required over existing spaceborne radiometer and spectrometer performances). The SORCE project is a PI-led mission, designed, managed and operated by LASP (Laboratory for Atmospheric & Space Physics) of the University of Colorado in Boulder, CO (PI: G. J. Rottman). The payload instruments are designed and built by CU/LASP.
The SORCE mission is part of NASA's EOS (Earth Observation System) program. NASA plans to transition many of the EOS key measurements into longer-term observations using NOAA operational satellites. The TIM and SIM instruments are currently planned to be flown on the future NPOESS series spacecraft. Within the context of NPOESS the TIM and SIM instruments are referred to as TSIS (Total Solar Irradiance Sensor); TSIS is being built by Northrop Grumman for LASP. 5)
Background: The SORCE program was developed between 1999 and the launch in 2003 - but originated in the early 1980's when NASA began developing the EOS program with a goal of determining the extent, causes, and regional consequences of climate change. EOS defined science and policy priorities based on recommendations of national and international programs including the IPCC (Intergovernmental Panel on Climate Change) and the CEES/CERN (Committee on Earth and Environmental Sciences/Committee on the Environment and Natural Resources). 6)
The SORCE minisatellite, developed and built at OSC (Orbital Sciences Corporation) of Dulles, VA, employs a Leostar-2 platform (of OrbView-4 heritage). The S/C is a three-axis stabilized; it is a momentum-bias system introducing the first redundant version of the Leostar-2 bus for nearly all subsystems: reaction wheel assemblies, active magnetic control (torque rods), star trackers, magnetometers, and redundant sun sensors (provided by the instrument payload). 7) 8)
The S/C dimensions are: 1.48 m in height and 1.12 m in diameter; power = 350 W (EOL), provided by six fixed GaAs solar arrays; a 23 Ah NiH2 battery provides power during eclipse phases. The C&DH (Command and Data Handling) subsystem controls all aspects of the bus operations. The OBC features RAD6000 processors. The onboard communication is via MIL-STD 1553B interface cards.
The deployed spacecraft measures 340 cm in diameter and 161 cm in height. The S/C dry mass is 290 kg, the payload mass is 86 kg, the S/C design life is six years. An on-board data storage of 1024 Mbit is provided (for more than 24 hours of S/C data).
The SORCE spacecraft consists of two integrated modules: the satellite bus and the instrument module that contains the instruments and their associated electronics. The bus module structure is a robust, hexagonal modular unit of aluminum honeycomb.
The spacecraft is sun-pointed during the dayside orbit and star-pointed during the nightside portion of the orbit (for stellar calibration purposes). The S/C pointing values are:
• Accuracy of better than 60 arcsec (pitch and yaw)
• Knowledge of better than 36 arcsec (pitch and yaw)
• Stability of better than 1 arcmin/min (pitch and yaw)
• Jitter of better than 20 arcsec/s (pitch and yaw)
• Slew rate is ≥ 1.1º/s.
SORCE was designed to perform its mission without the use of a gyro (due to cost limits), both for safe mode and for science mode. OSC and LASP developed and analyzed this gyroless mode of operations. 9)
• Safe mode is used to recover the S/C from arbitrary attitude and attitude rates, to safe the S/C into a power-positive orientation using minimum hardware. The safe mode relies on sensing by CSS (Coarse Sun Sensors) and TAM (Three-Axis Magnetometer)
• Science mode points the instruments at solar and stellar targets to collect solar radiance and calibration data. Two STR (Star Trackers), mounted orthogonally, and FSS (Fine Sun Sensor) provide inputs for deriving the attitude and rate estimates.
Figure 2: Artist's rendition of the deployed SORCE spacecraft in orbit (image credit: CU/LASP)
RF communications: Redundant onboard S-band transceivers and a pair of omni-directional antennas. The uplink data rate is 2 kbit/s and the downlink is 1.5 Mbit/s (maximum data volume of about 570 Mbit/day). In addition, there is a downlink of 4 kbit/s via TDRS.
