Designed to Meet the Salinity Remote-sensing Challenge

In an Oceanography article published 13 years ago, three of us identified salinity measurement from satellites as the next ocean remote-sensing challenge. We argued that this represented the next " zeroth order " contribution to oceanography (Lagerloef et al., 1995) because salinity variations form part of the interaction between ocean circulation and the global water cycle, which in turn affects the ocean's capacity to store and transport heat and regulate Earth's climate. Now, we are pleased to report that a new satellite program scheduled for launch in the near future will provide data to reveal how the ocean responds to the combined effects of evaporation, precipitation, ice melt, and river runoff on seasonal and interannual time scales. These measurements can be used, for example, to close the marine hydrologic budget, constrain coupled climate models , monitor mode water formation, investigate the upper-ocean response to precipitation variability in the tropical convergence zones, and provide early detection of low-salinity intrusions in the subpolar Atlantic and Southern oceans. Sea-surface salinity (SSS) and sea-surface temperature (SST) determine sea-surface density, which controls the formation of water masses and regulates three-dimensional ocean circulation. The 1995 Oceanography paper evidently marked a turning point for salinity remote sensing. Until that time, there was no organized program for developing a satellite capability to measure this important yet sparsely observed ocean variable, in contrast to the established satellite programs to observe surface temperatures, winds, sea level, and ocean color. The paper demonstrated that newly evolving technology could provide global salinity measurements at a similar, scientifically useful accuracy and spatio-temporal resolution, and it came at a time of growing scientific awareness of the need for the data. Vigorous efforts begun in the late 1990s by the community of scientists interested in salinity measurements have led to the development of the Aquarius/SAC-D mission (Figure 1), now targeted for launch in mid 2010. This mission's principal scientific objective is to provide monthly global measurements of SSS. Here, we describe the Aquarius/SAC-D mission and how its capabilities are designed to meet the salinity remote-sensing challenge, providing the ocean research community with a preview of the measurement characteristics and scientific applications of this pioneering mission.

The AquArius/sAC-D MissioN s p e C i A l i s s u e o N s A l i N i T y In an Oceanography article published 13 years ago, three of us identified salinity measurement from satellites as the next ocean remote-sensing challenge.We argued that this represented the next "zeroth order" contribution to oceanography (Lagerloef et al., 1995) because salinity variations form part of the interaction between ocean circulation and the global water cycle, which in turn affects the ocean's capacity to store and transport heat and regulate Earth's climate.Now, we are pleased to report that a new satellite program scheduled for launch in the near future will provide data to reveal how the ocean responds to the combined effects of evaporation, precipitation, ice melt, and river runoff on seasonal and interannual time scales.These measurements can be used, for example, to close the marine hydrologic budget, constrain coupled climate models, monitor mode water formation, investigate the upper-ocean response to precipitation variability in the tropical convergence zones, and provide early detection of low-salinity intrusions in the subpolar Atlantic and Southern oceans.Sea-surface salinity (SSS) and sea-surface temperature (SST) determine sea-surface density, which controls the formation of water masses and regulates three-dimensional ocean circulation.
The 1995 Oceanography paper evidently marked a turning point for salinity remote sensing.Until that time, there was no organized program for developing a satellite capability to measure this important yet sparsely observed ocean variable, in contrast to the established satellite programs to observe surface temperatures, winds, sea level, and ocean color.The paper demonstrated that newly evolving technology could provide global salinity measurements at a similar, scientifically useful accuracy and spatio-temporal resolution, and it came at a time of growing scientific awareness of the need for the data.
Vigorous efforts begun in the late 1990s by the community of scientists interested in salinity measurements have led to the development of the Aquarius/SAC-D mission (Figure 1), now targeted for launch in mid 2010.This mission's principal scientific objective is to provide monthly global measurements of SSS.Here, we describe the Aquarius/SAC-D mission and how its capabilities are designed to meet the salinity remote-sensing challenge, providing the ocean research community with a preview of the measurement characteristics and scientific applications of this pioneering mission.Gary Lagerloef (lager@esr.org)taken into account (e.g., Font et al., 2004;Sabia et al., 2006).The impacts of SMOS instrument biases and many geophysical error sources on the final measurement accuracy continue to be investigated.1).
In conjunction with the SSIWG, parallel efforts in both Europe and United States continued to study satellite sensor concepts and mission designs that could measure salinity either as a secondary or primary objective.Further analyses provided more rigorous assessment of the technical issues and feasibility (e.g., Yueh et al., 2001).A number of field campaigns were launched to obtain new data and test retrieval models (Le Vine et al., 1998;Wilson et al., 2001;Camps et al., 2004) 2. Improving ocean rainfall estimates and global hydrologic budgets: precipitation over the ocean is still poorly known and relates to both the hydrologic budget and to latent heating of the overlying atmosphere.The "ocean rain gauge" concept shows considerable promise in reducing uncertainties in the surface freshwater flux on climate time scales, given sss observations, surface velocities, and adequate mixed-layer modeling.

