Highly Elliptical Orbits for Arctic observations: Assessment of ionizing radiation

Abstract

The ionizing radiation environment was analyzed for a variety of potential Highly Elliptical Orbits (HEOs) with orbital periods ranging from 6h to 24h suitable to continuously monitor the Arctic region. Several models available from the ESA Space Environment Information System (SPENVIS) online tool were employed, including the new-generation AE9/AP9 model for trapped radiation. Results showed that the Total Ionizing Dose (TID) has a well-pronounced local minimum for the 14-h orbit, which is nearly identical to the overall minimum observed for the longest orbital period (24h). The thickness of slab aluminum shielding required to keep the annual TID below 10, 5 and 3.33krad (i.e. 150, 75 and 50krad for 15years of mission duration) for a 14-h orbit is 2.1, 2.7 and 3.1mm respectively. The 16-h orbit requires an additional 0.5mm of aluminum to achieve the same results, while the 24-h orbit requires less shielding in the order of 0.2–0.3mm. Comparison between the AE8/AP8 and AE9/AP9 models was conducted for all selected orbits. Results demonstrated that differences ranged from −70% to +170% depending on orbit geometry.The vulnerability to the Single Event Effect (SEE) was compared for all orbits by modeling the Linear Energy Transfer (LET) for long-term conditions and for the 5min “worst case” scenario. The analysis showed no preference among orbits with periods longer than 15h, and in order to keep the 14-h orbit at the same level, the shielding should be increased by ∼33% or approximately by 1mm. To keep the Single Event Upset (SEU) rate produced by the “worst case” event at the same order of magnitude as for the “statistical” long-term case, the thickness of aluminum should be as high as 22mm. The overall conclusion from a space environment point of view is that all HEO orbits with periods equal to or longer than 14h can be regarded as good candidates for operational missions. Therefore, selection of orbit should be based on other criteria, for example, uniformity of spatial coverage for meteorological imaging or the configuration of the ground network for data reception.