Table 1: Spacecraft parameters
Orbit: Circular orbit, altitude = 645 km, inclination = 40º, period = 97.19 minutes.
• June 30, 2019: The SORCE team hosted an Ice Cream Social at LASP to celebrate that SORCE has been in orbit for 6000 days at the end of June 2019! To recognize this milestone attendees topped their ice cream sundaes with 6000 M&Ms, which included some fun custom M&Ms (see photo). SORCE launched on Jan. 25, 2003 and reached 6000 days in orbit on June 30, 2019. This extremely successful mission is the product of spectacular engineering, dedicated managers, engineers, and scientists, and an amazing mission operations team that continues to make it happen!
Figure 3: SORCE PI Tom Woods (left) and the original SORCE PM Tom Sparn enjoying another SORCE milestone (image credit: LASP)
• June 15, 2019: Great news for the SORCE mission! After a Key Decision Point (KDP) Meeting regarding SORCE's Phase F, NASA Headquarters has decided to extend SORCE operations into January 2020. 11)
- SORCE has had battery issues for years and had some communication outages in April 2019. This later anomaly triggered NASA HQ to accelerate the SORCE decommissioning planning and to hold the SORCE KDP-F meeting on July 11, 2019. The objective for this meeting was to evaluate the SORCE decommissioning plan, schedule, and budget for mission close-out and decide between a mid July 2019 passivation (turn-off) or the original January 2020 passivation date.
- The final decision to stay with January 2020 passivation was based on SORCE's compelling science, than the potential battery and other spacecraft risks, which might force an early passivation. The current plan for SORCE is to continue operations until January 15, 2020, and SORCE Phase F will start the day after passivation and go through September 2020 to produce and archive the final data products.
• May-June 2018: As it approaches the sixteenth of a 5 year prime mission, NASA's SORCE (Solar Radiation and Climate Experiment) mission continues to meet and exceed all science requirements while operating with severely degraded batteries. By 2013, the batteries had degraded such that the On Board Computer (OBC) could not be powered through eclipse. This prevents science data collection in eclipse and erases over 99% of all stored science and engineering telemetry. To mitigate these problems, the FOT (Flight Operations Team) at the Laboratory for Atmospheric and Space Physics (LASP) adopted a new operations scheme. "Daylight Only Operations" (DO-OP) transitions the spacecraft from safemode to science mode every orbit. This involved heavily automating the transition process using ground autonomy to improve spacecraft recovery time to 6 minutes - down from 3 orbits of manual commanding. While highly successful, this method of operations poses daily challenges that must be overcome using increasingly complex ground software. 12) 13)
- Since 2014, SORCE is successfully operating in DO-OP mode and regularly exceeds all science requirements. With minimal adjustments, ground automation has successfully configured the spacecraft for over 2,000+ orbits and 60,000+ contacts. During this time, ground autonomy has correctly responded to several anomalies such as APE (Attitude and Power Electronics) brownout, UL Card brownout, battery degradation and recovery, unexpectedly high battery currents, various connection issues with ground stations, and TDRS anomalies. Without ground automation, DO-OP mode would require an unsustainable commitment from the FOT. By eliminating constant manual commanding and creating procedures that can completely configure the spacecraft by reacting to a variety of nominal and anomalous conditions, the FOT has developed a low risk solution to allow SORCE to continue science operations in a demanding environment. As components degrade, the FOT updates and adds additional capabilities to the ground automation to keep SORCE healthy and performing nominally.
• On January 25, 2016, the SORCE mission was 13 years on orbit (design life of 5 years). The SORCE mission has been extended through 2018 to overlap with the NASA TSIS (Total and Spectral Solar Irradiance Sensor) scheduled for deployment on the International Space Station in late 2017. To insure continuity in the TSI (Total Solar Irradiance) calibration scale between SORCE and TSIS, in the event that SORCE end-of-mission occurs prior to the launch of TSIS, the JPSS (Joint Polar Satellite System) TSI Calibration Transfer Experiment (TCTE) was launched on the US Air Force STPSat-3 (Space Test Program Satellite-3) in 2013. TCTE has also been extended to 2017 in order to overlap with TSIS. 14)
- With respect to the SSI (Solar Spectral Irradiance) data record continuity, the SORCE team evaluated the requirements for sufficient overlap between the SORCE and TSIS-1 SSI measurements. This analysis is more involved than the TSI continuity analysis due to the wavelength dependence of both the spectral variability as well as the instrument stability corrections.