Monitoring large-scale salinity events:
This effort may include ice melt, major river runoff events, or monsoons.in particular, tracking interannual sss variations in the Nordic seas is vital to long-time-scale climate prediction and modeling.high-latitude sss variations influence the rate of oceanic convection and poleward heat transport.These measurements will also be the most technically challenging because of the sss accuracy needed, and the relatively weaker radiometric signature at low sea temperature.spheric analyses or spaceborne precipitation measurements (Lagerloef, 2002;Stammer et al., 2002)    Swift" was derived more than 30 years ago from rather antiquated seawater dielectric measurements, yet has proven to be quite accurate relative to recent experiments (Wilson et al., 2004).A comprehensive new set of seawater dielectric measurements at 1.413 GHz are now being done to update the model (Lang et al., 2007).

Basic Measurement
The three Aquarius microwave radiometers will measure microwave brightness temperature in vertical and horizontal polarizations (T H and T V , respectively).The radiometers will also measure polarimetric channels to correct for the Faraday rotation of the signal as it passes through the ionosphere (Yueh, 2000).These sensors will be aligned with an offset 2.5-m-aperture antenna reflector to generate the three fixed beams at incidence angles of 28.7°, 37.8°, and 45.6°   Aquarius data processing will use the highest quality available SST fields obtained by other satellites (Donlon et al., 2007).The spread between the salinity isolines indicates the measurement sensitivity (∆T V /∆S).The salient feature is the decline in sensitivity with decreasing SST, reducing the ratio of ocean signal to the measurement noise in the higher latitude seas.This is partly offset by the increased sample density toward the poles as the ground tracks converge, allowing more samples to be averaged.
The net result is an error budget that varies with latitude (Table 2,  2 This sensor differs fundamentally from other orbiting scatterometers designed to measure ocean winds in that it does not provide multiple directional views to resolve wind direction. The coefficients a 0 , a 1 , a 2 , and a 3 are independent functions of SST and the individual beam incidence angle θ.These coefficients are presently derived by regression analysis with the simulated data and will be tuned with surface calibration during the mission.The sensitivity to wind roughness is less for T V than for T H (Camps et al., 2004).This  Aquarius science Algorithm simulation In addition to the NASA Aquarius instrument, the observatory will include five other scientific instruments and a data-relay system (Table 3).The CONAE Microwave Radiometer (MWR) will make complementary measurements of rain, wind, and sea ice with 23.8 and 36.5 GHz channels in an overlapping swath pattern (Figure 7).The MWR  refereNCe s figure 1. Artist's concept of Aquarius/ sAC-D satellite in orbit over southern patagonia.The background image is based on NAsA MoDis satellite data.
On the western side of the Atlantic Ocean, the Aquarius/SAC-D mission (http://aquarius.gsfc.nasa.gov) is being developed with the primary scientific objective to accurately measure ocean salinity from space.This program, a partnership between the United States National Aeronautics and Space Administration (NASA) and the Argentina Comisión Nacional de Actividades Espaciales (CONAE), is now planned to launch in mid 2010. 1 Although Aquarius/SAC-D is primarily designed to measure salinity, it will include several other remote-sensing measurements.These measurements include a number of CONAE and other European payloads (see below).The baseline mission is designed to operate for up to three years, with potential extended durations of two years or longer.The principal scientific objective is to make global SSS measurements over the ice-free oceans with 150-km spatial resolution, and to achieve a measurement error less than 0.2 (PSS-78 [practical salinity scale of 1978]) on a 30-day time scale, taking into account all sensor and geophysical random errors and biases.For comparison, the Global 1 NASA uses the term "Aquarius" for the mission within its ESSP (Earth System Science Pathfinder) program,where Aquarius is the prime instrument of the mission.At CONAE, the mission is referred to as "SAC-D" (Satélite Aplicaciones Científicas-D), in reference to its previous missions.The respective agency contributions are described in a later section of this article.Both agencies have committed the necessary resources to meet the recently revised launch date of May 2010, and construction phases for the instruments and satellite are on schedule.
As an indicator of the growing scientific interest, a special section of papers on ocean salinity appeared in Journal of Geophysical Research in 2002(Lagerloef, 2002).The importance of ocean salinity to understanding and predicting climate variability is further documented in the recent report of the US CLIVAR Salinity Working Group (US CLIVAR Office, 2007).The working group's activities included organizing a special session at the 2006 Ocean Sciences meeting, which was followed by an international workshop at Woods Hole, Massachusetts, in May 2006, held jointly with an Aquarius/SAC-D science workshop (Lagerloef and Schmitt, 2006).The resulting report identifies salinity as an important indicator of the strength of the hydrologic cycle because it tracks the differences created by varying evaporation and precipitation, runoff, and ice processes.These variables lead to important dynamical consequences for oceanic currents and mixing that influence the ocean's capacity to absorb, transport, and store heat, freshwater, and carbon dioxide.There is clear observational evidence in recent decades of large-scale, integrated changes in the cycling and distribution of freshwater, for example, in the form of decreasing salinity in the subpolar North Atlantic as well as in the Southern Ocean, while over the same time the near-surface salinity in the subtropics has been increasing.Many of the processes governing the role of salinity in the modulation of upper-ocean mixing in both tropical and high-latitude regions are neither well understood nor adequately represented in climate models.Model studies suggest that expanded monitoring of salinity (satellite and in situ) will improve climate forecasts on interannual to decadal time scales.The working group also recognized the limitations of historical observations and recommended the maintenance and expansion of the current in situ observing system, including the global Argo array and volunteer observing ships.In particular, they supported specific enhancements to the global observing system to improve SSS estimation by (a) expanding the Argo instrument suite to include salinity profiling within the upper 5 m to allow more precise calibration of satellite data (with optical depth of ~ 2 cm) and quantify "skin-bulk" SSS differences, and (b) developing and testing of SSS sensors for deployment on the surface drifters of the Global Drifter Program.In addition to these and other conclusions, the report emphasizes that: "The launch of Aquarius/SAC-D, as well as improvements in meteorological observing systems and models, offers the scientific community a unique opportunity to step beyond monitoring and to attempt to constrain the complete surface atmosphere/ocean hydrologic cycle based on observations." The report goes on to propose a coordinated field program in conjunction with the mission to attempt such a water budget closure experiment (see CLIVAR Salinity Working Group, this issue).Such closure studies are essential.A major unresolved problem for climate modeling and analyses is the large surface water flux adjustment required to bring standalone ocean general circulation model (OGCM) salt budgets in balance with the net air-sea flux derived from atmo-

