SciSat-1/ACE (Science Satellite/Atmospheric Chemistry Experiment)
SciSat-1/ACE is a Canadian atmospheric science mission. In the time frame 1996/97, CSA (Canadian Space Agency) initiated the SciSat program with the objective to provide opportunities for Canadian scientists to define and conduct space experiments in the following fields: Earth sciences, space astronomy, and solar-terrestrial relations. Mission selection procedures in the program were conducted via an AO (Announcement of Opportunity) process and peer reviews. 1) 2) 3) 4) 5) 6) 7) 8) 9)
In addition, the SciSat program is also part of a CSA/NASA collaboration program, consisting of two missions. Under the terms of the cooperative agreement, each agency provides a spacecraft and instrumentation, to be co-launched on an expendable vehicle. The AO for the Canadian elements of the first SciSat (SciSat-1) was released in 1997. The ACE mission was selected for flight in November of 1998.
The SciSat/ACE mission is based at the University of Waterloo, Waterloo, Ontario (Mission Scientist: Peter Bernath). The overall objective is to monitor and analyze the chemical processes that control the distribution of ozone in the upper troposphere and stratosphere. In particular, ACE is focussing on one important and serious aspect of the atmospheric ozone problem - the decline of stratospheric ozone at northern mid-latitudes and in the Arctic. A comprehensive set of simultaneous measurements of trace gases, thin clouds, aerosols and temperature are being collected by solar occultation from low earth orbit. More than 30 molecules have been detected including: O3, N2O, CH4, HNO3, H2O, HCl, HF, NO, NO2, ClNO3, CO, CO2, CCl3F, CCl2F2, and N2O5.
The SciSat/ACE minisatellite structure, designed, built and integrated by Canadian industry (prime contractor: Bristol Aerospace Ltd. of Winnipeg, Manitoba, a division of Magellan Aerospace Corporation), uses a circular instrument/component aluminum mounting plate (1.12 m in diameter) as the main structure of the platform. The spacecraft is 3-axis stabilized. Attitude control is based on a bias momentum stabilization approach. The subsystem consists of a momentum wheel, torque rods (MTR-30 of SSTL) along all three body-fixed axes, one fine sun sensor, a magnetometer and a set of six coarse sun sensors. All sensors and actuators are off-the-shelf components with flight heritage. Pointing control provides ±1º in pitch and yaw axis (3σ) and ±2º in the roll axis (3σ). The CALTRAC™ star trackers were manufactured by CAL Corp, Canada (now COM DEV International). 10)
Figure 2: Artist's rendering of the SciSat spacecraft (image credit: Bristol Aerospace)
In addition, the GyroWheelTM (developed at Bristol AeroSpace) is flown for technology validation. The GyroWheel has the ability to provide the S/C simultaneously with stored angular momentum, to function as a 3-axis torque actuator, and to measure the S/C angular rates in two axes. The GyroWheel is a CMG (Control Moment Gyroscope) device and as such an actuator/sensor demonstration experiment. The design is based on a spinning flex-gimbal system as opposed to the conventional non-spinning motor-driven gimbals. This innovation allows for maintaining the same three-axis momentum steering capability as a CMG. The primary benefit of the CMG design that it allows for substantial savings in mass (eliminating the need for multiple momentum wheels and gyros), power, and cost of the attitude control system. - Flight testing of GyroWheel was carried out mainly during periods when science is not being conducted. After its validation, the GyroWheel is expected to operate as the primary wheel and will used during science operations. The performance of the GyroWheel is now validated and it functions as a back-up system. The GyroWheel is functionally redundant with the momentum wheel. 11) 12)
Table 1: Characteristics of the GyroWheel
Figure 3: Illustration of the GyroWheel (image credit: Bristol AeroSpace)
The spacecraft is always in a sun-pointing configuration. S/C power (75 W orbital average, triple junction solar cells, GaIn/GaIn/Ge, one of them is a Si cell) is generated by a single body-mounted solar panel. In addition, there are Lithium-Ion batteries (13.6 Ah capacity) for the orbital eclipse phase operations.
On-board source data recording of up to 1.5 GByte is provided. The C&DH (Command & Data Handling) unit was developed for small satellites. It is responsible for all onboard data handling, monitoring and recording. The unit features low-power (≤ 10 W), low-mass (≤ 4 kg) and a radiation-tolerant core in a single string architecture (a UTMC 80C196 16-bit processor is used to perform all S/C operations). The instruments are connected to C&DH via a synchronous RS-422 interface. The S/C mass is 152 kg (payload mass = 47 kg), the design life is two years with a goal of five years. 13)
Figure 4: 2. The scientific instruments and some of the bus components aboard SciSat-1 (image credit: Bristol Aerospace)
Launch: A NASA-sponsored launch of SciSat/ACE took place on Aug. 13, 2003 (UTC) on a Pegasus-XL vehicle (air launch) from VAFB (Vandenberg Air Force Base), CA.
