1. Aquarius Instrument

2. Aquarius Radiometer • Aquaruis A microwave instrument designed to sense the salinity of the oceans. • Objective To investigate the effects on climate and buoyancy driven ocean circulation (thermohaline) due to variations in ocean salinity. [1] • Measuring salinity dates back to 1970’s.

3. Mission Sponsors • The Aquarius radiometer is the result of collaboration between two space agencies; NASA and CONAE. • The Argentine space program will be able to monitor the environment as well as detect hazards. • Launched from NASA’s Western Test Range using a Boeing Delta-II launch vehicle [1].

4. Importance of Ocean Salinity • Fresh water lenses Affect the exchange of energy between the atmosphere and the ocean, thus impacting monsoons and El Nino- Southern Oscillations.[1] • Helps track the global hydrologic cycle 86 % of total evaporation and 78% of total precipitation occur over the ocean.

5. Measurement Physics • Changes in salinity affect the emissivity of ocean water resulting in a change of about 0.5 K/psu(at L-band). • Operation in the L-band Radiometric sensitivity drops quickly above ~ 1 GHz.

6. Aquarius Specifications • The system is capable of completely mapping the Earth’s oceans (ice-free) in just seven days. • Achieve an average accuracy of 0.2 psu (0.1 K) and a spatial resolution of 150 km (limited by antenna size). Table 1. Parameters of the Aquarius Radiometer [1]

7. Limiting Factors • Faraday rotation Change/rotation of an electromagnetic wave’s polarization vector due to a magnetic field. • Surface Roughness (waves) Impacts the emissivity of the ocean and thus its brightness temperature.

8. Microwave Radiometry • The Rayleigh-Jean approximation can be used to determine the thermal radiation of a blackbody. Figure 1. Simplified geometry for remotely sensing the surface of a body of water.[1]

9. Ocean Water Emission • Brightness temperature is a function of the physical temperature of the ocean as well as salinity. • If we know TB and Tphys we can determine salinity. Figure 2. A plot relating brightness temperature to the physical temperature of sea water assuming a normal incidence angle [1]

10. Brightness Temperature Sensitivity • Brightness temperature is also a function of frequency, polarization and angle of incidence. • Sensing at ~ 600 MHz would be ideal, but is impractical due to manmade noise and limited BW. Figure 3. Magnitude of the sensitivity to changes in brightness temperature[1]

11. Minimizing Sources of Error • Antennas are directed so as to point away from the Sun. Solar flares can also increase the error in our measurements. • The presence of land will decrease the accuracy of our measurement. Figure 4. Remote sensing geometry.

12. Minimizing Error Due to Surface Roughness • The most difficult source of error to correct is surface roughness. • Accurately characterizing a rough surface is difficult. • A scatterometer (1.26 GHz) is used to measure the backscatter and with this data as well as data on wind speed from other satellites and weather models, it is possible predict surface roughness.

13. Man Made Interference • Computers, air traffic control radars, digital measuring equipment, etc… • Rapidly sample and retain only those samples that are least contaminated with RFI before determining the final average.

14. Aquarius/SAC-D Observatory Figure 4. Aquarius radiometer in its deployed configuration.[1] • The radiometer and scatterometer use the same feed. • The polarimetricchannel allows the radiometer to correct for Faraday rotation. • The dish-like collar is a sun shield (thermal control).

15. References [1] D. M. Le Vine, G. S. Lagerloef, and S. E. Torrusio, “Aquarius and Remote Sensing of Sea Surface Salinity from Space,” Proc. Vol. 97, No. 5, May 2010