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The Global Positioning System (GPS) satellite differential code bias (DCB) should be precisely calibrated when obtaining ionospheric slant total electron content (TEC). So far, it is ground-based GPS observations that have been used to estimate GPS satellite DCB. With the increased Low Earth Orbit (LEO) missions in the near future, the real-time satellite DCB estimation is a crucial factor in real-time LEO GPS data applications. One alternative way is estimating GPS DCB based on the LEO observations themselves, instead of using ground observations. We propose an approach to estimate the satellite DCB based on Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) and Challenging Minisatellite Payload (CHAMP) GPS observations during the years 2002-2012. The results have been validated through comparisons with those issued by Center for Orbit Determination in Europe (CODE). The evaluations indicate that: The approach can estimate satellite DCB in a reasonable way; the DCB estimated based on CHAMP observations is much better than those on COSMIC observations; the accuracy and precision of DCB show a possible dependency on the ionospheric ionization level. This method is significance for the real-time processing of LEO-based GNSS TEC data from the perspective of real-time applications. [ABSTRACT FROM AUTHOR]

The combination of GPS measurements and high-fidelity dynamic models via a Kalman filter/smoother, known as the reduced dynamic technique, allows 3D positioning of Low Earth Orbiters to the sub-decimeter level. Such accuracies can only be achieved if the GPS data are nearly continuous, post-processed and a dual-frequency receiver is utilized. The focus of this study is to quantitatively analyze the degradations in position accuracy in the presence of various limitations or constraints, which can be brought on by mission hardware limitations, for example, on micro- or nanosatellites. The constraints explored in this study are as follows: the use of single-frequency data only; real-time processing; limited dynamic modeling due to computing capabilities; and non-continuous GPS receiver operation due to power limits. The experiments are conducted with 6-h data arcs for 7 separate days using data from the CHAllenging Mini-Satellite Payload. A 3D root mean square (rms) error of 15 cm is observed in the best-case solution, in which dual-frequency data are post-processed with all available data. Various levels of accuracy degradations are observed as constraints are placed on this best-case solution. The 3D rms error of the post-processed, single-frequency solution is 68 cm and 1.3 m for the real-time, dual-frequency solution. In very challenging environments, for example, with the receiver on for only 10 min of a 90-min orbit, the 3D rms increases to 350 m. [ABSTRACT FROM AUTHOR]

Abstract: The analytical methods have nearly been replaced by the numerical methods due to their higher accuracy and accessibility of computation facilities. The semi-analytical Lagrange method of orbit propagation using f and g series is a competitive alternative to the numerical integration technique if the Lagrange coefficients are derived in a full gravitational field. In this paper, a generalization of the Lagrange method of orbit propagation is introduced. In other words, we introduce a complete form of the Lagrange coefficients in all force fields developed in the spherical harmonics for example full gravitational field of the Earth. The method is numerically compared with the numerical integration technique. In order to show the numerical performance of the method, it has been implemented for orbit propagation of a GPS-like MEO and CHAMP-like LEO satellites. Discrepancy at centimeter level for CHAMP-like and sub-millimeter accuracy for GPS-like satellites shows relatively high performance of the developed algorithm. Compared to integration method, the proposed Lagrange method is nearly faster by a factor two for small Nmax and four for large Nmax. [Copyright &y& Elsevier]

Abstract: The gravity field model AIUB-CHAMP02S, which is based on six years of CHAMP GPS data, is presented here. The gravity field parameters were derived using a two step procedure: In a first step a kinematic trajectory of a low Earth orbiting (LEO) satellite is computed using the GPS data from the on-board receiver. In this step the orbits and clock corrections of the GPS satellites as well as the Earth rotation parameters (ERPs) are introduced as known. In the second step this kinematic orbit is represented by a gravitational force model and orbit parameters. In order to ensure full model consistency the GPS satellite orbits and clock corrections, which have been used for the generation of the kinematic LEO trajectories, were taken from the Center for Orbit Determination in Europe (CODE), located at AIUB (). In recent years many changes have taken place in the processing chain of global navigation satellite system (GNSS) data, e.g., the implementation of absolute antenna phase center modeling. Therefore a reprocessing of the GPS data to obtain state-of-the-art GPS satellite orbits and clock corrections was performed. From these updated GPS products new kinematic orbits of the CHAMP satellite were derived for the years 2002–2007. From the updated CHAMP trajectories spherical harmonic (SH) coefficients of the Earth’s gravity field were determined in exactly the same way as from the original LEO orbit. This allowed us to study the impact of the improved LEO orbits on the derived gravity field parameters and the generation of the multi-year gravity field model AIUB-CHAMP02S. The change of the IGS standards creates an inconsistency to existing global gravity field models, which mainly affects the zonal coefficients of low even degrees. The inconsistency is caused by the change to the absolute antenna phase center model and can be reduced by estimating the phase center variation of the CHAMP GPS antenna. [Copyright &y& Elsevier]