• January 2016: The SORCE/TIM determines a new, lower value of TSI than previous measurements. A TSI of 1361 W/m2 has clearly been established over the past several years. Prior to the SORCE launch in 2003, on-orbit TSI instruments agreed with each other in measured TSI values near 1365 W/m2 near solar minima. The new SORCE/TIM, including many optical, electrical, and calibration improvements over these prior instruments, measured values 0.35% lower than the other on-orbit instruments (Figure 4). Initially disregarded by the community as an error in the TIM instrument, this difference has recently been shown to be due to uncorrected scatter causing erroneously high measurements by other instruments, all of which have an optical design that differs from the TIM by allowing two to three times the amount of light intended for measurement into the instrument. The TIM, placing the instrument's small precision aperture at the entrance, only allows the light intended for measurement into the instrument interior, and hence is much less susceptible to scattered light. 15) 16)
Figure 4: The TSI Climate Data Record now spans 36 years. Instrument offsets are unresolved calibration differences, much of which are due to internal instrument scatter (image credit: CU/LASP)
• Nov. 2015: Launched in 2003, the SORCE mission has been extended to overlap with ISS-TSIS in 2017-2018. The spacecraft and instruments are making routine daily measurements in its DO-Op (Day-Only Operations) mode. 17)
- SORCE operations are planned through 2018 so that SORCE has adequate overlap with ISS-TSIS (Total Solar Irradiance Sensor). The ISS-TSIS launch is planned for July 2017. The goal is to have a 12-month overlap of SORCE & ISS-TSIS.
Figure 5: Illustration of SORCE DO-Op (Day Only Operations) since 2014, image credit: CU/LASP
Figure 6: The SORCE spacecraft is stable in its DO-Op mode. The SORCE critical life-limiting item is the battery (image credit: CU/LASP)
• August 2014: Great news! SORCE successfully transitioned to a new "hybrid" operating mode on Monday, Feb. 24, 2014. The hybrid mode allows SORCE to take solar measurements again after an approximate 6-month break. After months of development and planning, the new version of flight software, making this all possible, was uploaded to the spacecraft on Feb. 21,2014 and then activated on Monday, February 24th. 18)
- It is called "hybrid" because every orbit is a combination of normal operating mode and safe-hold. The hybrid mode allows SORCE to make solar observations during the daylight part of the orbit, and then put itself into safe-hold every eclipse. Commands are sent to the spacecraft at sunrise from NASA's TDRSS to wake it up. It then initializes all of the instruments and starts taking data. Either a ground contact later in the orbit or a real-time TDRSS contact then captures the data. A few minutes before sunset, the spacecraft puts itself back into safe mode to conserve power during the night. No other spacecraft has ever operated like this before and the SORCE team is very excited.
- The first week in hybrid mode was very successful. Daily teleconferences were held between the LASP mission operations team, instrument scientists, Orbital Sciences Corp., and NASA for status updates. No serious issues surfaced during this trial first week. After one week, the SORCE team presented a Hybrid Mode summary confirming its wonderful success. SORCE has been able to make good solar measurements during almost every orbit in its new hybrid mode. The SORCE extended mission objectives will be fully addressed with the new SORCE hybrid operations.
- The SORCE team will next examine a potential option to operate in hybrid mode even with a spacecraft brown-out scenario (caused by another battery cell loss), with the goal being to operate SORCE until after the TSIS launch in 2017.
Table 2: Some background on the SORCE operations recovery 19)
• January 2014: The SORCE satellite is still being operated at LASP; NASA has extended the SORCE mission until 2015. 20)
- However, the battery had an anomaly on July 30, 2013 so that its voltage is too low to leave on instruments and one of the key spacecraft computers during orbit eclipse. Thus "normal" operations was stopped on July 31, 2013. The project is now operating in safe-hold mode while flight software is updated so that a return to daily solar observations can be accomplished sometime in early 2014.
