Aquarius is an L-band microwave instrument being developed to map the surface salinity field of the oceans from space. It is part of the Aquarius/SAC-D mission, a partnership between the USA (NASA) and Argentina (CONAE) with launch scheduled for early in 2009. The primary science objective of this mission is to monitor the seasonal and interannual variation of the large scale features of the surface salinity field in the open ocean with a spatial resolution of 150 km and a retrieval accuracy of 0.2 psu globally on a monthly basis

Dinnat, E.

Lagerloef, G. S. E.

LeVine, D. M.

Pellerano, F.

Yueh, S.

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NASA Goddard Space night Center 
Aquarius Mission Technical Ovehiew 
D.M. Le Vine' G.S.E. Lagerloep, S. Yueh3, F. Pellerano', E. Dinnat', F. Wentz4 
1. Goddard Space Flight Center, Greenbeltw 
Email: David.M.LeVine@nasa.gov 
2. Earth and Space Research, Seattle, WA 
3. Jet Propulsion Laboratory, Pasadena, CA 
4. Remote Sensing Systems, Santa Rosa, CA 
Abstract - Aquarius is an L-band microwave instrument being 
developed to map the surface salinity field of the oceans from 
space. It is part of the AquariudSAC-D mission, a partnership 
between the USA (NASA) and Argentina (CONAE) with launch 
scheduled for early in 2009. The primary science objective of this 
mission is to monitor the seasonal and interannual variation of 
the large scale features of the surface salinity field in the open 
ocean with a spatial resolution of 150 km and a retrieval 
accuracy of 0.2 psu globally on a monthly basis. 
Key words: Microwave Remote Sensing, Ocean Salinity 
I. INTRODUCTION 
Aquarius is a combination active/passive L-band microwave 
instrument designed to map the surface salinity field of the 
oceans fiom space. It will be flown on the AquariudSAC-D 
mission, a partnership between the USA space agency 
(NASA) and Argentine space agency (CONAE) with launch 
scheduled for early 2009. CONAE is providing the service 
platform, SAC-D, the forth spacecraft in the Satelite de 
Aplicaciones Cientificas (SAC) program and a complement of 
instruments. In addition, CONAE will provide the data link 
and mission operations using the facilities at the Mission 
Operations Center (MOC) at the Cordoba ground station. In 
addition to Aquarius, NASA will provide the launch, fiom 
NASA's Western Test Range at Vandenberg AFB, and the 
launch vehicle, Delta-II. Aquarius data will be sent from the 
MOC to the Goddard Space Flight Center for processing 
where salinity products will be produced. The salinity maps 
will be distributed to the public and eventually archived at the 
Physical Oceanography DAAC at the Jet Propulsion 
Laboratory. 
11. AQUARIUS CIENCE OBJECTIVES 
The primary objective of the Aquarius instnunent is to 
monitor the seasonal and interannual variation of the large 
scale features of the d a c e  salinity field in the open ocean. 
This will provide data to better understand 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. Salinity also plays an important role in 
energy exchange between the ocean and atmosphere (for 
example in the formation of stable buoyant layers at the 
surface). Salinity is also a surrogate for fieshwater flux. Zn 
particular, changes in surface salinity reflect changes in 
fieshwater (e.g. evaporation and precipitation, and also river 
run-off and ice melt). Hence, a map of the global salinity field 
will also help to better understand the global water cycle and 
how it is changing. The time and spatial scale of observations 
needed to improve understanding of these processes are 
relatively long but the salinity changes are relatively smdl 
(tenths of a psu) compared, for example, to processes in the 
coastal ocean. The goal for the Aquarius instrument is to 
provide global maps of the sea surface field in the open ocean 
on a monthly basis with an average accuracy of 0.2 psu and at 
a spatial resolution of 150 km. 
111. AQUARIUS INSTRUMENT 
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 prominent feature of 
this instrument is the antenna, a 2.5-m offset parabolic 
reflector with three feed horns. The three beams are arranged 
to image in pushbroom fashion pointed across track (roughly 
90 degrees with respect to the spacecraft heading) at incidence 
angles 25.8, 33.8 and 40.3 degrees with respect to the satellite 
nadir. The local incidence angles at the surface at beam center 
are 28.7, 37.8 and 45.6 degrees, respectively. Each feed is 
shared by both the radiometer and scatterometer. Each feed 
