A specific interest represents solar protons possessing energy enough to induce an atmospheric cascade in the Earth’s atmosphere, whose secondary particles reach the ground, eventually registered by ground-based detectors e.g. neutron monitors. This class of events is known as ground-level enhancements (GLEs). The solar cycle 23 provided several strong GLEs, the first observed on 14 July 2000 (the Bastille day event), while the last was observed on 13 December 2006. The systematic study of relativistic SEPs provides an important basis to understand their acceleration and propagation in interplanetary space, as well as to quantify the related space weather effects such as radiation dose at flight altitudes. The Easter event on 15 April 2001 is among the strongest and accordingly, it is the focus of this study. Here we performed a precise analysis of neutron monitor records and derived the spectral and angular characteristics of the solar energetic particles during this event. We modeled the particle propagation in the Earth’s magnetosphere and atmosphere using a newly computed and verified NM yield function computed at several altitudes above sea level. The solar protons spectra and pitch angle distributions were obtained in their dynamical development throughout the event. We assessed the radiation dose at flight altitude and compared the results with experimental measurements performed with the Liulin gamma probe.