CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations)
CALIPSO, alias PICASSO-CENA (Pathfinder Instruments for Cloud and Aerosol Spaceborne Observations / Climatologie Etendue des Nuages et des Aerosols), and alias ESSP-3 (Earth System Science Pathfinder-3), is a satellite science mission, a collaborative NASA/CNES project in the ESSP (Earth System Science Pathfinder) program of NASA. Other project partners (algorithm development, etc.) are Hampton University in Hampton, VA, and IPSL (Institut Pierre Simon Laplace), Jussieu, France.
The overall science objective of the mission is to profile the vertical distribution of clouds and aerosols and their role in the heating/cooling of the Earth (improve of estimates for direct and indirect radiative forcing, improve accuracy of long-wave radiative fluxes at the Earth's surface and within the atmosphere, assessment of cloud feed back in the climate system). The study of cloud and aerosols radiative impacts is within the GCRP (Global Climate Research Program) of WMO providing in particular inputs for GEWEX (Global Energy and Water Cycle Experiment) and CLIVAR (CLImate VARiability and predictability). 1) 2) 3) 4) 5) 6) 7) 8)
The mission is managed by NASA/LaRC. CNES supplies the S/C and an IR imaging sensor (of IASI heritage). NASA/LaRC provides the main instrument suite (a Ball-built lidar sensor) and S/C launch services aboard a Taurus launch vehicle. CNES provides S/C operations, the payload operations center is located at LaRC.
Figure 1: Artist's rendition of the CALIPSO spacecraft (image credit: CNES)
The S/C employes an enhanced Proteus bus (of Jason-1 heritage), developed by Thales Alenia Space (prime contractor, formerly Alcatel Alenia Space), and funded by CNES. The platform structure has has dimensions of 2.46 m (height) x 1.51 m x 1.91 m. When deployed the solar arrays extend to 9.72 m. All the equipment units are accommodated on four lateral panels; the hydrazine mono-propellant system with a 40 liter tank and four 1 N thrusters are positioned on and under the lower plate. 9)
The S/C is three-axis stabilized. The normal in-orbit platform attitude control is based on a gyro-stellar concept. Three accurate 2-axis gyroscopes are used for stability requirements and attitude propagation. Attitude acquisition is obtained using magnetic and solar measurements (two 3-axis magnetometers and eight coarse sun sensors). The AOCS can provide yaw steering. Four small reactions wheels generate a torque for attitude control, they are de-saturated using magnetic torquers. The pointing attitude restitution is 0.05º (3 σ) on each axis. Accurate attitude determination is based on two star trackers (nominal and redundant) measurements. Both star trackers are accommodated on the payload in the STA (Star Tracker Assembly), equipped with an autonomous thermal control (the star trackers, SED16, were built at EADS Sodern). Each star tracker has a 25º circular FOV and includes a CCD detector (1024 x 1024) which is regulated to a temperature of -10ºC by a Peltier device. A GPS receiver provides satellite position information for accurate orbit ephemeris determination and onboard time delivery.
Figure 2: Line drawing of the CALIPSO spacecraft (image credit: NASA)
Figure 3: Internal layout of the Proteus bus (image credit: Thales Alenia Space)
Figure 4: Functional block diagram of the Proteus bus (image credit: Alcatel Alenia Space)
Electric power is generated by two symmetric wing arrays attached near to the satellite center of mass with two single axis stepping motor mechanisms. Each wing is constituted of four 1.5 x 0.8 m panels covered with classical silicon cells. The power is distributed through a single non-regulated primary electrical bus (23/37 V with an average 28 V voltage), using a Li-Ion battery (9 series-3 parallel technology) developed by SAFT, France. Up to 16 switchable power lines (5 A max) can be provided to the payload.
The DHU (Data Handling Unit) performs its tasks through the OBC (MA 31750 processor). It supports all onboard functions including the management of the communication links with every satellite unit, including the payload, either via discrete point-to-point lines, or via a MIL-STD-1553-B bus. The S/C mass is 635 kg including 28 kg of hydrazine, power = 560 W (average). The design life is 3 years.
Figure 5: Illustration of the SED 26 star trackers (image credit: EADS Sodern)
RF communications: An onboard data storage capability of 60 Gbit is provided for the payload data and 2 Gbit for platform housekeeping data. TT&C communications are in S-band with data rates of 727 kbit/s for telemetry (QPSK modulation) and 4kbit/s for telecommands. The payload data are transmitted in X-band at 80 Mbit/s. The CCSDS packet standard protocol is used for telemetry encoding and telecommand decoding.
