One-Point Microwave Radiometer Calibration (original) (raw)
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Calibration of the MIRAS Radiometers
IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 2019
The microwave imaging radiometer with aperture synthesis (MIRAS) is formed by 69 total power radiometers, of which three are the noise-injection type. Their calibration is reviewed on the basis of the data gathered during more than eight years of operation. Internally calibrated gain and offset corrections with improved temporal stability are presented. New front-end loss characterization with lower seasonal dependence originated from external temperature swings is also proposed. Finally, a methodology to validate the external calibrations, with the instrument pointing to the cold sky, is developed. It seems to indicate that the change of orientation of the instrument, with associated thermal variations, may induce small changes in the radiometer front-end losses, thus introducing calibration errors.
Identification of Spaceborne Microwave Radiometer Calibration Sites for Satellite Missions
The first dedicated soil moisture satellite mission will be the European Space Agency's Soil Moisture and Ocean Salinity (SMOS) mission. This satellite, scheduled for launch in the second half of 2009, has a new type of satellite design that is based on the radio-astronomy technique of simulating a large antenna from a number of smaller ones placed some distance apart. Because of its unique design and the fact it is sensing in a currently unutilized frequency range makes it critical that on-orbit calibration targets be included in the calibration strategy. Consequently, targets such as the Antarctic, cold oceans, tropical forests and deserts are being considered. However, the large footprint size of passive microwave observations means that large scale homogeneous regions must be identified for calibration purposes. Moreover, these sites must also be either stable through time or the temporal variation easily described by models. In order to satisfy the calibration accuracy required by SMOS for soil moisture retrieval, such sites should be characterized with a brightness temperature uncertainty of less than 4K.
Long-term stability of ERS-2 and TOPEX microwave radiometer in-flight calibration
IEEE Transactions on Geoscience and Remote Sensing, 2005
The microwave radiometers on altimeter missions are specified to provide the "wet" troposphere path delay with an uncertainty of 1 cm or lower, at the location of the altimeter footprint. The constraints on the calibration and stability of these instruments are therefore particularly stringent. The paper addresses the questions of long-term stability and absolute calibration of the National Aeronautics and Space Administration Topography Experiment (TOPEX) and European Space Agency European Remote Sensing 2 (ERS-2) radiometers over the entire range of brightness temperatures. Selecting the coldest measurements over ocean from the two radiometers, the drift of the TOPEX radiometer 18-GHz channel is confirmed to be about 0.2 K/year over the seven first years of the mission, and the one of the ERS-2 radiometer 23.8-GHz channel to be 0 2 K/year.
Radiometric Performance of the SMOS Reference Radiometers—Assessment After One Year of Operation
IEEE Transactions on Geoscience and Remote Sensing, 2012
In this paper, we present an analysis of the radiometric performance of the three 1.4-GHz noise injection radiometers of the European Space Agency's Soil Moisture and Ocean Salinity (SMOS) satellite. The units measure the antenna temperature, which contributes to the average brightness temperature level of SMOS retrievals. We assess the radiometric resolution of the receivers, the similarity between their measurements, and their thermal stability. For these purposes, we use SMOS measurement data gathered during the first year of the orbital operations of the satellite, which was launched in November 2009. The main results from the analysis are that the units meet the design requirements with a margin. Also, we present a new thermal model for the radiometers to further enhance their stability.
