QuikSCAT (Quick Scatterometer Mission)
QuikSCAT is a NASA ESE (Earth Science Enterprise) program satellite built by BATC (Ball Aerospace & Technologies Corporation) of Boulder, CO. The QuikSCAT mission was initiated in the wake of the lost NSCAT (NASA Scatterometer) instrument measurements aboard NASDA's ADEOS-1 satellite. The ADEOS-1 S/C ceased functioning on June 30, 1997.
The overall objective is to restart NASA's ocean-wind measurement program, needed for improved weather forecasts and climate research. The NASA/GSFC contract to Ball was awarded under its new Rapid Spacecraft Acquisition (RSA) procurement program. QuikSCAT is a mission designed to complete turnaround from conception to launch in a very short period of time (one year). JPL's NSCAT/SeaWinds Program Office has been assigned responsibility and provides overall project management, as well as science, ground processing systems, and the SeaWinds instrument. NASA/GSFC manages the satellite development and operation. The spacecraft was built in a record time of 12 months. 1) 2)
The science objectives are:
• To acquire all-weather, high-resolution measurements of near-surface winds over global oceans
• To determine atmospheric forcing, ocean response, and air-sea interaction mechanisms on various spatial and temporal scales
• To combine wind data with measurements from scientific instruments in other disciplines to help us better understand the mechanisms of global climate change and weather patterns
• To study daily/seasonal sea ice edge movement and Arctic/Antarctic ice pack changes.
Note: QuikSCAT was the first satellite to use NASA's RSA procurement process and was built and delivered in less than a year. Ball Aerospace provided the spacecraft bus, launch interface systems, system integration, test and launch support. Ball Aerospace also performs mission operations with the University of Colorado's LASP (Laboratory for Atmospheric and Space Physics) as a subcontractor.
Figure 1: Artist's rendition of the QuikSCAT spacecraft (image credit: NASA)
The spacecraft uses a BCP 2000 (Ball Commercial Platform) series bus with dimensions: 2.2 m x 1.7 m x 1.4 m. The bus structure is made of aluminum honeycomb panels tied together by comer posts made of extruded aluminum. Most of the electronics are mounted inside the box, as are the propulsion subsystem, torque rods, reaction wheels, and inertial reference units. Subsystems mounted outside include various antennas including the rotating radar antenna, star trackers, and magnetometers. Most subsystems on the satellite are redundant, so that if one fails a backup unit can take over.
The S/C is three-axis stabilized. The ADCS (Attitude Determination and Control Subsystem) uses 2 star trackers, 14 sun sensors, a magnetometer, and an IRU (Inertial Reference Unit). Actuation is provided by 4 reaction wheels, 3 torque rods and 4 thrusters. A GPS receiver with C/A code capability provides onboard timing and orbit information. The pointing accuracy is ≤ 0.1º absolute per axis; the pointing knowledge is ≤ 0.05º per axis. The propulsion system uses anhydrous hydrazine (N2H4) blowdown. The solar array provides an average power of 874 W, a NiH2 battery (40 Ah capacity) is used for eclipse operations. 3)
S/C mass (wet) = 970 kg, payload mass = 205 kg, payload power = 250 W (average). Design life ≥ 2 years with 3 years for expendables.
Figure 2: Line drawing the the QuikSCAT spacecraft (image credit: NASA)
RF communications: The on-board data recorder has a capacity of 8 Gbit. Downlink/uplink communications are provided in S-band at a data rate of 2 Mbit/s for payload data. Housekeeping data are at data rates of 4, 16, and 256 kbit/s, the uplink data rate is 2 kbit/s. Data acquisition is facilitated at Wallops Flight Facility (WFF), Poker Flats (AGS), Svalbard Ground Station (SGS) Norway, and McMurdo (MGS) Ground Station, Antarctica, Hatoyama, Japan (contingency station).
Launch: The launch of the QuikSCAT spacecraft took place on June 19, 1999 atop a Titan II vehicle (LM) from VAFB, CA. 4)
Orbit: Circular sun-synchronous polar orbit with a local equator crossing time on the ascending node of 6:00 hours ± 30 minutes, altitude = 803 km, inclination = 98.6º, period = 102 min.
• In 2014, the QuikSCAT mission is in extended operations (extended through 2015). Due to technical failure (the antenna stopped rotating in November 2009), and the instrument no longer collects ocean wind vector data. However it still provides calibration data for other on-orbit scatterometers (OSCAT), which enables the continuation of a climate-quality wind vector dataset (Ref. 6).
