ISS Utilization: ISS-RapidScat
The ISS-RapidScat instrument is a speedy and cost-effective replacement for NASA's QuikSCAT Earth satellite, which monitored ocean winds to provide essential measurements used in weather predictions, including hurricane monitoring.
Background: After ten years of successful operations, in late 2009 the NASA SeaWinds instrument on the QuikScat satellite suffered a degradation that significantly decreased the amount of wind data it could collect over the oceans, leaving a hole in the global constellation of wind scatterometers. The QuikSCAT instrument is still able to operate collecting a small swath, and has been used successfully by NASA to provide a cross-calibration standard for the international scatterometer constellation of ISRO's OSCAT (OceanSat-2 Scatterometer) and EUMETSAT's ASCAT(Advanced Scatterometer). Although next-generation replacements to this satellite have been under study by NASA and NOAA, these instruments will not be readily available to mitigate the degradation of QuikScat in the near term.
To meet this challenge, the Jet Propulsion Laboratory (JPL), in partnership with NASA's International Space Station Program Office, will deploy the QuikScat engineering model, which had been used to test the basic functionality and performance of the instrument, on the ISS to continue and improve QuikSCAT's calibration standard across the present scatterometer constellation and demonstrate NASA's capability for fast response to science challenges in a cost constrained environment.
ISS-RapidScat will also exploit the special characteristics of the ISS orbit to advance our understanding of the Earth's winds. Current scatterometers are in polar sun-synchronous orbits, visiting each point on the Earth at approximately the same local time. Consequently, satellites in the scatterometer constellation have different local observation times, and products significant challenges in stitching the data from different satellites into a data record appropriate for monitoring subtle changes in the wind field across satellite records and over long periods. The ISS orbit, on the other hand, is not synchronized with the Earth's rotation and has a lower inclination than polar sun-synchronous satellites. This will cause the ISS orbit to intersect the orbits of every one of these sun-synchronous satellites approximately every hour, allowing winds to be estimated simultaneously by ISS-RapidScat and the other scatterometers. These simultaneous views will allow ISS-RapidScat to serve as the calibration golden standard that will enable improved calibration of the international scatterometer constellation.
The primary goal of this investigation is to provide a gap-filler ocean vector winds measurement capability to mitigate the loss of the NASA QuikSCAT scatterometer. Scatterometers are radar instruments that can measure near-surface wind speed and direction over the ocean, and have proved to be extremely valuable for weather forecasting, including hurricane monitoring, and for monitoring large-scale changes in the Earth's climate, such as El Niño. The ISS RapidScat instrument will provide wind measurements that will enhance the international scatterometer constellation, provide unique cross-calibration capabilities to extend the climate data record initiated by the QuikSCAT satellite. In addition, because of the unique orbit characteristics of the ISS, RapidSCAT will be enable the first measurements of the systematic diurnal changes of winds over the ocean. 1)
In 2013, NASA's ESD (Earth Science Division) is planning to use the ISS (International Space Station) more than ever as a platform for observing the Earth. In addition to those missions solely funded by the ESD, like SAGE III and OCO-3 (Orbiting Carbon Observatory-3), the CATS (Cloud Aerosol Transport System) and RapidSCAT were selected as part of an ISS Program science utilization solicitation. CATS for the ISS, is a cloud aerosol LIDAR that is a possible precursor for the upcoming ACE (Aerosol/Cloud/Ecosystems) Decadal Survey Mission. 2)
For the sun synchronous orbits, it is impossible to obtain this type of global collocation, since orbital overlaps tend to occur at very high latitudes, limiting coverage to either land or the Southern Ocean, where special conditions apply in terms of wind speed and stability that are not globally representative.
Figure 1 presents estimates of the standard error on the relative bias between RapidScat and ASCAT as a function of latitude. This type of cm/s accuracy is what is required to enable climate studies of wind variability.
Figure 1: Estimated standard error in the estimated bias between RapidScat and ASCAT as a function of latitude, and for a mission duration of one (dashed) and two (solid) years (image credit: NASA/JPL)
The proposed plan for the joint use of RapidScat and QuikSCAT will consist of using the QuikSCAT-RapidScat collocations to achieve cross-calibration between the two instruments on an ongoing basis during the RapidScat mission. The continuous calibration will alleviate any issues that might arise in RapidScat due to the special environment on the ISS. The calibrated RapidScat will then be used as the golden standard to develop bias corrections (as a function of wind speed, direction, and geographical location) so that all instruments on the constellation (ASCAT, OSCAT, QuikSCAT, and, potentially, OSCAT2) have a common reference frame for producing a consistent winds data set.
Diurnal studies: A hurdle in estimating the semi-diurnal observations from sun-synchronous scatterometers is due to the fact that biases between two scatterometers will alias into the semi-diurnal component, so that good relative calibration is required.
The ISS orbit, on the other hand, visits all points at latitudes smaller than 51.6o at all times of day over a period of roughly 2 months (Figure 2). This will allow, over a period of two years, the estimation of the semi-diurnal wind components from the RapidScat data alone (see Figure 3 for estimated accuracies). Furthermore, since RapidScat enables a consistent set of biases, other scatterometers in the constellation can also be used in obtaining this estimate, which will lead to improved precision in the estimates.