IEEE Transactions on Geoscience and Remote Sensing, 2013
A technique for comparing spaceborne microwave radiometer brightness temperatures (Tb) is described in the context of the upcoming National Aeronautics and Space Administration Global Precipitation Measurement (GPM) mission. The GPM mission strategy is to measure precipitation globally with high temporal resolution by using a constellation of satellite radiometers logically united by the GPM core satellite, which will be in a non-sun-synchronous medium inclination orbit. The usefulness of the combined product depends on the consistency of precipitation retrievals from the various microwave radiometers. The Tb calibration requirement to achieve such consistency demands first that Tb's from the individual radiometers be free of instrument and measurement artifacts and, second, that these self-consistent Tb's will be translated to a common standard (GPM core) for the unification of the precipitation retrieval. The intersatellite radiometric calibration technique described herein serves both the purposes by comparing individual radiometer observations to radiative transfer model (RTM) simulations (for "self-consistency" check) and by using a double-difference technique (to establish a linear calibration transfer function from one radiometer to another). This double-difference technique subtracts the RTMsimulated difference from the observed difference between a pair of radiometer Tb's. To establish a linear inter-radiometer calibration transfer function, comparisons at both the cold (ocean) and the warm (land) end of the Tb's are necessary so that, using these two points, slope and offset coefficients are determined. To this end, a simplified calibration transfer technique at the warm end (over the Amazon and Congo rain forest) is introduced. Finally, an error model is described that provides an estimate of the uncertainty of the radiometric bias estimate between comparison radiometer channels.
Detection of calibration drifts in spaceborne microwave radiometers using a vicarious cold reference
IEEE Transactions on Geoscience and Remote Sensing, 2000
The coldest possible brightness temperatures observed by a downward-looking microwave radiometer from space are often produced by calm oceans under cloud-free skies and very low humidity. This set of conditions tends to occur with sufficient regularity that an orbiting radiometer will accumulate a useful number of observations within a period of a few days to weeks. Histograms of the radiometer's coldest measurements provide an anchor point against which very small drifts in absolute calibration can be detected. This technique is applied to the TOPEX microwave radiometer (TMR), and a statistically significant drift of several tenths of a Kelvin per year is clearly detected in one of the channels. TMR housekeeping calibration data indicates a likely cause for the drift, as small changes in the isolation of latching ferrite circulators that are used in the onboard calibration-switch assembly. This method can easily be adapted to other microwave radiometers, especially imagers operating at frequencies in the atmospheric windows. In addition to detecting long-term instrument drifts with high precision, the method also provides a means for cross-calibrating different instruments. The cold reference provides a common tie point, even between sensors operating at different polarizations and/or incidence angles.
IEEE Transactions on Geoscience and Remote Sensing, 2003
A new type of calibration standard is presented which produces a pair of microwave noise signals to aid in the characterization and calibration of correlating radiometers. The Correlated Noise Calibration Standard (CNCS) is able to generate pairs of broad bandwidth stochastic noise signals with a wide variety of statistical properties. The CNCS can be used with synthetic aperture interferometers to generate specific visibility functions. It can be used with fully polarimetric radiometers to generate specific third and fourth Stokes parameters of brightness temperature. It can be used with spectrometers to generate specific power spectra and autocorrelations. It is also possible to combine these features and, for example, to generate the pair of signals that would be measured by a fully polarimetric, spectrally resolving, synthetic aperture radiometer at a particular pair of polarizations and antenna baselines for a specified scene over a specified frequency band. Algorithms are presented to construct signals with the desired statistical properties. Also presented is a description of the key hardware design challenges that were associated with fabrication of the first unit. CNCS performance is demonstrated by characterization tests of a pair of microwave interferometer radiometer receivers.
Radiometric Performance of the SMOS Reference Radiometers—Assessment After One Year of Operation
IEEE Transactions on Geoscience and Remote Sensing, 2012
In this paper, we present an analysis of the radiometric performance of the three 1.4-GHz noise injection radiometers of the European Space Agency's Soil Moisture and Ocean Salinity (SMOS) satellite. The units measure the antenna temperature, which contributes to the average brightness temperature level of SMOS retrievals. We assess the radiometric resolution of the receivers, the similarity between their measurements, and their thermal stability. For these purposes, we use SMOS measurement data gathered during the first year of the orbital operations of the satellite, which was launched in November 2009. The main results from the analysis are that the units meet the design requirements with a margin. Also, we present a new thermal model for the radiometers to further enhance their stability.