Abstract: Current and planned Earth observation missions are equipped with highly sensitive accelerometers. Before using the data, the instrument has to be calibrated by determining scale and bias parameters for each axis. Here, the accelerometer measurements are used in a GPS-based reduced-dynamic orbit determination approach, replacing the non-gravitational force models, and nominally daily calibration parameters are estimated. Additional empirical accelerations are estimated to account for deficiencies in the applied force models. This method is applied to 5 years of CHAMP and GRACE data, resulting in an orbit precision at the level of a few centimeters. In along-track direction the calibration parameters can be estimated freely, scale factors of 0.96±0.014 and 0.95±0.015 are obtained for GRACE A and B, and 0.85±0.024 for CHAMP. A constant scale factor results in the smoothest bias series, with clear trends and occasional jumps. In radial and cross-track direction tight constraints to a priori biases have to be applied. Furthermore, the determined orbits are analyzed with respect to reference trajectories, and SLR, phase and KBR residuals are presented. [Copyright &y& Elsevier]

Abstract: Ongoing and future geopotential space missions are equipped with one or more accelerometers. In order to use these observations, the accelerometer measurements have to be calibrated before processing them. In this paper they are introduced in the GPS based precise orbit determination, by replacing the non-gravitational force models for atmospheric drag and solar radiation pressure. Empirical accelerations are still estimated to account for deficiencies in the applied conservative force models. The in-orbit calibrated accelerometer observations are used to validate the accelerations determined by force modeling. In along-track direction they show the best agreement. During days of high solar activity the benefit of using accelerometer observations is clearly visible. The observations during these days show high frequency fluctuations which the modeled and empirical accelerations cannot follow. A long period of GRACE (second half of 2003) and of CHAMP (2004) data is processed. This results for GRACE in a mean orbit fit of a few centimeters with respect to high-quality JPL reference orbits, showing a slightly better consistency compared to the case when using force models, which is also supported by SLR residual analysis. The daily calibration factors determined with this technique show a small variation. When not including empirical accelerations in the estimation procedure, the calibration factors in radial and cross-track direction show a bigger spread, with an orbit fit below the decimeter level. [Copyright &y& Elsevier]

Abstract: Apart from the gradiometer as the core instrument, the first ESA Earth Explorer Core Mission GOCE (Gravity field and steady-state Ocean Circulation Explorer) will carry a 12-channel GPS receiver dedicated for precise orbit determination (POD) of the satellite. The EGG-C (European GOCE Gravity-Consortium), led by the Technical University in Munich, is building the GOCE HPF (High-level Processing Facility) dedicated to the Level 1b to Level 2 data processing. One of the tasks of this facility is the computation of the Precise Science Orbit (PSO) for GOCE. The PSO includes a reduced-dynamic and a kinematic orbit solution. The baseline for the PSO is a zero-difference procedure using GPS satellite orbits, clocks, and Earth Rotation Parameters (ERPs) from CODE (Center for Orbit Determination in Europe), one of the IGS (International GNSS Service) Analysis Centers. The scheme for reduced-dynamic and kinematic orbit determination is based on experiences gained from CHAMP and GRACE POD and is realized in one processing flow. Particular emphasis is put on maximum consistency in the analysis of day boundary overlapping orbital arcs, as well as on the higher data sampling rate with respect to CHAMP and GRACE and on differences originating from different GPS antenna configurations. We focus on the description of the procedure used for the two different orbit determinations and on the validation of the procedure using real data from the two GRACE satellites as well as simulated GOCE data. [Copyright &y& Elsevier]

Abstract: Extensive recovery experiments based on GPS satellite-to-satellite tracking data from the CHAMP and GRACE-A satellites show that the performance of GPS-based accelerometry is comparable for both satellites. Several different quality measures like the recovery error, correlation and the contribution measure are used to assess the performance of GPS-based accelerometry and these quality measures indicate that the best performance is obtained in along-track direction. In cross-track direction the performance is slightly worse and due to correlations between the accelerations and the initial conditions in cross-track direction, a bias in this direction seems hardly observable. Unfortunately, GPS-based accelerometry is hardly sensitive in radial direction, due to correlations between the accelerations in the radial direction and the initial conditions in radial and along-track direction. It is shown that predominantly the longer wavelengths are well determined and high-frequency accelerations, caused by e.g. geomagnetic storms, are not well recovered. [Copyright &y& Elsevier]