Orbit: The CALIPSO spacecraft is part of the formation flight in the so-called "A-Train" (Aqua in the lead and Aura at the tail) or afternoon constellation. Near sun-synchronous orbit, altitude = 705 km, inclination = 98.05º, the ascending node crossing time is around 13:30 hours.
All members of the A-train employ the WRS-2 (World Reference System-2), a global catalog notation system for data which enables a user to inquire about satellite imagery (ground coverage in any repeat cycle) over any portion of the world by specifying a nominal scene center designated by PATH and ROW numbers. However, CALIPSO and CloudSat are not using the WRS-2 reference. CALIPSO is controlled to a customized grid shifted 215 km east from the WRS-2. And CloudSat is flying in close formation with CALIPSO (12.5 ± 2.5 s ahead of it).
Observation strategy: The train consists of the following S/C: Aqua, CloudSat, CALIPSO, PARASOL, and Aura. The objective is to provide a coincident set of data on aerosol and cloud properties, radiative fluxes and atmospheric state essential for accurate quantification of aerosol and cloud radiative effects. The formation flight of CALIPSO with Aqua and CloudSat includes the following constraints:
• CALIPSO shall maintain the formation with Aqua
• CloudSat shall maintain the formation with CALIPSO.
The CALIPSO orbit is maintained so that a target area can be observed within 6 minutes of each other (CALIPSO and Aqua). The relevant Aqua instruments for CALIPSO are: CERES, MODIS, AIRS, and AMSR-E.
Figure 6: Illustration of formation flight configuration in the A-Train (image credit: NASA/CNES)
• September 27, 2018: CloudSat and CALIPSO: They were designed to complement each other in the 1990s. They launched together on the same rocket in 2006. Then they spent more than 10 years orbiting Earth in formation with a coterie of other satellites in what's known as the A-Train, or afternoon constellation. 12)
Figure 7: In February 2018, facing a mechanical challenge, CloudSat (pictured in background) had to exit the A-Train, or afternoon constellation, of Earth-orbiting satellites, and move to a lower orbit. Following an exit maneuver of its own in September, CALIPSO (foreground) has joined CloudSat, forming what NASA scientists are calling the C-Train (image credit: NASA)
- Flying together enables the A Train satellites to gather diverse measurements of the Earth below at nearly the same time as they circle the globe pole-to-pole, crossing the equator around 1:30 p.m. local time every day. The nearly simultaneous observations allow scientists to build a more sophisticated understanding of the Earth system than would be possible if the measurements were separated. Other members of the A Train include NASA's Aqua, Orbiting Carbon Observatory-2 and Aura spacecraft, along with the Japan Aerospace Exploration Agency's Global Change Observation Mission - Water (GCOM-W1) satellite.
- In the A-Train, measurements from CloudSat's radar and CALIPSO's lidar were combined to revolutionize the way scientists see clouds and the tiny particles suspended in the atmosphere collectively known as aerosols. Scientists have long known that there are important interrelationships between clouds and aerosols, but prior to the launch of CALIPSO and CloudSat, did not have the tools to understand how they interact.
- It was a beautiful partnership forged in labs around the world: At NASA's Langley Research Center in Hampton, Virginia, which provided the aerosol-measuring lidar carried by CALIPSO; at CNES ( Centre National d'études Spatiales) in Toulouse, France, which provided the CALIPSO spacecraft; at the CSA (Canadian Space Agency in Montreal, Canada and the JPL (Jet Propulsion Lab) in Pasadena, California, which built the radar on CloudSat; and at the labs of Ball Aerospace Corporation in Boulder, Colorado, which build the CloudSat spacecraft.
- CloudSat and CALIPSO orbited and took measurements together for almost 12 years in the A-Train. Then, in February 2018, facing a mechanical challenge, CloudSat had to exit the A-Train, moving to a lower orbit — leaving CALIPSO behind and the future of their partnership uncertain.
- Uncertain, that is, until now. -Following a recent exit maneuver of its own in September, CALIPSO has rejoined CloudSat. NASA scientists are referring to the new orbit, just a few miles below the A-train, as the C-Train — C being the first letter of each satellite.
- The decision for CALIPSO to leave the A Train was not made lightly. But the scientists on the sister missions knew that the value of their Earth observations was higher if they collected data from the same orbit as CloudSat.
