Tethered unmanned aerial platforms (tUAVs) offer distinct operational advantages compared to autonomous UAVs, particularly their ability to maintain continuous, long-duration missions through ground-based power supply. This capability makes them ideal for various applications where persistent aerial presence is required. However, ensuring operational reliability becomes critical during Global Navigation Satellite System (GNSS) signal degradation, which may occur due to environmental factors or intentional interference. This study presents a novel solution through the development of a laser-based Local Navigation System (LNS), with particular focus on its implementation for tethered high-altitude telecommunication platform. We conduct a comprehensive reliability assessment using established risk tree methodology, which enables quantitative modeling of risk event occurrence times, calculation of key statistical parameters (mean, variance) and estimation of potential damage impacts. The probabilistictemporal characteristics derived from this analysis provide system operators with critical reliability metrics for operational decision-making and risk mitigation.