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

RFI Statistical Distribution and Missed Detection in Aquarius Radiometer Measurements

Aquarius is an microwave active/passive sensor whose main goal is to globally estimate sea surface salinity from space. Two instruments, a radar scatterometer and a radiometer, operate at L-band observing the same surface footprint almost simultaneously. The sensitivity to sea surface salinity (SSS) is given by the radiometer, while the scatterometer measurements provide a correction for sea surface roughness. Although the primary objective is the measurement of SSS, the instrument combination operates continuously, acquiring data over land and sea ice as well. Radio Frequency Interference (RFI) can occur in both the radiometer and the scatterometer bands of operation, and for this reason detection and mitigation of RFI was included in the data processing of both active and passive instruments. This paper will focus on the RFI processing for the Aquarius radiometer only and provide an update on the efforts to reduce the amount of missed RFI detection

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

Weekly gridded Aquarius L-band radiometer/scatterometer observations and salinity retrievals over the polar regions – Part 1: Product description

Passive and active observations at L band (frequency ~1.4 GHz) from the Aquarius/SAC-D mission offer new capabilities to study the polar regions. Due to the lack of polar-gridded products, however, applications over the cryosphere have been limited. We present three weekly polar-gridded products of Aquarius data to improve our understanding of L-band observations of ice sheets, sea ice, permafrost, and the polar oceans. Additionally, these products intend to facilitate access to L-band data, and can be used to assist in algorithm developments. Aquarius data at latitudes higher than 50° are averaged and gridded into weekly products of brightness temperature (TB), normalized radar cross section (NRCS), and sea surface salinity (SSS). Each grid cell also contains sea ice fraction, the standard deviation of TB, NRCS, and SSS, and the number of footprint observations collected during the seven-day cycle. The largest 3 dB footprint dimensions are 97 km × 156 km and 74 km × 122 km (along × across track) for the radiometers and scatterometer, respectively. The data is gridded to the Equal-Area Scalable Earth version 2.0 (EASE2.0) grid, with a grid cell resolution of 36 km. The data sets start in August 2011, with the first Aquarius observations and will be updated on a monthly basis following the release schedule of the Aquarius Level 2 data sets. The weekly gridded products are distributed by the US National Snow and Ice Data Center at http://nsidc.org/data/aquarius/index.html

Statistical analysis and combination of active and passive microwave remote sensing methods for soil moisture retrieval

Knowledge about soil moisture and its spatio-temporal dynamics is essential for the improvement of climate and hydrological modeling, including drought and flood monitoring and forecasting, as well as weather forecasting models. In recent years, several soil moisture products from active and passive microwave remote sensing have become available with high temporal resolution and global coverage. Thus, the validation and evaluation of spatial and temporal soil moisture patterns are of great interest, for improving soil moisture products as well as for their proper use in models or other applications. This thesis analyzes the different accuracy levels of global soil moisture products and identifies the major influencing factors on this accuracy based on a small catchment example. Furthermore, on global scale, structural differences betweenthe soil moisture products were investigated. This includes in particular the representation of spatial and temporal patterns, as well as a general scaling law of soil moisture variability with extent scale. The results of the catchment scale as well as the global scale analyses identified vegetation to have a high impact on the accuracy of remotely sensed soil moisture products. Therefore, an improved method to consider vegetation characteristics in pasive soil moisture retrieval from active radar satellite data was developed and tested. The knowledge gained by this thesis will contribute to improve soil moisture retrieval of current and future microwave remote sensors (e.g. SMOS or SMAP)

Improved Monitoring of the Changjiang River Plume in the East China Sea During the Monsoon Season Using Satellite Borne L-Band Radiometers

Measurement of sea surface salinity (SSS) from Satellite borne L-band (1.4 GHz, 21cm) radiometers (NASA Aquarius/SAC-D and ESA SMOS) in the East China Sea (ECS) is challenging due to the uncertainty of SSS caused by land thermal emissions in the antenna side lobes and because of strong radio frequency interference (RFI) due to illegally emitted man-made sources. RFI contamination in the ECS has gradually decreased because of the on-going international efforts to eliminate broadcasts in the protected L-band radio-astronomy frequency band. The present dissertation focuses on carefully eliminating the remaining RFI contamination in retrieved SSS, and masking out regions close to the coast that are likely contaminated by thermal emissions from the land. Afterward, observation of SSS during the summer monsoon season in the ECS was conducted to demonstrate low salinity (\u3c 28 psu) Changjiang Diluted Water (CDW) which is a mixture of Changjiang River (CR) plume mixing and the ambient ocean water causing ecosystem disruptions as far east as the Korean peninsula. In this study, during southeasterly wind, CDW was observed to be horizontally advected east-northeastward due to Ekman flow. In addition, monthly averaged Aquarius SSS presented one-month lagged robust relationship with freshwater flux. Despite limits on temporal information of SMOS, the detachment of CDW from its formation region and northeastward advection was successfully observed after the arrival of the tropical storm Matmo in the mainland China