- "It's not that we're following one another just because we want to," said Chip Trepte, project scientist for CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation). "It's because staying together enables the shared science we want to do. It's been the driving part of our missions from the get-go."
- 'We Took Risks' — The get-go was in the early 1990s. That's when scientists came up with the innovative idea of putting two "active" sensors in space to take complementary measurements of Earth's atmosphere. CloudSat and CALIPSO are known as "active" sensors because they direct beams of energy at the Earth (Radio wave signals in the case of CloudSat and laser light in the case of CALIPSO) and measure how these beams reflect from the clouds and aerosols in the atmosphere. Other orbiting science instruments use "passive" sensors that rely on collecting reflected sunlight or radiation emitted from the Earth or clouds to make measurements.
- "No one had ever flown 94-gigahertz-frequency radar technology in space with high voltages," said Deborah Vane, current project manager and former deputy principal investigator for CloudSat. "And no one had ever flown a backscatter lidar like the CALIPSO lidar. So we took risks."
- And those risks paid off. After years of planning and development, the satellites launched together on a Delta II rocket April 28, 2006. Neither team expected to get more than a couple of years of science out of their sister satellites. They've ended up with almost 12 and they're still counting.
Figure 8: This scene shows two tropical cyclones from September 20, 2017 — Hurricane Maria near the Caribbean Sea and Tropical Storm José off the northeast U.S. coast. Data from the CALIPSO lidar and CloudSat radar appear as vertical slices in the atmosphere. CALIPSO lidar data is visualized as a bluish slice, with red and yellow colors denoting more scattering off of clouds and aerosols. CloudSat radar data is superimposed on the CALIPSO slice in brighter colors. Areas of heavier precipitation, found in each storm's spiral bands, appear in red and pink (image credit: NASA/Roman Kowch)
Table 1: Changing the Textbooks (Ref. 12)
- In addition to maintaining the French scientific community's dynamic and structure in the field of space lidar, 4 essential objectives have been identified to justify a 3-year extension:
a) Extending Calipso's timeframe: a major requirement for climate change studies. An observation period of over 10 years is necessary in order to detect potential trend on relevant variables. Contributing to World Climate Research Program (WCRP) projects such as CMIP6 is also a possibility. These exercises provide content for the Intergovernmental Panel on Climate Change (IPCC) reports. An extended observation period also allows for richer statistical sampling and increases the chances to observe punctual events such as volcanic eruptions.
b) Observational continuity with future missions: extending the Calipso mission to 2020 will ensure continuity with the CATS (NASA, lidar aboard the ISS, already in orbit), ADM-Aeolus (ESA, planned for launch in early 2018), and possibly EarthCARE (ESA, launch due in August 2019) missions. ESA missions Aeolus and EarthCARE will operate lidars on different wavelengths (355 nm instead of 532 and 1064 nm), so data will have to be recalibrated in order to provide continuous series from active sensors. A longer simultaneous observation period will provide a better comparison and analysis of specific data from each of these missions. Calipso will also improve these missions' Cal/Val exercises (as suggested in a co-written letter from the Aeolus and Earthcare mission scientists).
c) Contributing to several upcoming scientific campaigns: EUREC4A (S. Bony, the Field Study, 2020), EXAEDRE (Eric Defer, 2018), MOSAIC (Arctic, 2020)
d) Developing new applications: Two new research activities are growing rapidly: data mining using air quality prediction models (the Copernicus C3S and CAMS services, as well as the French national meteorological service Météo-France, have expressed great interest for this field), and assessing the oceanic surface layer's optical properties.
- When it comes to the space segment of the Calipso mission, the satellite has been in orbit for over 11 years; every redundancy is still in place and the payload is still operating at full capacity with comfortable leeway for both temperature and power. Calipso will be able to stay its course on the A-Train at least until 2019, and then slowly drift eastward from Aqua, but without any effect on the satellite or its mission. The WFC and IIR instruments remain fully operational and the lidar's performance is generally excellent. However, low-energy impulses can be observed in the SAA above the planet's poles, and the laser's pressure levels lead to Corona impulses such as the one observed on the primary laser in 2009. Therefore, the current laser should soon be switched off to be replaced by the primary laser, which should be out of the critical pressure zone.
Table 2: Mission extension recommendations of the NASA Earth Science Senior Review Subcommittee Report - 2017 (Ref. 14)
Notes on augmentation: Augment without an asterisk recommends sustaining operations beyond a currently planned termination date. Augmentations with an asterisk recommends funds for new and specific additional scope.