Figure 2: The local time sampling characteristics of the ISS are to revisit the same latitude at slightly different local times each orbit. To fully sample the diurnal and semi-diurnal cycles once globally requires at least 2 months of data. To estimate diurnal and semi-diurnal cycles accurately, on the order of 10 sets of observations (~2 years) will be required (image credit: NASA/JPL)
Figure 3: Upper bound on the estimated semi-diurnal cosine component of zonal and meridional wind components assuming inversion using RapidScat data alone (image credit: NASA/JPL)
Figure 4: Planned NASA Earth Science Instruments for the International Space Station(image credit: NASA) 3)
Launch: RapidScat and other cargo/instruments were launched on September 21, 2014 (05:52:03 UTC) on a Falcon-9v1.1 vehicle and the Dragon spacecraft of the SpaceX CRS-4 flight to the ISS. The launch site was SLC-40 at Cape Canaveral, FL. The Dragon spacecraft is scheduled to dock to the Harmony nadir port of the ISS on Sept. 23, 2014. 4) 5) 6)
SpaceX CRS-4 is a cargo resupply mission to the ISS (International Space Station), contracted to NASA. It is the sixth flight for SpaceX's uncrewed Dragon cargo spacecraft, and the fourth SpaceX operational mission contracted to NASA under a CRS (Commercial Resupply Services) contract.
On May 12, 2014, ISS-RapidScat arrived at KSC (Kennedy Space Center) in Florida to begin final preparations for launch. 7)
Orbit: Near-circular orbit of ISS, altitude range of 375-435 km, inclination = 51.6º. The prograde orbit of the ISS means that there will be an intersection with the orbits of every scatterometer in the constellation (ASCAT, OSCAT, QuikSCAT) once every revolution, and the likelihood of having nearly coincident temporal coverage (within 0.5 to 1 hours) is guaranteed. Furthermore, as the orbit moves over the year, the loci of these intersections will shift in latitude, yielding over time a global estimate of the relative wind and geographical biases between RapidScat and any other system in the constellation (Ref. 1).
Figure 5: The RapidScat instrument and its Nadir Adapter were strategically mounted inside the trunk (image credit: NASA/JPL, Ref. 27)
Dragon delivers ~2270 kg of supplies and payloads, including critical materials to support 255 science and research investigations that will occur during Expeditions 41 and 42. Dragon carries three powered cargo payloads in its pressurized section and two in its unpressurized trunk. — Dragon will return with about 1725 kg of cargo, which includes crew supplies, hardware and computer resources, science experiments, space station hardware, and four powered payloads (recovery in the Pacific Ocean ~ 700 km off the coast of California). 8) 9)
• RapidScat is the primary payload on this CRS-4 flight of SpaceX.
• 3D Print device of Made in Space Inc. of Mountain View, CA. (Note: the 3D Print device is described in a separate file on the eoPortal)
• New permanent life science research facility. The Bone Densitometer (BD) payload, developed by Techshot, will provide a bone density scanning capability on ISS for utilization by NASA and CASIS (Center for the Advancement of Science in Space). The system measures bone mineral density (and lean and fat tissue) in mice using DEXA (Dual-Energy X-ray Absorptiometry).
For the first time, Dragon will carry live mammals – 20 rodents will ride up in in NASA's Rodent Research Facility, developed by scientists and engineers at NASA's Ames Research Center. The rodent research system enables researchers to study the long-term effects of microgravity—or weightlessness—on mammalian physiology.
• Arkyd-3 is a 3U CubeSat technology demonstrator (4 kg) from the private company Planetary Resources Inc. of Bellevue, WA, USA, (formerly known as Arkyd Astronautics). The objective is to test the technology used on the future larger Arkyd-100 space telescope. The company has contracted with NanoRacks to take the Arkyd-3 nanosatellite to the ISS where it will be released from the airlock in the Kibo module. 10)
Figure 6: Illustration of the deployed Arkyd-3 nanosatellite (image credit: Planetary Resources)
• SpinSat (Special Purpose Inexpensive Satellite), a microsatellite (57 kg) of NRL (Naval Research Laboratory), Washington D.C. SpinSat is documented in a separate file on the eoPortal.
• SSIKLOPS (Space Station Integrated Kinetic Launcher for Orbital Payload Systems). This launcher will provide still another means to release other small satellites from the ISS. This system is also known by the name of Cyclops and is described in the SpinSat file on the eoPortal. SSIKLOPS will be used to deploy SpinSat from the ISS. 11)
Status of ISS-RapidScat mission:
• November 28, 2016: NASA's ISS-RapidScat Earth science instrument has ended operations following a successful two-year mission aboard the space station. The mission launched Sept. 21, 2014, and had recently passed its original decommissioning date. 12)
- ISS-RapidScat used the unique vantage point of the space station to provide near-real-time monitoring of ocean winds, which are critical in determining regional weather patterns. Its measurements of wind speed and direction over the ocean surface have been used by agencies worldwide for weather and marine forecasting and tropical cyclone monitoring. Its location on the space station made it the first spaceborne scatterometer that could observe how winds evolve throughout the course of a day.
