J. van den IJssel

Champ precise orbit determination using GPS data

Abstract Precise CHAMP orbits have been computed in the framework of the IGS/LEO CHAMP Orbit Comparison Campaign for a period of 11 days in May 2001. A reduced-dynamic orbit determination strategy has been applied based on ionospheric-free triple-differenced GPS phase measurements along with precise GPS orbits computed by the International GPS Service (IGS). The resulting CHAMP orbit accuracy is assessed using GPS observation residuals, orbit overlap statistics, independent satellite laser ranging (SLR) residuals and comparisons with orbits computed by other institutes. These evaluations indicate a 3D orbit accuracy of the DEOS orbit solution at the sub-decimeter level.

Verification of champ accelerometer observations

Abstract CHAMP is the first satellite that provides high-accuracy accelerometer observations with the aim of separating non-conservative accelerations from gravity when analyzing the CPS satellite-to-satellite tracking (SST) observations. An accuracy assessment of the accelerometer observations is required in order to find the best strategy for incorporation of these observations in the gravity field estimation. A number of methods has been investigated to estimate accelerometer bias and scale factor values. First, the accelerometer observations were compared with non-conservative accelerations predicted by an atmospheric density model. Second, bias and scale factors were estimated in CHAMP precise orbit determination where the accelerometer observations were used to complement the gravity and other conservative force models. Third, these accelerometer parameters were estimated in a gravity field model adjustment experiment. The sensitivity of all three methods with respect to the a priori gravity field model was investigated. It was found that the most stable accelerometer parameter values were obtained with the first method when using a priori gravity field models in which already use was made of CHAMP CPS SST and accelerometer data. In case of relatively large a priori gravity field model errors, the third method indicates that simultaneous gravity field and accelerometer parameter estimation is required in order to obtain reasonable bias and scale factor values, supporting the results obtained with the first method. All methods indicate that for the radial, along-track and cross-track accelerometer components different bias values need to be applied, but that the scale factors seem to converge to one common value equal to about 0.8.

Calibration and validation of individual GOCE accelerometers by precise orbit determination

The European Space Agency Gravity field and steady-state Ocean Circular Explorer (GOCE) carries a gradiometer consisting of three pairs of accelerometers in an orthogonal triad. Precise GOCE science orbit solutions (PSO), which are based on satellite-to-satellite tracking observations by the Global Positioning System and which are claimed to be at the few cm precision level, can be used to calibrate and validate the observations taken by the accelerometers. This has been done for each individual accelerometer by a dynamic orbit fit of the time series of position co-ordinates from the PSOs, where the accelerometer observations represent the non-gravitational accelerations. Since the accelerometers do not coincide with the center of mass of the GOCE satellite, the observations have to be corrected for rotational and gravity gradient terms. This is not required when using the so-called common-mode accelerometer observations, provided the center of the gradiometer coincides with the GOCE center of mass. Dynamic orbit fits based on these common-mode accelerations therefore served as reference. It is shown that for all individual accelerometers, similar dynamic orbit fits can be obtained provided the above-mentioned corrections are made. In addition, accelerometer bias estimates are obtained that are consistent with offsets in the gravity gradients that are derived from the GOCE gradiometer observations.

Aiming at a 1-cm orbit for low earth orbiters: Reduced-dynamic and kinematic precise orbit determination

The computation of high-accuracy orbits is a prerequisite for the success of Low Earth Orbiter (LEO) missions such as CHAMP, GRACE and GOCE. The mission objectives of these satellites cannot be reached without computing orbits with an accuracy at the few cm level. Such a level of accuracy might be achieved with the techniques of reduced-dynamic and kinematic precise orbit determination (POD) assuming continuous Satellite-to-Satellite Tracking (SST) by the Global Positioning System (GPS). Both techniques have reached a high level of maturity and have been successfully applied to missions in the past, for example to TOPEX/POSEIDON (T/P), leading to (sub-)decimeter orbit accuracy. New LEO gravity missions are (to be) equipped with advanced GPS receivers promising to provide very high quality SST observations thereby opening the possibility for computing cm-level accuracy orbits. The computation of orbits at this accuracy level does not only require high-quality GPS receivers, but also advanced and demanding observation preprocessing and correction algorithms. Moreover, sophisticated parameter estimation schemes need to be adapted and extended to allow the computation of such orbits. Finally, reliable methods need to be employed for assessing the orbit quality and providing feedback to the different processing steps in the orbit computation process.

Data analysis for the GOCE mission

We investigate the time-wise approach to the data anlaysis for the GOCE mission. The number of observations collected during the mission, the number of potential coefficients to be estimated, and the complexity of the mathematical model for the time-wise approach require a special strategy, which has to reduce the CPU-time and storage requirements considerably. Our approach is based on (1) the iterative solution of the normal equations using a Richardson-iteration scheme and (2) the approximation of the design matrix in order to assemble the right-hand side in each iteration step efficiently. We demonstrate the performance of the approach for white noise and coloured noise observations along a simulated GOCE orbit up to degree and order 180. We provide error estimates anal show that the solution is unbiased. We also prove that the method does not converge to the solution of the normal equations. However, the approximation error can be neglected in our simulations.

Global Ionospheric and Thermospheric Effects of the June 2015 Geomagnetic Disturbances: Multi‐Instrumental Observations and Modeling

Abstract By using data from multiple instruments, we investigate ionospheric/thermospheric behavior during the period from 21 to 23 June 2015, when three interplanetary shocks (IS) of different intensities arrived at Earth. The first IS was registered at 16:45 UT on 21 June and caused ~50 nT increase in the SYM‐H index. The second IS arrived at 5:45 UT on 22 June and induced an enhancement of the auroral/substorm activity that led to rapid increase of thermospheric neutral mass density and ionospheric vertical total electron content at high latitudes. Several hours later, topside electron content and electron density increased at low latitudes on the nightside. The third and much larger IS arrived at 18:30 UT on 22 June and initiated a major geomagnetic storm that lasted for many hours. The storm provoked significant effects in the thermosphere and ionosphere on both dayside and nightside. In the thermosphere, the dayside neutral mass density exceeded the quiet time levels by 300–500%, with stronger effects in the summer hemisphere. In the ionosphere, both positive and negative storm effects were observed on both dayside and nightside. We compared the ionospheric observations with simulations by the coupled Sami3 is Also a Model of the Ionosphere/Rice Convection Model (SAMI3/RCM) model. We find rather good agreement between the data and the model for the first phase of the storm, when the prompt penetration electric field (PPEF) was the principal driver. At the end of the storm main phase, when the ionospheric effects were, most likely, driven by a combination of PPEF and thermospheric winds, the modeling results agree less with the observations.