• December 20, 2016: A new study, using the NASA satellite instrument CALIOP orbiting Earth on the CALIPSO mission, has found that small, environmental changes in polar food webs significantly influence the boom-and-bust, or peak and decline, cycles of phytoplankton. These findings will supply important data for ecosystem management, commercial fisheries and our understanding of the interactions between Earth's climate and key ocean ecosystems. 15)
- "It's really important for us to understand what controls these boom-and-bust cycles, and how they might change in the future so we can better evaluate the implications on all other parts of the food web," said Michael Behrenfeld, a marine plankton expert at Oregon State University in Corvallis.
- Phytoplankton also influence Earth's carbon cycle. Through photosynthesis, they absorb a great deal of the carbon dioxide dissolved in the upper ocean and produce oxygen, which is vital for life on Earth. This reduces the amount of carbon dioxide in the atmosphere.
- Behrenfeld, along with scientists from NASA's Langley Research Center in Hampton, Virginia, and several other institutions collaborated on the study. The findings were published on Dec. 19 in Nature Geoscience. 16)
- Coastal economies and wildlife depend on what happens to tiny green plants, or phytoplankton, at the base of the ocean food chain. Commercial fisheries, marine mammals and birds all depend on phytoplankton blooms. The new study shows that accelerations in growth rate cause blooms by allowing phytoplankton to outgrow the animals that prey on them. When this happens, the phytoplankton populations rapidly increase.
- However, as soon as that acceleration in growth stops, the predatory animals catch up by eating the ocean plants and the bloom ends. This new understanding goes against traditional theories that blooms only occur when phytoplankton growth rates exceed a specific threshold of fast growth and that they end when these growth rates fall below that threshold again.
- Behrenfeld compares the new idea to two rubber balls connected by a rubber band. "A green ball represents the phytoplankton. A red one represents all the things that eat or kill the phytoplankton," he said. "Take the green ball and whack it with a paddle. As long as that green ball accelerates, the rubber band will stretch and the red ball won't catch the green ball. As soon as the green ball stops accelerating, the tension in the rubber band will pull that red ball up to it and the red ball will catch the green ball."
- NASA's CALIOP (Cloud-Aerosol LIdar with Orthogonal Polarization), an instrument aboard the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) satellite launched in 2006, uses a laser to take measurements. Scientists used the instrument to continuously monitor plankton in polar regions from 2006 to 2015.
- "CALIOP was a game-changer in our thinking about ocean remote sensing from space," said Chris Hostetler, a research scientist at Langley. "We were able to study the workings of the high-latitude ocean ecosystem during times of year when we were previously completely blind."
- Ocean ecosystems typically are monitored with satellite sensors that simply measure sunlight reflected back to space from the ocean. These instruments have a problem seeing the ocean plankton in polar regions because of limited sunlight and persistent clouds that obscure their view of the ocean surface. The lidar shines its own light – a laser – and can illuminate and measure the plankton day or night, in between clouds, and even through some clouds.
- The study also reveals that year-to-year variations in this constant push and pull between predator and prey have been the primary driver of change in Arctic plankton stocks over the past decade. In the Southern Ocean around Antarctica, though, changes in the ice cover were more important to phytoplankton population fluctuations than were differences in growth rates and predation. "The take home message is that if we want to understand the biological food web and production of the polar systems as a whole, we have to focus both on changes in ice cover and changes in the ecosystems that regulate this delicate balance between predators and prey," said Behrenfeld.
- The current CALIOP lidar was engineered to take atmospheric measurements, not optimized for ocean measurements. Nonetheless, the CALIOP ocean measurements are scientifically valuable, as demonstrated by the results of this study.
- New lidar technology is being tested that would allow scientists to better measure how phytoplankton are distributed through the sunlit layer of the ocean. This new capability will improve knowledge of phytoplankton concentrations and photosynthesis and will reveal more about the causes of phytoplankton blooms. This knowledge is critical for understanding cycling of ocean carbon, and for determining and managing the health of global ocean ecosystems.
