Global monitoring of Sea Surface Salinity with Aquarius

Abstract

Aquarius is a microwave remote sensing system designed to obtain global maps of the surface salinity field of the oceans from space. It will be flown on the Aquarius/SAC-D mission, a partnership between the USA (NASA) and Argentina (CONAE) with launch scheduled for late in 2008. The objective of Aquarius is to monitor the seasonal and interannual variation of the large scale features of the surface salinity field in the open ocean. This will provide data to address scientific questions associated with ocean circulation and its impact on climate. For example, salinity is needed to understand the large scale thermohaline circulation, driven by buoyancy, which moves large masses of water and heat around the globe. Of the two variables that determine buoyancy (salinity and temperature), temperature is already being monitored. Salinity is the missing variable needed to understand this circulation. Salinity also has an important role in energy exchange between the ocean and atmosphere, for example in the development of fresh water lenses (buoyant water that forms stable layers and insulates water below from the atmosphere) which alter the air-sea coupling. Aquarius is a combination radiometer and scatterometer (radar) operating at L-band (1.413 GHz for the radiometer and 1.26 GHz for the scatterometer). The primary instrument,for measuring salinity is the radiometer which is able to detect salinity because of the modulation salinity produces on the thermal emission from sea water. This change is detectable at the long wavelength end of the microwave spectrum. The scatterometer will provide a correction for surface roughness (waves) which is one of the greatest unknowns in the retrieval. The sensor will be in a sun-synchronous orbit at about 650 km with equatorial crossings of 6am/6pm. The antenna for these two instruments is a 3 meter offset fed reflector with three feeds arranged in pushbroom fashion looking away from the sun toward the shadow side of the orbit to minimize sunglint. The mission goal is to produce maps of the salinity field globally once each month with an accuracy of 0.2 psu and a spatial resolution of 100 km. This will be adequate to address l&ge scale features of the salinity field of the open ocean. The temporal resolution is sufficient to address seasonal changes and a three year mission is planned to-collect sufficient data to look for interannual variation. Aquarius is being developed by NASA as part of the Earth System Science Pathfinder (ESSP) program. The SAC-D mission is being developed by CONAE and will include the space craft and several additional instruments, including visible and infrared cameras and a microwave radiometer to monitor rain and wind velocity over the oceans, and sea ice.

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Global monitoring of Sea Surface Salinity with Aquarius

G.S.E. Lagerloef1, D.M. Le Vine2, Y. Chao3, R. Colomb4, I. Nollman5

1. Earth and Space Research, 1910 Fairview Ave E, Suite 210, Seattle WA 98102-3620 E-mail:
lager@esr.org
2. Goddard Space Flight Center Greenbelt, MD, 20771 E-mail: David.M.LeVine@nasa.gov
3. Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109-8099 E-mail:
Yi.Chao@jpl.nasa.gov
4. Comisión Nacional de Actividades Espaciales (CONAE), Buenos Aires, AR E-mail:
rcolomb@conae.gov.ar
5. Comisión Nacional de Actividades Espaciales (CONAE) , Buenos Aires, AR E-mail:
nollmann@conae.gov.ar

ABSTRACT

Aquarius is an L-band microwave instrument to make precise sea surface salinity measurements from
space. The objective is to study interactions between the ocean circulation, water cycle and climate by
monitoring large scale seasonal and interannual salinity variations in the open ocean. Aquarius combines
an ultra-stable radiometer as the primary instrument for measuring salinity with a scatterometer to provide a
surface roughness correction. The instrument, which is being developed under NASA’s Earth System
Science Pathfinder program, is part of the Aquarius/SAC-D mission, a partnership between the USA
(NASA) and Argentina (CONAE).

