Radio-echo sounding is a standard technique for imaging the englacial stratigraphy of glaciers and ice sheets. In most cases, internal reflection horizons (IRHs) represent former glacier surfaces, comprise information about past accumulation and ice deformation, and enable the linking of ice core chronologies. IRHs in the lower third of the ice column are often difficult to detect or coherently trace. In the polar ice sheets, progress in IRH detection has been made by using multistatic, phase-coherent radars, enabling focused synthetic aperture radar (SAR) processing. However, these radar systems are often not suitable for deployment on mountain glaciers. We present a proof-of-concept study for a lightweight, phase-coherent and ground-based radar system, based on the phase-sensitive radio-echo sounder (pRES). To improve the detectability of IRHs we additionally adapted a layer-optimized SAR processing scheme to this setup. We showcase the system capability at Colle Gnifetti, Switzerland and Italy, where specular reflections are now apparent down to the base of the glacier. Compared to previously deployed impulse radar systems, with the mobile pRES the age of the oldest continuously traceable IRH could be increased from 78±12 to 288±35 a. Corresponding reflection mechanisms for this glacier are linked to stratified acidic impurities which in the upper part were deposited at a higher rate due to increased industrial activity in the area. Possible improvements to the system are discussed. If successfully implemented, these may provide a new way to map the deep internal structure of Colle Gnifetti and other mountain glaciers more extensively in future deployments.