[1] Densities in the 400–500 km height region inferred from accelerometer measurements on the CHAMP and GRACE satellites are utilized to study the response to the isolated and severe geomagnetic storm of 20–21 November 2003. The CHAMP and GRACE satellites provide data at approximate local times of 1110/2310 and 1430/0230 hours, respectively. In a global sense, density increases of order 300–800% occur during this storm, with relatively little time delay at high latitudes and with about 4-hour delay at the equator. Significant latitudinal asymmetries in the response are discussed in the context of neutral wind patterns and enhanced summer versus winter Joule heating rates at high latitudes. Comparisons with the NRLMSISE-00 empirical model densities during this period show marked differences in amplitude of the response, as well as latitudinal and temporal structures. Evidence for a tight coupling between solar wind and neutral density variability is found for the high-latitude summer hemisphere near noon, in that densities near 410 km are very responsive to increases in solar wind dynamic pressure, even during periods when both Bz and By are near zero or positive. Filtering of the data reveals regional-scale (∼1000–2000 km) density anomalies that are common between the CHAMP and GRACE measurements. During the geomagnetic disturbance, alternating regions of density enhancements (∼50%) and depressions (∼50%) exist between the pole and low to middle latitudes that have the appearance of a standing wave pattern. During magnetically quiet intervals before and after the storm, daytime density depressions (∼4–8%) are seen that track the magnetic equator, while density enhancements (∼10%) track the latitudes of the ionospheric Appleton anomaly peaks. Large-scale (≥1000 km) wave-like structures are also prevalent during both day and night during the magnetic disturbance and extend throughout both hemispheres, consistent with the concept that these are associated with the so-called “direct” gravity waves forced in the auroral regions and propagating far from the source. Substantial variability is also present at medium scales (∼300–500 km), but these structures are confined mainly to middle to high latitudes, in accord with theoretical expectations. GRACE-A and GRACE-B data are used in a feasibility study of the detection of small-scale wave activity by taking advantage of the 30-s separation of the satellites in the same orbital plane. Despite the large amount of maneuvers, which are not always recorded, this first analysis is promising. Amplitude variations of order 10–20% and wave speeds of 1000–1500 m s−1 are observed.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005JA011284

Thermosphere density response to the 20-21 November 2003 solar and geomagnetic storm from CHAMP and GRACE accelerometer data

Dedicated gravity field missions—principles and aims

The EGM2008 model is nowadays one of the description of the global gravitational field at the highest resolution. It is delivered with two, not fully consistent, sources of information on its error: spherical harmonic coefficient variances and a geographical map of error variances, e.g. in terms of geoid undulation. In the present work, the gravity field information derived from a GOCE satellite-only global model is used to improve the accuracy of EGM2008 model in the low to medium frequencies, especially in areas where no data were available at the time of EGM2008 computation. The key issue is to set up the error covariance matrices of the two models for an optimal least-squares combination: the full error covariance matrix of GOCE spherical harmonic coefficients is approximated by an order-wise block-diagonal matrix, while for EGM2008, the pointwise error variances are taken from the provided geoid error map and the error spatial correlations from the coefficient variances. Due to computational reasons the combination is directly performed in terms of geoid values over a regular grid on local areas. Repeating the combination for overlapping areas all over the world and then performing a harmonic analysis, a new combined model is obtained. It is called GECO and extends up to the EGM2008 maximum degree. Comparisons with other recent combined models, such as EIGEN-6C4, and a local geoid based on new gravity datasets in Antarctica are performed to evaluate its quality. The main conclusion is that the proposed combination, weighting the different input contributions not only on a global basis but also according to some local error information, can perform even better than other more sophisticated combinations in areas where the input global error description is not reliable enough.

GECO: a global gravity model by locally combining GOCE data and EGM2008

JPL's BlackJack receiver currently represents the most widely used geodetic grade GPS receiver for space applications. Using data from the CHAMP science mission, the in-flight performance of the BlackJack receiver has been assessed and the impact of various software updates performed during the 2.5 years since launch is described. Key aspects of the study comprise the channel allocation, anomalous data points, and the noise level of the code and carrier data. In addition, it has been demonstrated that the code measurements collected onboard the CHAMP satellite are notably affected by multipath errors in the aft-looking hemisphere, which can be attributed to cross-talk between the occultation antenna string and the primary precise orbit determination antenna. For carrier smoothed 10 s normal points, the code noise itself varies between a minimum of 5 cm at high elevations and 0.5 m (C/A) to 1.0 m (P1, P2) at 10° elevation. Carrier-phase data exhibit representative errors of 0.2 to 2.5 mm. The results of the CHAMP GPS data analysis contribute to a better understanding and possible improvement of the BlackJack receiver and support the design of optimal data editing and weighting strategies in precise orbit determination applications.

/pdf/in-flight-performance-analysis-of-the-champ-blackjack-gps-10tgauna05.pdf

In-flight performance analysis of the CHAMP BlackJack GPS Receiver

First experience with near real-time water vapor estimation in a German GPS network

CHAMP Phase B: Executive Summary

. CHAMP has so far been the most successful magnetic field mission. For achieving that, special effort had to be invested in building a magnetically clean spacecraft. The magnetic moment of the spacecraft is about 1 Am2. In this article we introduce a new method that allows the determination of the magnetic moment during the mission from the average current strength of the magneto-torquers. In order to achieve precise field vector data, the readings of the fluxgate magnetometer are routinely calibrated against the absolute Overhauser measurements. A reanalysis of all the magnetic field data is performed which takes also into account small disturbances from the power system. Uncertainties of the final magnetic field data are estimated to be of the order of 0.1 nT.

/pdf/magnetic-properties-of-champ-and-their-effects-on-in-orbit-2uhbaubtnx.pdf

Magnetic properties of CHAMP and their effects on in-orbit calibration

CHAMP PhaseB - Executive Summary

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CHAMP Phase B: Executive Summary

Magnetic properties of CHAMP and their effects on in-orbit calibration

CHAMP PhaseB - Executive Summary