Fourth
Assessment reports released by the Intergovernmental Panel on Climate Change (IPCC) in early 2007 verified that the Arctic is experiencing the most rapid and widespread climate change of any large region around the globe.A more recent study (Overland and Wang, 2007) predicts that the summer sea ice will disappear by mid century, some decades earlier than recently forecast by the IPCC.The late summer ice cover in September 2007 was dramatically reduced by another ~ 25% relative to the previous low record in 2005 (http://nsidc.org).These astonishing changes indicate that there will be considerably more open water in the Arctic Ocean than was ever envisioned when Aquarius/SAC-D and SMOS were conceived.The dwindling ice cover suggests that the ice-albedo positive feedback is rapidly taking hold (as ice covered areas with high albedo shrink, open water area with low albedo expands, absorbing more solar radiation, melting more ice cover).In the Arctic Ocean, sea ice melting and continental runoff maintain a shallow cold fresh layer, which sustains a halocline that separates the surface layer from warmer and saltier water below.The recent trends raise several timely scientific questions to which Aquarius/ SAC-D data can be applied, such as: Will the halocline break down where the ice cover recedes?Will there be measurable open-water salinity and temperature changes at the surface?How will these influence the summer melting rate and the re-forming of seasonal winter ice?What is the fate of the summer melt water?The Aquarius/SAC-D data coverage will extend to about 88°N latitude.It is increasingly evident that the Aquarius/SAC-D mission comes at a time when we are continuing to observe and identify significant changes in the climate system related to ocean salinity.The potential scientific applications are already beginning to reach beyond the still-important oceanographic science objectives first put forth by the SSIWG.Aquarius/SAC-D's overarching scientific goal is to quantify and understand the linkages among ocean circulation, the global water cycle, and climate by accurately measuring surface salinity.

