A review of RFI mitigation techniques in microwave radiometry

Radio frequency interference (RFI) is a well-known problem in microwave radiometry (MWR). Any undesired signal overlapping the MWR protected frequency bands introduces a bias in the measurements, which can corrupt the retrieved geophysical parameters. This paper presents a literature review of RFI detection and mitigation techniques for microwave radiometry from space. The reviewed techniques are divided between real aperture and aperture synthesis. A discussion and assessment of the application of RFI mitigation techniques is presented for each type of radiometer.Peer ReviewedPostprint (published version

Rf interference analysis in aperture synthesis interferometric radiometers: application to l-band miras instrument

Current spaceborne radiometers do not achieve the required spatial resolution demanded by the scientific community due to antenna-size technological limitations. In recent years, several space agencies have been studying aperture synthesis interferometric radiometers as a way of overcoming these limitations, which are more evident at low microwave frequencies (e.g., at L-band), where sea surface salinity and soil moisture can be monitored. Interference is an important issue in any remote sensing instrument, but it is crucial in microwave radiometers, since the signal being measured is the spontaneous thermal noise emission. Interference analyses already exist for classic radiometers. The objective of this paper is the analysis of RF interference on interferometric radiometers. The study involves the analysis of possible interference sources that may affect the performance of such systems at L-band: (1) nearby emissions from radars, non-Geo-Stationary Orbit (GSO) and Mobile Satellite Services (GSO-MSS), (2) harmonics of lower frequency emissions, and (3) possible jamming.Peer ReviewedPostprint (published version

RF interference analisys in aperture synthesis interferometric radiometers: Application to L-band MIRAS instrument

Current spaceborne radiometers do not achieve the required spatial resolution demanded by the scientific community due to antenna-size technological limitations. In recent years, several space agencies have been studying aperture synthesis interferometric radiometers as a way of overcoming these limitations, which are more evident at low microwave frequencies (e.g., at L-band), where sea surface salinity and soil moisture can be monitored. Interference is an important issue in any remote sensing instrument, but it is crucial in microwave radiometers, since the signal being measured is the spontaneous thermal noise emission. Interference analyses already exist for classic radiometers. The objective of this paper is the analysis of RF interference on interferometric radiometers. The study involves the analysis of possible interference sources that may affect the performance of such systems at L-band: (1) nearby emissions from radars, non-Geo-Stationary Orbit (GSO) and Mobile Satellite Services (GSO-MSS), (2) harmonics of lower frequency emissions, and (3) possible jamming.Peer Reviewe

Radiometric Sensitivity Computation in Aperture Synthesis Interferometric Radiometry

This paper is concerned with the radiometric sensitivity computation of an aperture synthesis interferometric radiometer devoted to Earth observation. The impact of system parameters and the use of simultaneous redundant measurements are analyzed. The interferometric radiometer uncertainty principle is presented; it quantifies the relationship between radiometric sensitivity and angular resolution.Peer Reviewe

Radiometric sensitivity computation in aperture synthesis interferometric radiometry

This paper is concerned with the radiometric sensitivity computation of an aperture synthesis interferometric radiometer devoted to Earth observation. The impact of system parameters and the use of simultaneous redundant measurements are analyzed. The interferometric radiometer uncertainty principle is presented; it quantifies the relationship between radiometric sensitivity and angular resolution.Peer Reviewe

A pre-correlation RFI mitigation algorithm for L-band interferometric radiometers

Radio Frequency Interference (RFI) is a major concern for both real and synthetic aperture radiometers. After the lessons learnt from SMOS, ESA is preparing the next generation of L-band interferometric radiometers with RFI mitigation integrated into the cross-correlators. This work presents a preliminary design and results of a pre-correlation RFI mitigation algorithm tailored for interferometric radiometers. The results show that the correlation error introduced by the RFI is reduced on average to the half, with peaks of 20 dB of mitigation.Peer ReviewedPostprint (author's final draft

Experimental validation of radiometric sensitivity in correlation radiometers

The prediction of the radiometric sensitivity of correlation radiometers based on the system parameters requires knowledge of the standard deviation of each measurement due to the finite integration time. Theoretical formulations have been experimentally validated showing the dependence of the standard deviation with the measurement and extended for arbitrary over sampling factors.Peer Reviewe

Correlators for Interferometric Radiometry in Remote Sensing Applications, A Scaling Perspective

Correlators are extensively used in the field of radio interferometry. Two different types are considered for two applications; autocorrelators for spectrometry and cross-correlators for aperture synthesis. We concentrate on satellite-based applications where power budgets are very restrictive. Several satellites are already employing correlators for interferometric measurements, and future projects are targeting even larger systems in terms of spectral channels in the case of spectrometry and baseline counts in the case of aperture synthesis. Thus, it is important to develop correlators with increasing channel count, either using ASIC technology scaling or by constructing larger systems from several ASICs. Building on earlier ASIC designs, we examine how larger correlator systems can be constructed and the implications this has, in terms of power dissipation, system complexity, and ASIC count. Our findings indicate that, for large systems, having a very high channel count per ASIC is indeed of interest for keeping system complexity and power dissipation down by reducing both ASIC and I/O count, especially for cross-correlators

Automatic calibration of channels frequency response in interferometric radiometers

The large field of view required in Earth observation interferometric radiometers does not enable the use of a variable delay to compensate for different transit times, as in radioastronomy. A technique is presented to characterise the influence of a channels frequency response automatically in the shape of the cross-correlation function.Peer Reviewe

RFI detection and mitigation for advanced correlators in interferometric radiometers

This work presents the first RFI detection and mitigation algorithm for the interferometric radiometers that will be implemented in its correlator unit. The algorithm operates in the time and frequency domains, applying polarimetric and statistical tests in both domains, and exhibiting a tunable and arbitrary low probability of false alarm. It is scalable to a configurable number of receivers, and it is optimized in terms of quantization bits and the implementation of the cross-correlations in the time or frequency domains for hardware resource saving. New features of this algorithm are the computation of the Stokes parameters per frequency bin in the Short-Time Fourier Transform and a new parameter called Polarimetric Kurtosis. If RFI is detected in one domain or in both, it is removed using the calculated blanking masks. The optimum algorithm parameters are computed, such as length of the FFTs, the threshold selection for a given probability of false alarm, and the selection of the blanking masks. Last, an important result refers to the application of Parseval’s theorem for the computation of the cross-correlations in the frequency domain, instead of in the time domain, which is more efficient and leads to smaller errors even when using moderate quantization levels. The algorithm has been developed in the framework of the ESA’s technology preparation for a potential L-band radiometer mission beyond SMOS. However, it is also applicable to (polarimetric) real aperture radiometers, and its performance would improve if more than one bit is used in the signal quantization.This research was funded by ESA, grant number ITT AO9359, by project SPOT: Sensing with Pioneering Opportunistic Techniques grant RTI2018-099008-B-C21/AEI/10.13039/501100011033, and the grant for recruitment of early stage research staff of the Agència de Gestió d’Ajuts Universitaris i de Recerca (AGAUR) Generalitat de Catalunya, Spain (FISDUR2020/105).Peer ReviewedPostprint (published version