• On April 28, 2016, the CALIPSO spacecraft was 10 years on orbit. Leaders from the cooperative mission of NASA and CNES (Centre National d'Etudes Spatiales), the space agency of France, gathered on April 21, at NASA/LaRC (Langley Research Center) in Hampton, Virginia, to celebrate this remarkable and durable record of success. 17)
- Since its launch in 2006, CALIPSO has orbited the Earth tirelessly, using its lasers to take more than 5.7 billion lidar (light detection and ranging) measurements probing the vertical structure and properties of clouds and other particles — such as dust, sea salt, ash and soot — in the air. For atmospheric scientists, the ability to regularly and accurately examine clouds and particles at different heights was something new. Gathering that data over such a long time period has contributed to a scientific bonanza.
- Also, scientists are able to combine CALIPSO's measurements with those of other Earth-observing satellites circling the globe in a cooperative path called the A-Train constellation. CALIPSO and CloudSat, a satellite containing a cloud-profiling radar, launched together on a Delta II rocket, now fly in formation with satellites named Aqua, Aura, GCOM-W (Global Change Observation Mission-Water) and OCO-2 (Orbiting Carbon Observatory-2). Combining measurements from these A-Train satellites offers a clearer view of what's happening in the atmosphere.
- "CALIPSO, in complement with the other instruments flying in the A-Train, has provided observations in areas that had never been observed before," said Pierre Tabary, program manager for atmosphere, meteorology and climate for CNES, the French space agency. He said scientists have gained knowledge about clouds, and their impact on incoming solar radiation, as well as on infrared radiation coming from the Earth's surface, known as the greenhouse effect. "There were no other instruments providing such observations before," Tabary said. "All those data have been used to improve our understanding of how climate works."
- The winning streak that began with the CALIPSO satellite's launch on April 28, 2006, shows no sign of ending. After 10 years, CALIPSO continues to overachieve in its mission to gather unique data about our home planet's clouds and other tiny particles suspended in the atmosphere. Originally designed as a three-year mission, it is now past its 10th anniversary without missing a step.
• May 2015: After 9 years of exploitation, the satellite and payload performances are still excellent and the CALIPSO mission is considered as a real success. Thanks to their high precision vertical resolution, CALIPSO observations are a recognized and unique contribution to the study of clouds and aerosols, and in the validation of climate models. The latest GIEC (Groupe d'experts intergouvernemental sur l'évolution du climat) repport highlights it. A new extension by 2 years of the CALIPSO mission in the A-TRAIN is confirmed for 2016-2017, it will enable: 18)
- Long term observation needed to understand the role of clouds and aerosols on the Earth's climate
- CALIPSO/OCO-2 coupled observations to study the influence of the aerosols on the CO2 restitution
- An overlap or continuity with the CATS, ADM-Aeolus, EarthCARE and SAGE-III missions
- And to estimate the contribution of the Lidar measurements to the ocean color thematic, by a measurement campaign using the 30 º depointing capability in order to observe the phytoplankton while minimizing the specular reflections.
• March-April 2015: The CALIPSO satellite has quantified in three dimensions how much dust makes this trans-Atlantic journey. Scientists have not only measured the volume of dust, they have also calculated how much phosphorus—remnant in Saharan sands from part of the desert's past as a lake bed—gets carried across the ocean from one of the planet's most desolate places to one of its most fertile. 19)
- The productivity of the Amazon rainforest is constrained by the availability of nutrients, in particular phosphorus (P). Deposition of long-range transported African dust is recognized as a potentially important but poorly quantified source of phosphorus. This study provides a first multiyear satellite-based estimate of dust deposition into the Amazon Basin using three-dimensional (3-D) aerosol measurements over 2007–2013 from the CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) instrument. The dust depositio20)n shows significant interannual variation that is negatively correlated with the prior-year rainfall in the Sahel. 21)
Figure 9: An example of a cross-section, or "curtain," of data from the CALIOP instrument aboard the CALIPSO satellite, which sends out pulses of light that bounce off particles in the atmosphere and back to the satellite. CALIOP can distinguish dust from other particles based on optical properties (image credit: NASA's Scientific Visualization Studio)
- In Feb. 2015, NASA produced an animation showing that wind and weather pick up on average 182 million tons of dust each year and carry it past the western edge of the Sahara (at 15° W longitude). This volume is the equivalent of 689,290 semi trucks filled with dust. The dust then travels 2500 km across the Atlantic Ocean, though some drops to the surface or is flushed from the sky by rain. Near the eastern coast of South America, at 35° W longitude, 132 million tons remain in the air, and 27.7 million tons (enough to fill 104,908 semi trucks) fall to the surface over the Amazon basin. About 43 million tons of dust travel farther to settle out over the Caribbean Sea - past 75° W longitude (Ref. 19).