figure 2 .
figure 2. The Aquarius/sAC-D mission concept.The satellite will be in a polar sun-synchronous orbit crossing the equator at 6 p.m. (ascending) and 6 a.m.(descending) local time.The Aquarius sensor views continually away from the sun to avoid signal contamination from solar l-band energy flux.The three beams vary in width and incidence angle to form a 390-km-wide ground swath.The orbit has an exact repeat ground track every seven days, with 390-km track spacing at the equator, ensuring complete global coverage of the swath.salinity data calibration and validation will be based on available in situ surface measurements by ships, buoys, and Argo floats.

figure 3 .
figure 3. illustration of the basin-scale spatial resolution provided by the Aquarius footprint dimensions.Top: snapshot of a 1/8-degree ocean General Circulation Model sea surface salinity (sss) field.Middle: The same field with a 150-km Gaussian filter applied to simulate the Aquarius spatial resolution, removing much of the eddy-scale structure while preserving good spatial resolution of basin-and gyre-scale structures.Bottom: Mean annual sss from World ocean Atlas 2005.
relative to the ocean surface to form the three distinct footprints aligned across the ~ 390-km swath width (Figure2).A detailed technical description of the instrument is given by LeVine et al. (2007).The Aquarius microwave radiometers have very exacting requirements for low noise and calibration stability, and will be the most accurate (by about an order of magnitude) ever developed for Earth remote sensing.

figure 4 .
figure 4. The Aquarius beam pattern superimposed on a bathymetry map of the labrador sea.Colors indicate the three Aquarius beams: inner (green), middle (red), and outer (blue).Ascending tracks are oriented southeast to northwest.

Figure 5
Figure 5 illustrates the basic model function showing T V as a function of SST for constant values of open ocean salinity (32 to 37) and a flat surface (no waves) at the middle incidence angle.The variation is nonlinear with temperature, but the gradient with SSS is quite linear at any given SST.Note that SSS is determined unambiguously from T V as long as SST is independently known.
figure 5. 1.413 Ghz v-polarized brightness temperature (T V ) as a function of sea surface temperature (ssT) for constant values of open ocean salinity (32 to 37) and a flat surface (no waves), when viewed at the 37.8° incidence angle (Aquarius middle beam), based on the klein and swift (1977) model.The family of curves is very similar for the other incidence angles and polarization, but the ordinate scales are offset.
geophysical error sources are added, and then the inverse calculation is performed to compare with the input SSS field.The difference field shown in Figure 6 simulates one complete seven-day cycle.As expected, based on the model function and Table 2, the smallest errors appear in the tropical and mid latitudes, and the largest in the subpolar regions.The global RMS is about 0.4.However, these simulated statistics apply to individual measurements (every 6 sec) along the swath and will be less than 0.2 when averaged monthly on 150-km scales.One further check is made by analyzing the impact of a long baseline instrument calibration error on a monthly gridded analysis.We also factored in a significantly reduced number of independent samples for the geophysical errors to account for their longer correlation scales.In this analytical calculation, the resultant global RMS error for a monthly gridded salinity analysis is ~ 0.18.Based on all these assessments, we are confident that the Aquarius/SAC-D observatory will do better than the required 0.2 global monthly RMS salinity error.The error tables and many of the foregoing assessments assume vertical polarization because the radiometric sensitivity to salinity is somewhat greater than for horizontal polarization at offnadir incidence angles.Nevertheless, horizontally polarized data contain useful information, and the Aquarius baseline algorithm and science simulator utilize both channels in the basic retrieval model (1).
condition allows the possible tuning of coefficients a 1 and a 2 to offset the roughness effect to some degree.W represents an independent wind parameterization that can be input based on the radar scatterometer data or data from an ancillary source such as a numerical analysis.The present simulator uses ancillary wind fields, and the "at launch" processor will do likewise until both the radiometer and radar sensors have been calibrated on-orbit and the correction algorithms tuned accordingly.The model can also be expanded to include nonlinearities and additional ancillary terms such as wave height, rain rate, or wind direction.We intend to publish a comprehensive simulated data set on the Aquarius Web site (http://aquarius.gsfc.nasa.gov)before the end of 2008.The sAC-D science payloads The Aquarius/SAC-D mission also serves many of Argentina's national goals for satellite Earth observations, and it provides a flight of opportunity for two additional European instruments.The mission is the fourth Satélite Aplicaciones Científicas (SAC) developed by Argentina in partnership with the United States.The SAC-D scientific objectives are to conduct local measurements over Argentina and contribute to global investigations of the atmosphere, the oceans, and the effects of human and natural processes on the environment as defined in the Strategic Plan of the Argentine National Space Program.

