ACE (Atmospheric Climate Experiment) was an ESA Earth Explorer Opportunity concept studied from 1999 at Phase-A level and retained as a hot-spare atmospheric sounding mission. It would exploit refraction of GNSS navigation-satellite signals to provide globally distributed temperature and humidity observations in the upper atmosphere, improving long-term climate monitoring and atmospheric-model knowledge through radio occultation. ESA material documents ACE as a research concept rather than an implemented programme.