- An average of 27.7 million tons of dust per year – enough to fill 104,908 semi trucks – fall to the surface over the Amazon basin. The phosphorus portion, an estimated 22,000 tons per year, is about the same amount as that lost from rain and flooding. The finding is part of a bigger research effort to understand the role of dust and aerosols in the environment and on local and global climate. 22)
- This transcontinental journey of dust is important because of what is in the dust. Specifically the dust picked up from the Bodélé Depression in Chad, an ancient lakebed where rock minerals composed of dead microorganisms are loaded with phosphorus. Phosphorus is an essential nutrient for plant proteins and growth, which the Amazon rain forest depends on in order to flourish.
• Nov. 27, 2014: CALIPSO just passed the 5 billion of laser shootings: 1.62 billion for the primary laser and 3.38 for the secondary laser (nominally used since 2009), thus a total of 100 seconds of light emission for 8 years and 7 months of data in orbit! 23)
• The CALIPSO mission exploitation has been extended until the end of 2015. Beginning in 2015, another 2 years extension will be studied by the 2 agencies (Ref. 25).
• The CALIPSO spacecraft and its payload are operating nominally in 2012. 25)
• The CALIPSO spacecraft and its payload are operating nominally in 2011.
The CALIPSO mission has provided the first multi-year global dataset of lidar aerosol and cloud profiles. CALIOP active profiling in conjunction with A-Train and CALIPSO passive observations are opening new fields of investigation into the role of aerosols and clouds in the climate system, from direct comparison with model outputs and ongoing developments in data assimilation. 27)
• In late April 2010, the CALIPSO spacecraft completed its 4th year in orbit. Since the laser switch in March 2009, the mission continues nominally and the laser performances are excellent. 28)
• The spacecraft and its payload are operating nominally in 2010. NASA requested a mission extension in the A-Train to the end of 2011. In 2009 the Senior Review Panel recommended to NASA that an extended mission status will lead to more fundamental science results. In addition, the synergy between CALIPSO and other A-train components is vital and the measurements are crucial as a bridge to the next satellite lidar mission: the ADM (Atmospheric Dynamics Mission), the European Space Agency lidar scheduled for launch in 2011, as well as EarthCARE (2013) and ACE (2015). 29) 30) 31)
• In April 2009, CALIPSO resumed operations after switching from its primary to its backup laser in March 2009. The backup laser was designed into CALIPSO to make it robust, in case the primary laser became unreliable. The value of the planning came to the forefront early in 2009 as the primary laser began to behave erratically, due to a slow pressure leak in the laser's canister. The backup laser provided its first observation data on March 12, 2009. The instrument then resumed normal operations and is undergoing a calibration review now. The release of standard data products should resume in late April 2009. 32) 33) 34)
• In Dec. 2006, the CALIPSO mission started the distribution of its data products. This data release consists of data beginning in mid June 2006 and includes Level 1 radiances from each of the instruments. This release also includes the lidar Level 2 vertical feature mask and cloud and aerosol layer products. The CALIPSO data are available through the Atmospheric Science Data Center (ASDC) at NASA/LaRC.
• Since mid-June 2006, CALIPSO has joined the A-train and the satellite is in routine operations. CALIPSO is controlled to ± 10 km at equator crossing.
• After launch, the spacecraft underwent a commissioning phase of 38 days. Based on telemetry data, the pointing performance of the spacecraft is < 0.02º about each axis. 35)
Sensor complement: (CALIOP, IIR, WFC)
The payload consists of three co-aligned nadir viewing instruments: CALIOP (Cloud-Aerosol LIdar with Orthogonal Polarization), IIR (Imaging Infrared Radiometer), and WFC (Wide-Field Camera).
Figure 10: Overview of the CALIPSO payload (image credit: NASA)
Figure 11: Payload elements and configuration (image credit: BATC)
Figure 12: The CALIPSO payload consists of the CALIOP lidar and two passive sensors (image credit: NASA/LaRC, BATC)
• CALIOP has a two-wavelength laser transmitter and a three-channel receiver
• The IIR is a three-channel infrared radiometer
• The WFC is a visible imager with a single channel.
The two passive sensors image a 60 km swath centered on the lidar footprint. 36)
CALIOP (Cloud-Aerosol LIdar with Orthogonal Polarization):
CALIOP is provided by NASA/LaRC (built by Ball Aerospace and Technologies Corporation). The objective is to acquire vertical profiles of elastic backscatter (distributions of aerosols and clouds, cloud particle phase) at dual-wavelength frequencies from a nadir-viewing geometry during the day and night phases of the orbit.