- "As a first-of-its-kind mission, ISS-RapidScat proved successful in providing researchers and forecasters with a low-cost eye on winds over remote areas of Earth's oceans," said Michael Freilich, director of NASA's Earth Science Division. "The data from ISS-RapidScat will help researchers contribute to an improved understanding of fundamental weather and climate processes, such as how tropical weather systems form and evolve."
- The agencies that routinely used ISS-RapidScat's data for forecasting and monitoring operations include NOAA (National Oceanic and Atmospheric Administration) and the U.S. Navy, along with European and Indian weather agencies. It provided more complete coverage of wind patterns far out to sea that could build into dangerous storms. Even if these storms never reach land, they can bring devastating wave impacts to coastal areas far away.
- "The unique coverage of ISS-RapidScat allowed us to see the rate of change or evolution in key wind features along mid-latitude storm tracks, which happen to intersect major shipping routes," said Paul Chang, Ocean Surface Winds Science team lead at NOAA's Center for Satellite Applications and Research. "ISS-RapidScat observations improved situational awareness of marine weather conditions, which aid optimal ship routing and hazard avoidance, and marine forecasts and warnings."
- During its mission, ISS-RapidScat also provided new insights into research questions such as how changing winds over the Pacific drove changes in sea surface temperature during the 2015-2016 El Niño event. Due to its unique ability to sample winds at different times of day, its data will be useful to scientists for years to come.
- ISS-RapidScat was born out of ingenuity, expertise and a need for speed. It was constructed in less than two years to replace its widely valued predecessor, NASA's decade-old QuikSCAT scatterometer satellite, at a fraction of the cost of the original — largely by adapting spare parts from QuikSCAT.
- On Aug. 19, a power distribution unit for the space station's Columbus module failed, resulting in a power loss to ISS-RapidScat. Later that day, as the mission operations team from NASA's Jet Propulsion Laboratory in Pasadena, California, attempted to reactivate the instrument, one of the outlets on the power distribution unit experienced an electrical overload. In the following weeks, multiple attempts to restore ISS-RapidScat to normal operations were not successful, including a final attempt on Oct. 17.
- NASA currently does not plan to launch another scatterometer mission. However, the loss of ISS-RapidScat data will be partially mitigated by the newly launched SCATSat ocean wind sensor, a mission of ISRO (Indian Space Research Organization).
- ISS-RapidScat was the first continuous Earth-observing instrument specifically designed and developed to operate on the International Space Station exterior, but it's no longer the only one. The CATS (Cloud-Aerosol Transport System) joined the space station in January 2015 to provide cost-effective measurements of atmospheric aerosols and clouds in Earth's atmosphere. Two more instruments are scheduled to launch to the space station in 2017 — one that will allow scientists to monitor the ozone layer's gradually improving health, and another to observe lightning over Earth's tropics and mid-latitudes. Following that, two additional Earth science instruments are scheduled for launch in 2018 and 2019.
• Sept. 9, 2016: Mission managers at NASA/JPL and NASA/MSFC are assessing two power system-related anomalies affecting the operation of NASA's ISS-RapidScat instrument aboard the International Space Station. RapidScat measures surface wind speeds and directions over the ocean. 13)
- RapidScat is currently deactivated and in a stable configuration. A RapidScat project anomaly response team has been formed, working in conjunction with the space station anomaly response team. RapidScat will remain deactivated as the investigation continues.
- On Aug. 19, the RapidScat team was notified by the International Space Station payload operations center at Marshall that the station's Columbus Module experienced an anomaly with one of the two units aboard the station that distribute electrical power to the module. The anomaly resulted in the loss of power to several payloads aboard the space station, including RapidScat. Later that day, as JPL mission managers attempted to reactivate RapidScat, one of the outlets on the power distribution unit experienced an electrical overload. That outlet powers the station's RapidScat, HDEV (High-Definition Earth Viewing Experiment) and SOLAR (Solar Monitoring Observatory) payloads.
- On Aug. 23, the crew manually isolated RapidScat's external payload site from its Columbus module power circuit. Ground teams then successfully restored power to the affected power distribution unit outlet, and SOLAR and HDEV powered up successfully, with no sign of electrical overload. This action isolated the outlet overload to the RapidScat site. It is not yet known if the fault is on the Columbus or RapidScat side of the power supply interface.
- Mission managers are handling the incidents as two separate anomalies: loss of power to multiple payloads connected to the power distribution unit, and the electrical overload on the unit's outlet during the attempted reactivation of RapidScat. RapidScat's survival heaters are currently on (the heaters receive power from a different Columbus power circuit). The heaters are designed to keep the instrument within allowable flight temperatures indefinitely.
• March 7, 2016: During this season of El Niño influenced Pacific storms, NASA has been analyzing the storms that brought rain and snow to the U.S. West Coast. — NASA's RapidScat instrument spied tropical-storm-force winds in a weather system affecting the Pacific Northwestern U.S. and southwestern Canada on Sunday, March 6 and Monday, March 7 (Figure 7). NOAA's GOES-West satellite provided an infrared look at the clouds associated with the system that blanketed the U.S. West Coast (Figure 8). 14)
- In addition, NASA's IMERG (Integrated Multi-satellitE Retrievals for GPM) precipitation product added up the large rainfall totals that the west coast received from Feb. 29 to early March 7, 2016 as the latest system was bringing more rain and snow (Figure 9).
