Thermospheric behavior at low altitude during the deep minimum of solar cycle 24: some operational experiences

Pardini C., Anselmo L.

Thermosphere  Density models  Low Earth orbit  Satellite decay  Solar Cycle 24  Minimum  Biases 

At the beginning of 2018 (Tiangong-1) and 2019 (Electron Second Stage 2018-010D) we had the occasion of participating to a couple of international reentry prediction test campaigns, carried out during a low minimum of solar activity, characterized by a nearly stable solar flux at 10.7 cm of about 70 standard flux units. With these environmental conditions, the accuracy of the reentry predictions was significantly affected by the more or less predictable occurrence of minor - G1 class - geomagnetic storms, and the operational impact of these uncertainties, in particular for events with civil protection implications, as that involving the Chinese space station, will be presented. In the case of Tiangong-1, due to the relevance of the object, we started systematic reentry predictions at the beginning of 2017, i.e. at an altitude (> 350 km) much higher than the initial one generally found in typical reentry prediction campaigns. This offered the occasion to "probe" the behavior of some atmospheric density models - in particular between 350 km and 250 km, where Tiangong-1 spent more than 1 year - in conditions of quite low solar activity and cold thermosphere. Estimates of the residual lifetime during 2017 and until February 2018 were performed using the NRLMSISE-00 model. Since 1 February 2018, also the GOST2004 model was employed for comparison reasons and for checking the mutual consistency of the forecasts. Concerning the predictions carried out with NRLMSISE-00 during the year 2017 and until the beginning of February 2018, they systematically obtained a nominal re-entry epoch earlier than the actual one. Therefore, a post-reentry analysis was performed to investigate whether the cause of the systematic earlier predicted reentry times was due to a bias in the density model used, or to errors in the forecasts of solar and geomagnetic activity. The second possibility was first verified by replacing the predicted values of the solar and geomagnetic indices with the observed ones, and hence by re-propagating the trajectory under true environmental conditions. In this case, still using NRLMSISE-00, the re-computed nominal reentry times were indeed delayed with respect to the original predictions. Therefore, a "true" representation of the environment would have led to an improvement of the forecasts, by obtaining nominal reentry times closer to the actual one. However, all the revised predictions adopting the real space weather conditions were still systematically anticipated with respect to the actual reentry epoch. The other test consisted in changing the density model used for the thermosphere. For such a posteriori analysis, also JB2008 and GOST2004 were considered, in addition to NRLMSISE-00, to re-calculate the nominal reentry times. Nevertheless, the three density models behaved in a similar way, with a maximum discrepancy, in terms of residual lifetime, of about 9%. In other words, all three found a residual lifetime systematically shorter than the actual one. A possible explanation might be that, under the environmental conditions encountered by Tiangong-1 during its uncontrolled decay (i.e. a deep minimum of the solar activity cycle), each atmospheric model analyzed was probably affected by an intrinsic density bias, with a negative derivative with respect to the geodetic altitude, at least down to 270 km. This lead to a percentage increase in the estimation of the atmospheric density as the spacecraft height decreased, neutralizing the absorption of the bias at higher altitudes through the previous ballistic parameter calibrations and causing the systematic underestimation of the residual lifetime recorded up to the beginning of February 2018.

Source: COSPAR 2021 - 43rd COSPAR Scientific Assembly 2021 (Hybrid), Sydney, Australia, 28/01/2021 - 04/02/2021