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 11).
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)
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)
Launch: 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)
• 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. 16)
- 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 4: 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. 17)
- 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.
Table 1: Mission extension recommendations of the NASA Earth Science Senior Review Subcommittee Report - 2017 (Ref. 17)
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. 18)
- 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.
Table 2: Overview of the science objectives for the CloudSat and CALIPSO missions (Ref. 18)
- 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 5 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 5: 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 6). 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 6: 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. 18)
• 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. 19)
- MODIS on NASA's Aqua satellite captured the infrared (Figure 7) image taken a few minutes before the CloudSat overpass (Figure 8) denoted by the blue line A->B). The enhanced infrared VIIRS imagery of Suomi-NPP, Figure 9, 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 8) 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 7: MODIS IR imagery with CloudSat track (image credit: NOAA LAADS Web)
Figure 8: CloudSat overpass track (image credit: CloudSat Data Processing Center)
Figure 9: Enhanced infrared VIIRS image (image credit: RAMMB)
Figure 10: Cross section of CloudSat and GOES imagery (image credit: NASA/JPL-Caltech/CloudSat Data Processing Center, Ref. 19)
• 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. 20)
• 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. 21)
• 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. 22)
- 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. 23)
• 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. 24) 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. 29).
• 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. 25) 26)
• 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. 29): 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. 27)
• 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. 27).
• 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. 27).
• In June 2011, the NASA Earth Science Senior Review recommended an extension of the CloudSat mission up to 2015. 28)
• 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. 29)
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. 30) 31)
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. 32)
• 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. 33) 34)
• 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 11: Overview of CloudSat mission partners (image credit: NASA)
Sensor complement: (CPR)
CPR (Cloud Profiling Radar):
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 12: The CPR assembly with a schematic of the antenna and HPA subsystems (image credit: NASA)
Note: The inset figures of Figure 12 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). 38) 39)
Table 3: CPR instrument parameters
Figure 13: Simplified block diagram of CPR
Figure 14: The HVPS (High-Voltage Power Supply) device of CPR (image credit: NASA/JPL)
Figure 15: Illustration of the CPR antenna subsystem (image credit: NASA/JPL)
Figure 16: Illustration of the CloudSat spacecraft (image credit: NASA/JPL)
Legend of Figure 16: 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.
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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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).