Figure 7: RapidScat saw sustained surface winds associated with a low pressure area near 46.9 mph (21 meters per second/75.6 kph) as it approached western Canada and the U.S. Pacific Northwest coast (image credit: NASA/JPL, Doug Tyler)
Figure 8: This NOAA GOES-West infrared image from March 7 at 15:41 UTC shows the clouds associated with the weather system over the U.S. Pacific Northwest (image credit: NASA/NOAA GOES Project)
Figure 9: An IMERG rainfall analysis from Feb. 29 to Mar. 7 showed heavy rainfall that occurred near the California and Oregon border over that eight-day period. Rainfall totals for this period were estimated by IMERG to be over 8 inches (203 mm) in some areas (image credit: NASA, JAXA, Hal Pierce, Ref. 14)
• January 25, 2016: NASA's ISS-RapidScat instrument on the ISS provided a look at the strong winds that led to coastal flooding in southern New Jersey during the historic winter storm that blanketed much of the U.S. East Coast, starting Jan. 23, 2016. At 18:00 UTC on Jan. 22, RapidScat showed sustained winds as strong as 45 m/s (162 km/hr) along the coast of southern New Jersey. Many beachfront towns in this region were flooded as winds pushed ocean waters inland. The town of Cape May, New Jersey, reported a flood level of 2.74 m — the highest on record. 15)
- RapidScat is an important tool for meteorologists because maximum sustained winds are not always equally distributed in a storm. RapidScat shows forecasters the location of the strongest winds in different quadrants of a storm, indicating locations that could be facing greatest impacts.
Figure 10: This image shows ocean winds near the surface off the U.S. East Coast, from the hook of Cape Cod at top center to the South Carolina coastline at bottom left. Southern New Jersey and the northern edge of Delaware are directly in the path of the highest wind speeds (image credit: NASA/JPL)
• Nov. 6, 2015: NASA's ISS-RapidScat instrument, which last month celebrated its one-year anniversary, helps make the ocean wind measurements to enhance weather forecasting and understanding of climate. The instrument was first activated on the ISS (International Space Station) on Oct. 1, 2014. - In its first year in action, the instrument has collected data on many severe storms, including typhoons and tropical cyclones. RapidScat has proven valuable for tracking the Southern Hemisphere's hurricane season and the Northern Hemisphere's winter storm season. 16)
- Most recently, RapidScat played a role in tracking Hurricane Patricia, which loomed over Mexico in October (Figure 11). Patricia was the strongest hurricane ever recorded in the Western Hemisphere, with maximum winds of 320 km/h. When it first made landfall on the Pacific coast of Mexico on Oct. 23, it was a destructive Category 5 storm.
- Worldwide, many meteorological agencies include RapidScat data in the ensemble of data sets used to create forecasts. The agencies include the U.S. Navy, NOAA (National Oceanographic and Atmospheric Administration), and EUMETSAT (European Organization for the Exploitation of Meteorological Satellites). High wind warnings, to which RapidScat data contribute, are especially important for anyone involved in shipping and sailing. Wind information from RapidScat can also be useful for enthusiasts of water sports.
- RapidScat measures winds that are just above the ocean surface. The instrument is a Ku-band scatterometer that transmits pulses of microwave energy toward Earth. The surface of Earth reflects this signal, and RapidScat measures the strength of the pulse that comes back. Stronger return signals from the ocean indicate larger waves. The return signal also carries information about wind direction.
- Most scatterometers are launched in sun-synchronous orbits, such that each time they fly over the same place on Earth, it's at the same local time. But because RapidScat is mounted on the space station, which is not in a sun-synchronous orbit, it sees different places at different local times. The instrument samples all local times of day over the course of about two months, allowing scientists to learn more about how winds vary over the course of a day for a given location.
Figure 11: NASA's RapidScat's antenna, lower right, was pointed at Hurricane Patricia as the powerful storm approached Mexico on Oct. 23, 2015 (image credit: NASA)
• August 2015: RapidScat started delivering data to operational agencies in November 2014, and from December 2014 delivers science data to the global community through NASA's PODAAC (Physical Oceanography Distributed Active Archiving Center), as well as through KNMI (Royal Netherlands Meteorological Institute) and NOAA. Although similar in design to QuikSCAT, the RapidScat non-sun synchronous sampling opens up new opportunities to explore new science and extend the climate data record. 17)
- One of the major limitations of past cross-calibration exercises between different scatterometers is the fact that sun-synchronous orbits with different orbit ascending nodes rarely see each other, and when they do, it tends to be at high latitudes where special conditions may apply. The ISS 56.1º inclination orbit guarantees that RapidScat will be able to have near-simultaneous (within about 1/2 an hour) with every other scatterometer in the constellation, including the two ASCAT's, QuikSCAT, and the upcoming ScatSat-1 mission of ISRO, currently scheduled to launch in 2016.
