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Satellite Missions Catalogue


May 29, 2012



Cloud profile and rain radars




Cloud type, amount and cloud top temperature


Launched in April 2006, CloudSat is an imaging radar satellite whose operation is led by the National Aeronautical and Space Administration (NASA) and is assisted by the Canadian Space Agency (CSA) and the United States Department of Defence (DoD). By collecting data concerning clouds, it improves the way clouds are represented in global models and improves predictions of cloud activity.

Quick facts


Mission typeEO
AgencyNASA, CSA, DoD (USA)
Mission statusOperational (extended)
Launch date28 Apr 2006
Measurement domainAtmosphere
Measurement categoryCloud type, amount and cloud top temperature, Liquid water and precipitation rate, Atmospheric Temperature Fields, Cloud particle properties and profile, Radiation budget
Measurement detailedPrecipitation Profile (liquid or solid), Downward long-wave irradiance at Earth surface, Cloud cover, Cloud optical depth, Precipitation intensity at the surface (liquid or solid), Cloud type, Cloud ice content (at cloud top), Cloud ice (column/profile), Cloud imagery, Cloud base height, Cloud liquid water (column/profile)
InstrumentsCPR (CloudSat)
Instrument typeCloud profile and rain radars
CEOS EO HandbookSee CloudSat summary

Artist's rendition of the CloudSat spacecraft (Image credit: NASA)



Mission Capabilities

CloudSat has a single instrument onboard - a Cloud Profiling Radar (CPR). The nadir-pointing radar measures the power of microwaves backscattered by clouds as a function of distance from the radar. Using this data it is able to measure the vertical cloud structure and ice/water content. This data is used to improve the way clouds are represented in global models, improve our predictions of cloud activity, and improve our understanding of the role of clouds in climate change.

Performance Specifications

For backscatter detection, CPR emits W-band frequency microwaves of 94.05 GHz with a bandwidth of 0.3 MHz. It has a vertical spatial resolution of 485 m, a cross-track resolution of 1.4 km, and an along-track resolution of 1.9 km.
CloudSat undertakes a sun-synchronous orbit with an altitude of 685 km, an inclination of 98.2°, and a period of 98.3 minutes. The satellite is part of a constellation that follows an identical orbit and is within seconds of each other. This constellation called C-Train comprises both CloudSat and another cloud observing satellite named CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations).

Space and Hardware Components

CloudSat implements an RS2000 bus built by BATC (Ball Aerospace & Technology Corporation). It was launched aboard NASA’s Boeing Delta II launch vehicle with CALIPSO at the Vandenberg Air Force Base in California, America. Both satellites joined the A-Train, a constellation of satellites that follow the same orbit in formation. However, in February 2018, CloudSat experienced a mechanical error causing both CloudSat and CALIPSO to leave the A-Train to a lower orbit and form the C-train.
Radio communications with CloudSat are controlled from a United States Air Force facility with their satellite control network. The payload data is being transmitted at an S-band frequency at a rate of 1 Mbit/s.


Spacecraft    Launch   Mission Status    Sensor Complement   References

A NASA/CSA (USA/Canada) cooperative research mission in the ESSP (Earth System Science Pathfinder) program to study the effects of clouds on climate and weather. CloudSat's primary goal is to furnish data needed to evaluate and improve the way clouds are represented in global models, thereby contributing to better predictions of clouds and thus to their poorly understood role in climate change and the cloud-climate feedback [focused on understanding the role of optically thick clouds on the Earth's radiation budget (a balance of solar energy reaching the Earth and lost to space that ultimately controls the temperature of the Earth)]. 1) 2) 3) 4) 5) 6) 7) 8) 9)

Although the original CloudSat concept included the combination of lidar and radar and even precipitation measurements (GEWEX 1994), this proved too costly. Also due to cost constraints imposed by ESSP, contributions to specific portions of the mission were required.

In the cooperative arrangement, NASA provides the spacecraft (RS2000, a variant of the BCP 2000 bus of BATC), major portions of the cloud-profiling radar including integration and test of the radar system, and the S/C launch, while CSA (Canadian Space Agency) provides additional components of the advanced cloud-profiling radar. Other partners in the program are: CIRA (Cooperative Institute for Research in the Atmosphere) of CSU (Colorado State University) at Fort Collins, CO handles data processing and distribution during the mission operations phase (archiving after the mission will be the responsibility of a NASA DAAC); ground operations and communications are provided the USAF/STP (Space Test Program), and the U.S. Department of Energy (DOE) provides ground and aircraft-based observations for validation. In addition, members of the Science Team, including non-USA members, are contributing additional ground and aircraft-based measurements for validation. The CloudSat mission and payload development is managed by NASA/JPL (Figure 13).

CloudSat fills a significant gap in the existing and planned Earth observation missions by measuring the vertical profile of clouds using active remote sensing (94 GHz radar). The CloudSat S/C flies on-orbit in tight formation with a NASA/CNES lidar satellite CALIPSO, which carries a dual-wavelength backscattering (polarization-sensitive) lidar. The CloudSat orbit will be adjusted to hold a fixed distance of about 460 km (about 60 s delay) with respect to CALIPSO, with an option to decrease this distance to a 15 second separation. The sensors of both S/C point into the same ground track. The footprint of the radar is expected to overlay the footprint of the lidar by about 50%, or more, of the time and will always be within 2 km. It is planned that the two satellites fly in formation with the Aqua S/C (EOS-PM) such that the radar and lidar footprints will always fall in the central 36 km of the Aqua-MODIS swath.

Figure 1: Artist's rendition of the CloudSat spacecraft (image credit: NASA)
Figure 1: Artist's rendition of the CloudSat spacecraft (image credit: NASA)


The CloudSat S/C, built by BATC (Ball Aerospace & Technology Corporation), uses a BCP 2000 series bus of QuikSCAT and ICESat heritage (also referred to as RS 2000 bus). The S/C is three-axis inertially stabilized (momentum biased), the ADCS (Attitude Determination and Control Subsystem) uses a star tracker (one redundant) and 14 coarse sun sensors, which also provide sun direction for solar array control, and three two-axis magnetometers. Actuation is provided by 3 dual-winding torque rods and 4 reaction wheels. The pointing accuracy is ≤ 0.07º, two-axes, 99.7% probable; the geolocation knowledge of the radar footprint position is better than 1 km on the geoid, two-axes, 99.7% probable. Two global positioning system receivers (C/A code capability) provide GPS time and position and velocity to the onboard location determination process.