RF communications are provided in S-band using the CCSDS protocol suite. Variable-rate telemetry downlinks can be supported (4, 2, 1, 0.5 , or 0.04 Mbit/s). The downlink uses OQPSK (Offset Quadrature Phase Shift Keying) modulation, and the uplink is compatible with NASA's STDN (Satellite Tracking and Data Network) standards. The maximum data rate (occurring during an occultation) is about 9.6 Mbit/s, and almost all of it come from the ACE-FTS instrument. The uplink data rate is 4 kbit/s. Reed Solomon (RS) encoding is used on the downlink to achieve a 1 x 10-9 bit error rate. Spacecraft tracking and orbit determination is done using coherent Doppler tracking. The ACE-FTS raw data volume is about 2 GByte/day.
Orbit: Circular high-inclination orbit, altitude = 650 km, inclination = 73.9º, period = 97.7 minutes. No onboard propulsion is available for orbit maintenance. The ACE orbit was selected so that the latitude coverage repeats annually (Figure 5).
Figure 6: Illustration of the SciSat-1 spacecraft and payload, three views (image credit: ABB Bomem)
• February 8, 2021: SciSat is still operating nominally in its 18th year on orbit. CSA awarded the SciSat project at the University of Waterloo an extension contract for 3 more years starting on April 1, 2021. 14)
- The Atmospheric Chemistry Experiment (ACE) satellite measures infrared transmission spectra of the atmosphere with a Fourier Transform Spectrometer (FTS) using the Sun as a light source. ACE provides a global view of atmospheric composition from altitude profiles of volume mixing ratios of 44 molecules starting in February 2004. The current version of ACE-FTS processing is 4.1 released in July 2020. Compared to v.4.0, the trends and altitude-latitude distributions have changed only slightly. Quarterly altitude-latitude distributions have been computed to highlight seasonal effects. Generally, the tropospheric volume mixing ratios of v.4.1 agree well with surface measurements made by the NOAA (National Oceanic and Atmospheric Administration) flask network and the AG AGE network. The revised ACE trends provide a quantitative state-of-the-atmosphere report. 15)
- The Atmospheric Chemistry Experiment is a satellite-based mission that has been probing the Earth’s atmosphere via solar occultation since February 2004. Instruments on board include a high resolution Fourier transform spectrometer (ACE-FTS) and a pair of filtered imagers. A new processing version (version 4, with version 4.1 representing the most recent update) has been implemented for these instruments. Analysis for the ACE-FTS instrument makes use of the latest spectroscopic information and features improved accuracy in forward model calculations, including a new instrumental line shape and employing a 100 m altitude sub-grid within the tangent layer of the 1 km altitude grid employed in previous processing versions. Changes were made in the handling of solar and deep space calibration spectra to avoid systematic errors that impacted previous processing versions. Emphasis was placed on improving software robustness, as well as minimizing occurrences of unphysical oscillation in retrieved profiles. Seven new molecules and three new isotopologues were added to the list of atmospheric constituents retrieved from the previous processing version (version 3.5/3.6) for a total of 44 molecules and 24 isotopologues. For the imagers, forward model calculations were changed to a 100 m altitude grid (rather than a 1 km grid) in version 4 processing. 16)
• August 2020: Satellite remote sensing of the Earth’s atmosphere offers a global perspective that is not available from ground-based or air- borne instruments. The Atmospheric Chemistry Experiment (ACE) satellite (a.k.a. SCISAT) measures the composition of the atmosphere by recording the absorption of 44 molecules plus 24 isotopologues by using the solar occultation method. During sunrise and sunset, the center of the solar radiance is tracked, and its attenuation is measured with a high-resolution infrared spectrometer as well as with a visible near infrared spectrophotometer. 17)
- These solar occultation spectra are processed on the ground and yield altitude profiles of volume mixing ratios (VMRs or mole fractions) with a typical vertical resolution of about 3 km from an altitude of 5 km (or the cloud tops) in the troposphere up to about 120 km in the lower thermosphere for CO2. The long horizontal pathlength associated with the limb-viewing geometry of ACE improves the detection limits compared to nadir-viewing satellites. Although nadir-viewing satellite instruments such as IASI (Infrared Atmospheric Sounding Interferometer) on MetOp-A offer exceptional global coverage and relatively high horizontal resolution (e.g., 12 km at nadir), they lack the vertical resolution of limb sounders and typically provide total column densities for trace gases rather than profiles.