- To look at the potential for this data for cross-calibration, Figure 12 is presented for a comparison of ASCAT morning and evening speed climatologies with the climatologies for the same times derived from the first three months of data. Since RapidScat spends less time at these local times than ASCAT, its climatology is noisier, but already similarities and differences are becoming apparent. By the time these results will be presented, the project expects to be able to present a global comparison of differences in wind speed, wind stress, U and V components, and curl and divergence derived from both missions and to examine the geographical biases between the two, with recommendations on how to conduct the cross-calibration.
Figure 12: Comparison of ASCAT (left) and RapidScat (right) wind speed climatologies for 9 to10 hours local time (top) and 21 to 22 hours (bottom), when the two systems are nearly coincident. The climatologies are made for the first three months of data (image credit: NASA/JPL, Caltech)
- Diurnal and subdiurnal variability: One of the exciting possibilities opened up by the ISS orbit is the sampling of diurnal and sub-diurnal cycles and subdiurnal variability associated with local time of day. Gille et al., have used the 8 months of coincident SeaWinds and QuikSCAT data to derive the first estimates of the diurnal cycle for the tropics. 18) Lee has used the same data set to study the impact of subdiurnal variability on ocean models. 19) The project expects that the RapidScat data will offer significant advantages in advancing similar studies. As a preview of upcoming results, Figure13 shows the climatologies of the difference between RapidScat and similarly sampled ECMWF U components for four different averaging periods spanning the day. It is clear, even at this early stage, that there seem to be semi-diurnal differences between the two data sets, although wind variability is still visible in the image. These systematic differences will be refined by the additional data collected from now until the meeting in seven months time.
Figure 13: Climatologies of U-component wind differences between RapidScat and similarly sampled ECMWF data after dividing the day in 6-hour intervals. Semi-diurnal differences are starting to be seen in these differences, as well as some diurnal signatures. These figures will be refined with the additional data the mission will collect in the next seven months (image credit: NASA/JPL, Caltech)
• April 21, 2015: The ISS-RapidScat instrument has been in orbit seven months, and forecasters are already finding this new eye-in-the-sky helpful as they keep watch on major storms around the globe. RapidScat has kept busy in 2015's already active Southern Hemisphere hurricane season and the Northern Hemisphere's winter storm season. 20)
- According to Bryan Stiles, lead for RapidScat science data processing at NASA's Jet Propulsion Laboratory, Pasadena, California, "RapidScat data is now being used by meteorological agencies around the world, including NOAA and EUMETSAT, among others," he said. "Wind data obtained by RapidScat have been used by NOAA to detect gale force and storm force conditions and issue warnings to shipping. The wind data is available to both forecasters and scientists. RapidScat data is used to support real-time weather prediction and to improve the models scientists use to predict both short-term weather and long-term climate trends."
- From the space station, this orbiting scatterometer instrument uses radar pulses reflected from the ocean's surface from different angles to measure ocean surface roughness, which is then used to determine surface wind speed and direction. This vantage point, combined with the fact that the space station orbits Earth every 90 minutes, also allows RapidScat to provide observations on how ocean winds vary over the course of the day.
- "Most Earth observing satellites are in polar, sun-synchronous orbits, meaning they observe the same locations at the same two local times of day with a regular repeating pattern," said Doug Tyler of the RapidScat team at JPL. "Because of the unique orbit of the space station, RapidScat observations occur at varying times of day with an irregular repeat period. RapidScat sometimes sees things several times in a row. The ISS orbit provided three overpasses of [Tropical Cyclone Nathan] in 23 hours, allowing RapidScat to capture changes in wind speed and direction as the storm developed."
- The Joint Typhoon Warning Center (JTWC) forecasts tropical cyclones in several oceans and is also using ISS-RapidScat data. On March 15, 2015, at 0428 UTC, JTWC noted, "RapidScat showed that [Tropical Cyclone Nathan's] strongest winds (still assessed at 40 knots) remain in the northern periphery of the system, with significantly weaker winds in the southern portion." It's helpful to know where the strongest winds are in the system to enhance warnings, especially if they are affecting land or in a shipping channel.
- The same week, RapidScat provided surface wind data on the most powerful tropical cyclone to affect the Southern Pacific island nation of Vanuatu. After Tropical Cyclone Pam became an extra-tropical cyclone and moved near New Zealand, RapidScat continued to provide the location and speed of the strongest surface winds, which assisted with warnings.
- Earlier this year, RapidScat also provided wind data on a nor'easter that affected New England and triggered blizzard warnings on Jan. 27 and 28. The wind data captured on the intense system showed the strongest winds on the first day near 78 miles per hour (35 meters per second/126 kilometers per hour) as it moved along the coast, stretching from eastern Long Island, New York, to southern Nova Scotia, Canada.
Figure 14: On Jan. 28, 2015 from 2:41 to 4:14 UTC, ISS-RapidScat saw the nor'easter's strongest sustained winds (red) near 35 m/s (126 km/h) just off-shore from eastern Cape Cod (image credit: NASA JPL, Doug Tyler)
• November 10, 2014: The RapidScat mission was declared operational by the RapidScat team. The data of the ISS-RapidScat mission are being released, two months ahead of schedule. Working at an accelerated pace, scientists and engineers have successfully cross-calibrated ISS-RapidScat's ocean winds data with data from NASA's QuikSCAT satellite and validated the data against ground measurements. The team reports the RapidScat data are meeting all planned wind performance requirements and are ready to begin extending the long-term climate data record of ocean-surface winds begun by QuikSCAT in 1999. 21)
- The RapidScat team includes scientists and engineers from NASA's Jet Propulsion Laboratory, Pasadena, California; the National Oceanic and Atmospheric Administration; Brigham Young University, Provo, Utah; and Remote Sensing Systems, Santa Rosa, California.
