Metop-A Lifetime Extension - Surviving on a Drifting LTAN

摘要

In order to maximize the utilization of existing assets and help guarantee the transition to next generation systems, there is a desire to extend the lifetime of the Metop-A spacecraft, which along with Metop-B and the soon to be launched Metop-C comprises the space segment of the current EUMETSAT Polar System (EPS). Despite being specifically designed for a Sun-Synchronous orbit with an LTAN of 21:30 and having the fuel to maintain that LTAN for at least more 10 years, a combination of system level constraints and a desire to adhere to ISO 24113 debris mitigation guidelines means that the only feasible strategy for a lifetime extension is to allow Metop-A's LTAN to drift outside its design envelope. This paper identifies the main impacts of operating on a drifting LTAN, with detailed analysis concentrated on the critical Power and Thermal aspects in order to determine the permissible lifetime extension. A Key finding presented in this paper is that while the LTAN is drifting from 21:30 to earlier LTANs, the integrated solar flux falling on the spacecraft body and solar arrays remains almost constant, and there is very little overall impact: Shortening eclipses almost exactly compensate for the less favorable angle of incidence of sunlight. This indicates that there are generally regions of LTAN where the Power and Thermal situation can be inherently stable for any polar orbiting spacecraft. Once eclipses disappear however, both energy margins and temperatures will drop very rapidly with small changes in LTAN, and seasonal variations can dramatically exacerbate this effect - there are situations for example where the Metop-A orbital energy margins would make the transition from having been continuously at near nominal values to critical within just 6 weeks. Due to the uncertainty outside the inherently stable LTAN region which has been identified, the Metop-A Lifetime is extension is currently targeted for the end of 2021, which will mean a total of 15 years in orbit. Options to extend to 2022 and beyond are pending more detailed analysis.