- The Canadian ACE satellite was launched by NASA in August 2003 and was designed for a two-year operational lifetime. Now in its 17th year, ACE still works well and has a time series of data starting in February 2004. ACE’s longevity provides an opportunity to measure the change in atmospheric composition of 44 gases as a function of latitude, longitude and altitude for more than 16 years. Anthropogenic chlorofluorocarbons (CFCs) and related gases are responsible for the destruction of stratospheric ozone which protects us from deleterious ultraviolet radiation. ACE monitors the progress of the Montreal Protocol, which controls the production of these long-lived halogen-containing gases. Non- condensable greenhouse gases such as CO2, CH4 and N2O are the “control knob governing Earth’s temperature”. Air quality is affected by the oxidation of organic molecules such as C2H6 (ethane), CH3OH (methanol) and CH3C(O)CH3 (acetone) catalyzed by nitrogen oxides (NO and NO2). ACE has a global data set for 16 + years for H2O, O3, N2O, NO, NO2, HNO3, N2O5, H2O2, HO2NO2, O2, N2, SO2, HCl, HF, ClO, ClONO2, CFC-11, CFC-12, CFC-113, COF2, COCl2, COFCl, CF4, SF6, CH3Cl, CCl4, HCFC-22, HCFC-141b, HCFC- 142b, HFC-134a, HFC-23, CO, CH4, CH3OH, H2CO, HCOOH, C2H2, C2H6, OCS, HCN, CH3C(O)CH3, CH3CN, PAN (CH3C(O)OONO2), high and low altitude CO2 as well as pressure and temperature, http://www.ace.uwaterloo.ca/molecules.php.
Figure 7: Altitude-latitude distribution for CFC-11 (CCl3F) (left), and trends for ACE-FTS and from Montzka et al. (right). The ACE-FTS VMR trend values are the average of data from 60ºS to 60ºN latitude and 5.5 to 10.5 km in altitude (image credit: University of Waterloo)
• February 3, 2020: The SciSat/ACE mission is still working well without problems (Ref. 22). The current version of ACE-FTS processing is 4.0. Version 4.0 looks good and we now have 44 routine molecules and 24 isotopologues (http://www.ace.uwaterloo.ca/molecules.php). Perhaps the most interesting new molecule (in view of climate change) is low altitude CO2. 18)
- A Special Issue of JQSRT (Journal of Quantitative Spectroscopy and Radiative Transfer) entitled the Golden Age of Satellite Remote Sensing: Honoring the 15th anniversary of ACE on orbit has been published in Nov. 2019 (https://www.sciencedirect.com/journal/journal-of-quantitative-spectroscopy-and-radiative-transfer/vol/238/suppl/C) with 9 ACE-related papers.
- Another feature is near-real time (3 day delay) availability of ACE-FTS data, with some sample plots posted daily to the web site (http://www.ace.uwaterloo.ca/), tabs on the left.
- The best overview of the ACE mission is still the paper: P. F. Bernath, ”The Atmospheric Chemistry Experiment (ACE),” JQSRT, Volume 186, January 2017, pp.3-16, http://dx.doi.org/10.1016/j.jqsrt.2016.04.006
• January 30, 2019: The SciSat/ACE mission is still working well with no major technical issues in spite of more than 15 years on orbit. The project at Waterloo University has a contract from CSA (Canadian Space Agency) for another 2 years of mission operations and their strong support for continued funding (Ref. 22).
• September 4, 2018: Launched by NASA on board the Canadian satellite SciSat in 2003, the Atmospheric Chemistry Experiment (ACE) was intended for a two-year mission. Fifteen years later, ACE is still providing excellent spectra that provide vital chemical and physical data about our atmosphere, particularly the ozone layer. 19)
- “ACE monitors the global distribution of more than 35 different species including CFCs, hydrochloric acid, and ozone - in other words, nearly all molecules specified by the Montreal Protocol and associated with the Antarctic ozone hole,” says Peter Bernath, ACE Mission Scientist and team lead for ACE’s Science Operations Center headquartered in the Department of Chemistry at the University of Waterloo. “ACE represents quite an achievement in terms of return on investment, both for science and policy.”
- The 1987 Montreal Protocol has been hailed as the world’s most successful international environmental treaty. It phases out the production of chlorofluorocarbons (CFCs) and other substances shown to deplete Earth’s protective ozone layer. ACE is monitoring the decline of these banned source gases in the lower atmosphere and of product gases such as hydrochloric acid in the stratosphere.
- “We’re the only ones in orbit doing this, and in real time as a function of altitude,” says Bernath. “You can actually watch the ozone hole forming on our website where we post near-real time data every day.”
- ACE is not only known for monitoring the ozone hole; scientists worldwide have published more than 430 papers using ACE data sets. For example, ACE data were used to show how the Asian monsoon directly injects combustion-generated pollution into the upper atmosphere by tracking hydrogen cyanide gas produced mainly by fires. 20)
- ACE data were also used to prove solar activity acts as an additional source of atmospheric nitrous oxide in the upper atmosphere. Previously, the only known natural source of nitrous oxide was denitrifying bacteria living in soils at the Earth’s surface. Nitrous oxide is not only an important greenhouse gas; it’s also a powerful ozone-depleting molecule.