The S/C wet mass is about 995 kg, the S/C power is 700 W average (> 800 W peak), a battery capacity of 40 Ah is provided for eclipse operations. The main structure has a size of 2.54 m x 2.03 m x 2.29 m. The wing span of the deployed solar array is 5.08 m from tip to tip (total array area of 6.4 m2). The design life of both, the spacecraft and radar, exceeds two years, and consumables are carried for a three-year mission, enabling an extended mission. 10)

Propulsion: The spacecraft uses an all-welded, chemical, blow-down monopropellant system for attitude control and translational maneuvers. The propellant is hydrazine and the pressurant is gaseous nitrogen. There are four thruster assemblies, each rated at 4.45 N (newton) with an Isp of 220 seconds. Other propulsion system components include the fill/drain valve, fill/vent valve, pressure transducer and two latching isolation valves.

Figure 2: Line drawing of the CloudSat spacecraft (image credit: NASA)
Figure 2: Line drawing of the CloudSat spacecraft (image credit: NASA)


A launch of CloudSat on a Delta 7420-10C launch vehicle from VAFB, CA took place on April 28, 2006. CloudSat was co-manifested with the NASA lidar mission CALIPSO (formerly named ESSP-3). 11)

Orbit: Sun-synchronous near-circular “frozen” orbit [same orbit as Aqua (EOS/PM)], altitude = 705 km, inclination = 98.2º.

RF communications: CloudSat is being monitoring and controlled from a USAF facility using the Air Force Satellite Control Network (AFSCN). The payload data is downlinked in S-band at 1 Mbit/s and relayed to the control center, where it is sent to CIRA for processing into data products. 12)

Formation flying: CloudSat is going to be part of the A-train (Aqua in the lead and Aura at the tail), consisting of the following S/C sequence: Aqua, CloudSat, CALIPSO, PARASOL, and Aura. Formation flying is a navigation strategy that enables CloudSat to closely track the groundtracks of the lidar spacecraft CALIPSO and Aqua in a very precise way. After launch, maneuvers within the first 45 days of the mission will bring the CloudSat spacecraft into formation with the other two spacecraft. The CloudSat orbit will be adjusted and monitored to hold the CloudSat spacecraft at a fixed distance/time from CALIPSO. CloudSat is to trail Aqua by ≤ 120 s and will maneuver to just 15 s ahead of CALIPSO. CloudSat will then maintain a tight formation with CALIPSO by controlling its cross-track motion to within ±1 km of the CALIPSO ground track. This is achieved by placing CloudSat in a small circulation orbit relative to CALIPSO and contained within CALIPSO's control box. The chosen delay is a compromise between the desire to minimize the time delay between the radar and lidar measurements and the need to hold the frequency of formation flying maintenance maneuvers to an interval of once per week or longer. In this way, the radar footprint will spatially overlay the lidar footprint much of the time, creating coordinated and essentially simultaneous measurements. The mean separation time of CloudSat from CALIPSO is 15 seconds. 13) 14)

The addition of the CloudSat and CALIPSO satellites into the constellation, and the controlled maintenance of this formation of satellites, brings the opportunity of combining active (the CloudSat radar and the CALIPSO lidar) with the more traditional passive measurement approaches and Aqua, Aura and PARASOL.

Figure 3: Illustration of the A-train formation flight configuration with the GCOM-W1 (Shizuku) satellite joining the A-train constellation as last entry (image credit: NASA, JAXA) 15)
Figure 3: Illustration of the A-train formation flight configuration with the GCOM-W1 (Shizuku) satellite joining the A-train constellation as last entry (image credit: NASA, JAXA) 15)


Mission Status

• 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. 16)

Figure 4: 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)
Figure 4: 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 5: 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)

Figure 5: 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)

Changing the Textbooks (Ref. 16)


In the A-Train, which orbits approximately 705 km above Earth's surface, the satellites initially flew one after the other, about 15 seconds apart. That allowed them to take near-simultaneous measurements of clouds. The combination of high-powered radar and lidar gave researchers an unprecedented view into those clouds' vertical structures.

"Before CALIPSO and CloudSat were launched, we really didn't know what the vertical structure of clouds looked like over the globe," said Trepte. "We've really changed the textbooks."

For instance, some passive sensors have difficulty "seeing" through wispy, high-altitude cirrus clouds that might mask thicker clouds below. But CALIPSO's lidar is able to see the vertical structure of the cirrus clouds and much of the thicker clouds below, while CloudSat's radar probes the even thicker and rain-producing clouds below. Those kinds of views have been helpful in determining what role clouds play in Earth's weather/climate system.

"By having this vertical profile information, we can really understand the albedo [or reflective] effect of clouds versus their opposing greenhouse effect," said Graeme Stephens, CloudSat principal investigator, referring to the capability of clouds to trap heat emitted from the Earth's surface. "This is something CloudSat and CALIPSO have uniquely delivered, and now we can say something quantitative about these processes and how they affect Earth’s climate."

The satellites also have complemented each other by revealing important insights about the effects of aerosols on clouds and precipitation. That had been a point of great uncertainty.

"We now have a much deeper understanding of that interaction and a much clearer idea of how those interactions affect Earth's climate system — we now know where and why our models were wrong in the way they represent aerosol influences on clouds and precipitation," said Stephens. — Data products from both CALIPSO and CloudSat have been cited in thousands of scientific publications.

The Big Move

In 2017, one of CloudSat's four reaction wheels, which help it maneuver, became unusable. Though CloudSat can still operate with only three reaction wheels, the potential loss of a second wheel could have caused it to drift too close to another A-Train satellite.

That left only one clear safe option, so in February CloudSat fired up its thrusters and moved to a "safe-exit orbit" below the A-Train. The question then: What about CALIPSO?

CloudSat scientists hoped if they offered to move their satellite into CALIPSO's disposal orbit — which it would have to enter in the near future anyway — they could entice the CALIPSO team to join them. There, they could spend the beginning years of a decades-long descent into Earth's atmosphere squeezing out every last possible drop of coordinated data and joint science.

At a meeting of the CloudSat and CALIPSO science teams to determine whether CALIPSO should join its sister, there was no debate. "Literally no one came forward and said, 'I have an absolute burning need for CALIPSO to stay in the A-train,' " said Charles Webb, who oversees NASA's fleet of operating Earth-orbiting satellites.