INTRODUCTION

Aquarius is a microwave remote sensing system designed to obtain global maps of the surface salinity field
of the oceans from space. It will be flown on the Aquarius/SAC-D mission, a partnership between the
USA (NASA) and Argentina (CONAE) with launch scheduled for late in 2008 or early 2009. The
objective of Aquarius is to monitor the seasonal and interannual variation of the large scale features of the
surface salinity field in the open ocean. This will provide data to address scientific questions associated
with ocean circulation and its impact on climate. For example, salinity is needed to understand the large
scale thermohaline circulation, driven by buoyancy, which moves large masses of water and heat around
the globe. Of the two variables that determine buoyancy (salinity and temperature), temperature is already
being monitored. Salinity is the missing variable needed to understand this circulation. Salinity also has an
important role in energy exchange between the ocean and atmosphere, for example in the development of
fresh water lenses (buoyant water that forms stable layers and insulates water below from the atmosphere)
which alter the air-sea coupling.

Aquarius is a combination radiometer and scatterometer (radar) operating at L-band (1.413 GHz for the
radiometer and 1.26 GHz for the scatterometer). The primary instrument for measuring salinity is the
radiometer which responds to salinity because of the modulation salinity produces on thermal emission
from sea water. This is illustrated in Figure 1 which shows brightness temperature at L-band (vertical axis)
for an ideal (flat) surface as function of sea surface temperature (SST) for constant values of salinity. The
response to salinity decreases rapidly with frequency and is quite small at C-band. The scatterometer will
provide a correction for surface roughness (waves) which is one of the greatest unknowns in the retrieval.

PARTNERSHIP

This remote sensing mission is a partnership between the USA space agency, NASA, and the Argentine
space agency, CONAE. The mission is called Aquarius/SAC-D. It consists of two parts: (a) Aquarius, the
radiometer/scatterometer instrument for measuring salinity, which is being developed by NASA as part of
the Earth System Science Pathfinder (ESSP) program; and (b) SAC-D, the forth spacecraft in the CONAE
Satelite Aplicaciones Cientificas (SAC) program. NASA will provide the launch vehicle (Delta-II) and
CONAE the data link. In addition to the SAC-D spacecraft, CONAE will be providing several additional

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remote sensing instruments including visible and infrared cameras and a microwave radiometer to monitor
rain and wind velocity over the oceans, and sea ice. These are listed in Table I.

Table I: SAC-D Instruments

Instrument
Objective


MWR: Microwave
Radiometer ice concentration, water vapour polarized; 390 km swath

NIRST: New IR
Sensor Technology
Sea Surface Temperature
Swath: 180 km;

HSC: High
Sensitivity Camera
Snow cover Swath: 700 km;

DCS: Data
Collection System

ROSA: Radio
Occultation Sounder
for Atmosphere

CARMEN 1:
(ICARE and
SODAD)
SODAD: Distribution micro-
particles and space debris
sensors

TDP: Technology
Demonstration Pkg Inertial angular velocity
Inertial reference unit



Figure 2 shows the Aquarius/SAC-D observatory in its deployed configuration. The Aquarius instrument
package consists of a 2.5 meter offset fed reflector with three beams. It is the large structure to the left in
Figure 2. Aquarius is mounted at the end of the SAC-D opposite the solar panels. SAC-D and the several
instruments in Table I are the cylindrical structure in Figure 2.

AQUARIUS

Aquarius is a combination radiometer and scatterometer (radar) operating at L-band (1.413 GHz for the
radiometer and 1.26 GHz for the scatterometer). The antenna is an offset fed reflector with three feeds that
are shared by the two instruments. The feeds produce three beams that image pushbroom-style. The beams
look to the right across-track (i.e. 90 degrees with respect to the spacecraft heading) with look angles
(direction measured at the spacecraft) of 25.8, 33.8 and 40.3 degrees. The sensor will be in a sun-
synchronous orbit at an altitude of about 657 km with equatorial crossings of 6am/6pm (ascending at 6
pm). Thus, the spacecraft will fly near the day-night terminator and the three beams are positioned to look
away from the sun toward the shadow side of the orbit to minimize sun glint.

The three beams are positioned to map a swath of approximately 390 km and the orbit will produce global
coverage in a 7 day exact repeat cycle. To reduce measurement noise, the weekly maps will be averaged to
produce maps of the salinity field globally once each month with an accuracy of 0.2 psu and a spatial
resolution of 150 km. The spatial resolution will be adequate to address large scale features of the salinity
field of the open ocean. The temporal resolution is sufficient to address seasonal changes. A three year
mission is planned to collect sufficient data to look for interannual variations.