figure 6 .
figure 6. preliminary results of the Aquarius science algorithm simulator over a seven-day orbit cycle, showing root mean square retrieval errors (top) between the reference (middle) and simulated retrieval (bottom).key assumptions and inputs: solar max conditions, worst-case faraday rotation, radiometer six-second average sensor noise of 0.08 k, ssT input error 0.3°C, wind speed error 1.0 m/s.
data will be used by the Aquarius data processor for rain and sea-ice flags, and as supplementary rain and surface windspeed correction algorithms.The New InfraRed Scanner Technology (NIRST) camera is a narrow swath imager intended to detect forest fires and other thermal events on land.It can be tilted to observe preferred targets, and on occasion will be used to map SST within one Aquarius footprint.A High Sensitivity Camera (HSC) is designed to make nighttime images of urban lighting and fires, as well as aurora events.The data collection system (DCS) relays scientific data from autonomous ground stations (similar to Argos).Also included among the instruments on SAC-D is a GPS atmospheric occultation experiment called ROSA, for measuring atmospheric temperature and humidity profiles, which will be provided by the Italian Space Agency (ASI) and the ICARE/ SODAD instrument, called CARMEN1, provided by the French Centre National d'Études Spatiales (CNES) to measure the radiation and microparticle environment in space.Data processing, Distribution, and science education NASA and CONAE will share the data processing and distribution activity.CONAE will manage the telemetry from the satellite and data processing and distribution from the SAC-D instruments.The raw Aquarius data will be transmitted to the NASA Goddard Space Flight Center in Greenbelt, Maryland, where the Aquarius data processing system (ADPS) will generate salinity products and provide access for the science community.The eventual Aquarius data archive will be with NASA's Physical Oceanography Distributed Active Archive Center (PO-DAAC) at the Jet Propulsion Laboratory.The Aquarius validation data system (AVDS) is being developed to accumulate and format in situ SSS and SST data from a variety of platforms such as Argo floats, volunteer observing ships, research ships, moored buoys, and drifting buoys.This consolidated AVDS data set will be tailored to calibrate and validate the Aquarius data, and will also be available to the science community as an independent resource.Aquarius will collect data continuously over all ocean, land, and ice surfaces in its field of view.The ADPS, and eventually PO-DAAC, will retain all lower-level data for use by researchers in other disciplines.Preliminary plans are also developing for the ADPS to process a land soil moisture data set with algorithms provided by a separate working group.

figure 7 .
figure 7. schematic illustration of Argentina's Comisión Nacional de Actividades espaciales microwave radiometer swaths overlapping with Aquarius along the respective ground tracks.The microwave radiometer has eight offset 45-km beams arrayed across two 390-km swaths, with the 36.5 Ghz channel swath ahead and the 23.8 Ghz channel behind the Aquarius swath as the satellite moves along the orbit.The red box symbolizes the New infrared scanner Technology field of view.
is Principal Investigator, NASA Aquarius Mission, Earth & Space Research, Seattle, WA, USA.F. Raul Colomb is SAC-D Principal Investigator for the Argentine Space Agency, Comisión Nacional de Actividades Espaciales (CONAE), Buenos Aires, Argentina.David Le Vine is Deputy Principal Investigator, NASA Aquarius Mission, Instrument Sciences Branch, NASA Goddard Space Flight Center, Greenbelt, MD, USA.
is Senior Science Educator, Bigelow Laboratory for Ocean Sciences, West Boothbay Harbor, ME, USA.Gene Feldman is Program Manager, SeaWiFS, NASA Goddard Space Flight Center, Greenbelt, MD, USA.Calvin Swift is Professor, Electrical and Computer Engineering, University of Massachusetts, Amherst, MA, USA.

. Improving seasonal to interannual [ENSO] climate predictions: This
effort involves the effective use of sss data to initialize and improve the coupled climate forecast models, and to study and model the role of freshwater flux in the formation and maintenance of barrier layers and mixed layer heat budget in the tropics.

Table 2 .
Aquarius salinity retrieval error TableThe margin is the additional root sum square (rss) error that could be included before exceeding the allocation.The bottom portion shows the monthly root mean square (rMs) salinity error by latitude band based on the total error allocation of 0.38 k for each data point.These figures include the measurement sensitivity as it varies with sea surface temperature (ssT) and the number of samples averaged per latitude band.The global rMs error allocation over the latitude bands is 0.2 (pss-78) and CBe 0.14 (pss-78).