Joel T. Johnson

Radio-Frequency Interference Mitigation for the Soil Moisture Active Passive Microwave Radiometer

The Soil Moisture Active Passive (SMAP) radiometer operates in the L-band protected spectrum (1400-1427 MHz) that is known to be vulnerable to radio-frequency interference (RFI). Although transmissions are forbidden at these frequencies by international regulations, ground-based, airborne, and spaceborne radiometric observations show substantial evidence of out-of-band emissions from neighboring transmitters and possibly illegally operating emitters. The spectral environment that SMAP faces includes not only occasional large levels of RFI but also significant amounts of low-level RFI equivalent to a brightness temperature of 0.1-10 K at the radiometer output. This low-level interference would be enough to jeopardize the success of a mission without an aggressive mitigation solution, including special flight hardware and ground software with capabilities of RFI detection and removal. SMAP takes a multidomain approach to RFI mitigation by utilizing an innovative onboard digital detector back end with digital signal processing algorithms to characterize the time, frequency, polarization, and statistical properties of the received signals. Almost 1000 times more measurements than what is conventionally necessary are collected to enable the ground processing algorithm to detect and remove harmful interference. Multiple RFI detectors are run on the ground, and their outputs are combined for maximum likelihood of detection to remove the RFI within a footprint. The capabilities of the hardware and software systems are successfully demonstrated using test data collected with a SMAP radiometer engineering test unit.

Radiometric Approach for Estimating Relative Changes in Intraglacier Average Temperature

We investigate the degree to which ultrahigh frequency radio emission can be used to estimate subsurface physical temperature in the polar ice sheets. We combine electromagnetic emission forward models with plausible models of depth-dependent physical properties in the ice sheet. Temperature models are parameterized with variables including accumulation rate, geothermal heat flux, and surface temperature. Scattering is parameterized using empirical observations of grain growth combined with measured densities. Electromagnetic absorption is modeled using dielectric dispersion processes and semiempirical models based on observations. Our models illustrate that information about East Antarctic ice sheet temperature from near the surface to near the base can be gleaned from ultrawideband radiometer data. Based on our modeling study, we illustrate an instrument concept to measure ice sheet temperature profiles comprising a novel ultrawideband radiometer.

A Study of SMOS RFI Over North America

The European Space Agencys Soil Moisture and Ocean Salinity (SMOS) mission has been providing L-band brightness temperature observations of the Earth since its launch in November 2009. Radio frequency interference (RFI) is clearly present in SMOS data, and RFI detection and mitigation are a challenging problem. Furthermore, the interferometric nature of SMOS observations can cause RFI artifacts in SMOS measurements. This letter reports an analysis of the characteristics of SMOS RFI in North America, including a study of RFI artifacts and a method for their removal. Polarimetric properties and statistics of the resulting observations after artifact removal are also examined as an initial step in characterizing the “true” RFI sources observed in North America.

A Comparative Analysis of Low-Level Radio Frequency Interference in SMOS and Aquarius Microwave Radiometer Measurements

Measurements of both the Soil Moisture and Ocean Salinity (SMOS) and Aquarius L-band microwave radiometers show a significant presence of radio frequency interference (RFI), although they operate in a protected frequency band where transmission is prohibited. RFI detection and mitigation remain a challenging problem for both missions, especially for low or moderate (i.e., on the order of 10 K or less) amplitude contributions. An algorithm for low-level source detection and mitigation is already included in Aquarius data sets, and both Aquarius and SMOS have distinct attributes that can potentially enable further improvements in detection and mitigation of these sources to some degree. The combination of SMOS and Aquarius data sets may enable further future improvements as well. Initial efforts toward this goal are reported in this paper. Similarities and differences in RFI effects on SMOS and Aquarius are examined, with a particular focus on instrument properties that cause differences in received RFI power in SMOS and Aquarius observations of a specific source. A study is also performed of SMOS observations for regions reported by Aquarius to contain “low-level” RFI. It is shown that the detection of these sources in the SMOS data set is challenging and that the dependence of the SMOS third and fourth Stokes parameters on incidence angle makes the polarimetric features of SMOS difficult to utilize for low-level source detection. However, an angular fitting procedure suggested previously in the literature can, in some cases, detect such sources in horizontal and vertical polarizations.

Studies of radio frequency interference in SMOS observations

ESAs SMOS mission has been providing L-band brightness temperature observations of the Earth since launch in Nov. 2009. Radio frequency interference (RFI) is clearly present in SMOS data, and RFI detection and mitigation remains a challenging problem. The interferometric nature of SMOS observations also causes some RFI artifacts in SMOS measurements that do not reflect the actual brightness temperature values. This paper reports on an analysis of SMOS RFI in North America, including a study of RFI artifacts and a method for their removal. Properties of the remaining RFI sources are then examined after artifact removal.