- The dissemination of these data to the international operational weather and marine forecasting communities ensures that RapidScat's benefits will be felt throughout the world. The agencies that will receive near-real-time RapidScat data include NOAA, the U.S. Navy, EUMETSAT (European Organization for the Exploitation of Meteorological Satellites), ISRO (Indian Space Research Organization) and KNMI (Royal Netherlands Meteorological Institute). The data will also be distributed to RapidScat science team members. The spatial sampling – how far apart the samples are — of RapidScat data is currently 25 km. A version to be released in the near future will double its spatial resolution – meaning it will shrink the distance between samples by half.
- ISS-RapidScat's berth on the space station's Columbus module puts it in an orbit unique from any other wind-measuring instrument currently in orbit. This vantage point will give scientists the first near-global direct observations of how ocean winds vary over the course of the day due to solar heating. Because it crosses the path of every other scatterometer currently in orbit, ISS-RapidScat will also be able to ensure that all spaceborne scatterometer data sets are accurate and consistent with each other, a process known as cross-calibration.
Figure 15: ISS RapidScat observation of North Atlantic extratropical storm at 23:00 UTC, Oct. 13, 2014 (image credit: NASA/JPL-Caltech, NOAA)
Legend to Figure 15: ISS-RapidScat data on a North Atlantic extratropical cyclone, as seen by the NCEP/AWIPS (National Centers for Environmental Prediction/Advanced Weather Interactive Processing System), used by weather forecasters at the NOAA's Ocean Prediction Center.
• October 6, 2014: ISS-RapidScat was installed on the ISS and is collecting its first science data on ocean wind speeds and direction following its successful installation and activation on the exterior of the station's Columbus module. 22) 23)
- On Sept. 29-30, 2014, ground controllers at NASA's Johnson Space Center in Houston robotically assembled the RapidScat instrument and its nadir adapter, which orients the instrument to point at Earth. On Oct. 1, the instrument was powered on, its antenna began spinning and it started transmitting and receiving its first winds data. The team then began checking out the instrument, a process expected to take about two weeks. Checkout activities to date are proceeding nominally. Following instrument checkout, the team will perform two weeks of preliminary calibration and validation of science data. RapidScat will then be ready to begin its two-year science mission.
Figure 16: Photo of Canadarm2 and Dextre installing the RapidScat experiment, remotely controlled from NASA/JSC, onto Columbus EF (image credit: ESA, NASA, Alexander Gerst)
- On Oct. 3, mission scientists processed their first winds data and produced their first uncalibrated images: a partial global map of wind speeds and a close-up image of what was then Tropical Storm Simon, brewing off the west coast of Mexico, showing its wind speeds and wind directions at approximately 7 p.m. local time (Figure 17).
- RapidScat is the first science payload to be robotically assembled in space since the space station itself. Following inspections of RapidScat from cameras installed in the Dragon's trunk and on the station's robotic arm, ground controllers at Johnson Space Center used the DEXTRE manipulator on the station's robotic arm to pluck RapidScat's nadir adapter from the Dragon trunk on Sept. 29. An intricate set of maneuvers by the robotic arm then followed, leading to the adapter's successful mechanical and electrical connection to the Columbus module's External Payload Facility SDX site five hours later. The robotic arm was then released from the adapter.
About 15 hours later, the RapidScat team was back at work again, using the robotic arm to remove the RapidScat instrument itself from the Dragon's trunk and install it onto the nadir adapter. The installation went so well that a process expected to take five hours was completed in just two hours and 20 minutes. Following this first payload-to-payload mate in the history of the space station program, RapidScat then began drawing its power from the space station for the first time. RapidScat is an autonomous payload that requires no interaction from space station astronauts.
RapidScat will boost global monitoring of ocean winds for improved weather and marine forecasting, including hurricane monitoring, as well as climate studies. From the unique vantage point of the space station, this space-based scatterometer instrument will use radar pulses reflected from the ocean's surface from different angles to calculate ocean surface wind speeds and directions.
