Auto-tracking antennas are a key component of broadband mobile satellite communications. In particular, low-profile antennas are required for high-speed trains to reduce wind noise. We present an auto-tracking receiving antenna system with an ultra-low-profile (90-mm height) configuration. The antenna has high G/T, wideband capacity, and highly accurate tracking performance. We tested the antenna on an experimental microbus. Results demonstrate the antenna can be used for mobile satellite communications. This paper describes the configuration and performance of the antenna system.