has a dedicated radiometer; however, there is only one 
scatterometer which cycles between the three feeds. The 
resolution of the three beams is approximately the same for 
the radiometer and scatterometer and ranges at the 3 dB level 
from an ellipse of 76 x 94 km for the inner beam to 97 x 157 
km for the outer beam. Together the three beams provide 
coverage of a swath of about 390 km. 
The radiometers are Dicke radiometers with noise injection for 
internal calibration (Wilson et al, 2005). The radiometer 
switching sequence between antenna, Dicke load, and noise 
diode has been optimized to maximize antenna observations 
and minimize NEDT. The radiometers measure power at 
both polarizations and also the sum and difference of the fields 
(voltages) at the two polarizations from which the third 
Stoke's parameter can be obtained. 
The scatterometer transmits a 1 ms pulse with a PRF of 100 
Hz. This is designed so that the scatterometer pulses and the 
radiometer observations of the surface are interleaved (an 
individual radiometer observation is 9 ms long for a basic 
period of 10 ms). The scatterometer measures power at VV, 
https://ntrs.nasa.gov/search.jsp?R=20070015930 2019-08-30T00:40:14+00:00Z
VH, H V ,  HH and also background noise (no transmit pulse) 
which is part of calibration. Internal calibration is 
accomplished with a loop-back circuit which couples a portion 
of the transmitted power through the receiver chain. 
The inversion from brightness temperature to salinity depends 
(except at nadir) on polarization. To help correct for this 
potential source of error, the radiometer in Aquarius includes a 
polarime~c channel and will use the measured third Stoke’s 
parameter and an 
retrieve the angle o 
a significant sourc 
2005) and to avoid 
main beam of the 
synchronous orbit near 
pm equatorial crossing) with the antenna beams poin 
toward the night time side of the orbit. Finally, the response 
to changes in salinity depends on incidence angle (the angle 
relative to the local normal to the surface). With large antenna 
footprints such as are being used with Aquarius, changes in 
incidence angle across the footprint can be important @innat 
and Le Vine, 2006). Figure 2 is an example showing the 
difference between the brightness temperature at boresight and 
the value obtained by the actual antenna (Le. after integration 
over the antenna pattern). As a consequence, careful attention 
needs to be given to the antenna pattern correction. One 
promising possibility is to use a regression derived from the 
observations in which a linear combination of the observed 
values are used to approximate the value at boresight or a 
related parameter (e.g. the equivalent TB that yields the 
average S S S  over the footprint). 
Figure 1 shows Aquarius in its stowed (left) and deployed 
(right) configuration. The parabolic reflector is deployed with 
a single fold mechanism and is the largest size that could fit 
inside the Delta-II launch vehicle fairing. The collar at the 
junction between Aquarius and the service platform (right) is a 
sun shield to help with thermal control. The feed structure 
consists of three feed-OMT assemblies mounted together near 
the focus of the reflector. The radiometer front end and 
associated electronics (diplexers that couple with the 
scatterometer, couplers for the correlated noise diode, etc) are 
mounted around the OMT and temperature controlled to 
provide a thermally stable environment for the critical RF 
components. 
The parameters of the mission and the Aquarius sensor system 
are listed in Table I. The sensor will be in a sun-synchronous 
orbit with a 6 pm (ascending) equatorial crossing and altitude 
of 657 km. This orbit places the spacecraft close to the day- 
night terminator and the three beams will point to the night- 
time side to avoid sun-glint and to provide good coverage of a 
potential calibration site in the Antarctic. The 390 km swath 
will permit complete coverage of the globe in 7 days. The 
goal is monthly maps and, in order to achieve an accuracy of 
0.2 psu, averaging is necessary to reduce noise (e.g. Yeuh et 
al, 2001). An orbit with a 7-day repeat cycle has been selected 
to assist in the averaging. 
v. REFERENCES 
Camps, A.; Font, J.; Vall-llossera, M.; et al., “The WISE 2000 and 2001 
field experiments in support of the SMOS mission: sea surface L-band 
brightness temperature observations and their application to sea surface 
salinity retrieval”, IEEE Trans. Geosci. Remote Sens., 42 (#4),804 - 
823,2004. 