Not that CALIPSO didn't have value in the A-train. It just has more value paired up with CloudSat. The two were meant to be together.

"Everybody said go with CloudSat," said Webb. "It was unanimous."

CALIPSO began its exit from the A-Train Sept. 13 and entered its new orbit Sept. 20. The C-Train orbit is approximately 688 km above the Earth's surface. It'll take some time to get the satellites in phase with one another and they'll be a little farther apart than they once were — about 60 seconds. But once they're completely settled in — likely in late October — they'll continue to provide indispensable, complementary views of clouds and aerosols for what scientists hope will be another two years or more.

Then, about 25 years from now, long after both sister satellites have stopped working, they'll burn up and disintegrate as they reenter the very atmosphere they spent so long observing together.

"I was a little sad to leave the A-Train, but I'm overwhelmingly happy that we'll continue this work together," said Vane. "It's been an amazing opportunity for the science community that we've been able to develop and fly these missions and that we extended their lifetimes way beyond anything we ever dreamed of."


• February 23, 2018: Mission managers at NASA's Jet Propulsion Laboratory in Pasadena, California, this week lowered the orbit of the nearly 12-year-old CloudSat satellite following the loss of one of its reaction wheels, which control its orientation in orbit. While CloudSat's science mission will continue, it will no longer fly as part of the A- Train (Afternoon Constellation) — six Earth-monitoring satellites that fly in a coordinated orbit to advance our understanding of how Earth functions as a system. 17)

- CloudSat launched in 2006 to improve understanding of the role clouds play in our climate system. It joined the A-Train about a month later. In April 2011, the spacecraft experienced a technical issue affecting the ability of the battery to provide enough current to power all spacecraft systems during the time in each orbit when the spacecraft is on the dark side of the planet and the spacecraft's solar panels are not illuminated. In response, spacecraft engineers at Ball Aerospace in Boulder, Colorado, developed a new operational mode for CloudSat that enabled it to continue science operations, but only during the part of each orbit when the spacecraft is in sunlight.

Figure 6: Artist's concept of NASA's CloudSat spacecraft, which provides the first global survey of cloud properties to better understand their effects on both weather and climate (image credit: NASA/JPL)
Figure 6: Artist's concept of NASA's CloudSat spacecraft, which provides the first global survey of cloud properties to better understand their effects on both weather and climate (image credit: NASA/JPL)

- Recognizing the vulnerable nature of the spacecraft battery and the age of other spacecraft systems, the CloudSat project developed a set of criteria under which they would exit the A-Train. One criterion was the loss of one of CloudSat's four reaction wheels. Although CloudSat can conduct science operations using only three reaction wheels, a subsequent loss of a second reaction wheel could leave the spacecraft unable to maneuver or change its orientation. Without the capability to maneuver, the satellite could drift too close to another A-Train satellite.

- In June 2017, one of CloudSat's reaction wheels displayed significant friction. It was subsequently determined that the wheel would no longer be usable, thus triggering preparations to exit the A-Train.

- On 22 Feb. 2018, CloudSat successfully executed two thruster burns, placing the satellite in an orbit below the altitude of the A-Train. After telemetry has been analyzed, mission managers will determine if a third orbit trim burn is necessary. CloudSat will remain in this "safe-exit orbit" while the project studies orbit options for continuing science operations even farther below the A-Train.

- CloudSat is the first satellite to use an advanced cloud-profiling radar to "slice" through clouds to see their vertical structure, providing a completely new observational capability from space. The mission furnishes data that evaluate and improve the way clouds and precipitation are represented in global models, contributing to better predictions of clouds and their role in climate change.

- Among the mission's many science accomplishments to date, CloudSat has provided the capability to look jointly at clouds and at the precipitation that comes from them, spotlighting flaws in climate model physics: models produce precipitation too frequently, and the modeled precipitation is lighter than actual observations. CloudSat directly quantified, for the first time, global snowfall and found that climate models overestimate Antarctic snowfall, many by more than 100 percent.

- The A-Train satellites rush along together like a train on a "track" 705 km above Earth's surface, flying minutes, and sometimes seconds, behind one another. Together, the satellites and their more than 15 scientific instruments work as a united, powerful tool to examine many different aspects of our home planet. The A-Train has proven to be a successful integrated approach to observing Earth because it allows multiple instruments to observe the same location on Earth nearly simultaneously as they pass overhead. In addition to CloudSat (a partnership with the Canadian Space Agency and the U.S. Air Force), the other satellites currently in the A-Train include NASA's Aqua, Orbiting Carbon Observatory-2 and Aura spacecraft; the NASA/CNES CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) spacecraft; and JAXA's GCOM-W1 (Global Change Observation Mission - Water 1) satellite.

- With CloudSat now in an orbit below the A-Train, it will occasionally pass beneath the constellation, enabling the mission to collect data in support of some of its pre-A-Train-exit data products.

• June 2017: CloudSat’s primary instrument is the CPR (Cloud Profiling Radar), a radar operating at 94 GHz to sense cloud-sized particles (i.e. cloud ice, snow, cloud droplets and light rain). CloudSat continues to be only satellite data source for combined vertical profiles of global cloud liquid and ice water content. Although it produces a radar-only cloud water product (both liquid and ice), CloudSat has been strategically leveraged in combination with other A-Train satellite sensors to yield higher quality cloud properties than would be possible with CloudSat alone. By combining Aqua/MODIS visible optical depth, CALIPSO lidar and/or Aqua/AMSR microwave measurements, different particle-size regions of the PSD (Particle Size Distribution) are resolved, allowing additional cloud properties to be retrieved that correspond to these PSD regions. Excellent progress has been demonstrated in validating sensor retrievals against (1) similar satellite retrieval products (from different observations and physics) and (2) validation of CloudSat retrievals against aircraft or ground-based measurements. Based on our review, the probability for technical and scientific success of an extended mission appears very high. 18)

- The Subcommittee recommends approving the proposed FY22 CloudSat budget (continued with augmentation) since the NASA budget guideline of $4.4M would be insufficient to sustain operations, based on the actual cost history in previous years, and on Sect. 2.4 of the CloudSat proposal, which states that without the requested amount for FY22, mission termination would begin in FY21.

