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.
Figure 1: Artist's view of the SciSat spacecraft (image credit: Bristol Aerospace)
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). 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.
Ground segment: 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.
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 5: Latitude coverage of the ACE-FTS for one year (image credit: University of Waterloo)
Figure 6: Illustration of the SciSat-1 spacecraft and payload, three views (image credit: ABB Bomem)
• 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. 16).
• 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. 14)
- 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. 15)
• In April 2013, the SciSat/ACE spacecraft and its payload are operating nominally. 16)
• 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. 17)
- 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. 19). 18)
• The SciSat-1 spacecraft and its payload are operating nominally in 2011. 19)
• 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. 20) 21) 22)
• 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. 23) 24) 25) 26) 27)
• 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). 28)
• 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 9).
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. 29) 30) 31) 32) 33) 34) 35)
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 7: The FTS interferometer (image credit: ABB Bomem)
Figure 8: Illustration of the optics side of the ACE-FTS instrument (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 9. 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 41 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.
Figure 9: Optical layout of the ACE instrument (image credit: ABB BOMEM)
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. 36) 37)
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 10: 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 11: 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.
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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.