Figure 17: Wind profile of the Tropical Storm Simon acquired with ISS-RapidScat on Oct. 3, 2014 (image credit: NASA)
• Sept. 23, 2014: After the launch of the SpaceX-CRS-4 (Commercial Resupply Services-4) Dragon capsule from Cape Canaveral on Sept. 21 (05:52:03 UTC) and a two day chase through space, the Dragon cargo capsule arrived at the ISS, following a carefully choreographed series of thruster firings that brought the vessel to within a capture distance of some 10 m. For the rendezvous, ESA astronaut Alexander Gerst and NASA astronaut Reid Wiseman had set up a temporary robotics workstation in the Station's Cupola observatory module to monitor the approaching commercial spacecraft until it stopped 10 m from the Station. Working two joysticks, Alexander moved Canadarm2 to hold Dragon-4 as the Station soared some 415 km above the Pacific Ocean. From there, the spacecraft was berthed to the Space Station's Harmony module. 24)
The ISS-RapidScat instrument will utilize the spare engineering model used to test the QuikSCAT scatterometer, modified to operate from the ISS to provide a low-cost mission to mitigate the significant loss of measurement capability by QuikSCAT in 2009. The resulting instrument package will produce ocean vector winds similar in accuracy to QuikSCAT, but with a measurement swath on the ground smaller by a factor of two due to the lower ISS orbit. This swath width will be similar to the EUMETSAT ASCAT (Advanced Scatterometer) flown on MetOp, and the two data sets will complement each other to achieve coverage similar to QuikSCAT. Using engineering models in space, like on ISS-RapidSCAT, represents a low cost approach to acquiring valuable wind vector data. It does come with technical and programmatic risks. The hardware was not directly fabricated for space and will require rework in order to prepare for the rigors of space travel and operation. Meeting the cost commitment will require new and innovative approaches to development.
1) To provide ocean vector wind data for a period of two years to mitigate the loss of QuikSCAT to scientists and weather forecasters. -The ISS orbit will enable coincident measurements in space and time with each of the satellites in the constellation ASCAT, OSCAT (OceanSat-2 Scatterometer).
2) To serve as a cross-calibration standard to the international scatterometer constellation, enabling the continuation of the QuikSCAT data record, and enabling monitoring of climate variability and change over multiple decades.
3) To study the systematic variation of ocean winds as a function of time of day. These variations are important in understanding the dynamics and interactions of the ocean and atmosphere in the tropics, where current climate models still exhibit shortcomings, and which play a significant role in governing the Earth's energy and water budgets.
Figure 18: Artist's view of the ISS-RapidScat observation geometry and accommodation on the Columbus module (image credit: NASA) 28)
ISS-RapidScat will have measurement accuracy similar to QuikScat's and will survey all regions of Earth accessible from the ISS orbit. The instrument will be launched to the space station aboard a SpaceX Dragon cargo spacecraft. It will be installed on the end of the station's Columbus laboratory of ESA as an autonomous payload requiring no interaction by station crew members. It is expected to operate aboard the station for two years.
ISS-RapidScat will take advantage of the space station's unique characteristics to advance understanding of Earth's winds. Current scatterometer orbits pass the same point on Earth at approximately the same time every day. Since the space station's orbit intersects the orbits of each of these satellites about once every hour, ISS-RapidScat can serve as a calibration standard and help scientists stitch together the data from multiple sources into a long-term record.
Figure 19: Flight system overview (image credit: NASA/JPL, Ref. 25)
Figure 20: ISS-RapidScat versus ASCAT coverage (image credit: NASA, Ref. 26)
Inherited hardware constraints (Ref. 25):
• Pulse Repetition Interval (PRI) commandable between 5 and 6 ms
• Fit within the dynamic range of the receiver
- Roughly 45 dB instantaneous
- Attenuator can be set over 20 dB range
• Pulse width commandable between 0.5 and 1.5 ms
- Bandwidth is tied to pulse width via constant chirp rate of 250 kHz/ms. Reducing the pulse width from the 1.5 ms used with QuikSCAT reduces the bandwidth and range resolution
- Frequency resolution of processed slices is tied to length of the data collection, increasing range gate width decreases resolution.
• Antenna spin rate 18 rpm or 19.8 rpm
• Range delay and Doppler frequency offset are commanded via tables
- The variation of these parameters over a 360º scan has to be represented as A + B cos(θ+phase)
- Maximum Doppler 600 kHz, max range delay 12.75 ms
• SeaWinds EM is being used as is with the exception of the antenna system, thus the design space consists of:
- Selection of operating parameters within existing constraints (ISS and existing hardware)
- Design of antenna characteristics for desired performance
• Timing is a central concern in our analyses due to ISS altitude and attitude constraints
• SeaWinds was designed to transmit on one beam and then receive on the other (interleaved), operating at 800 km altitude
• At 400 km altitude the roundtrip time to match the inner OceanSat-2 angle is 4.0 ms; to match outer is 4.7 ms
• These are within the commandable PRI range of 5-6 ms
• To match Seawinds interleaved operational design, the PRI would need to be reduced to less than 3 ms – not feasible
• Hence, timing must be modified to transmit and receive on the same beam.
ISS constraints on instrument design and performance:
• EMC requirements
- Fields from RapidScat must on all ISS equipment and on ISS solar panels must be below required limits
• ISS altitude and attitude variations
- Expected altitude range: near 410 km mean altitude, plan for 375 km to 435 km (variation over an orbit ~ 20 km)
- Pitch can be roughly 0º to -10º, depending on visiting vehicles and presence of MLM (Multipurpose Laboratory Module)
- Roll should generally be less than 1º
- Yaw around -6º
- Control is to LVLH system; adds max of about 0.2º error to project desired geodetic nadir (based on project calculations).