CALIOP consists of a laser transmitter subsystem, a receiver subsystem, and the payload controller (PLC) subsystem. The laser transmitter subsystem consists of two redundant lasers, each with a beam expander, and a beam steering system ensuring alignment between the transmitter and receiver. The Q-switched Nd:YAG lasers use a crossed porro prism resonator design to minimize alignment sensitivity and polarization outcoupling to provide a highly polarized output beam. Frequency doubling produces simultaneous pulses at 1064 nm and at 532 nm. Each laser produces 110 mJ of energy at each of the two wavelengths at a pulse repetition rate of 20.16 Hz, although only one laser is operated at a time. Beam expanders reduce the angular divergence of the laser beam to produce a footprint of 70 m diameter on the Earth's surface.
Figure 13: Photo of the CALIOP instrument (image credit: BATC, NASA)
Figure 14: CALIOP transmitter and receiver subsystems (image credit: NASA/LaRC)
Two orthogonal polarization components (parallel and perpendicular to the polarization plane of the transmitted beam) are measured at 532 nm. Vertical profiles are measured from the surface to 40 km with the 532 nm channel, with the upper region used for normalization/calibration. The other two channels span 0 to +26 km, covering the cloud and aerosol measurement region with polar stratospheric clouds occurring toward the top of the range. The maximum vertical resolution is 30 m, the footprint diameter on the ground is 70 m. The horizontal spacing between footprint centers is 333 m (along-track). Low altitude data are being downlinked at full resolution. At altitudes above 5 km, resolutions are reduced by on-board averaging. 37) 38) 39) 40) 41) 42)
Table 3: Parameters of the CALIOP instrument
Figure 15: Simplified block diagram of the CALIOP instrument (image credit: NASA/LaRC)
Calibration: CALIOP is calibrated in three steps.
• First, the 532 nm parallel channel signal is calibrated to the predicted molecular volume backscatter coefficient in the 30-34 km region. The molecular backscatter coefficient can be accurately estimated using temperature and pressure profiles from a gridded meteorological analysis product. The 30-34 km region was chosen as the aerosol backscatter in that region is insignificant with respect to molecular backscatter and the molecular density does not exhibit large variations. The parallel-polarized component of the molecular backscatter is derived from the estimate of total molecular backscatter by taking into account the bandwidth of the receiver optical filters. Independent estimates of the 532 nm parallel channel calibration constant are computed at approximately 700 km intervals over the dark side of each orbit and interpolated to the day side.
• Second, the calibration of the 532-nm parallel channel is transferred to the perpendicular channel via insertion of a pseudo-depolarizer in the receiver optical path upstream from the polarization beamsplitter (Figure 15). The pseudo depolarizer ensures that, regardless of the polarization state of the backscatter incident on the receiver, an equal amount of light is sent to the parallel and perpendicular channels of the receiver downstream of the depolarizer. The pseudo depolarizer will be inserted periodically during the mission, to track any relative change in sensitivity of the parallel and perpendicular channel detectors.
• Third, the calibration of the 532 nm channels is transferred to the 1064 nm channel via comparison of the return signals from high-altitude cirrus clouds. Cirrus cloud particles are large compared to the transmitted wavelengths, so the backscatter coefficients will be nearly equal at 532 nm and 1064 nm. By choosing clouds for which the ratio of particulate to molecular scattering is 50 and above, the calibration can be transferred with high accuracy. This calibration can be performed on both the dark and daylight side of the orbit, wherever cirrus of sufficient backscatter strength exist.
Figure 16: View of the instrument locations on CALIPSO (image credit: NASA)
IIR (Imaging Infrared Radiometer):
IIR is provided by CNES and developed at EADS Sodern, France. The objective is to measure calibrated radiances at 10.5 and at 12 µm over a 40 km swath (the two wavelengths are chosen to optimize the joint Lidar/IIR retrievals of cirrus emissivity and particle size).