- In addition, the Subcommittee collectively agreed that additional funds should be allocated to finance cross-calibration activities between CloudSat and EarthCARE and moving the CloudSat data to a climate quality record. Cross-calibration requires a non-trivial amount of work to account for absolute calibration, instrument sensitivity, measurement resolution, and sampling time differences. It is possible to produce merged cloud products in the future but it will take significant work, as the instruments are fairly different in various ways. Simultaneous orbit-intersections for cross-calibration are limited – rare circumstances (once in 3 days) and only at 12 degrees latitude. So this is not going to be a source of calibration per se, but the overlap will provide estimates of the impact of the diurnal cycle on the data record, since there are differences in sampling time (though only 30 minutes off). CloudSat can calibrate on ocean surface targets. Producing a climate quality data record is of high importance for NASA science and should be done while the operations team is functional and the mission is active.

- The mission indicated that mechanisms exist to fund a US science team, but currently there are no plans to develop an EarthCARE-ROSES call. The Senior Review Subcommittee recommends such a call before the time of the EarthCARE launch in 2019.



Suggested Change in Scope










Stop TES Operations/Finalize Dataset












Cross-calibration for climate record




Additional Competed Science Needed








Refine XCO2 algorithm for flux req'ts


Continue/Augment for 1 year/2 years depending on ScatSat Stability

















MISR algorithm update

Table 1: Mission extension recommendations of the NASA Earth Science Senior Review Subcommittee Report - 2017 (Ref. 18)

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.

• On April 28, 2016, the CloudSat and CALIPSO missions, both members of the ”A-Train Constellation”, celebrated their 10th anniversary on orbit. Both spacecraft have long exceeded their prime missions (which were intended to show the value of active remote sensing instruments in space) and are in extended operations. Both missions have had to overcome some significant performance challenges over the past ten years. Nevertheless, both missions continue to collect important scientific data that improve our understanding of the roles clouds and aerosols play in regulating Earth’s climate and weather. As they enter their second decades, these two satellites have more than met their initial science goals, and groundbreaking results are being shared even today. 19)

- Unlike earlier passive remote sensing instruments, both the CloudSat radar and CALIPSO lidar have provided the first long-term spaceborne active remote sensing observations. These active instruments radiate pulses of energy towards Earth’s surface. Scientists then use the timing of the returned energy to deter-mine the distance to a target, and the strength of the returning signal to determine the properties of a target. This allows scientists to observe vertical profiles of clouds and aerosols throughout the atmosphere, thereby reducing the uncertainty in our knowledge of these two important atmospheric constituents.

Overview of the science objectives for the CloudSat and CALIPSO missions (Ref. 19)


The science objectives for CloudSat are to better understand the vertical structure and the physical and chemical properties of clouds. Specifically, they include:

1) Profiling the vertical structure of clouds. Measurements of the vertical structure of clouds are fundamentally important to improving our understanding of how clouds affect both local- and large-scale environments.

2) Measuring the profiles of cloud liquid water and ice water content. These two quantities, predicted by cloud-resolving models and global-scale models alike, determine practically all other cloud properties, e.g., precipitation and cloud optical properties.

3) Retrieving profiles of cloud optical properties. These measurements, when combined with water and ice content information, provide critical tests of key cloud process parameterizations—i.e., bulk estimates of processes too small to be resolved by a given model but necessary to accurately represent the science.


CALIPSO’s overall goal was to provide global, vertically-resolved measurements of aerosol and thin cirrus distribution. The specific objectives focus on improving scientists’ understanding of the direct and indirect roles aerosols play in Earth’s climate system through direct observation of aerosols or their influences on cloud properties. They include:

1) Developing a global suite of measurements from which the first observationally-based estimates of aerosol direct radiative forcing can be determined. As they absorb and reflect visible and infrared radiation in the atmosphere, aerosols affect how the sun’s energy moves through Earth’s energy system. The exact nature of those effects, referred to as the aerosol direct effect, depends on the size and composition of the aerosols. CALIPSO is designed to measure those aerosol properties to quantify the aerosol direct effect over a variety of locations and surface types.

2) Dramatically improving the empirical basis for assessing aerosol indirect radiative forcing of the Earth’s climate system. Aerosols also affect the energy budget of the planet by changing the size and number of droplets or ice crystals in clouds, which can then lead to clouds becoming less transparent to electromagnetic radiation and affecting the formation of precipitation. This is known as the aerosol indirect effect.

3) Improving the accuracy of satellite estimates of longwave radiative energy fluxes at the Earth’s surface and in the atmosphere. The infrared energy that flows into and out of the surface of the Earth is hugely impacted by clouds and aerosols. With CALIPSO’s improved observations, estimates have improved significantly over the past ten years.

4) Creating a new ability to assess cloud-radiation feedback in the climate system. Clouds and the radiant energy budget of the climate are intimately connected. Changes in cloud properties lead to changes in the amount of energy passing through the system, and that in turn influences where and when clouds form. The study of these feedback mechanisms requires accurate measurements of clouds and radiant energy, available from the A-Train satellites.

- CloudSat and CALIPSO Data Products: From their inception, the capabilities of CloudSat and CALIPSO were intended to complement one another. This is reflected in each mission’s list of data products which, while supporting the objectives of the individual mission, also enhances the other, often shoring up a data gap in the counterpart mission. For example, while CloudSat can penetrate thicker clouds to give scientists the best cloud-detection data available from space, it is not as effective at detecting thin cirrus clouds. CALIPSO, on the other hand, has superior thin-cloud detection capabilities, and thus complements CloudSat. Combined, they allow detection of almost all clouds from the thinnest clouds in the stratosphere down to 500 m above the surface in all but the heaviest rain conditions.

- The designed complementarity of CloudSat and CALIPSO also benefits the observing capabilities of other missions in the A-Train. For example, one of the CALIPSO products is designed to be used in concert with the two CERES6 instruments on Aqua. It works both ways as well, since many of the other sensors that are sensitive to the presence of clouds use CloudSat and CALIPSO’s superior cloud-detection ability to filter out any data contaminated by clouds. The data products have allowed the CloudSat and CALIPSO science teams to push the boundaries of what we know about aerosols and clouds and their impacts on the Earth-atmosphere system.

