Thermospheric Mass Density Retrieved From Satellite Precise Orbit Determination Data

Thermospheric variations can significantly affect the determination and prediction of satellite orbits.Empirical models,which are considered to be with large error,are widely used to calculate the atmospheric perturbation force on a low Earth orbit satellite.In this work,we mainly explored the methods of deriving thermospheric density from satellite precise orbit determination(POD)data and accelerometer data.Furthermore,we estimated the influence of the possible bias in the temperature from the MSIS00 model on the calculation of drag coefficient and the density retrieval.Accelerometer and POD data from CHAMP and GRACE satellites were used to retrieve thermospheric density in this work.A new method is introduced to retrieve atmospheric density from the POD data on the basis of orbital energy balance.Without numerical integration,this method has high computational efficiency,and the retrieval process can be used to calibrate CHAMP accelerometer data.Furthermore,the calibrated accelerometer data is used to retrieve thermospheric density with high precision.Since the accelerometer data is of high accuracy and space-time resolution,the thermospheric densities derived from accelerometer data are considered as the reference densities,which are compared with those from POD data.The accuracy of density retrieval from POD data based on energy balance method depends on integration time span.With a long time interval leading to large energy decay,the impact of orbit determination error is relatively small,although the spatial-temporal resolution is reduced.It was found that the optimal time interval for density retrieval from CHAMP POD data during 2007-2009 is about 20 minutes,while it is about 5 minutes for GRACE POD data,since the orbit determination accuracy is higher.The accuracy of POD density retrieval is also associated with satellite altitude and local time.The accuracy becomes higher when the satellite flies around local noon and when the altitude is low,because of the high atmospheric density causing large energy decay.Thermospheric densities retrieved from POD data are used to study the thermospheric response to geomagnetic activity,and the results are consistent with those from accelerometer data.Our results show that thermospheric densities from both POD data and accelerometer data are about 25%higher compared with those from the MSIS00 model.This indicates that exospheric temperature from the MSIS00 model is possibly overestimated during 2007-2009.Given that the exospheric temperature is an important parameter to calculate atmospheric drag coefficient,it is necessary to analyze the impact of exospheric temperature deviation from the MSIS00 model on the density retrieval.The densities derived from accelerometer data are used to calculate the corresponding exospheric temperature.The exospheric temperature from accelerometer density,which is about 15%higher than the MSIS00 model,is then used to re-calculate the drag coefficient.The densities retrieved from accelerometer data are also updated using the modified drag coefficient.Our results show that the resultant error due to this prior temperature effect in the retrieved density is about 1?2%,and the maximum difference reaches as large as 5%.Thus,the density retrieval error associated with this temperature bias could have a significant effect on the study of long-term variation of the thermosphere and satellite orbit prediction for a long period.