Note: The ISS-RapidScat instrument is a speedy and cost-effective replacement for NASA's QuikSCAT Earth satellite. The ISS-RapidScat instrument is slated to launch in 2014 and will fly aboard the International Space Station to measure Earth's ocean surface wind speed and direction. 5)
• 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. 6)
- The extended QuikSCAT mission will also continue to use and improve adapted QuikSCAT algorithms to produce climate quality OSCAT (OceanSat-2 Scanning Scatterometer) ocean vector winds and ice products that continue the high quality QuikSCAT time series. This approach is viewed as the optimal way to continue the science- and climate-quality data record, since the ISRO mission is directed at near real time operational applications. Without appropriate calibration and data processing, these data will not be useful for climate and cryosphere research. ISRO and NASA have demonstrated successful collaboration to achieve these goals. QuikSCAT has been extremely stable in its calibration, and the radar instrumentation shows no indication of either calibration drift or deterioration, making this instrument ideal for calibration of these Ku-band scatterometers.
• 2013: QuikSCAT) is currently being used for the intercalibration of other scatterometer space missions following the age-related failure in 2009 of the mechanism that spins its scatterometer antenna.
• In 2012, the SeaWinds radar continues to operate normally and is collecting calibrated sigma naught (σο) measurements on a significantly reduced swath. The new QuikSCAT mission goal is to provide a facility for cross-calibration of multiple Ku-band scatterometers to a known, well calibrated source, enabling climate data consistency. The QuikSCAT spacecraft is in its 13th year on orbit (design life was three years). 7)
In June 2011, the NASA Earth Science Senior Review recommended an extension of the QuikSCAT mission for cross-calibration services of OSCAT and ASCAT up to 2013. QuikSCAT has been extremely stable in its calibration, and the radar instrumentation shows no indication of either calibration drift or deterioration worthy of concern; therefore, long-term stability of the QuikSCAT backscatter is anticipated and makes this instrument ideal for calibration of future Ku-band scatterometers. This approach allows for a common model function to be applied to the intercalibrated backscatter, which is important for long-term consistency. 8)
• The satellite image of Hurricane Irene (Figure 3), showing the storm's ocean surface wind speed and direction, was acquired at 1:07 a.m. EDT on Aug. 27, 2011 approximately six hours before it hit the North Carolina coast. The data are provided courtesy of the Indian Space Research Organization (ISRO) from the OSCAT instrument on ISRO's OceanSat 2 spacecraft, launched in September 2009. Wind vector data processing was performed at NASA/JPL, Pasadena, CA. The OSCAT winds are obtained at a resolution of 25 km x 25 km and do not resolve the hurricane's maximum wind speeds, which occur at much finer scales.
Since NASA's SeaWinds instrument on QuikSCAT ceased nominal operations in November 2009, scientists and engineers from NASA, JPL, and NOAA (National Oceanic and Atmospheric Administration) have collaborated with ISRO in ongoing efforts to calibrate and validate OSCAT (OceanSat-2 Scanning Scatterometer) measurements in order to ensure continuous coverage of ocean vector winds for use by the global weather forecasting and climate community. 9)
Figure 3: NASA-ISRO image shows Irene's winds before landfall on Aug. 27, 2011 (image credit: NASA, ISRO)
Legend to Figure 3: Hurricane Irene made landfall early Saturday morning, Aug. 27, 2011, just west of Cape Lookout, NC (USA), as a category one hurricane with maximum sustained winds of 136 km/h (75 knots). It is currently over eastern North Carolina and is forecast to gradually weaken as it moves northward along the East Coast of the United States over the next two days.
• NOAA has been receiving day-old OSCAT data via the ISRO dedicated FTP server since September 2010. 10)
• Since the QuikSCAT spacecraft and the scatterometer instrument (SeaWinds) themselves remained in otherwise good health after the spin mechanism failure, the scatterometer now tracks an operational data path swath significantly reduced from its original capability (the instrument is still producing data along its pencil beam). These data are continuing to provide an accurate and reliable transfer standard for cross-calibration of other ocean vector winds sensors, and for establishing the measurement stability needed for continuity with future scatterometer missions.