Statistical analysis and combination of active and passive microwave remote sensing methods for soil moisture retrieval

Knowledge about soil moisture and its spatio-temporal dynamics is essential for the improvement of climate and hydrological modeling, including drought and flood monitoring and forecasting, as well as weather forecasting models. In recent years, several soil moisture products from active and passive microwave remote sensing have become available with high temporal resolution and global coverage. However, for the improvement of a soil moisture product and for its proper use in models or other applications, validation and evaluation of its spatial and temporal patterns are of great importance. In chapter 2 the Level 2 Soil Moisture and Ocean Salinity (SMOS) soil moisture product and the Advanced Scatterometer (ASCAT) surface soil moisture product are validated in the Rur and Erft catchments in western Germany for the years 2010 to 2012 against a soil moisture reference created by a hydrological model, which was calibrated by in situ observations. Correlation with the modeled soil moisture reference results in an overall correlation coefficient of 0.28 for the SMOS product and 0.50 for ASCAT. While the correlation of both products with the reference is highly dependent ontopography and vegetation, SMOS is also strongly influenced by radiofrequency interferences in the study area. Both products exhibit dry biases as compared to the reference. The bias of the SMOS product is constant in time, while the ASCAT bias is more variable. For the investigation of spatio temporal soil moisture patterns in the study area, a new validation method based on the temporal stability analysis is developed. Through investigation of mean relative differences of soil moisture for every pixel the temporal persistence of spatial patterns is analyzed. Results indicate a lower temporal persistence for both SMOS and ASCAT soil moisture products as compared to modeled soil moisture. ASCAT soil moisture, converted to absolute values, shows highest consistence of ranks and therefore most similar spatio-temporal patterns with the soil moisture reference, while the correlation of ranks of mean relative differences is low for SMOS and relative ASCAT soil moisture products. Chapter 3 investigates the spatial and temporal behavior of the SMOS and ASCAT soil moisture products and additionally of the ERA Interim product from a weather forecast model reanalysis on global scale. Results show similar temporal patterns of the soil moisture products, but high impact of sensor and retrieval types and therefore higher deviations in absolute soil moisture values. Results are more variable for the spatial patterns of the soil moisture products: While the global patterns are similar, a ranking of mean relative differences reveals that ASCAT and ERA Interim products show most similar spatial soil moisture patterns, while ERA and SMOS products show least similarities. Patterns are generally more similar between the products in regions with low vegetation. [...

Performance and requirements of GEO SAR systems in the presence of Radio Frequency Interferences

Geosynchronous Synthetic Aperture Radar (GEO SAR) is a possible next generation SAR system, which has the excellent performance of less than one-day revisit and hundreds of kilometres coverage. However, Radio Frequency Interference (RFI) is a serious problem, because the specified primary allocation frequencies are shared by the increasing number of microwave devices. More seriously, as the high orbit of GEO SAR makes the system have a very large imaging swath, the RFI signals all over the illuminated continent will interfere and deteriorate the GEO SAR signal. Aimed at the RFI impact in GEO SAR case, this paper focuses on the performance evaluation and the system design requirement of GEO SAR in the presence of RFI impact. Under the RFI impact, Signal-to-Interference-plus-Noise Ratio (SINR) and the required power are theoretically deduced both for the ground RFI and the bistatic scattering RFI cases. Based on the theoretical analysis, performance evaluations of the GEO SAR design examples in the presence of RFI are conducted. The results show that higher RFI intensity and lower working frequency will make the GEO SAR have a higher power requirement for compensating the RFI impact. Moreover, specular RFI bistatic scattering will give rise to the extremely serious impact on GEO SAR, which needs incredible power requirements for compensations. At last, real RFI signal behaviours and statistical analyses based on the SMOS satellite, Beidou-2 navigation satellite and Sentinel-1 A data have been given in the appendix

Scanning L-Band Active Passive (SLAP) - Recent Results from an Airborne Simulator for SMAP

Scanning L-band Active Passive (SLAP) is a recently-developed NASA airborne instrument specially tailored to simulate the new Soil Moisture Active Passive (SMAP) satellite instrument suite. SLAP conducted its first test flights in December, 2013 and participated in its first science campaign-the IPHEX ground validation campaign of the GPM mission-in May, 2014. This paper will present results from additional test flights and science observations scheduled for 2015