- Meanwhile, this science mission continues to evolve and improve. The high resolution spectroscopic data gathered by ACE allows the Waterloo ACE team to forensically identify new species and then quantify their global concentration trends going back the entire 15 years of the project.
- “We recently received a request to provide data on two hydrofluorocarbons (HFCs) that are being regulated as part of the 2016 Kigali Amendment to the Montreal Protocol,” says Bernath. “We were able to isolate the signal in the spectra and within a month provide an entirely new data product. The unique algorithms the Waterloo team continue to develop are only possible with this project’s longevity.”
Figure 8: The ACE Team poses with a copy of the ACE instrument. Left to Right back: Peter Bernath, Dennis Cok, and Scott Jones; Left to Right front: Johnny Steffen and Chris Boone (image credit: University of Waterloo)
• July 27, 2018: Canada's SciSat satellite has detected pollutants in the atmosphere that have never been identified before from space while conducting the Atmospheric Chemistry Experiment (ACE). One of these is HCFC-142b, a refrigerant gas that is being used to replace ozone-depleting chlorofluorocarbons. Although the radiation force of HCFCs is 10 times weaker than that of CFCs, they still contribute to the greenhouse effect. SCISAT is the only space-based instrument that can measure these pollutants to assess their effects on the environment. 21)
- The satellite has also analyzed the pollutants in the atmosphere caused by the burning of biomass, such as forest fires, and many other constituents, such as methanol. This toxic pollutant is present in very low concentrations throughout the atmosphere. "The data can help us refine global air pollution models," says Dr. Peter Bernath, principal investigator for the mission.
- The ACE confirmed the models used to study and make projections of ozone depletion. "We knew ozone was being depleted, and we had an idea how, but the data to prove our theories was limited," says Professor Jack McConnell of York University, a project participant. "ACE observations have given us a better understanding of why and how ozone depletion is occurring. Now it's easier to make predictions."
- SCISAT continues to operate well after 15 years and the Canadian Space Agency (CSA) extended its mission until 2021.
• The SciSat/ACE mission is fully operational in February 2018. Now in its 15th year since launch (Aug. 13, 2003) ,the satellite is still working well and the project anticipates many more years of science operations. The longevity of SciSat/ACE makes the data particularly valuable for monitoring changes in atmospheric composition as a function of altitude on a global scale. For example, ACE measures the halogenated gases associated with stratospheric ozone depletion and thus monitors the effectiveness of the Montreal Protocol. Long-lived halogenated gases are also important climate change gases because of their large global warming potentials. 22)
- Indeed SciSat/ACE measures all major greenhouse gases and aerosols that drive climate change, and provides data to test and validate climate prediction models. The mission also measures the major organic molecules and nitrogen oxides responsible for air pollution in the troposphere. As “chemical weather forecasting” becomes increasingly sophisticated, SciSat/ACE provides an extensive data set to test the chemical transport models used to make air pollution predictions.
• December 5, 2017: From February 23 to April 1, 2017, a group of eight researchers was collecting data on atmospheric conditions from the PEARL (Polar Environment Atmospheric Research Laboratory) facilities in Nunavut. These measurements, taken with state-of-the-art instruments installed on the ground and aboard balloons, are used to validate data from Canadian instruments on the SCISAT/ACE and Odin satellites. The 2017 validation campaign is supported by CSA (Canadian Space Agency), Environment and Climate Change Canada, the Natural Sciences and Engineering Research Council of Canada, and the Northern Scientific Training Program. 23)
Figure 9: A multi-institutional arctic campaign was conducted in 2017 to validate the data of the Odin and SciSat missions (image credit: CSA)
• In January 2017, the SciSat/ACE mission continues to operate nominally according to Peter F. Bernath, who heads the SciSat science team. The project at the University of Waterloo has a contract with CSA (Canadian Space Agency) until 2018 (but this will very likely be extended). The SciSat mission provides data to academia, space agencies, governmental and scientific organizations around the world. 24)
- The ACE (Atmospheric Chemistry Experiment) on SciSat measures chemical molecules that influence the distribution of stratospheric ozone, particularly in the Arctic. These data are making an important contribution to international environmental policy making aimed at protecting the ozone layer, such as the Montreal Protocol that bans certain CFCs. ACE measures the absorption of solar light by the atmosphere at sunrise and sunset. Different atmospheric constituents absorb different wavelengths of light in characteristic ways—a signature by which they can be identified. This technique is what allows ACE to make extremely accurate measurements. 25)
- With its instrument performing so well, SCISAT is now moving beyond its original mandate and providing excellent data related not only to ozone depletion, but also to climate change, and air quality and pollution. "There are many things we're doing now that we really didn't know we could do," Bernath said.