- One of the main charges of the CALIPSO mission was to assess the impact of aerosols on Earth’s radiation budget through the aerosol direct and indirect effects. CALIPSO data have been used to estimate the global aerosol direct radiative effect, in both clear and cloudy skies, as well as above and below clouds for the first time. These results are more representative than previous measurements attempted with passive satellite observations [e.g., MODIS (Moderate Resolution Imaging Spectroradiometer) onboard NASA’s Terra and Aqua platforms] largely because of assumptions that had to be used to estimate aerosol effects near clouds. While there is still no agreement on the size of the effect of aerosols near clouds, scientists are for the first time able to base their estimates on observations rather than assumptions. Figure 7 illustrates where this effect is strongest, typically downwind of areas of biomass burning, sources of dust and sand, and anthropogenic air pollution.

- In turn, the aerosol indirect effect is believed to be caused by two distinct mechanisms: A cloud polluted with additional aerosols will have many more droplets of water, each of which is relatively small compared with the droplets found in a non-polluted cloud. Such clouds with smaller droplets appear to be thicker to visible-light wavelengths, giving rise to what scientists call the first aerosol indirect effect, reducing the amount of sunlight that reaches the surface. Clouds that are more polluted also are believed to be less likely to produce rain or snow. Thus, the second aerosol indirect effect results in clouds that last longer, thereby reducing the amount of sunlight reaching the surface over time.

Figure 7: Map of the annual mean, all-sky (including clouds) direct radiative effect (DRE) of aerosols at the top of the atmosphere (TOA), as observed by CloudSat and CALIPSO over a 10-year period. Negative numbers indicate that aerosols are reducing the amount of solar radiation penetrating the atmosphere, which should indicate a cooling effect on the climate (image credit: Alexander Matus)
Figure 7: Map of the annual mean, all-sky (including clouds) direct radiative effect (DRE) of aerosols at the top of the atmosphere (TOA), as observed by CloudSat and CALIPSO over a 10-year period. Negative numbers indicate that aerosols are reducing the amount of solar radiation penetrating the atmosphere, which should indicate a cooling effect on the climate (image credit: Alexander Matus)

- Clouds in the polar regions may be more important than we thought. Another important aspect of CloudSat and CALIPSO (as well as the other missions in the A-Train) is that they are capable of measuring clouds over Arctic and Antarctic climate zones. As a result, data from these missions have provided a treasure trove of insights into the unique roles that polar clouds play in influencing the climate of these regions. For example, clouds over the Arctic region have been shown to have a major impact on summer sea ice extent in the Arctic Ocean. CloudSat and CALIPSO observations revealed a 16% decrease in cloudiness from 2006 to 2007 that contributed enough extra solar energy to melt an additional 0.3 m of ice, or to warm ocean water by 2.4 °C over the three summer months. A follow-on study used similar data to show that in the Arctic fall, low clouds increase in response to the warmer ocean waters, marking the first direct observational evidence of clouds changing in response to anthropogenic greenhouse gases and climate change. These studies both help to explain many of the key features of how the relationships between the atmosphere and sea-ice coverage respond to climate changes.

- Finally, the impact of clouds on Earth’s climate extends well above Earth’s surface. PSCs (Polar Stratospheric Clouds) are a combination of water, sulfuric acid, and nitric acid that form in the very cold polar stratosphere. Scientists have long known that PSCs play a significant role in ozone-hole chemistry due to their ability to catalyze reactions with chlorine-bearing compounds that can destroy ozone. In another recent study, CALIPSO data were used to monitor the evolution of different kinds of PSCs and their relative ability to catalyze such ozone-destroying reactions. These observations have the potential to address hypotheses about the kinds of clouds involved in ozone chemistry—phenomena that had been heretofore very challenging to observe directly.

- One of the most remarkable achievements by the CloudSat science team is the development of precipitation profile products. Over the years, rainfall modeling has been challenging—and snowfall modeling has been even more difficult. CloudSat observations have changed that. CloudSat rainfall data have been available since 2007; more recently, the first-ever global estimate of snowfall has become available. Both products are now standard data outputs from CloudSat and CALIPSO observations and have opened the doors to new insights on how and where precipitation falls, globally.

- For example, from the CloudSat rainfall data, scientists have learned that, averaged globally, clouds produce rain nearly 20% of the time, with a significant amount of that rainfall falling as lighter precipitation from shallow cumulus clouds over the tropical oceans. By comparison, heritage passive measurements of rainfall often grossly underestimate the amount of rain falling around the globe—in some cases by almost 60%—because they miss the precipitation from these shallower cumulus clouds.

- Putting it all together - the global energy balance of the Earth-atmosphere system: Perhaps the most compact way to summarize the importance of the scientific achievements from the first decade of the CloudSat and CALIPSO missions is by revisiting the planetary global energy balance. Clouds play several important roles in modulating how energy moves through Earth’s atmosphere: by reflecting sunlight, emitting infrared energy back to the surface as part of Earth’s greenhouse effect, and releasing energy into the atmosphere as water vapor condenses to form cloud droplets and precipitation. Other instruments (e.g., CERES) have also been able to measure total energy flows into and out of the atmosphere. But CERES data suffered from the limitations common to all passive measurements. Until the arrival of CloudSat and CALIPSO, the impacts of clouds on radiant energy remained difficult to quantify in part because not all clouds could be identified from space and, in part, because passive measurements could not always accurately characterize the composition of those clouds.

- Scientists have developed an updated set of estimates of the global energy budget, which incorporates data from CloudSat and CALIPSO (Figure 8). The estimates of energy flowing to the surface were improved by using satellite data, including CloudSat- and CALIPSO-derived heating rates, to make up for the lack of surface observations over the ocean. As a result, estimates of the longwave downwelling radiation—infrared radiation entering the surface from the atmosphere—increased by over 10 W/m2, mostly due to improvements in representing the impacts of clouds in calculations of how energy flows through the Earth-atmosphere system. This increase in surface energy is balanced largely by increased evaporation of water vapor into the atmosphere, and consequently by the increased precipitation observed by CloudSat.

Figure 8: The global annual mean energy budget of Earth for the approximate period 2000-2010 — updated to include information from CloudSat and CALIPSO. All fluxes are measured in W/m2. Solar fluxes are depicted in yellow [left], while infrared fluxes are depicted in pink [right]. The four flux quantities [purple-shaded rectangles] represent the principal components of the atmospheric energy balance (image credit: Graeme Stephens, Ref. 19)
Figure 8: The global annual mean energy budget of Earth for the approximate period 2000-2010 — updated to include information from CloudSat and CALIPSO. All fluxes are measured in W/m2. Solar fluxes are depicted in yellow [left], while infrared fluxes are depicted in pink [right]. The four flux quantities [purple-shaded rectangles] represent the principal components of the atmospheric energy balance (image credit: Graeme Stephens, Ref. 19)

• The CloudSat satellite and its payload are operational in January 2016.

