The 3rd generation partnership program (3GPP) has in recent years started working on integratingnon-terrestrial networks (NTN) into the 5G eco-system. This thesis focuses on the mobility between NTN and TN, which is of great importance if 5G NTN is to provide seamless and limitless connectivity. The target of this thesis is to understand and improve the mobility performance ofnon-terrestrial and terrestrial networks in a heterogeneous scenario. We first analyze data from system-level simulations of rural deployment scenarios when altering the parameters of the A3 measurement event and and then we further evaluate the use of a new NTN-specific distance-based measurement event, the D1 measurement event. We also evaluate the impact of needing toperform GNSS measurements when performing handovers from a terrestrial to a non-terrestrialnetwork. The results show that acquiring GNSS data during the handover procedure significantly increases handover delay time but does not heavily impact overall network performance. Additionally, the results show that by changing the parameters of the A3 measurement event and using the D1measurement event, ping-pong events between NTN-TN and unnecessary handovers to NTN canbe significantly reduced and improve the overall network performance.