- ACE observations are also useful in climate studies. For example, they've shown that previously observed increases in the amount of water vapor being injected into the stratosphere have stopped. Water vapor is the most powerful natural greenhouse gas and plays a key role in the Earth's climate. "No one knew why it was increasing and we don't know why it stopped, so there are quite a few mysteries left," said Bernath.
• In October 2015, Canada's SciSat mission is 12 years on orbit with its 2 instruments, ACE-FTS and MAESTRO. The high-resolution ACE-FTS (Atmospheric Chemistry Experiment -Fourier Transform Spectrometer) is operating in the solar occultation mode in Earth-orbit. ACE-FTS retrieves vertical profiles of temperature, pressure, and volume mixing ratio of 38 molecular species, and relies on international cooperation to validate its data products. Data availability from international Earth-observing missions is vital to interpreting domestic results, and ACE-FTS validation uses data products from 12 instruments and 7 space agencies. 26)
- The data of the Taiwan/US FormoSat-3/COSMIC constellation mission are being used and incorporated into the ACE-FTS data validation campaign. The FormoSat-3/COSMIC mission of six small satellites uses signals from GPS satellites to measure water vapor pressure and temperature via radio occultation. These data are compared to those retrieved by the ACE-FTS instrument on SciSat along with a newly developed algorithm applied to ACE-FTS spectra. The new algorithm to retrieve vertical profiles of temperature and pressure from high-resolution solar transmission spectra was developed in support of a partnership between the CSA and NASA/JPL (Jet Propulsion Laboratory) to place an FTS in orbit around Mars as part of the ESA and NASA's joint ExoMars mission (NASA since withdrew). — This algorithm exploits the temperature dependence of individual absorption lines in an infrared vibration-rotation band. ACE-FTS makes multiple measurements during an occultation, separated by 1.5-5 km, and the project analyzes 10 CO2 vibration-rotation bands at each altitude, each with a different usable altitude range. The retrieved profiles have no seasonal or zonal biases, but do have a warm bias in the stratosphere and a cold bias in the mesosphere, with mean differences less than 5 K when compared to ACE-FTS. The FormoSat-3/COSMIC comparisons are done below 40 km where the best agreement prevails with FTS-ACE data and the mean differences are less than 3 K. The H2O comparisons between ACE-FTS and FormoSat-3/COSMIC show good agreement in the stratosphere, and higher concentrations retrieved by COSMIC in the troposphere.
- Over its lifetime of 12 years in orbit, ACE-FTS has been well validated by satellite, ground-based, and balloon-borne instruments to establish the accuracy and reliability of the ACE-FTS data products. Published temperature validation results used three satellite instruments, nine ground-based instruments, one balloon instrument and sondes launched from several sites. The satellite instruments were: SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) instrument on the TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics) spacecraft, MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) on Envisat, and the HALOE (Halogen Occultation Experiment) on UARS (Upper Atmosphere Research Satellite). TIMED and HALOE are NASA instruments, while MIPAS was developed by ESA (European Space Agency).
Legend to Figure 10: Shown are: number of coincidences, correlation coefficient, mean relative differences, and the standard deviation of the mean relative differences. Instruments used in the validation are: HALOE, MAESTRO, MIPAS, MLS, POAM III, SAGE III, SMR, and COSMIC. MIPAS has data products from two retrieval software versions by ESA and IMK (Institute of Meteorology and Climate Research), formerly the Karlsruhe Research Center, Karlsruhe, Germany.
• Nov. 6, 2014: Thanks to Canada’s SciSat mission, an international team of scientists has discovered a recent and unexpected increase in stratospheric hydrogen chloride (HCl) in the Northern Hemisphere. 27) 28)
- Information from SciSat along with other satellite data and ground-based measurements showed the scientific team that the increase in stratospheric HCl is due to a slowdown in the atmospheric circulation of the Northern Hemisphere. This discovery could impact how scientists will analyze the evolution of the stratospheric ozone layer going forward.
- Since 1989, the implementation of the UN’s Montreal Protocol has led to a reduction in chlorofluorocarbons (CFCs) around the globe. These CFCs are responsible for the depletion of the ozone layer that protects us from ultraviolet radiation. CFCs break-up in the stratosphere and release chlorine atoms that then form HCl. Under certain conditions, HCl can be transformed into other chlorine-containing molecules that destroy ozone.
- The SciSat data was essential in identifying the altitudes at which this change was taking place. Without reliable HCl measurements in the lower stratosphere provided by the Canadian instrument, we could not have reached this conclusion. No other single instrument has provided such a wide range of data products for such a long time.