- The project at LASP did have a 7-day solar observing campaign in December 2013 for overlap with the TCTE (TSI Calibration Transfer Experiment) TIM new measurements of TSI (Total Solar Irradiance) - this campaign was during a time of short eclipse periods and thus the battery voltage was high enough for normal operations to get through the short eclipse periods. 21)
- The project is updating the flight software so that the spacecraft computer and instruments can autonomously be activated to make solar observations during just the dayside of the orbit. Once these updates are installed, SORCE will return to "normal" operations with daily solar observations (Ref. 20).
• August 19, 2013: With the loss of another battery cell, SORCE is now operating in a new configuration. 22)
SORCE spacecraft is currently operating in an ‘emergency' mode.
- SORCE has experienced a cell failure for CPV6 (each nickel hydride common pressure vessel, CPV, has two cells, and SORCE has 11 CPVs). This CPV6 failure caused the end-of-orbit discharge voltage to fall below the voltage level needed for operation of critical flight functions controlled by the Onboard Computer (OBC). This ‘brown-out' condition causes the system to revert to a spacecraft safe-hold condition under control of a more limited satellite controller called the APE (Attitude and Power Electronics). The APE system controls basic functionality of the spacecraft in safe-hold mode until the OBC is able to resume command.
- Current efforts are in progress at Orbital and LASP for an update to the APE flight software so that safe-hold mode operation does not require 24/7 operations. This update will enable more heating on the orbit day side and turning off non-essential components before the orbit eclipse begins (night side).
- The SORCE flight operations and science teams are now developing new operations and science concepts for the continuation of the mission given the current status of the SORCE power systems.Prior to the onset of the CPV6 failure all the instruments were powered-off during eclipse and making solar measurements during orbit day side. This mode of operation has been on-going for several months prior to the CPV6 failure.
- The key impacts on instrument operations in this power-cycling mode are:
1) Larger thermal variations for all instruments – SIM data quality is impacted more than other instruments
2) SOLSTICE no longer has stellar calibration observations
3) TIM no longer has dark calibrations (night side).
- While these impacts are significant, they do not exclude the acquisition of useable science data. With the current spacecraft performance and software configuration, we believe that future science data acquisition will be limited to times of the year where the eclipse period is less than 23 minutes. This is necessary to keep the end-of-eclipse voltage above 19 volts. These short eclipse periods will limit the data to time periods in June and December where the minimum eclipse duration is about 15 minutes. Consequently, these time periods are the best opportunities for SORCE data campaigns.
- SORCE will perform measurement campaigns in these time periods with priority given to TIM TSI observations to provide overlap with the TCTE mission scheduled for launch in November 2013. As the orbit day side is not power limited, the project expects to make SSI measurements by SIM, SOLSTICE, and XPS during these SORCE campaigns.
• June 2013: The 2013 Senior Review evaluated 13 NASA satellite missions in extended operations: ACRIMSAT, Aqua, Aura, CALIPSO, CloudSat, EO-1, GRACE, Jason-1, OSTM, QuikSCAT, SORCE, Terra, and TRMM. The Senior Review was tasked with reviewing proposals submitted by each mission team for extended operations and funding for FY14-FY15, and FY16-FY17. Since CloudSat, GRACE, QuikSCAT and SORCE have shown evidence of aging issues, they received baseline funding for extension through 2015. 23)
- The SORCE Mission has provided a decade of solar irradiance observations (TSI and SSI) with high precision and stability and these results have been and will be used by the Earth Science and Solar Irradiance communities. The results from this mission will be important to the development of future climate models that include both TSI and SSI (Solar Spectral Irradiance) as well as studies of the sources and properties of solar variability.