- Agreements are in place to allow the QuikSCAT team access to measurements from the ISRO (Indian Space Research Organization) OceanSat-2 scatterometer known as OSCAT, which was launched on September 23, 2009. The NASA/ISRO partnership is a long-term collaboration between the two agencies, and provides for a direct cross-calibration of QuikSCAT’s SeaWinds scatterometer with the OSCAT data to assist in the production of an ongoing ocean vector winds climate series. - Continuing the QuikSCAT mission remains vital to NASA’s science objectives and societal needs, and ongoing QuikSCAT observations will help satisfy the requirement for contiguous overlap and cross-calibration of ocean scatterometer climate data records. 11)
- While NASA and NOAA provided the QuikSCAT Ku-band scatterometer observations to the to the international operational ice monitoring community for the past decade - the new NASA/NOAA agreement with ISRO, reached in 2010, makes near-real-time, routine access to Oceansat-2 scatterometer (OSCAT) Level-1B data available. 12) 13)
Figure 4: Overview of missions with scatterometer instruments for global ocean wind vector observations (image credit: CEOS) 14)
• Following the age-related failure of the mechanism that spins the scatterometer antenna, NASA mission managers started to assess the options for future operations of the venerable QuikSCAT satellite. This spinning antenna had been providing near-real-time ocean- surface wind speed and direction data over 90% of the global ocean every day. 15)
• On November 23, 2009, the rotating scatterometer antenna mechanism of the SeaWinds instrument stopped spinning after more than a decade of operations, leaving weather forecasters without a critical tool to measure winds inside distant hurricanes. QuikSCAT has been used as an operational resource by meteorologists around the world. It has proven particularly invaluable in gaging the location, size and strength of hurricanes in the open ocean, far from land-based radars and outside the range of reconnaissance aircraft. During its nominal mission, QuikSCAT was a primary data source for science applications and studies involving climate models, interactions between the atmosphere and ocean, and weather/climate phenomena such as hurricanes and El Niño. 16) 17) 18)
The QuikSCAT mission was launched with a two-year mission goal. Its radar instrument spin mechanism was designed to last five years. - All attempts to restart the antenna failed so far. Should engineers be unable to restart the antenna, QuikSAT will be unable to continue its primary science mission, as the antenna spin is necessary to estimate wind speed and direction and form the wide data swath necessary to obtain nearly global sampling.
• The QuikSCAT mission is in its extended mission phase and is operational in 2009 (>10 years of operations). The NASA budget provides for continuing the QuikSCAT mission through FY2009.
Since 2007, NASA and NOAA are trying hard to get a budget for a QuikSCAT follow-on mission. The vector wind data has become a potent tool of the weather services to track severe weather systems. In 2009 the perspectives are that a follow-on mission cannot be launched before 2013.
• In 2006, the QuikSCAT spacecraft, instrument and ground system continue to function well and are meeting mission requirements. The aging of the scatterometer antenna bearings is the highest mission risk. 19)
QuikSCAT has revolutionized the analysis and short-term forecasting of winds over the oceans at the NOAA Ocean Prediction Center (OPC). The success of QuikSCAT in OPC operations is due to the wide 1800 km swath width, large retrievable wind speed range ( 0 to in excess of 30 m/s), ability to view QuikSCAT winds in a comprehensive form in operational workstations, and reliable near-real-time delivery of data. Prior to QuikSCAT, marine forecasters at the OPC made warning and forecast decisions over vast ocean areas based on a limited number of conventional observations or on the satellite presentation of a storm system. Today, QuikSCAT winds are a heavily used tool by OPC forecasters. 20)
• Starting in early 2002, the US and Europe integrated the scatterometer data of SeaWinds into their operational global weather analysis and forecast systems. 21)
• In the fall of 2003, NASA extended the on-orbit operations of the QuikSCAT satellite - based on its consistent performance in delivering important weather data to users around the world. 22)
• The QuikSCAT spacecraft was commissioned (i.e., declared operational) on July 27, 1999.
As of mid-2007, US forecasters are nervous because QuikSCAT is now operating on its backup downlink transmitter - and the Bush Administration has no plans to develop a backup or replacement for the mission. 23)
Figure 5: QuikSCAT spacecraft in the thermal vacuum chamber at BATC (image credit: BATC)
Sensor complement: (SeaWinds)
The SeaWinds instrument is also referred to as NASA Scatterometer-II (NSCAT-II). PI: M. H. Freilich, NASA/JPL and Oregon State University, Corvallis, OR). Objective: to acquire accurate, high-resolution, global measurements of sea-surface wind vectors in 1 to 2 day repeat cycles and fast delivery of its data. Applications: studies of tropospheric dynamics and air-sea interaction processes, including air-sea momentum transfer. 24) 25) 26) 27)
The instrument is an active microwave radar (a conically scanning pencil-beam scatterometer) with dual-beam, 40º (inner beam) and 46º look angle from nadir (outer beam), conical scan 1 m diameter reflector (rotating dish) antenna, operating in Ku-band at 13.402 GHz (nominal peak power of pulse =110 W, 189 Hz PRF (Pulse Repetition Frequency).
Measurement technique: The SeaWinds instrument transmits microwave pulses to the ocean surface and measures the backscattered power received. The sea surface radar cross-section, referred to as ”σo”, is measured for several different azimuth angles and for both horizontally and vertically polarized radiation. The wind vector is retrieved by fitting these measurements to the NSCAT-2 geophysical model function that describes the expected σo as a function of wind speed, wind direction relative to the look angle, and the incidence angle.