• Oct. 2014: SciSat has been operating more than 5 times its design lifetime. The mission has been highly successful and its scientific return far exceeded initial hopes. Starting from a dedicated ozone characterization experiment, the SciSat data are now used worldwide to monitor many chemical species in the atmosphere impossible to observe with any other instruments. Hence, there is a strong interest and value in assuring the continuity of the measurements made with SciSat. 29)
- The ACE-FTS was designed for a 2-year mission. The parts selection & qualification level, the redundancy scheme, the shielding, the life testing of critical elements have all been done with a 2-year mindset and a scientific mission risk profile. After more than 11 years on orbit, the SNR performance still exceeds the original requirement (100) by a factor of up to 4x in certain spectral regions. No anomaly was observed to date despite the fact that early during its mission life, the SciSat-1 spacecraft had to face a violent and severe solar storm. SciSat remains the only sensor in orbit providing long term trending of low concentration molecules within the atmosphere. This is a key contribution to the assessment of climate change.
- Various studies have been conducted for potential SciSat follow-on. One of these missions is the CASS (Chemical and Aerosol Sounding Satellite) mission. The goals of the CASS mission are to further expand our understanding of:
1) How the Earth’s atmospheric composition is evolving and responding to both natural and anthropogenic changes.
2) The coupling between the recovery of the ozone layer from the effect of ozone-depleting substances and climate change.
3) The processes and dynamics that control the role of the upper troposphere/lower stratosphere (UT/LS) region and the links with the surface climate.
CASS also has for objectives to fill a potential data gap between the current limb sounding mission and the future limb sounding missions. To achieve these goals, CASS is planned to carry an improved version of the ACE-FTS with enhanced capabilities.
The Phase 0 of CASS was completed in Spring 2012 and experimental demonstration of critical subsystem was completed in the summer of 2014. - The CSA (Canadian Space Agency) has also been considering many other missions involving FTS technology in the past years. None of these missions are currently funded but preliminary studies have been conducted for them and discussions for international collaboration have been taking place (Ref. 29).
• January 2014: The SciSat/ACE spacecraft and its payload continue to operate nominally. The current extension of the mission is to July 2014. However, ACE is currently undergoing a major review, and a further extension of the mission may be the outcome (Ref. 32).
• In August 2013, the SciSat/ACE spacecraft was completing 10 years on orbit. The project team and CSA are proud of the unique measurement capabilities of this small Canadian satellite and of the large role that this space mission plays in monitoring stratospheric ozone and its associated chemistry. SciSat helps a team of Canadian and international scientists improve their understanding of the depletion of the ozone layer, with a special emphasis on the changes occurring over Canada and in the Arctic. 30)
- SciSat has surpassed expectations by lasting 10 years to date. It delivers valuable data on climate change, air quality and pollution in support of international environmental policy aimed at protecting the ozone layer. The tenth anniversary of the first science data downloaded from SCISAT will be marked by a scientific workshop held at York University in Toronto from October 23 to 25, 2013.
- Originally planned as a two-year mission, SciSat’s instruments continue to provide information about more than 30 different molecular species, which is more than have ever been thoroughly measured from space. SciSat delivers excellent data related not only to ozone depletion, but also to climate change, air quality and pollution. Undoubtedly, SciSat’s mission is a great Canadian success story.
- On October 22, 2013 at the University of Toronto, scientists, government representatives and industry partners participated in a media event to celebrate a decade of success for Canada's SciSat/ACE satellite mission. The tenth anniversary of the first science data observations from SciSat/ACE will also be marked by a scientific workshop at York University in Toronto from October 23-25, 2013. 31)
• In April 2013, the SciSat/ACE spacecraft and its payload are operating nominally. 32)
• On August 13, 2012, the SCISAT/ACE project marked the 9th anniversary of the mission in orbit (2 year design life). Since launch, the satellite and instrument operations are nominal. 33)
- On 8 June 2012, SCISAT completed its 47,500th orbit!
- Profiles available for ~29,000 occultations
- ~50% of occultations occur in polar regions (> 60 degrees)
- Operation of ACE mission approved until the end of March 2014.
• In 2012, the SCISAT/ACE instruments and satellite are continuing to function nominally and produce excellent results (deriving altitude profiles of over 30 different atmospheric trace-gas species, temperature and pressure) after more than 8 years on orbit (Ref. 35). 34)
• The SciSat-1 spacecraft and its payload are operating nominally in 2011. 35)
• In the early part of 2010 (February 20 -April 1), the “Canadian ACE Arctic Validation Campaign” was conducted with a suite of ground-based instruments which were deployed making measurements of trace gases to assess the measurements of ACE-FTS on SciSat. The campaign took place at the PEARL (Polar Environment Atmospheric Research Laboratory) facility located in Eureka, Nunavut (Canada).