• Sept. 29, 2015: CloudSat overpassed Tropical Storm Joaquin in the Caribbean on September 29, 2015 at 1810 UTC. Joaquin contained maximum sustained winds of 100 km/hr with a minimum central pressure of 992 mb. CloudSat overpassed directly through the center on convection of Tropical Storm Joaquin. 20)

- MODIS on NASA's Aqua satellite captured the infrared (Figure 9) image taken a few minutes before the CloudSat overpass (Figure 10) denoted by the blue line A->B). The enhanced infrared VIIRS imagery of Suomi-NPP, Figure 11, also shows the CloudSat track during the same time period. This imagery distinguishes between cold and warm cloud top temperatures using IR imagery (higher cloud tops -> colder temperatures).

- CloudSat intersected Tropical Storm Joaquin just as the storm was starting to become better organized. The upper level wind environment was becoming more favorable for storm development along with warm SSTs (Sea Surface Temperatures) to fuel intensification. The CloudSat profile reveals a broad area of deep convection in the northern section of the storm. The CloudSat overpass (Figure 10) intersects a dense section of moderate to heavy rain and deep convection with some of the stronger convective cores bubbling through the cirrus shield (light pink and dark red areas indicate the largest amount of ice/water). The CloudSat cloud profiling signal attenuates in areas of moderate to heavy precipitation (diminished signal from the surface to 5-6 km).

- Figure 3 reveals the VIIRS (Enhanced IR Visible Infrared Radiometer Suite) overpass of the same area taken about 4 minutes prior to the CloudSat overpass. At 750 m resolution the imagery emphasizes the colder temperatures and highest cloud tops. Areas of bubbling convection are distinguishable (-80ºC) that are yellow in color.

Figure 9: MODIS IR imagery with CloudSat track (image credit: NOAA LAADS Web)
Figure 9: MODIS IR imagery with CloudSat track (image credit: NOAA LAADS Web)
Figure 10: CloudSat overpass track (image credit: CloudSat Data Processing Center)
Figure 10: CloudSat overpass track (image credit: CloudSat Data Processing Center)
Figure 11: Enhanced infrared VIIRS image (image credit: RAMMB)
Figure 11: Enhanced infrared VIIRS image (image credit: RAMMB)
Figure 12: Cross section of CloudSat and GOES imagery (image credit: NASA/JPL-Caltech/CloudSat Data Processing Center, Ref. 20)
Figure 12: Cross section of CloudSat and GOES imagery (image credit: NASA/JPL-Caltech/CloudSat Data Processing Center, Ref. 20)

• The CloudSat spacecraft and its payload are operating nominally during the sunlit portion of the orbit in 2015. Due to a battery degradation in April 2011, the CPR instrument is turned off during the eclipse phase of the orbit.

• April 29, 2014: The CloudSat spacecraft systems are performing nominally and, with the exception of a few hours surrounding the 29 April Solar Eclipse, Daylight Only Operations (DO-Op) continued according to plan. Two routine formation flying maneuvers were successfully executed last week, on 22 and 26 April, to position the satellite within its Constellation Control Box, while also maintaining ground track overlap with CALIPSO. 21)

• The CloudSat spacecraft and its payload are operating nominally during the sunlit portion of the orbit in 2014.

• December 11, 2013: CloudSat successfully executed an 'Orbit Lower Maneuver' on 6 December, 2013. The spacecraft is well positioned in it's constellation control box and is in good ground track alignment with CALIPSO. It is expected that this was the last maneuver of 2013, baring any unexpected atmospheric behavior. 22)

• June 2013: The 2013 Senior Review evaluated 13 NASA satellite missions in extended operations: ACRIMSAT, Aqua, Aura, CALIPSO, CloudSat, EO-1, GRACE, Jason-1, OSTM, QuikSCAT, SORCE, Terra, and TRMM. The Senior Review was tasked with reviewing proposals submitted by each mission team for extended operations and funding for FY14-FY15, and FY16-FY17. Since CloudSat, GRACE, QuikSCAT and SORCE have shown evidence of aging issues, they received baseline funding for extension through 2015. 23)

- Continuing the CloudSat mission carries a number of benefits: 1) allowing for new science in the context of weather and climate variability and also enabling new products, 2) uninterrupted applications to aviation and weather forecasting, 3) improved understanding of seasonal and inter-annual variations in cloud behavior, 4) enhanced data for evaluating model behavior, and 5) providing calibration for future missions such as EarthCARE (2016) and GPM (2014). There is also strong synergy with the future OCO-2 mission as the oxygen A-band from OCO-2 provides complementary information on clouds.

• May 2013: Despite a serious battery anomaly (in April 2011), CloudSat has returned to normal operations and has also rejoined the A-Train. The team had to extensively modify the method of operations, including the plans for performing burns. On-orbit data has demonstrated a robust system that is capable of maintaining formation relative to CALIPSO. CloudSat expects to be operational for many additional years – the remaining fuel onboard the vehicle should last for approximately another 7 years, and no other components on the spacecraft have shown any measurable degradation. 24)

• The CloudSat spacecraft and its payload are operating nominally during the sunlit portion of the orbit in 2013 (Ref. 4). CloudSat is in formation with Aqua and CALIPSO in the A-Train. 25)

• On July 24, 2012, the CloudSat spacecraft successfully executed the first Drag Make-Up (DMU) maneuver, since returning to the A-Train. This 12.5 cm/s maneuver successfully adjusted the position of the spacecraft within it's control box and aligned the ground track for formation flying with Calipso [Ref. 25) Update of July 26, 2012] .

• On July 17, 2012, the CloudSat spacecraft successfully executed an “Inclination Increase Maneuver”. The results indicate very small underperformance, which will be corrected by a DMU (Drag Make Up) maneuver on 25 July. Last night's maneuver successfully positioned the CloudSat spacecraft 108 seconds behind CALIPSO and in alignment with the other A-Train (Afternoon Constellation) satellites. This completes the maneuver campaign to return CloudSat to the A-Train (update of July 19, 2012, Ref. 30).