- The SORCE spacecraft has battery issues, and the mission team acknowledged that the additional staffing provided in the last Senior Review panel was very helpful in mitigating these issues. However, it must be recognized that the SORCE mission is rated as High Risk for technical failure within the mission extension despite the excellent efforts being made by the mission team to mitigate problems and prolong battery performance. It is quite possible that SORCE will fail to overlap with the future TSI missions planned for late 2013 and 2016. In spite of this risk, the contribution of the measurements to the historical data base of solar flux is invaluable and these measurements should be continued until spacecraft failure in order to help prevent data gaps in this climate-critical data product. Thus, the Senior Review Panel recommends continuation of this mission at the baseline budget.
Note: The TCTE (TSI Calibration Transfer Experiment) is part of the STPSat-3 sensor complement which was launched on November 20, 2013. The launch was part of the ORS-3 (Operationally Responsive Space-3) enabler launch mission.
Figure 7: This timeline shows various milestones in the development of solar irradiance measurements relevant to the SORCE mission (image credit: LASP, NASA)
- The SORCE mission continues a seamless, long-term TSI (Total Solar Irradiance) data record that has been compiled by NASA and NOAA since 1978. Although SORCE's battery is expected to last for at least one more year, LASP is preparing to pass SORCE's TSI-recording responsibilities on to a new NOAA mission in the fall of 2013: The TCTE (Total Solar Irradiance Calibration Transfer Experiment) onboard the STPSat-3 (Space Test Program Satellite-3)spacecraft of the US Air Force SMC (Space and Missile Systems Center) will carry a LASP-built TIM (Total Irradiance Monitor), similar to the TIM onboard SORCE, which measures TSI. The overlapping missions (TIM on SORCE and TCTE on STPSat-3) will prevent a gap in this critical 34-year NASA/NOAA climate data record (following the loss of the NASA Glory mission in 2011 with TIM onboard) as SORCE continues to age. 26)
Figure 8: Long-term summary of contributions to the rising global surface temperature (image credit: Kopp, Lean)
Legend to Figure 8: Contributions to the rising global surface temperature can be broken down based on observations: El Niño Southern Oscillation (purple), volcanic eruptions (blue), anthropogenic effects (red), and solar irradiance (green). Continuation of the solar irradiance data will help ensure the robustness of this long-term data record; this, in turn, will improve our understanding of climate change.
The extended mission has 3 primary objectives:
4) Continue to measure TSI with high precision and provide a contiguous extended climate record of TSI
5) Make daily measurements of the solar spectral irradiance
6) Improve the understanding of how and why solar irradiance varies, and estimate future and past variations, and investigate the climate response.
• The SORCE spacecraft and its instruments are operating nominally in 2011 (the mission completed 8 years on orbit). A new TIM instrument is slated to launch on the Glory satellite in February 2011.
In a new study of satellite data, researchers report a lower value of the total solar irradiance than previously measured. The TIM instrument was newly calibrated which significantly improved the accuracy and consistancy of such measurements. 29)
Continuing the SORCE (Solar Radiation and Climate Experiment) irradiance observations into cycle 24 is important to improving our understanding of the solar variations in total and spectral irradiance, thus improving modeling of the solar cycles.
• The SORCE mission reached its extended design life (6 years) in January 2009. The Glory mission of NASA, which a launch scheduled for Q4 of 2009, carries a TIM instrument to continue the long-term measurements of TSI (Total Solar Irradiance). 32)
• As of spring 2007, the project management of the 5 year SORCE mission, launched on Jan. 25, 2003, is looking for extended funding starting in January 2008. 33)
• The spacecraft has performed exceptionally well with no hardware anomalies and continues operations on the primary side of all redundant components. Similarly, the instruments are operating flawlessly and have returned over 35 GB of data used to create the high quality science data products (as of end 2006).
Sensor complement: (TIM, SOLSTICE, SIM, XPS)
The instrument module (IM) features a graphite-epoxy optical bench assembly to provide a thermally stable mounting platform for the SORCE instruments and the two star trackers. The optical bench is kinematically supported by structural elements of the IM. The design's stiff structure provides a stable platform for the instruments, which is essential for science missions requiring fine precision pointing performance, rapid slew rates, and short settling times. - The modular IM concept allowed the independent development, integration, and test of the instrument module and spacecraft bus prior to observatory integration.