Abstract: Ground- and space-based GPS receiver observations together with non-GPS observations can be adjusted in an integrated solution estimating orbits and Earth parameters simultaneously in a dynamic model. Especially reference frame and low degree gravity field parameters profit in accuracy when compared to a solution where GPS ephemerides and clocks are separately estimated before treating the low Earth orbiters (LEOs). In this analysis, we integrate GPS constellation observations with CHAMP, GRACE and SAC-C data. Procedural problems are discussed as well as results concerning LEO orbit and Earth system parameter estimates. Based on 7 months of CHAMP data, time series of the geocenter location and the Earth’s principal axes of inertia are computed and the results discussed. It turns out that the time series of the X and Y components of the geocenter and the orientation of the Earths’ primary axis of inertia can be resolved showing geophysically meaningful sizes and frequencies. [Copyright &y& Elsevier]

Abstract: Extensive recovery experiments based on real CHAMP GPS satellite-to-satellite tracking (SST) data show that instead of directly measuring non-gravitational accelerations using an onboard accelerometer, it is also possible to derive these accelerations indirectly from GPS SST observations. The recent gravity field model improvements from the CHAMP and GRACE missions contribute significantly to a more accurate recovery. Using the first published GRACE models, non-gravitational along-track accelerations were recovered with a precision better than 10% of the total signal at a time resolution of 1min. The quality of the recovered accelerations depends strongly on proper regularisation of the associated normal equations. The possibility of using overlap analysis for deriving the regularisation parameters was assessed, indicating that for the moment this method is sub-optimal. [Copyright &y& Elsevier]

The availability of GPS (Global Positioning System) radio signals has introduced a new promising remote sensing technique for the Earth’s atmosphere. GPS-based radio occultation exploits GPS signals received onboard a Low Earth Orbiting (LEO) satellite for atmospheric limb sounding. Temperature and water vapour profiles with high accuracy and vertical resolution can be derived from these measurements. The GPS radio occultation technique requires no calibration, is not affected by clouds, aerosols or precipitation, and the occultations are almost uniformly distributed over the globe. The radio occultation experiment aboard the German CHAMP (CHAllenging Minisatellite Payload) satellite provided its first measurements in February 2001. Since then up to 250 daily occultation measurements were performed. About 70% of these were successfully processed to yield atmospheric temperature and water vapour profiles. The CHAMP radio occultation experiment demonstrated the immense potential of this technique for the global monitoring of stratospheric temperature, the tropopause region and the global humidity distribution in the troposphere. All of these are important variables of the climate system. In this paper the GPS radio occultation experiment onboard CHAMP will be described. At GeoForschungsZentrum (GFZ) Potsdam an operational system was established to process CHAMP radio occultation data, orbit data, and GPS ground station observations in an automatic and operational way to derive profiles of atmospheric parameters. The CHAMP temperature bias compared to ECMWF analyses is less than 1.5 K between the tropopause layer and the 30 hPa level, and less than 0.5 K between 12 and 20 km at mid and high latitudes. Further comparisons between CHAMP atmospheric profiles and radiosonde data as well as applications of CHAMP radio occultation data for atmospheric research and climate monitoring are given. [Copyright &y& Elsevier]

A method for kinematic point positioning of satellites in low Earth orbit (LEO) based on single frequency GPS measurements is discussed. It makes use of a ionosphere-free combination (GRAPHIC) of C/A code and L1 phase observations and can thus be employed with the majority of non-geodetic spaceborne GPS receivers. Other than dynamical Kalman filter schemes estimating position, velocity and clock parameters along with a set of biases for all tracked channels, a purely kinematic approach is examined in this study. It comprises a global adjustment of positions, clock offsets and pass-by-pass biases from the observations. As such, it is free of any assumptions on the motion of the user vehicle and can be applied for free-flying as well as maneuvering spacecraft. Sample results are shown for LEO GPS data collected with a low-cost single frequency receiver in a signal simulator test bed as well as flight data from the CHAMP mission. In both cases, a purely kinematic 3D position accuracy of 1–1.5 m could be demonstrated in comparison with concise reference trajectories. [Copyright &y& Elsevier]