• The CloudSat spacecraft returned to the A-Train with an orbit raise maneuver on May 15, 2012. CloudSat is now in a position about 100 seconds behind CALIPSO. An inclination-increase maneuver will be performed in mid-July to achieve footprint overlap between CALIPSO's lidar and CloudSat's radar instruments. 26) 27)

• In January 2012, CloudSat continues to fully operate in the mode (DO-OP) and all issues with the SSR (Solid State Recorder) operation have been resolved. CloudSat plans on rejoining the A-Train constellation orbit beginning February 3, 2012 to be completed by the end of February (Ref. 30): update for January 23, 2012).

• In November 2011, NASA/JPL declared CloudSat fully operational in the DO-OP (Daylight-Only Operation) mode per the revised CONOPS. The spacecraft cycles its subsystems on and off in Sun and eclipse portions of the orbit via weekly command sequences from the RSC. CloudSat is collecting science data during the sunlit portions of the orbit, below the A-Train, and hibernates in a stable spin during eclipse, to recover and return to point at the sun as it emerges from the dark side of the Earth. 28)

• This new CONOPS (Concept of Operations) requires constant care and monitoring of the thermal and power profiles, as well as more intensive commanding for the CloudSat operators. Though CloudSat will never be a fully nominal mission again, it is collecting data for 54 out of the 65 sunlit minutes in its orbit, and the CIRA (Cooperative Institute for Research in the Atmosphere) has begun distribution of science data to the CloudSat community once again. DO-OP is in use today, and maneuvers are currently being executed to return CloudSat to the A-Train, where it will fly 88 along-track seconds behind CALIPSO and resume its role in the A-Train constellation (Ref. 28).

• On Oct. 7, 2011, CloudSat successfully executed an Orbit Lowering ΔV Maneuver. As planned, telemetry indicated a burn duration of 398.6 s for a ΔV of -1.4 m/s. Attitude control was nominal during the burn. Following the maneuver, the spacecraft successfully transitioned back into point standby mode. Analysis of the post-maneuver orbit, indicates that the spacecraft achieved a ΔV of 1.3965 m/s, lowering the semi-major axis by 2.63 km. The CloudSat team is extremely pleased that this maneuver executed as planned, demonstrating the spacecrafts ability to perform maneuvers in advance of returning to a science orbit for the remainder of the mission. - With this demonstration of maneuverability, the Project had the confidence to recommend to NASA that CloudSat be allowed to return to the A-Train. A plan for this return is in place and currently executing toward completion (Ref. 28).

• In June 2011, the NASA Earth Science Senior Review recommended an extension of the CloudSat mission up to 2015. 29)

• CloudSat is operating nominally in early 2011. However, on April 17, 2011, CloudSat suffered a battery anomaly and has not been transmitting data since then. 30)

Although at about the same altitude, CloudSat is no longer part of the A-train of satellites, and hence no longer has the old 16-day repeating orbit. Orbital elements (the so-called TLE) may be downloaded and used as before and the NASA orbital website is still operational. Eventually, CloudSat may return to the A-Train.

• CloudSat is operating nominally in 2010 (beyond the design life of 2 years). NASA granted a mission extension to the end of 2011. In 2009 the Senior Review Panel recommended to NASA to keep CloudSat operational until the EarthCARE mission comes along in 2013. 31) 32)

CloudSat is beginning to meet its mission objectives: evaluating the representation of clouds in global models; evaluating the energy balance for different cloud systems; evaluating the cloud retrievals of other satellite instruments; improving the understanding of aerosol indirect effects. Its data products are highly relevant to NASA’s mission, especially in the area of the role of clouds in climate. In the short term, CloudSat products are being used to test a wide range of global atmospheric models, leading to improved simulation of clouds and cloud processes. In the longer term, CloudSat’s products could be assimilated into weather prediction models to improve forecasts.

• In June 2009 CPR is currently operating at better than the required performance. 33)

• In 2008, the CloudSat project has received authorization from NASA for an extension of its nominal 22-month mission through FY11. Continuing the mission beyond 22 months will produce many other science benefits and opportunities. 34) 35)

• CloudSat has provided the first real information on the fraction of clouds that produce precipitation. Over the Earth's oceans, CloudSat has shown that precipitation is much more common than was previously thought, due to the fact that precipitation over oceans is extremely hard to measure and that the light rain that often falls has been completely missed by satellite observations until now. CloudSat has shown that almost 15% of all oceanic clouds produce rain that falls to the surface. This is a fraction larger than previously believed and has much significance for improved understanding of the Earth's hydrological cycle. Globally averaged, approximately 12% of all clouds are producing rain. This quantity was unknown previous to CloudSat observations.

• On July 4, 2007 CloudSat executed a maneuver designed to avoid a close approach with the Iranian satellite, SINAH-1. The close approach was predicted to occur on July 6, 2007. Within the A-Train formation, CloudSat maintains a separation of 75 to 112 km with CALIPSO, its nearest neighbor in the constellation. The maneuver on July 4 was similar to the maneuvers CloudSat has conducted every few weeks in order to maintain this formation with CALIPSO. CloudSat conducted another maneuver on July 7, after the conjunction, to reverse the drift caused by the maneuver on July 4 and maintain the formation with CALIPSO.

• With the successful completion of OR2 (orbit raise maneuver no. 2 - segments A & B) on 27 May, 2006, the CloudSat spacecraft is now part of the A-Train constellation.

• The 94 GHz Cloud Profiling Radar (CPR) of the CloudSat mission was successfully transitioned to “operate mode” on May, 20, 2006. Science data have been collected since June 2, 2006.

Figure 13: Overview of CloudSat mission partners (image credit: NASA)
Figure 13: Overview of CloudSat mission partners (image credit: NASA)


Sensor Complement

CPR (Cloud Profiling Radar)

CPR is a joint development by NASA/JPL and CSA (CSA is providing EIK and RFES). The objective is to provide information on the vertical structure of all cloud systems. 36) 37) 38)

CPR is a 94 GHz nadir-looking millimeter-wave radar that measures the power backscattered by clouds as a function of distance from the radar (clouds are weak scatterers of microwave radiation especially in contrast to the reflection of the underlying Earth's surface - hence, maximum sensitivity of the CPR is required). The overriding requirement on CPR was to achieve a minimum detectable cloud reflectivity factor (Ze) of -28 dBZ.