Description Resolution Source


CONAE


CONAE


40 km


350 meters

Precipitation, wind speed, sea

23.8 GHz and 37 GHz; Dual

Hot spots (fires);

Bands: 3.8, 10.7 and 11.7 µm

Urban lights; Lightning

Bands: 450-900 µm;

200-300 m

CONAE

Environmental data collection

Band: 401.55 MHz uplink

2 contacts/day w
200 platforms

Hor: 300 km
Vert: 300 km

CONAE

Properties of Atmospheric

GPS occultation

ASI
(Italy)

ICARE: Effect of cosmic
radiation on electronics;

ICARE: Three depleted Si
and Si/Li detectors
SODAD: Four SMOS

I: 256 channels
S: 0.5 µ at 20
km/s sensitivity

CNES
(France)

Position, velocity and time;

GPS receiver;

20 meters
1-2 m/sec

CONAE

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Details of the Aquarius mission and sensor system are listed in Table II and Figure 3 shows one of the
antenna feed systems with the radiometer mounted around OMT. There are three radiometers, one for each
of the beams (feeds). The radiometers are fully polarimetric. They make measurements at horizontal
polarization, vertical polarization and at ± 45 degrees. The 3rd Stoke’s parameter (obtained from the later
measurement) will be used to derive a correction for Faraday rotation [2]. The internal radiometer sample
interval is 10 ms and a pair of noise diodes are used for internal calibration. The design is quite
demanding, calling for a ∆T = 0.06K and stability of better than 0.15K over 7 days [3]. There is one
scatterometer which is dual polarized and samples sequentially among the three beams. The radar
transmits between the radiometer samples (i.e. PRF = 100 Hz).


Table II: Parameters of the Aquarius Mission

Orbit

Altitude 657 km Main Reflector
Sun-synch 6 pm ascending Beams
Inclination 98 degrees Resolution
Coverage 7 days global Swath


Radiometer

Frequency 1.413 GHz Frequency
Polarization Fully Polarimetric Polarization
Sample time 20 ms PRF
Integration 6 seconds Pulse width


REFERENCES

[1]. L.A. Klein and C.T. Swift, “An improved model for the dielectric constant of sea water at microwave
frequencies”, IEEE trans. Antennas Propag., Vol AP-25, pp 104-111, 1977.
[2]. S.H. Yeuh, “Estimates of faraday rotation with passive microwave polarimetry for microwave remote
sensing of earth surfaces”, IEEE trans. Geosci Remote Sens., Vol 38 (#5), pp 2434-2438, 2000.
[3]. F.A. Pellerano, K.A. Horgan, W.J. Wilson, A.B. Tanner, “Development of a high-stability microstrip-based L-
band radiometer for ocean salinity measurements”, Internat. Geosci and Remote Sens Sympos. IGARSS '04.
Proceedings, Volume 2, pp 774 – 776, Sept., 2004.
















Fig 1: Level curves of constant salinity as function of sea surface temperature (abscissa) and microwave brightness
temperature (ordinate) for L-band (1.4 GHz) and normal incidence. The model for dielectric constant in [1] was used
to obtain these curves.

Antenna
2.5 m offset fed
25.8, 33.8, 40.3 degrees (at spacecraft)
42, 50, 61 km (equiv radius)
390 km


Scatterometer




1.26 GHz
HH, VH, HV, VV
100 Hz
1 ms (BW = 4 MHz)

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Fig 2: The Aquarius/SAC-D hardware in the deployed configuration. SAC-D is the spacecraft (cylindrical structure
plus solar panels) and also includes the several instruments listed in Table I. Aquarius consists of the large offset-fed
reflector and feed system mounted to the left at the end of the spacecraft.





























Fig 3: Drawing showing the Aquarius antenna feed system with the radiometer hardware mounted around the
orthomode transducer (OMT).