• The SciSat-1 spacecraft and its payload are operating nominally in 2010 (in its 7th year of operations, 2 years of nominal design life). CSA intends to keep the very successful mission operating. 36) 37) 38)
• In 2009 the spacecraft is operating nominally, there is no degradation in performance of the FTS or of the satellite bus. The only issue is for the wavelength limits of the MAESTRO instrument: they are now decreased to 450-1000 nm. Fortunately this does not have much impact on the primary MAESTRO science (NO2 and O3 profiles plus atmospheric extinction). - CSA has committed to supporting the mission at least until March 31, 2010.
• SciSat provides high-precision information on the condition of the ozone layer and atmospheric changes. In 2006, data collected by SciSat played a key role in helping scientists better understand the loss of ozone over the Northern hemisphere.
• As of mid-April 2006, the ACE satellite instruments had made more than 10,000 occultation measurements. No degradation of ACE-FTS instrument performance or functionality was observed since launch. 39) 40) 41) 42) 43)
• In August 2005, SciSat-1 met its mission life requirements of 2 years. CSA decided to extend the funding for SciSat-1/ACE mission operations (mission life) for two more years.
• After 6 months of commissioning and test phase the spacecraft was declared fully operational on February 27, 2004 (start of science mission). 44)
• On October 22, 2003, the first data observations from SciSat/ACE were acquired.
Sensor complement: (ACE-FTS, MAESTRO)
ACE-FTS (Atmospheric Chemistry Experiment-Fourier Transform Spectrometer)
ACE-FTS is the prime instrument of the SciSat mission. ACE has been built by ABB Bomem Inc. of Quebec City, Quebec. The objective is to measure the vertical distribution of atmospheric trace gases, in particular of the regional polar O3 budget, as well as pressure and temperature (derived from CO2 lines). The instrument is an adapted version of the classical sweeping Michelson interferometer, using an optimized optical layout (Figure 13).
ACE consists of the following components: the FTS, a VNIR (Visible Near Infrared) imager, a sun tracker, the instrument electronics, and a power supply. An SNR > 100 is achieved; IFOV (FTS) = 1.25 mrad; a telescope aperture diameter of 100 mm and a measurement period of 2 s. The instrument includes a suntracker, which provides fine pointing toward the radiometric center of the sun with a stability better than 15 µrad, to both the infrared spectrometer and the imager during solar occultation of the Earth's atmosphere (there are about 30 sun occultation periods per day). Measurements can be made in the altitude range 5-150 km. The FTS is coupled with an auxiliary 2-channel VNIR imager. 45) 46) 47) 48) 49) 50) 51)
The operation of the ACE-FTS in solar occultation provides a reproducible evaluation of the temperature profile. In fact, since the radiance of the sun is used as the radiometric reference for the instrument, the temperature sounding is much less sensitive to manufacturing variability from one unit to the other or to the ageing of the hardware. This is a key advantage for global climatology where trends over decades must be accurately measured.
• The FTS spectrometer looks at the sun through the atmosphere (occultation or limb-viewing geometry) at different tangent heights, providing a series of spectra that are used to deduce the vertical distribution of trace gases and temperature. The spectral range of the instrument is from 2-13 µm (750 - 4100 cm-1) in two bands, and the maximum resolution is 0.025 cm-1. InSb (1800-4100 cm-1) and HgCdTe (750-1800 cm-1) detectors are used. Both detectors are cooled below 110 K. The spectrometer transforms the spectra into a modulated signal, the interferogram, in which all of the IR bands are present simultaneously. The spectrometer output consists of such interferograms for each observed scene. The interferograms are Fourier-transformed into spectra on the ground to provide vertical profiles of atmospheric constituents at vertical resolutions of 3-4 km.
• VNIRI (Visible Near Infrared Imager). Objective: monitoring of aerosols using the method of extinction of solar radiation. Two filtered detectors at 1.02 and 0.525 µm are employed. VNIRI provides sun images in two spectral bands at 0.525 µm and at 1.02 µm. Refractive index distortion of the solar image for low altitude measurements is monitored with a large CMOS photodetector array of 128 x 128 effective elements covering 30 mrad with a pixel separation of 0.25 mrad (the IFOV is more than four times smaller than the IFOV of the FTS). These measurements have an SNR>100 for all sun-illuminated pixels in a two-second observation time. The sun tracker keeps the instruments (FTS and VNIRI) automatically pointed at the sun's radiometric center.