Instrument commands are passed directly to the IM microprocessor for instrument operations via the MIL-STD-1553B data bus. Instrument and spacecraft data are stored within the DRAM card and processed through the downlink card for RF transmission to the ground. The spacecraft has several modes of operation to support the solar measurements required by the SORCE mission, the stellar calibrations required for the instruments, and contingency operations in the event of a mission anomaly. 34)
Figure 9: Illustration of the SORCE instrument module (image credit: LASP, OSC)
All instruments were designed, developed and calibrated at LASP of the University of Colorado, Boulder, CO. 35)
TIM (Total Irradiance Monitor):
TIM is an absolute active cavity radiometer of ACRIM heritage, but with significant improvements in sensor and electrical design, particularly in thermal control. Objective: Measurement of TSI directly traceable to SI units (Système International d'Units) with an absolute accuracy of 0.03% and relative accuracy of 0.001% per year. The complete wavelength range is covered. 36) 37)
Figure 10: Schematic of an ESR (Electrical Substitution Radiometer) design (image credit: LASP)
The TIM instrument design features a cylindrical housing assembly containing four identical, right circular, 15º apex angle, conical cavity detectors cantilevered from a single support. The mount provides thermal impedance between the cavities and the heat sink which surrounds them. One cone, the measurement cavity, views the total solar disk through a precision aperture, and any of the other cones may be used as the reference cavity. The cones are made from 1 mm thick electro-deposited 99.99% pure silver. The cone interiors and the heat sink are coated with a Ni-P black and etched to produce a diffuse black surface which has a nominal visible reflectance of 0.3%.
Figure 11: Top and bottom view of cone housing assembly (image credit: LASP)
Figure 12: Illustration of a single conical cavity detector (image credit: LASP)
Six miniature (1 mm2) diamond-substrate chip heaters are soldered to the back of each cone. Gold electrode spinel chip thermistors mounted between the apex of each cone and its support structure are used as sensors for the electrical bridge circuit which controls the measurement cavity temperature. Baffles, located behind the primary aperture, shield the measurement cavity from earth albedo and off axis infrared radiation. Small Si and InAs photodiodes, located just behind the heat sink, view the measurement cavity at an oblique angle to measure the change in its reflectance. The heat sink is thermally isolated from the TIM channel case and its temperature is controlled to 0.001ºC.
The ESR (Electrical Substitution Radiometer) of TIM has dual bolometers with autobalancing through an AC, digital, feedback loop.
TIM incorporates four cavities (cones) and adheres to the basic concepts of Electrical Substitution Radiometers (ESRs), but employs modern, state-of-the-art electronics and materials. The four cavities provide multiple redundancy and added duty-cycling capability. TIM looks at the sun every spacecraft orbit. Each measurement consists of multiple samples taken over the course of a single orbit, providing 15 measurements per day. The TIM shutters, one for each radiometer, are driven by brushless torque motors. In its normal operational mode, the TIM shutter is cycled 50% open and 50% closed every 100 seconds throughout the orbit. Periodic (approximately once per week) field of view maps are obtained by offset pointing the S/C by ±15º about the sun vector. - Instrument mass = 7.9 kg, power = 14 W (average), size = 17.7 cm x 27.9 cm x 27.2 cm (H x W x D), nominal data rate = 539 bit/s.
Figure 13: Illustration of the TIM instrument (image credit: LASP)
Figure 14: Cutaway view of the TIM instrument (image credit: LASP)
Figure 15: The concept of electrical substitution radiometry (image credit: LASP)
Figure 16: Schematic depiction of global energy flows in the sun-climate system by Kiehl and Trenberth (Bulletin of the American Meteorological Society, 1997)
SOLSTICE (Solar/Stellar Irradiance Comparison Experiment):
This 2nd generation instrument is of SOLSTICE heritage flown on UARS (launch Sept. 12, 1991). SOLSTICE is a two-channel grating spectrometer with the objective to measure UV radiation in the spectral coverage of 115 to 320 nm with a spectral resolution between 0.1 nm and 0.2 nm. The device compares the ultraviolet output of the sun with similar radiation produced by about 20 stable, bright blue stars. The stars constitute the standards (calibration) against which the solar irradiance is measured. The standard may therefore serve to directly compare the solar observations done by SORCE as well as by UARS, or by any other future SOLSTICE-type observation.