The antenna is conically scanned such that each point on the Earth within the inner 700 km of the swath is viewed from four different azimuth directions (twice by the inner beam looking forward then aft and twice by the outer beam in a similar fashion). Measurement of wind speeds between 3-20 m/s to an accuracy of 2 m/s, wind vector directions to an accuracy of 20º. The dish antenna is rotated about the satellite nadir axis at 18 rpm. Data is collected in a continuous 1800 km swath, centered about nadir (about 400,000 measurements daily covering 90% of Earth's surface). Spatial resolution = 50 km; FOV = ±52º from nadir.
Instrument mass = 205 kg; power = 250 W (orbital average); duty cycle = 100%; average data rate = 40 kbit/s; thermal operating range is 5-40ºC; pointing knowledge to 500 arcseconds. - SeaWinds data products consist of global multiazimuth normalized radar cross section measurements and 50 km resolution ocean vector wind maps.
Figure 6: Schematic block diagram of the SeaWinds system (image credit: Brigham Young University) 28)
Figure 7: Illustration of the dual-beam scanning geometries (image credit: NASA)
Figure 8: Alternate view of the SeaWinds dual-beam scanning concept (image credit: University of Central Florida) 29)
Figure 9: Detailed view of SeaWinds scanning geometry (image credit: Brigham Young University)
The antenna subsystem consists of a 1m diameter parabolic reflector antenna mounted to a spin activator assembly, which causes the reflector to rotate at 18 rpm. The activator assembly provides very accurate spin control and precise position or pointing information to the CDS (Command and Data Subsystem). The antenna spins at a very precise rate, and emits two beams about 6º apart, each consisting of a continuous stream of pulses. The two beams are necessary to achieve accurate wind direction measurements. The pointing of these beams was calibrated before launch for accurate echo location determination.
Figure 10: Schematic view of the SeaWinds scatterometer elements (image credit: NASA/JPL)
Table 1: Overview of SeaWinds performance parameters
Data processing in the QuikSCAT ground segment:
The QuikSCAT NRT (Near-Real-Time) processing of SeaWinds data is being provided at NOAA/NESDIS (operational since February 2000). The data processing for QuikSCAT science applications is provided by JPL. 30) 31)
Within the NGN (NASA Ground Network), the SAFS (Standard Autonomous File Server) function has been implemented to support QuikSCAT processing. SAFS is actually an intermediary between NGN and the satellite data customers whose latency requirements cannot be met by media distribution. For SAFS purposes, file latency is defined to be the time from the start of satellite data downlink to the availability of the file to the customer. Telemetry processors at the ground stations acquire raw data from these downlinks and provide data files to the SAFS system for later customer consumption.
The operational design for NGN support incorporates distributed SAFS systems at ground stations on closed networks for file acquisition from telemetry processors (TMP), and a centralized SAFS at NASA Goddard Space Flight Center (GSFC) on open networks for file distribution to project customers. The Central SAFS provides a single point of contact for customers and isolates the ground stations from customer interactions. At each ground station, multiple TMP's supporting multiple antennas and/or multiple projects, acquire downlinked satellite data that is processed into files and sent to the ground station SAFS.
Each of these SAFS systems uses FASTCopy to automatically push these files to the central SAFS, where they are made available to each project's customers. Figure 11 shows the general flow of SeaWinds QuikSCAT data from acquisition through processing. The original operational mission requirement was to produce wind retrievals in 25 km resolution Wind Vector Cells (WVC) on an orbit-by-orbit basis within three hours of observation and to make them available in BUFR (Binary Universal Form for the Representation of meteorological data) format. The MGDR (Merged Geophysical Data Record) product contains both the wind retrievals along with the σo values (radar backscatter) for each wind vector cell.
Figure 11: QuikSCAT data flow diagram (image credit: NOAA)
Figure 12: The QuikSCAT processing flow diagram (image credit: NOAA)
The OPC (Ocean Prediction Center) of NOAA is responsible for issuing marine wind warnings and forecasts of winds and seas for the extratropical High Seas and Offshore waters of the Atlantic and Pacific Oceans. OPC wind warnings and forecasts, in part, fulfill the United States requirement to provide marine warnings and forecasts under the International Safety of Life At Sea Convention. 32) 33) 34) 35)
Participants in the QuikSCAT program include the US National Centers for Environmental Prediction (NCEP), a branch of the National Weather Service (NWS), Washington DC, and the European Centre for Medium-Range Weather Forecasts (ECMWF), Reading, UK. These organizations' decision to assimilate and turn QuikSCAT data into operational information culminates an intense inter-agency and international cooperative effort among NASA, NOAA, and European countries to demonstrate and validate QuikSCAT's potential impact on weather forecasting.
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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.