- Visiting vehicles and solar arrays are within the RapidScat main beam (E-field limits exceed specs by up to 9.5 dB)
• Impact (if not mitigated)
- While ISS solar arrays can handle the radiation level, they do create a blockage in RapidScat's FOV (worst case = 8%)
- Reducing tx level by 9.5 dB for visiting vehicle safety not feasible; would need to turn off instrument during all visits
- Estimate about 300 days of visiting vehicles at Node 2 during RapidScat's 2-year on-orbit period; about half time not operating.
- Implementation of a sector blanker; the RF energy will be blanked over a sector of up to 60º on every rotation of the antenna
- The sector blanker will affect performance of the radar by reducing the swath up to approximately 50 km.
ISS attitude and altitude variability:
• The ISS can have significant changes in attitude (primarily pitch) due to docking of vehicles
• Additionally, the MLM (Multipurpose Laboratory Module) is scheduled for launch and to be installed on ISS in December 2014: will shift station's pitch bias by -4º
• The future Russian module will shift station's pitch bias by another -2º; however it is not likely to be installed till the middle or end of the RapidScat mission
• Previous history of attitude variation has been evaluated from ISS data and from HICO RAIDS data
- Study found good agreement between the two sources
- Attitude mean difference is typically less than 0.1º
• Future attitude variations are predicted by JSC (Johnson Space Center).
ISS attitude (pitch) change effects:
• Excessive ISS attitude changes throughout the RapidScat ops period can lead to RapidScat performance loss (echo overlap with nadir return or next transmitted event)
- The radar can compensate for small changes by adjusting the pulse width and data range window
- Excessive change is beyond radar's capability to compensate and will lead to loss of signal.
Figure 21: Schematic view of ISS pitch angle change effects (image credit: NASA, Ref. 25)
ISS attitude variability: mitigation
• PRI will be set to 6 ms; this gives maximum timing margin between end of received pulse and start of new transmit event
• Pulse width is reduced to 1 ms (or less) for timing margin
• Rx gate width to 1.4 ms for 10 km slice width (based on SeaWinds processor operation); maximized to allow for pitch variations
• Frequent update (daily-weekly) of range delay and Doppler tables (which tell the radar when to start recording data and what frequency offset to use for compensating Doppler shift)
- Tables are designed to hold parameters for an orbit
- The largest short-term variation in attitude is due to orbital motion, so this must be predicted with sufficient accuracy
- Due to on-board implementation of Doppler shift update a maximum of ±12.5 kHz error can be expected in the return echo in addition to errors from attitude variation.
• Mount radar with forward pitch offset (+2.5º or +5.5º) to reduce pitch bias over mission
• Roll bias is expected to be small; yaw bias of 6º will not have significant impact on conically scanning instrument
• Simulations indicate that for expected altitude and attitude range, data loss should be small (less than a few %).
• Initial assessment of system performance is based on simple spreadsheet calculations of the the noise equivalent sigma0
• For parameters shown here it was assumed that the station's orbit and attitude were perfectly known and stable i.e. no variability in pitch or altitude.
- Direct assessment of the scatterometer's wind retrieval ability relies on a simulation that uses a wind field as input.
Table 1: System performance parameter comparison of RapidScat and QuikSCAT
- Measure antenna gain, antenna pointing relative to alignment feature (e.g., cube or tooling balls)
- Calibrate antenna encoder (measure actual antenna position versus digital readout)
- Measure spin axis orientation relative to CEPA (Columbus External Platform Adapter)
- Measure receiver gain and noise over expected temperature range
- Measure transmit power over expected temperature range
- Update tables used by ground processor with these measurements.
- Absolute calibration, slice balance, sigma0 from all slices match the sigma0 from the whole footprint, sigma0 vary appropriately along the scan (Amazon rain forest to determine scan bias)
- Use data to estimate pointing and update Doppler and range tables
- Use data to update gain and update calibration tables
- Solar array positions used to discard affected data (<8%).
• RapidScat can be accommodated at the ISS SDX (Starboard Attach Point) site
- Requires two FRAM-based (Flight Releasable Attachment Mechanism) units
- Becomes an oversized payload once installed on orbit
• Partial FOV blockage leads to some degradation in performance
- In worst case, when solar arrays are in FOV, up to 8% of the scan is blocked
• Radiation level exceeding ISS subsystem safe level requires sector blanking (up to 60º)
- Radiation on ISS solar arrays not a concern
- Mitigation to allow ops when visiting vehicles are present will reduce swath and may impact calibration
• RapidScat measurements will provide vector winds retrieval to the accuracies comparable to those from QuikSCAT
- Requires pitch pointing offset to counter ISS attitude bias variations
- Design provision to set pitching pointing offset as late as feasible during the integration period
- If ISS pitch bias is incorrectly predicted performance is degraded
1) Meet stated performance 99% of the time if pitch bias predicted correctly
2) If assumed presence of MLM is incorrect, performance met 32% of the time.
Figure 22: Measurement geometry of RapidScat (image credit: NASA)
Table 2: RapidScat instrument parameters
Table 3: ISS accommodation of RapidScat
Figure 23: Flight system configuration, block diagram, instrument constraints (image credit: NASA, Ref. 25)
Figure 24: Illustration of the flight system (image credit: NASA, Ref. 25)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).