The SOLSTICE design features two identical spectrometers, mounted at right angles to each other to use perpendicular dispersion directions in order to facilitate verification of stellar pointing . A plane mirror reflects the beam onto one of two diffraction gratings mounted in a precision single-axis gimbal. Rotating the gimbal to a specified angle causes the illuminated grating to diffract a small wavelength band of the original beam toward a second plane mirror. The diffraction grating disperses the radiation and directs a specific wavelength onto one of the two photomultiplier detectors. - About 15 solar spectra and multiple stellar observations are provided each calendar day, resulting from about 15 orbits/day with measurements taken in the daytime (sun) and nighttime (stars) portions of the orbit. 38) 39) 40)
Figure 17: Two views of the SOLSTICE spectrometer (image credit: LASP)
Figure 18: Optical layout of a single SOLSTICE instrument (image credit: LASP)
Figure 19: Optical-mechanical configuration of a single SOLSTICE I channel (image credit: LASP)
Table 3: SOLSTICE instrument parameters
In-flight instrument calibration includes: 1) long-term multi-wavelength observations of bright, blue stars, 2) redundant channel capability, and 3) anticipated underflight calibrations by almost identical instruments on sounding rockets. The wavelength calibration for the in-flight data uses the many well-known solar emission features to determine the wavelength scale.
SIM (Spectral Irradiance Monitor):
The objective of SIM is the measurement of the solar spectral irradiance (SSI) in the 200 to 2000 nm range. SIM is a dual-redundant spectrometer with a single optical element; it provides spectral measurements by rotating a small prism and recording the spectrum at the exit slit using a new high-sensitivity ESR (Electrical Substitution Radiometer). The ESR measurement technique measures heat flux. Two identical sensors, one active and the other used as reference, are connected so that they are in the same environment and at the same temperature. Joule heat is supplied to each sensor by an "actively controlled" heater circuit. The sensors have very high absorptivity in order to efficiently collect radiation, so that all photon energy incident on the detector is converted into heat. As radiation is allowed to fall on the active sensor, a corresponding amount of Joule heat must be removed from that sensor in order to restore the heat flux balance. This change in Joule heat to the active sensor is equivalent to the amount of radiation incident upon it. 41) 42) 43) 44)
Figure 20: Cross-sectional view of the SIM ESR (image credit: LASP)
The SIM instrument consists of two identical mirror-image spectrometers (A and B). This dual-channel design provides redundancy, self-calibration capability, and duty cycling necessary to meet the science objectives. In normal operation, spectrometer A is used for daily solar irradiance measurements while spectrometer B is used to make solar measurements on a much lower duty cycle (perhaps once per month). Additionally, a periscope system couples the two spectrometers and is used in conjunction with two photodiodes (Si and extended InGaAs) on a rotation turntable to provide an in-flight measurement of prism.
Table 4: SIM instrument S-channel specification
Figure 21: Block diagram of SIM (image credit: LASP)
Figure 22: Illustration of the SIM instrument (image credit: LASP)
In normal operation with the shutter open, sunlight enters the 0.3 mm x 7 mm entrance slit and follows a f/115 beam to the prism, 400 mm from the slit. The focusing prism forms a nearly flat spectrum at the focal plane. For each spectrometer, four separate detectors measure the solar irradiance; the primary detector is an Electrical Substitution Radiometer (ESR), and additional silicon and extended InGaAs photodiodes are used for the UV, Visible, and the IR portion of the spectrum respectively. The ESR and the photodiodes, behind separate exit slits, simultaneously measure radiation at four neighboring wavelengths.
