Preliminary Studies on CIMR Antenna Pattern Brightness Temperature Compensation

Abstract

Spaceborne microwave radiometry provides an essential contribution to monitoring the Earth with varying spatial resolution both related to the reflector dimension and the frequency of operation. The ESA's Copernicus imaging microwave radiometer (CIMR) mission aims at collecting the geophysical observables at a spatial resolution ranging from 60 km in L band to 4 km in Ka band. This goal can be achieved by equipping CIMR with a large unfurlable mesh reflector antenna. A limitation of the antenna design is that the antenna pattern includes grating lobes that contaminate the scene measurement with contributions originated far from the nominal footprint. This effect introduces inaccuracies in brightness temperature measurements, particularly when facing radiometric discontinuities, e.g., near the coastlines and sea ice edges, which can be greater than the mission required maximum of 0.5 K. The aim of this article is to assess a technique which will be able to correct the effects of antenna pattern and obtain reliable T B measurements. The analyzed simple technique is based on a regularized deconvolution of the antenna pattern to reconstruct the actual brightness temperatures. The technique was tested over a synthetic scenario that mimics both steep and smooth variations in spatial and thermal domains.

An Antenna Pattern Correction Algorithm for Conical Scanning Spaceborne Radiometers: The CIMR Case

“…The CIMR instrument is designed as a conically scanning imaging microwave radiometer that acquires continuous and contiguous measurements along the forward and backward scan. As reported in [5], the CIMR antenna foresees a deployable mesh reflector with a diameter of about 7 m, to accommodate the required coverage and spatial resolution within the applicable mass and volume constraints.…”

“…Nevertheless, the measurements collected by the spaceborne radiometer are characterized by undesired contributions: 1) contamination coming from the region outside the IFOV (e.g., cosmic microwave background, lunar and solar contributions, and galaxy emissions); 2) crosspolarization coupling; and 3) energy collected by side and grating lobes. In order to mitigate contributions of types 1) and 2), antenna pattern correction (APC) algorithms were developed [6], [7], [8], [9] and are recalled in [5], to which the reader is referred to for deeper insight. More specifically, a preliminary approach to the correction of these effects is presented in [5], but with simplified assumptions about the acquisition geometry.The mitigation of the grating lobe contamination, singling out the brightness temperature T b of the scenario within the IFOV, was obtained from the antenna temperature associated with the entire radiation pattern, i.e., over the 4π solid angle, through the implementation of a well-established image processing approach and based on the use of deconvolution techniques [10], [11].…”

“…In order to mitigate contributions of types 1) and 2), antenna pattern correction (APC) algorithms were developed [6], [7], [8], [9] and are recalled in [5], to which the reader is referred to for deeper insight. More specifically, a preliminary approach to the correction of these effects is presented in [5], but with simplified assumptions about the acquisition geometry.The mitigation of the grating lobe contamination, singling out the brightness temperature T b of the scenario within the IFOV, was obtained from the antenna temperature associated with the entire radiation pattern, i.e., over the 4π solid angle, through the implementation of a well-established image processing approach and based on the use of deconvolution techniques [10], [11]. The method This work is licensed under a Creative Commons Attribution 4.0 License.…”

“…As in [5], the proposed APC only considers the most energetic contributions of the antenna pattern, which are defined and limited by a proper threshold value of the normalized antenna gain. The choice of this threshold value is heuristically driven, but it can be adapted to other cases, depending on the antenna characteristics and the error and accuracy constraints on the brightness temperature and/or the EO products.…”

“…The choice of this threshold value is heuristically driven, but it can be adapted to other cases, depending on the antenna characteristics and the error and accuracy constraints on the brightness temperature and/or the EO products. In this present article, the chosen threshold value is −52 dB, with respect to the normalized maximum, so that the contributions to the measurements, besides the main beam, include the sidelobes and the more energetic grating lobes (up to >99% of the radiated energy) [5].…”