Understanding SMOS data in Antarctica

Since the SMOS satellite launch in 2009, its L-band radiometer data have been analyzed in depth by scientists worldwide and have resulted in significant steps forward in different disciplines. As primary objectives of the mission, the main research focus has been related to soil moisture and ocean salinity. However, the availability of a complete long-term, all-weather time-series of calibrated global brightness temperature data has enabled much broader research investigations on other topics such as the Cryosphere. SMOS data collected over central of Antarctica were also analyzed and whereas Tb is in general very stable in time it presents some intriguing spatial variations which are not yet fully explained. Using electromagnetic model simulations, and ancillary data for describing the physical parameters of the ice sheet, the observed variability and features in SMOS data are reproduced and explained.

L-Band Radio-Frequency Interference Observations During the SMAP Validation Experiment 2012

Radio-frequency interference (RFI) observations for L-band microwave radiometry during the SMAP Validation Experiment 2012 (SMAPVEX12) airborne campaign are reported in this paper. The soil moisture measurement campaign was conducted in summer 2012 near Winnipeg, MB, Canada, with additional RFI flights over Denver, CO, USA. The Passive Active L-Band sensor (PALS) radiometer of the Jet Propulsion Laboratory was used with a full-bandwidth direct sampling digital backend to measure and store predetection data that is fully resolved in time and frequency. Overviews of SMAPVEX12 and the receiver and digital backend used to collect data are presented, along with the data processing techniques used for RFI detection. Properties of the observed RFI are examined and compared with the results of previous studies. Finally, implications of the results are explained considering current missions such as NASAs Soil Moisture Active Passive Mission.

SMAP RFI mitigation algorithm performance characterization using airborne high-rate direct-sampled SMAPVEX 2012 data

The SMAP RFI detecting digital backend performance is characterized using real-environment L-band RFI data from the SMAPVEX 2012 campaign. Various types of RFI signals are extracted from the airborne campaign dataset and fed to the SMAP radiometer using an Arbitrary Waveform Generator (AWG). The backend detection performance is tested, and missed-detections are further investigated. Initial results indicate RFI detection performance for the SMAP digital backend is acceptable.

Joint analysis of radio frequency interference from SMOS measurements and from airborne observations

The Soil Moisture and Ocean Salinity (SMOS) mission has been providing L-band brightness temperature observations since its launch in November 2009. SMOS observations are being utilized for soil moisture and ocean salinity estimation. However, the mission has been affected by significant levels of radio frequency interference caused by anthropogenic sources of radiation in the 1400–1427 MHz band. SMOS has only one frequency channel and measures brightness temperatures with 1.2 seconds time resolution, therefore the detection and mitigation of RFI is challenging, especially for low-level RFI. One possible approach for RFI detection involves use of the fully polarimetric properties of SMOS measurements. In this paper, an examination of the characteristics of SMOS measurements over the USA is first reported. SMOS measurements from Fall 2010 are then compared with RFI source information obtained from a Fall 2008 airborne campaign called SMAPVEX08 (which supported NASAs Soil Moisture Active/Passive (SMAP)). The SMAPVEX08 “ground truth” information on RFI sources is used to examine possible RFI detection strategies using SMOS polarimetric data. Although the study is limited by possible changes in source properties in the intervening two year period, the results suggest that polarimetry can be used as an indication of some RFI sources, but may not reflect the RFI characteristics of a region in general.

Soil Moisture Active Passive (SMAP) microwave radiometer radio-frequency interference (RFI) mitigation: Algorithm updates and performance assessment

The Soil Moisture Active Passive (SMAP) mission, launched January 31, 2015, provides global observations of 1.4 GHz Earth thermal emissions from space through its L-band radiometer. Although SMAPs radiometer passband lies within the protected 1.4-1.427 GHz band, both unauthorized in-band transmitters as well as out-of-band emissions from transmitters operating at frequencies adjacent to this allocated spectrum have been documented as sources of radio frequency interference (RFI) to the L-band radiometers on SMOS and Aquarius. Low level RFI (0.1-10 Kelvin) is especially problematic as it can be mistaken for natural variability and if left unmitigated can corrupt radiometer measurements leading to flawed retrievals. SMAP has an aggressive approach to RFI mitigation using an advanced digital microwave radiometer to provide time and frequency measurements as well as a comprehensive ground processing algorithm.