No abstract

The Chinese Fengyun–4A geostationary meteorological satellite was successfully launched on 11 December 2016, carrying an Advanced Geostationary Radiation Imager (AGRI) to provide the observations of visible, near infrared, and infrared bands with improved spectral, spatial, and temporal resolution. The AGRI infrared observations can be assimilated into numerical weather prediction (NWP) data assimilation systems to improve the atmospheric analysis and weather forecasting capabilities. To achieve data assimilation, the first and crucial step is to characterize and evaluate the biases of the AGRI brightness temperatures in infrared channels 8–14. This study conducts the assessment of clear–sky AGRI full–disk infrared observation biases by coupling the RTTOV model and ERA Interim analysis. The AGRI observations are generally in good agreement with the model simulations. It is found that the biases over the ocean and land are less than 1.4 and 1.6 K, respectively. For bias difference between land and ocean, channels 11–13 are more obvious than water vapor channels 9–10. The fitting coefficient of linear regression tests between AGRI biases and sensor zenith angles manifests no obvious scan angle–dependent biases over ocean. All infrared channels observations are scene temperature–dependent over the ocean and land.

No abstract

No abstract

ConGalSAR is a novel SAR constellation which has outstanding performance of revisit and economy. In the system, with MirrorSAR technology, one geostationary illuminator and several economical low Earth orbit transponders work together to achieve short revisiting interval. Because of the huge difference in the distances of transmitting and receiving, the spatial weighting of the antenna patterns on the ground are quite different, which leads to the deterioration of ambiguity to signal ratio. Multiphase center and digital beam forming antennas can suppress the ambiguity energy; however, they are too expensive and heavy to be equipped on the dozens of transponding nano-satellites. In this article, a comprehensive ambiguity suppression method including reflector antenna design and signal processing is proposed to reduce the ambiguity ratio, which well solves the ambiguity problem under the condition of low manufacturing costs and lightweight transponders. In terms of hardware design, the multifeed reflector antenna is applied on the transponder, in which the feeds receive echo together. Based on this, the low sidelobes beamforming is achieved to well suppress the ambiguity. Meanwhile, amplitude-modulated chirps are also used here aiming at the residual ambiguity in the large dynamic scene. Through spectral selection and extrapolation processing, range ambiguity energy is suppressed more while preserving the resolution. Digital simulations show that the comprehensive method can realize-20 dB ambiguity ratios at least, which conforms to ambiguity-to-signal ratio design of ConGaLSAR.