1 An overview and issues of the sky radiometer technology and SKYNET Teruyuki Nakajima 1 , Monica Campanelli 2 , Huizheng Che 3 , Victor Estellés 2,4 , Hitoshi Irie 5 , Sang-Woo Kim 6 , Jhoon Kim 7 , Dong Liu 8 , Tomoaki Nishizawa 9 , Govindan Pandithurai 10 , Vijay Kumar Soni 11 , Boossarasiri Thana 12 , Nas-Urt Tugjsurn 13 , Kazuma Aoki 14 , Makiko Hashimoto 1 , Akiko Higurashi 9 , 5 Stelios Kazadzis 15 , Pradeep Khatri 16 , Natalia Kouremeti 15 , Rei Kudo 17 , Franco Marenco 18 , Masahiro Momoi 5,19 , Shantikumar S. Ningombam 20 , Claire L. Ryder 21 , Akihiro Uchiyama 9 1 Japan Aerospace Exploration Agency; 2-1-1 Sengen, Tsukuba, Ibaraki 305-8505, Japan 2 Consiglio Nazionale delle Ricerche, Istituto Scienze dell'Atmosfera e del Clima; via Fosso del Cavaliere, 100, 00133 - Roma, Italy 10 3 Centre for Atmosphere Watch And Services, CMA Chinese Academy of Meteorological Sciences; 46 Zhong-Guan-Cun S. Ave., Beijing 100081, China 4 Dept. Física de la Terra i Termodinàmica, Universitat de València; Burjassot, Valencia, Spain 5 Center for Environmental Remote Sensing, Chiba University; Chiba 263-8522, Japan 6 School of Earth and Environmental Sciences, Seoul National University; 08826, Seoul, Republic of Korea 15 7 Dept. of Atmospheric Sciences, Yonsei University, Seoul 03722, Republic of Korea 8 Center for Atmospheric Optics, Anhui Institute of Optics and Fine Mechanics, Hefei Institutes of Physical Science, Chinese Academy of Sciences; Hefei, Anhui 230031, China 9 National Institute for Environmental Studies; 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan 10 Centre for Climate Change Research, Indian Institute of Tropical Meteorology; Pune 411 008, India 20 11 Environment Monitoring & Research Centre, India Meteorological Department; Mausam Bhawan, Lodi Road, New Delhi-110003, India 12 Thailand Global Warming Academy, Napamitr Foundation; 234/88 Asoke-Din Daeng Road, Bang Kapi Sub-district, Huai Khwang District, Bangkok 10310, Thailand 13 Physics department, Mongolian University of Science and Technology; 216046, Ulaanbaatar, Mongolia 25 14 Graduate School of Science and Engineering (Science), University of Toyama; 3190 Gofuku, Toyama 930-8555, Japan 15 Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, Dorfstrasse 33, CH-7260 Davos Dorf, Switzerland 16 Center for Atmospheric and Oceanic Studies, Graduate School of Science, Tohoku University, Sendai, Japan 17 Meteorological Research Institute, Meteorological Agency; Nagamine, Tsukuba 305-0052, Ibaraki, Japan 30 18 Satellite Applications, Met Office, United Kingdom 19 Graduate School of Science, Tokyo University of Science, Tokyo 162-8601, Japan 20 Indian Institute of Astrophysics, 2nd Block Koramangala, Bangalore 560034, India 21 Department of Meteorology, University of Reading, Reading, RG6 6BB, United Kingdom Correspondence to: T. Nakajima (terry-nkj@nifty.com) 35 Abstract. This paper overviews the progress in the sky radiometer technology and development of the network called SKYNET. It is found that the technology has produced useful on-site calibration methods, retrieval algorithms, and data analyses from the sky radiometer observation of aerosol, cloud, water vapor and ozone. Increasing collaborations of users in the SKYNET community are becoming a useful platform for research and operation. The paper also presents issues of the technology for future development. 40 https://doi.org/10.5194/amt-2020-72 Preprint. Discussion started: 16 March 2020 c  Author(s) 2020. CC BY 4.0 License.