Table 2: Characteristics of the ACE-FTS instrument
Figure 11: The FTS interferometer (image credit: ABB Bomem)
Figure 12: Illustration of the optics side of the ACE-FTS instrument, front cover removed (image credit: ABB Bomem)
The main design drivers of the ACE-FTS instrument are sensitivity (SNR), spectral resolution and large spectral coverage. The spectrometer is an adapted version of the classical Michelson interferometer using an optimized optical layout. The instrument optics are based on a highly folded design and results in a very compact high performance instrument as shown in Figure 13. The first optical component is the suntracker module that tracks the radiometric center of the sun. The infrared and visible signals are then directed to a 5X magnification telescope primary mirror. A small bandpass filter, mounted on the primary telescope mirror, transmits the 1.52 µm to 1.59 µm spectral range to a quad cell (used as the feedback source for the suntracker module) and reflects the remaining spectrum to the VIS/NIR imager. The primary mirror reflects the signals through the aperture and field stops to the secondary collimation mirror. Then, the collimated beam is directed towards the interferometer. A filter is installed between the input optics and the interferometer to minimize the thermal load on the interferometer. The output of the interferometer is then condensed to the InSb/MCT detector assembly using an off-axis parabola. The ACE-FTS instrument has a mass of about 35 kg, power 40 W operating, and 15 W in standby.
Note: The ACE-FTS mission based on ATMOS (Atmospheric Trace Molecule Spectrometer), a JPL instrument which flew four times on the Space Shuttle (1985, 1992, 1993, and 1994). However, the ACE-FTS instrument has been miniaturized by nearly a factor of 10 in terms of mass, power and volume as compared to ATMOS.
MAESTRO (Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation)
MAESTRO was designed and built in a partnership between MSC (Meteorological Service of Canada), EMS Technologies of Ottawa, and the University of Toronto. The design of MAESTRO is of CPFM (Composition and Photodissociative Flux) heritage, an airborne instrument developed at the Meteorological Service of Canada, which has flown on numerous ER-2 aircraft missions. 52) 53)
MAESTRO is a dual-channel optical spectrometer in the spectral region of 285-1030 nm. The objective is to measure ozone, nitrogen dioxide and aerosol/cloud extinction (solar occultation measurements of atmospheric attenuation during satellite sunrise and sunset with the primary objective of assessing the stratospheric ozone budget). Solar occultation spectra are being used for retrieving vertical profiles of temperature and pressure, aerosols, and trace gases (O3, NO2, H2O) involved in middle atmosphere ozone distribution. - The use of two overlapping spectrometers (280 - 550 nm, 500 - 1030 nm) improves the stray-light performance. The spectral resolution is about 1-2 nm.
Figure 14: Illustration of the MAESTRO instrument (image credit: MSC)
The detectors are linear EG&G Reticon photodiode arrays with 1024 elements. The instrument design is based on a simple concave grating with no moving parts. The entrance slit is held horizontal to the horizon during sunrise and sunset by controlling the spacecraft roll with a startracker and a momentum wheel on the satellite bus. The vertical resolution of the MAESTRO data is about 1 km; the SNR is >1000. The high vertical resolution may help to distinguish between various atmospheric layers. The oxygen A-band at 762 nm (as well as the B-band and gamma-band) will be used to make an independent determination of atmospheric temperature and pressure. MAESTRO is also able to make some near-nadir solar backscatter measurements with a separate backscatter port. The mass of the instrument is about 8 kg, power = 15 W (operating), 7 W (standby), the data rate = 3 Mbit/s.
Figure 15: Schematic of the MAESTRO instrument (image credit: MSC)
The measurements obtained by the ACE-FTS and MAESTRO instruments are being combined with data gathered by ground-based, balloon-based and other space-based projects to obtain the best possible information to predict future trends relating to the ozone layer and its depletion.
The SciSat ground segment consists of a SOC (Science Operations Centre), based at the University of Waterloo, and a MOC (Mission Operations Centre) at CSA in Saint-Hubert (along with a ground station), Quebec, and a second ground station in Saskatoon, Saskatchewan. In addition, there is an ESA ground station at Kiruna, Sweden, and a NASA ground station at Fairbanks, AK.
At the SOC the data is archived and transformed into data products for distribution to the science team members. The data analysis of SciSat is based on the cooperation of many national and international partners. The science team includes researchers from Canada, USA, Belgium, Japan, France, and Sweden.
Raw data is transformed first into reflectance spectra as function of altitude (Level 1) and then into vertical profiles of concentration for various atmospheric constituents (Level 2). Data processing is conducted by the ACE Science Team based at the University of Waterloo. The following paragraph gives a list of the molecules retrieved routinely by the ACE-FTS Level 2 processing (version 3.0): 54)
H2O, O3, N2O, CO, CH4, NO, NO2, HNO3, HF, HCl, N2O5, ClONO2, CFC-12 (CCl2F2), CFC-11 (CCl3F), COF2, HCFC-22 (CHClF2), HDO, SF6, OCS, HCN, CF4, CH3Cl, C2H2, C2H6, N2, CO2, H2CO, H2O2, O2, HO2NO2, HCOOH, CH3OH, COCl2, CCl4, COClF, HCFC-142b (ClF2C-CH3), CFC-113 (Cl2FC-CClF2), HCFC-141b (Cl2FC-CH3), as well as isotopologues for some of these molecules.
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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).