During explosive eruptions, the real-time estimation of the mass eruption rate (MER) is challenging although crucial to mitigate the impact of erupted tephra. Microwave radar techniques at L and/or X bands, as well as thermal infrared imagery, can provide a reliable MER estimation in real time. Using lava fountains of 3-5 December 2015 at Mt. Etna (Italy) as test cases, we investigate the differences among all these remote sensing methods and introduce a new approach, called the Near Source Approach (NSA) using only X-band radar data. We also extend the Volcanic Advanced Radar Retrieval (VARR) methodology to estimate the gas-tephra mixture density near the volcanic crater. The analysis of uncertainty is carried out comparing the NSA with the Mass Continuity Approach (MCA), Top Plume Approach (TPA) and Surface Flux approach (SFA), already used to estimate the MER of other Etna explosive events. The analysis allows us to identify the optimal real-time MER retrieval strategy, showing the potential and limitations of each method. We show that the MCA method, entirely based on the Xband radar data processing, is the best strategy with a percentage uncertainty in the MER estimation of 22.3%, whereas other approaches exhibit e a higher uncertainty (26.4% for NSA, 30% for TPA and 31.6% for SFA).