Next-generation air-shower detectors, such as the Global Cosmic Ray Observatory (GCOS) and the Probing Extreme PeVatron Sources (PEPS) experiment, are considering water-Cherenkov detectors as a base design. A key factor in improving the sensitivity to ultra-high-energy gamma rays and to the mass composition of ultra-high-energy cosmic rays is the ability to measure the muonic content of air showers. To address this, a layered water Cherenkov tank design has been previously proposed. The water volume of the tank is divided into two optically separated layers. The electromagnetic component of the shower is mostly absorbed in the top layer, while the bottom layer records the light produced by through-going muons. Two prototype tanks were deployed at the Pierre Auger Observatory site in 2014 and have been recording data for more than 10 years. We present the performance of the prototype tanks and compare it with simulations, focusing mostly on the calibration. We investigate different dimensions for the water volumes. For the GCOS Observatory, one important challenge is to cover extremely large surfaces of 40000 km² to 60000 km² and achieve 100% efficiency at 10 EeV. Based on the size of the footprint of air-showers, we compute the number and spacing of detectors needed to fulfill the GCOS requirements.
Parenti et al. (Wed,) studied this question.