Dinnat, E.P. and D.M. Le Vine, “Effects of the Antenna Aperture on 
Remote Sensing of Sea Surface Salinity at L-band”, Proc. MicroRad06, 
Feb28-Mar3, San Juan, Puerto Rico, 2006. 
Etcheto, J., E. innat, J. Boutin, A. Camps, J. iller, S. Contardo, J. 
Wesson, J.Font and D.Long, “Wind speed effect on L-band brightness 
temperature inferred from EuroSTARRS and WISE2001 field 
experiments”, IEEE Trans. Geosci. & Remote Sens., vol. 42, no. 10, 
Oct, 2004. 
Klein, L.A. and C.T. Swift, “An improOved model for the dielectric 
constant fo sea water at microwave frequencies”, IEEE Trans. Antennas 
and Propag., Ap-25, 104-1 1 1, 1977. 
Lagerloef, G.S.E, C. Swift, D. Le Vine, 1995, Sea surface salinity: The 
next remote sensing challenge, Oceanogr. 8,44-50. 
Le Vine, D.M. and S. Abraham, “The effect of the ionosphere on 
remotes ensing of sea surface salinity from space: Absorption and 
emission at L-band”, IEEE Trans. Geosci. Remote Sens., 40 (M), 771- 
782,2002. 
Le Vie ,  D.M., S. Abraham, F. Wentz, G.S.E. Lagerloef, “Impact of the 
Sun on Remote Sensing of Sea S&e Salinity from Space”, Proc. 
Internat. Geosci. & Remote Sens. Symp., IGARSSOS, Seoul Korea, July, 
2005.. 
Wilson, W. J., Tanner, A., Pellerano, F., and Horgan, K. Ultrastable 
Radiometers for Future Sea Surface Salinity Missions, Jet Propulsion 
Laboratory Report D-3 1794, April 2005. 
Yueh, S.H., “Estimates of Faraday Rotation with Passive Microwave 
Polarimetry for Microwave Remote Sensing of Earth Surfaces”, IEEE 
Trans. Geosci Remote Sens., Vol38 (#5), pp 2434-24-38,2000. 
Yueh, S.H., R West, W.J. Wilson, F.K. Li, E.G. Njoku, and Y. Rahmat- 
Samii, “Error Sources and Feasibility for Microwave Remote Sensing of 
Ocean Surface Salinity”, IEEE Trans. Geosci and Remote Sensing, Vol 
39, #S, pp 1049-1059, May, 2001. 
1. 
IV. SALINITY RETRIEVAL 
2. 
3. 
The range of salinity to be encountered in the open ocean is on 
the order of 32-37.5 psu. The associated change in brightness 
temperature is small but measurable with modern radiometers 
(Klein and Swift, 1977; Lagerloef et al, 1995), and the 
window at 1.41 3 Ghz (L-band) where Aquarius will operate, 
is very near the peak in sensitivity of brightness temperature to 
changes in salinity. On the other hand, when actually making 
the remote sensing measurement from space, there are a 
number of complicating issues that must be taken into 
account. For example, surface roughness (e.g. waves) can 
also cause changes in the observed brightness temperature, 
and with an order of magnitude comparable to the salinity 
signature (Camps et al, 2004; Etcheto et al, 2004). It is for 
this reason, to correct for the contribution of roughness, that 
Aquarius includes a scatterometer. The scatterometer 
(backscatter) responds directly to surface roughness, and in 
the design of Aquarius, the radiometer and scatterometer 
operate at nearly the same fiequency, share the same antenna 
feed and look at the same pixel with approximately the same 
footprint. Another issue to be taken into account is Faraday 
rotation (rotation of the polarization vectors as the radiation 
propagates from the surface through the ionosphere) which 
can be significant at L-band (Le Vine and Abraham, 2002). 
4. 
5. 
6. 
7. 
8. 
9. 
10. 
2 
Figure 1: Aquarius in the stowed configuration (left) and 
deployed (right). The deployed configuration shows Aquarius 
mounted on the SAC-D spacecraft. 
.. .... " .-.._ T"-"r-.--s.. *""* .... " .... * " ' * - . . ~ *  .... "G..".". II 4 Jq".-..- ... *.l."'(-.".* .... .( 
Figure 2: The difference between the brightness temperature 
obtained by the actual antenna (i.e. after integrated over the 
antenna pattern) and the value at boresight. Results at the top 
are for vertical polarization and at the bottom for horizontal 
polarization. The three curves are for the three beams: 25.8 
(red), 33.8 (green) and 40.3 (blue). There are two curves for 
each case for SSS of 31 psu and 38 psu. 
' TABLE I 
Parameters of the Aquarius Mission 
Orbit 
Altitude 657 km 
Sun-s ynchronous 6 pm ascending 
Inclination 98 deg 
Coverage 7 days global 
Radiometer 
Frequency 1.413 GHz 
Polarization Polarimetric 
Sample time 10 ms 
Integration timehample 9 ms 
NEDT (5.76 seconds) 0.06K 
Calibration Stability 0.13K 
Antenna 
Main Reflector 2.5 m offset 
Beam look angles (deg) 25.8,33.8,40.3 
Local incidence (deg) 28.7,37.8,45.6 
Res (eff radius) 
Swath 390 km 
42,50, 61 km 
Scatterometer 
Frequency 1.26 GHz 
Polarization HH, VH, HV, w 
PRF 100 Hz 
Pulse width 0.98 ms 
Calibration 0.1 dB 
3 


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