Prism drive: The solar spectral intensity at S-channel resolution typically varies 3% per slit width (average value over the entire spectral range), requiring that the prism drive reproducibility be better than 1 µm (one slit width/300). The prism drive achieves 0.3 µm precision by rotating the prism on a flex pivot with a flex-suspended voice coil motor. Closed-loop control of prism rotation uses a small concave mirror on the prism table to focus a second, reference solar beam back to the focal plane and onto a 12,000 element (78 mm active length), 6.5 m2 pixel size, 6.5 m2 pitch linear CCD (Thomson TH7834C). The CCD detector at the focal plane compensates for motion and any bending of the optical bench. For ESR operation, spectra are measured in a step-stop-integrate mode with 3 steps per slit width (100 m2 steps) to eliminate spectral aliasing of the integrated spectrum. The steep portions of the solar spectrum are benchmarks for wavelength adjustment, and the local maxima and minima in the spectrum provide the most reproducible measurements of spectral irradiance.
The ESR provides a long-term, absolute detector for the S-channel and maintains the sensitivity standard for the working photodiodes. The ESR optical head consists of dual bolometers each at the center of an optical quality isolation hemisphere, which reflects light back onto and thermally shields the detectors.
XPS (XUV Photometer System):
XPS is of SXP (Solar X-ray Photometer) heritage flown on SNOE (launch Feb. 26, 1998); XPS is also part of the SEE (Solar EUV Experiment) instrument flown on the TIMED mission of NASA (launch Dec. 7, 2001).
The XPS objective is to measure the XUV (Extreme Ultraviolet) solar irradiance from 1 to 35 nm. The instrument package is a set of filter photometers consisting of twelve silicon XUV photodiodes. Each photodiode has a thin-film filter to provide an approximately 5 nm spectral bandpass. These thin film filters are deposited directly on the photodiode to avoid using delicate metal foil filters which are difficult to handle, prone to develop pin holes, and degrade with time. The set of twelve XUV photometers is packaged together with a common filter wheel mechanism, which can rotate a closed aperture, a fused silica window, or an open aperture in front of any given photometer. The fused silica windows on this filter wheel permit accurate subtraction of the background signal, if any, from visible and near UV light. 45) 46)
Figure 23: Block diagram of the XPS instrument
Table 5: XPS instrument parameters
The 12 XUV photometers (XPs) are grouped into three sets. Each set of four XPs is arranged in a circle for use with the filter wheel mechanism. The filter wheel, which has 3 different rings of filters for the three sets of XPs, has 8 positions : four blocked for dark measurements, two clear for solar XUV measurements, and two with fused silica windows for solar visible background measurements. An observation run is a sequence of measurements from five consecutive filter wheel positions, normally starting and ending with dark measurements. The electronics for each photodiode include only a current amplifier and a voltage-to-frequency (VTF) converter.
Calibration: The XPs are silicon photodiodes with thin film filters to measure the integrated solar UV irradiance from 0.1 to 35 nm with a typical bandpass of 5 nm. The XPS instrument has redundant XPs in order to have a working photometer for daily measurements and a lower duty cycle (reference) photometer for degradation analysis of the working photometer. The reference photometer is used for a solar measurement every week; therefore, the cadence of the reference photometer is once per 105 measurements by the daily channel assuming 15 orbits per day.
Figure 24: Illustration of the XPS instrument (image credit: LASP)
Mission operations are performed from LASP, using two ground stations located at Wallops Island, VA (WGS), and at Santiago, Chile [AGO (Argentina Ground Station)]. Nominal SORCE operations include data downloads twice a day (the NASA Space/Ground Network provides the communication link to the satellite). During the initial phase of the mission, the HBK (Hartebeesthoek Ground Station) at Johannesburg, SA, was used for additional support. NASA's DMSC (Data Services Management Center) at White Sands provides ground station scheduling.
The Science Operations Center (SOC), also located at LASP, is able to access the Level 0 and Level 1 data products in the database and generate processed science data. The science data are then stored in the publicly accessible DAAC located at GSFC. 47) 48) 49)
Figure 25: SORCE operations support network (image credit: LASP)
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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).