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The present contribution investigates the possibility to obtain thermospheric neutral density estimates using satellite laser ranging (SLR) observations of low Earth orbiters (LEOs). This approach is based on the analysis of the satellite atmospheric drag, driven by the fact that the drag force is the largest non-gravitational perturbation acting on LEOs. Due to the uncertainty of current thermospheric models, it is the main error source in the LEO orbit determination process. Moreover, the drag is physically related to the thermospheric density distribution, the interaction of the satellite surface with the surrounding thermosphere and thermospheric winds. For this investigation, a spherical satellite called "Atmospheric Neutral Density Experiment-Pollux" (ANDE-P) developed by the Naval Research Laboratory (USA) is adopted as a case study. The satellite flew at the very low altitude of about 350 km. The most important perturbing acceleration at this altitude, the atmospheric drag, is easier to model for a spherical satellite like ANDE-P than for a satellite of complex geometry. A precise orbit determination of ANDE-P was performed with the DGFI Orbit and Geodetic parameter estimation Software (DOGS) over a period of 49 days (16 August 2009 until 3 October 2009) using SLR observations to this satellite and different thermospheric models. In total, we tested four thermospheric models, namely CIRA86, NRLMSISE00, JB2008 and DTM2013. Correspondingly, scale factors of these reference models are estimated with a 6-h resolution. The results confirm that the estimation of force model parameters from SLR measurements of the ANDE-P satellite is sensitive to differences in the density distributions provided by different models. As a consequence, information on the discrepancies between the various models and the true density can be derived from SLR measurements. Moreover, it is found that SLR observations to LEO satellites at very low altitudes are capable to estimate corrections to (scale factors of) the integrated thermospheric density if all other perturbing accelerations are modelled with sufficient accuracy. We derived time series of estimated scale factors of the thermospheric densities provided by the models and obtained the following mean values during the processed period of time at the ANDE-P altitude: 0.65±0.26 for CIRA86, 0.65±0.25 for NRLMSISE00, 0.79±0.24 for DTM2013, and 0.89±0.27 for JB2008. This suggests that all models overestimate the true thermospheric density along the ANDE-P trajectory during the processed period to a certain extent. The thermospheric densities need to be scaled downwards to fit ANDE-P SLR observations with JB2008 requiring the least amount of scaling. [ABSTRACT FROM AUTHOR]

For low Earth orbiting satellites, non-gravitational forces cause one of the largest perturbing accelerations. During a precise orbit determination (POD), the accurate modeling of the satellite-body attitude and solar panel orientation is important since the satellite's effective cross-sectional area is directly related to the perturbing acceleration. Moreover, the position of tracking instruments that are mounted on the satellite body are affected by the satellite attitude. For satellites like Jason-1/-2/-3, attitude information is available in two forms—as a so-called nominal yaw steering model and as observation-based (measured by star tracking cameras) quaternions of the spacecraft body orientation and rotation angles of the solar arrays. In this study, we have developed a preprocessing procedure for publicly available satellite attitude information. We computed orbits based on Satellite Laser Ranging (SLR) observations to the Jason satellites at an overall time interval of approximately 25 years, using each of the two satellite attitude representations. Based on the analysis of the orbits, we investigate the influence of using preprocessed observation-based attitude in contrast to using a nominal yaw steering model for the POD. About 75% of all orbital arcs calculated with the observation-based satellite attitude data result in a smaller root mean square (RMS) of residuals. More precisely, the resulting orbits show an improvement in the overall mission RMS of SLR observation residuals of 5.93% (Jason-1), 8.27% (Jason-2) and 4.51% (Jason-3) compared to the nominal attitude realization. Besides the satellite orbits, also the estimated station coordinates benefit from the refined attitude handling, that is, the station repeatability is clearly improved at the draconitic period. Moreover, altimetry analysis indicates a clear improvement of the single-satellite crossover differences (6%, 15%, and 16% reduction of the mean of absolute differences and 1.2%, 2.7%, and 1.3% of their standard deviations for Jason-1/-2/-3, respectively). On request, the preprocessed attitude data are available. [ABSTRACT FROM AUTHOR]

Continuous monitoring of the perpetual changing Earth is one of the key topics of present Earth System research. In the past, the satellite mission Gravity Recovery And Climate Experiment (GRACE) was measuring the time-variable gravity field of the Earth with outstanding performance in accuracy and reliability. On October 27 in 2017, GRACE de-orbited and this very successful geodetic satellite mission ended. On May 22 in 2018, the successor mission GRACE-FO (Follow On) was launched and scientific data will be provided soon to the users. Between the two satellite missions, there is a gap of more than six months. Beside the GRACE and other satellite missions, several analysis centers (ACs) worldwide process laser ranging (SLR) data to spherical satellites orbiting the Earth which allow a very accurate estimation of the low-degree spherical harmonics of the Earth's gravity field. Up to now, the available time series are stand-alone solutions which co-exist to each other.In this investigation, we will combine the SLR-derived time series of low-degree spherical harmonics from different SLR analysis centers and, moreover, combine them with the GRACE data in order to extend the continuous time series of the monthly gravity field in the GRACE/GRACE-FO gap and into the past (before the launch of GRACE in 2002). We will compare the combined SLR time series to GRACE monthly solutions and evaluate the impact of different solution setups like the SLR-orbit length, a priori models, etc. on the results. Additionally, we study the benefit of a combined SLR time series compared to the AC stand-alone solutions and motivate the advantages of such a combination in the future! [ABSTRACT FROM AUTHOR]