The design of the CPR consists of the following subsystems: RFES (Radio Frequency Electronics Subsystem), HPA (High Power Amplifier), Antenna Subsystem (Quasi-Optical Transmission Line), and DSS (Digital Subsystem). The RFES consists of an up-converter which generates a pulsed signal and up-converts it to 94 GHz. The signal is amplified to about 200 mW by a state-of-the-art MMIC power amplifier. The receiver portion of the RFES down-converts the signal to an IF (Intermediate Frequency). The IF signal is detected using a logarithmic amplifier (high dynamic range). The receiver noise level is critical in achieving the required sensitivity.

The HPA (High-Power Amplifier), which amplifies the transmitted pulse to a nominal power level of 1.7 kW, consists of an EIK (Extended Interaction Klystron) and a high-voltage power supply (HVPS). Both a primary and a backup HPA are used to enhance system reliability. EIK differs from standard klystrons by using resonated bi-periodic ladder lines as a replacement for conventional klystron cavities. The HVPS provides 20 kV needed to operate the EIK and provides telemetry data necessary to system needs. The design uses a boost supply to minimize input current transients during the pulsing period and control EMC problems. Both the 94-GHz EIK and the 20-kV HVPS on CloudSat are the first of their kinds being flown in space. The EIK tube is manufactured by Communications and Power Industries, Canada, Inc.

Figure 14: The CPR assembly with a schematic of the antenna and HPA subsystems (image credit: NASA)
Figure 14: The CPR assembly with a schematic of the antenna and HPA subsystems (image credit: NASA)

Note: The inset figures of Figure 14 are actual photographs of flight hardware.

The CPR antenna is a fixed 1.85 m diameter reflector of composite graphite material to reduce mass. The antenna provides ≥ 63.1 dBi gain, has a half-beamwidth of ≤ 12º, and has sidelobes < about -50 dB for angles ≥ 7º from boresight. The quasi-optical transmission line (QOTL) replaces the conventional waveguide and circulator for sending power from the HPA to the reflector and sending received power to the receiver subsystem. Waveguides are replaced by mirrors and free-space propagation, while the circulator duplexing function is handled by a polarizer and Faraday rotator. The advantage of the QOTL over conventional waveguide and circulator is reduced loss, important for meeting sensitivity requirements. The DSS provides the command, control, and telemetry interface to the S/C. It includes a Control and Timing Unit and a data handling unit that accepts the analog signal from the RFES logarithmic detector. It digitizes it and performs the required sample averaging of 0.16 s.

To detect the low reflectivity of clouds, the CPR averages many samples of the measured power and subtracts the estimated system noise level. The number of independent samples can be increased by increasing the PRF (Pulse Repetition Frequency). However, the maximum PRF is given by the range ambiguity considerations. For CPR, the nominal range window size is set to 30 km, permitting the capture of the surface return and cloud return up to an altitude of 25 km. The system noise level is estimated using the clear air radar return from 25 to 30 km altitude. The radar footprint is 1.4 km, and is averaged over 0.16 seconds to produce an effective footprint of 3.5 km (along-track) by 1.2 km (cross-track). 39) 40)


Short Pulse (SP) Operation

Nominal frequency

94.05 GHz (W-band, corresponding to 3 mm wavelength)

Pulse width

3.3 µs

PRF (nominal)

4300 Hz

Data window

0-30 km

Vertical range resolution (6 dB)
Cross-track resolution
Along-track resolution

485 m
1.4 km
1.9 km

Along-track sampling

2 km

Antenna size (diameter) limited by launch constraints

1.85 m

Antenna gain

63.1 dBi

Antenna sidelobes

-50 dB @ θ > 7º

Minimum detectable reflectivity

-30 dBZ

Dynamic range

70 dB

Peak power (nominal)

1.7 kW


0.3 MHz

Integration time (single beam)

0.16 s

Along-track sampling

1.1 km

Data rate

15 kbit/s

Instrument mass, power

230 kg, 270 W

Table 2: CPR instrument parameters
Figure 15: Simplified block diagram of CPR
Figure 15: Simplified block diagram of CPR
Figure 16: The HVPS (High-Voltage Power Supply) device of CPR (image credit: NASA/JPL)
Figure 16: The HVPS (High-Voltage Power Supply) device of CPR (image credit: NASA/JPL)
Figure 17: Illustration of the CPR antenna subsystem (image credit: NASA/JPL)
Figure 17: Illustration of the CPR antenna subsystem (image credit: NASA/JPL)
Figure 18: Illustration of the CloudSat spacecraft (image credit: NASA/JPL)
Figure 18: Illustration of the CloudSat spacecraft (image credit: NASA/JPL)

Legend of Figure 18: This artist's concept shows NASA's CloudSat spacecraft and its Cloud Profiling Radar using microwave energy to observe cloud particles and determine the mass of water and ice within clouds.

The CPR instrument is the first-ever millimeterwave and the most sensitive radar so far launched into space. Its -30 dBZ detection sensitivity is enabling the first global view of the vertical structure of the atmospheric clouds at 500 m resolution. The CloudSat mission also provides an important demonstration of the 94 GHz radar technology in a spaceborne application.


Validation Campaigns

The validation of the data products uses remote sensing measurements from surface and airborne platforms together with in-situ aircraft measurements of relevant cloud parameters as well as matching aircraft data with satellite data after launch. The validation strategy also involves the exploitation of existing cloud data bases. The CloudSat validation plan benefits from the systematic measurement programs of ARM as well as selected sites within Europe, regular aircraft radar measurement activities within the USA, Japan and Europe, measurement capabilities at a number of universities, and field-experiment activities representing targets of opportunity are planned in the coming years.


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39) Eastwood Im, Simone Tanelli, Stephen L. Durden, Kyung Pak, “Cloud Profiling Radar Performance,” Proceedings of IGARSS 2007 (International Geoscience and Remote Sensing Symposium), Barcelona, Spain, July 23-27, 2007

40) E. Im S. L. Durden, S. Tanelli, K. Pak, “Early Results on Cloud Profiling Radar Post-launch Calibration and Operations,” Proceedings of IGARSS 2006 and 27th Canadian Symposium on Remote Sensing, Denver CO, USA, July 31-Aug. 4, 2006

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 (

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