P. de Matthaeis

The Aquarius Simulator and Cold-Sky Calibration

A numerical simulator has been developed to study remote sensing from space in the spectral window at 1.413 GHz (L-band), and it has been used to optimize the cold-sky calibration (CSC) for the Aquarius radiometers. The celestial sky is a common cold reference in microwave radiometry. It is currently being used by the Soil Moisture and Ocean Salinity satellite, and it is planned that, after launch, the Aquarius/SAC-D observatory will periodically rotate to view “cold sky” as part of the calibration plan. Although radiation from the celestial sky is stable and relatively well known, it varies with location. In addition, radiation from the Earth below contributes to the measured signal through the antenna back lobes and also varies along the orbit. Both effects must be taken into account for a careful calibration. The numerical simulator has been used with the Aquarius configuration (antennas and orbit) to investigate these issues and determine optimum conditions for performing a CSC. This paper provides an overview of the simulator and the analysis leading to the selection of the optimum locations for a CSC.

Aquarius Active/Passive RFI Environment at L-Band

Active/Passive instrument combinations (i.e., radiometer and radar) are being developed at L-band for remote sensing of sea surface salinity and soil moisture. Aquarius is already in orbit and SMAP is planned for launch in the Fall of 2014. Aquarius has provided for the first time a simultaneous look at the Radio Frequency Interference (RFI) environment from space for both active and passive instruments. The RFI environment for the radiometer observations is now reasonably well known and examples from Aquarius are presented in this manuscript that show that RFI is an important consideration for the scatterometer as well. In particular, extensive areas of the USA, Europe and Asia exhibit strong RFI in both the radiometer band at 1.41 GHz and in the band at 1.26 GHz employed by the Aquarius scatterometer. Furthermore, in areas such as the USA, where RFI at 1.4 GHz is relatively well controlled, RFI in the scatterometer band maybe the limiting consideration for the operation of combination active/passive instruments.

Aquarius: Status and recent results

Aquarius is a combination active/passive instrument at L band designed to map sea surface salinity globally from space. The radiometer (passive) is the primary instrument for retrieving salinity, and the scatterometer (active) provides information to correct for a major source of error, sea surface roughness (waves). In addition, the radiometer includes a number of special features designed to meet the goal for this challenging measurement, including measurement of the third Stokes parameter to help with the correction for Faraday rotation and rapid sampling to help with the mitigation of radio frequency interference. Aquarius was launched on 10 June 2011 aboard the Aquarius/SAC-D observatory and has been working well. The salinity retrieval continues to improve, and the special features suggest the potential for new applications of remote sensing from space at L band.

Impact of Antenna Pattern on Measurement of the Third Stokes Parameter From Space at L-Band

The third Stokes parameter will be observed from space for the first time at L-band by the Soil Moisture and Ocean Salinity and Aquarius/SAC-D satellites. The correlation between polarizations, which is the source of the third Stokes parameter, is of interest at L-band to measure Faraday rotation and also to indicate novel features of the surface. However, spurious signals (false indication of correlation) can occur in the third Stokes parameter. For example, this happens when the radiometer crosses boundaries associated with a large change in brightness temperature, such as land-water boundaries. In this paper, calculations with the Aquarius radiometer antennas will be used to show that these spurious signals are due to the cross-polarization coupling and large beamwidth associated with realistic L-band antennas in space.

Comparison of Surface and Volume Currents Models for Electromagnetic Scattering From Finite Dielectric Cylinders

The accuracy of modeling plane wave scattering by dielectric cylinders of finite length and circular cross section for microwave remote sensing applications is investigated. Exact expressions for their scattering cross section do not exist, leading to the use of approximate analytical methods. Two widely used models, based on the approximation of the induced currents, are considered here, and the validity of their results is evaluated by comparison with a corresponding numerical solution.

Aquarius overview and up date

Aquarius is an L-band instrument designed to map the surface salinity field of the global oceans. It consists of three L-band (1.41 GHz) radiometers and an L-band (1.26 GHz) scatterometer. The radiometers are the primary instruments for measuring salinity and the scatterometer provides a correction for surface roughness. Aquarius was launched in June 2011 and has been mapping the surface salinity field since it was turned on in August. In addition, Aquarius is now producing maps of radio frequency interference (RFI), Faraday rotation and soil moisture.

Aquarius RFI detection and mitigation

Aquarius is an L-band instrument designed to map sea surface salinity from space. Monitoring salinity from space is a particularly sensitive measurement and RFI is a concern, even in the protected band at 1.4 GHz where the Aquarius radiometers operate. To protect against RFI, the Aquarius radiometer samples rapidly and a glitch detection algorithm is employed to check each sample for RFI. This strategy has worked well over oceans, but there are large areas over land, especially in Asia and Europe, where contamination by RFI affects most samples.

Spurious signal in measurement of the third Stokes parameter from space at L-band

Spurious spikes in the third Stokes parameter have been observed in numerical simulations of the signal expected from the L-band radiometers to be flown as part of the Aquarius instrument. These signals are present over scenes with large contrast such as land water boundaries and are due to cross polarization coupling and the relatively large footprint of the antennas.