IIR is a nadir-viewing, non-scanning imager having a 64 km x 64 km swath with a pixel size of 1 km. The CALIOP beam is nominally aligned with the center of an IIR image. IIR uses a single microbolometer detector array (uncooled), with a rotating filter wheel providing measurements at three channels in the window region, on either side of the O3 absorption band at 9.6 µm. The central wavelengths (and spectral widths) of the three channels are 8.65 µm (0.9 µm), 10.6 µm (0.6 µm), and 12.05 µm (1.0 µm). These wavelengths were selected to optimize joint CALIOP/IIR retrievals of cirrus cloud emissivity and particle size. 43) 44) 45) 46)
Figure 17: Illustration of the IIR instrument (image credit: EADS Sodern)
Figure 18: Photo of the assembled IIR instrument (image credit: EADS Sodern)
The IIR device is composed of:
• The ISM (Imaging Sensor Module), constituted of an objective (aperture 0.75) optimized in the thermal infrared, a microbolometer detector array, specific electronics, a passive cooler to refrigerate the whole, and some mechanical pieces
• A filter-carrying wheel enabling to insert alternately 3 spectral filter in front of the camera
• A black body to calibrate the camera
• A pointing mirror rotating to sequentially select between the Earthview, the black body and the direction of the cold space (second source for calibration).
Figure 19: Schematic view of the ISM (image credit: CNES)
On-orbit calibration: The calibration procedure requires viewing scenes at two different known temperatures. A cold scene calibration (4 K) is achieved by observing deep space every 8 seconds, while a hot scene calibration (300 K) is performed every 40 seconds using an internal blackbody source.
Table 4: IIR specifications
Note: The IIR instrument is also referred to as CIM-02 (Caméra Infra-rouge Multimission) on the CALIPSO mission. CIM-02 is of CIM-01 heritage flown on the MetOp-01 mission of EUMETSAT (CIM-01 is also referred to as the "IASI Infrared Imager"). CIM02 differs from CIM01 through its optical interface, the objective and the radiator have been re-designed.
Figure 20: Schematic of the IIR instrument (image credit: CNES)
Figure 21: Electronic architecture of the IIR instrument (image credit: CNES)
WFC (Wide-Field Camera):
WFC was built by Ball Aerospace (a modified version of a commercial star camera, CT-633, a pushbroom type device with a CCD detector array of 512 x 512 pixels). The objective is to provide meteorological context and highly accurate spatial registration between the CALIPSO and the Aqua mission. 47) 48)
WFC is a digital CCD camera with a spectral coverage of 620-670 nm (single band) providing images of a 61 km swath with a spatial resolution of 1 km. In a 5 km central band, the spatial resolution is 125 m. Although the WFC comes with a 512 x 512 CCD array, it essentially operates in the pushbroom line array fashion by reading out only one row of pixels per image frame. Nominally, 488 of the 512 pixels in the target row are being utilized. To minimize smearing effects during readout operations, most of the CCD array is masked off except about 30 rows near the center.
The source data rate is 26 kbit/s. The WFC imagery is also being used to assess the cloud fraction within 1 km IIR pixels to enhance the retrieval of cloud properties from the IIR data.
Table 5: Performance specifications of WFC
Figure 22: Schematic of the WFC instrument (image credit: CNES)
Figure 23: Illustration of the WFC imager (image credit: BATC)
Figure 24: View of WFC swath in relation to laser footprint (image credit: NASA/LaRC)
The CALIPSO ground segment is comprised of two main components:
1) SOGC (Satellite Operations Ground System), consisting of:
• SOCC (Satellite Operations Control Center), located in Toulouse (France)
• TT&C S-band ground stations at Kiruna (Sweden) and Aussaguel (France)
• A digital communication network that provides communication between ground segment components.
2) MOGS (Mission Operations Ground System), dedicated to mission management, detailed payload data processing, payload programming and monitoring and preparation of commands. It consists of:
• The payload data delivery system, including the X-band ground stations located in Alaska (prime), and Hawaii (backup)
• MOCC (Mission Operations Control Center), in Hampton, VA (NASA/LaRC).
Figure 25: Overview of the CALIPSO ground segment (image credit: CNES)
Data and quick-looks of the CALIPSO mission are archived at the NASA/LaRC Atmospheric Sciences Data Center (ASDC). A mirror data site, part of the French ICARE structure, is located in Lille and operated by CNES, CNRS, the University of Lille, and the Région Nord-Pas-de-Calais. 49)
The thematic pole ICARE is managing the production and distribution of the scientific outputs of PARASOL and CALIPSO through its data processing and management center in Lille (CGTD). In the case of CALIPSO, it acts as a mirror site of NASA. In late 2009, the CGTD ICARE was managing the distribution of four TB of data per month, for the benefit of 380 registered users.
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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 (email@example.com).