Jose van den IJssel

Neutral Density and Crosswind Determination from Arbitrarily Oriented Multiaxis Accelerometers on Satellites

An iterative algorithm for determining density and crosswind from multiaxis accelerometer measurements on satellites is presented, which works independently of the orientation of the instrument in space. The performance of the algorithm is compared with previously published algorithms using simulated data for the challenging minisatellite payload. Without external error sources, the algorithm reduces rms density errors from 0.7 to 0.03% andrmswinderrorsfrom38to1 m=sinthistest.However,theeffectsoftheerrorsintheinstrumentcalibrationand the external models that are used in the density and wind retrieval are dominant for the challenging minisatellite payload. These lead to mostly systematic density errors of the order of 10–15%. The accuracy of the wind results whenusingthenewalgorithmisalmostfullydeterminedbythesensitivityofthecross-trackaccelerationcomponent to the calibration and radiation pressure modeling errors. The applicability of the iterative algorithm and the accuracyofits resultsaredemonstrated bypresenting challenging minisatellite payload datafromaperiodin which thesatellitewascommandedto flysidewaysandbycomparingthedensityandwindresultswiththosefromadjacent days for which the satellite was in its nominal attitude mode. These investigations result in recommendations for the design of future satellite accelerometer missions for thermosphere research.

The Swarm Satellite Constellation Application and Research Facility (SCARF) and Swarm data products

Swarm, a three-satellite constellation to study the dynamics of the Earth’s magnetic field and its interactions with the Earth system, is expected to be launched in late 2013. The objective of the Swarm mission is to provide the best ever survey of the geomagnetic field and its temporal evolution, in order to gain new insights into the Earth system by improving our understanding of the Earth’s interior and environment. In order to derive advanced models of the geomagnetic field (and other higher-level data products) it is necessary to take explicit advantage of the constellation aspect of Swarm. The Swarm SCARF (SatelliteConstellationApplication andResearchFacility) has been established with the goal of deriving Level-2 products by combination of data from the three satellites, and of the various instruments. The present paper describes the Swarm input data products (Level-1b and auxiliary data) used by SCARF, the various processing chains of SCARF, and the Level-2 output data products determined by SCARF.

Thermospheric density and wind retrieval from Swarm observations

The three-satellite ESA Swarm mission aims at mapping the Earth’s global geomagnetic field at unprecedented spatial and temporal resolution and precision. Swarm also aims at observing thermospheric density and possibly horizontal winds. Precise orbit determination (POD) and Thermospheric Density and Wind (TDW) chains form part of the Swarm Constellation and Application Facility (SCARF), which will provide the so-called Level 2 products. The POD and TDW chains generate the orbit, accelerometer calibration, and thermospheric density and wind Level 2 products. The POD and TDW chains have been tested with data from the CHAMP and GRACE missions, indicating that a 3D orbit precision of about 10 cm can be reached. In addition, POD allows to determine daily accelerometer bias and scale factor values with a precision of around 10–15 nm/s2 and 0.01–0.02, respectively, for the flight direction. With these accelerometer calibration parameter values, derived thermospheric density is consistent at the 9–11% level (standard deviation) with values predicted by models (taking into account that model values are 20–30% higher). The retrieval of crosswinds forms part of the processing chain, but will be challenging. The Swarm observations will be used for further developing and improving density and wind retrieval algorithms.

Determination of Non-Conservative Accelerations from Orbit Analysis

It is shown by means of an extensive simulation study as well as an experiment using real CHAMP data that it is feasible to accurately estimate non-conservative accelerations from precise GPS-based orbit perturbations. Assuming the availability of high-precision gravity field models, such as anticipated for GRACE and GOCE, an accuracy of better than 50 nm/s2 seems possible for 30-seconds averaged accelerations. The remaining dominant error sources seem to be GPS receiver carrier-phase noise and GPS ephemeris errors.

Swarm accelerometer data processing from raw accelerations to thermospheric neutral densities

Abstract The Swarm satellites were launched on November 22, 2013, and carry accelerometers and GPS receivers as part of their scientific payload. The GPS receivers do not only provide the position and time for the magnetic field measurements, but are also used for determining non-gravitational forces like drag and radiation pressure acting on the spacecraft. The accelerometers measure these forces directly, at much finer resolution than the GPS receivers, from which thermospheric neutral densities can be derived. Unfortunately, the acceleration measurements suffer from a variety of disturbances, the most prominent being slow temperature-induced bias variations and sudden bias changes. In this paper, we describe the new, improved four-stage processing that is applied for transforming the disturbed acceleration measurements into scientifically valuable thermospheric neutral densities. In the first stage, the sudden bias changes in the acceleration measurements are manually removed using a dedicated software tool. The second stage is the calibration of the accelerometer measurements against the non-gravitational accelerations derived from the GPS receiver, which includes the correction for the slow temperature-induced bias variations. The identification of validity periods for calibration and correction parameters is part of the second stage. In the third stage, the calibrated and corrected accelerations are merged with the non-gravitational accelerations derived from the observations of the GPS receiver by a weighted average in the spectral domain, where the weights depend on the frequency. The fourth stage consists of transforming the corrected and calibrated accelerations into thermospheric neutral densities. We present the first results of the processing of Swarm C acceleration measurements from June 2014 to May 2015. We started with Swarm C because its acceleration measurements contain much less disturbances than those of Swarm A and have a higher signal-to-noise ratio than those of Swarm B. The latter is caused by the higher altitude of Swarm B as well as larger noise in the acceleration measurements of Swarm B. We show the results of each processing stage, highlight the difficulties encountered, and comment on the quality of the thermospheric neutral density data set.

Validation of fast pre-mission error analysis of the GOCE gradiometry mission by a full gravity field recovery simulation

Abstract Spherical harmonic error analysis fully relies on the validity of the a priori observational and stochastic models. In this paper we validate error analysis results of the gradiometer mission GOCE by a full-fledged spherical harmonic coefficient recovery. Both methods (least squares error analysis and full recovery) are based on a semi-analytical approach. The results compare very well in spectral and spatial domains. Thus, it is demonstrated that, besides being fast, the least squares error analysis is a reliable premission error assessment tool.