Pieter Visser

Precise orbit determination and gravity field improvement for the ERS satellites

The radial orbit error has long been the major error source in ERS-1 altimetry, crippled by having only satellite laser ranging for precise tracking and relying on insufficiently accurate general-purpose gravity field models. Altimeter crossovers are used very effectively as additional tracking data to laser ranging. The ERS Tandem Mission even provides the unique possibility to simultaneously determine orbits of two similar satellites flying the same orbit. Altimeter crossovers between the two satellites then link the two orbits into a common reference frame. Tailoring of the Joint Gravity Model 3 (JGM 3) is another step to reduce orbit errors. This technique is aimed at the reduction of the geographically anticorrelated orbit error (observed in the crossover height differences) through the adjustment of selected gravity field parameters. The resulting Delft Gravity Model (DGM)-E04 has reduced this part of the orbit error by a factor of 2, performs even better with respect to the ESA-provided orbits, and also outperforms the recent Earth Geopotential Model EGM96 in this respect. ERS-1 and ERS-2 orbits for the entire Tandem Mission are computed and studied in detail, and orbit errors due to the gravity field and nonconservative forces are identified. Analyses systematically show that the orbits computed with JGM 3 have a radial root-mean-square orbit accuracy of 7 cm, with DGM-E04 5 cm.

Zonal winds in the equatorial upper thermosphere: Decomposing the solar flux, geomagnetic activity, and seasonal dependencies

Using 3 years (2002–2004), over 16,400 orbits of measurements from the accelerometer on board the CHAMP satellite, we have studied the climatology of the equatorial zonal wind in the upper thermosphere. Several main features are noticed. The most prominent one is that the solar flux significantly influences both the daytime and nighttime winds. It overrides the geomagnetic activity effect, which is found to be rather limited to the nightside. An elevation of the solar flux level from F10.7 ? 100 × 10?22 W m?2 Hz?1 to F10.7 ? 190 × 10?22 W m?2 Hz?1 produces an eastward disturbance wind up to ?110 m s?1. This consequently enhances the nighttime eastward wind but suppresses the daytime westward wind. A seasonal variation with weaker wind (by over 50 m s?1 at night) around June solstice than in other seasons has been observed regardless of solar flux and geomagnetic activity levels. The zonal wind is eastward throughout the night except around June solstice, where it ebbs to almost zero or turns even westward in the postmidnight sector at low solar flux level. The daytime wind is found to be generally more stable than the nighttime wind, particularly unresponsive to geomagnetic activities. Predictions from the Horizontal Wind Model find good agreement with the CHAMP?observed wind at high solar flux levels during nighttime. At low solar flux levels, however, the model strongly underestimates the westward wind during morning hours by 50–120 m s?1 depending on season. The major difference between the HWM?predicted and the CHAMP?observed wind is seen in the phase of its diurnal variation. The CHAMP?observed wind turns eastward around 1200–1300 MLT instead of 1600–1700 MLT predicted by the model. Comparisons with ground FPI observations and the NCAR Thermosphere?Ionosphere?Electrodynamics General Circulation Model (TIEGCM) predictions show that the solar flux effect obtained from CHAMP is consistent with that modeled by TIEGCM. The solar flux dependence of zonal wind found here together with that of the zonal ion drift found in previous studies reflect the relative importance of the E? and F?region wind dynamo in the thermosphere?ionosphere coupling process. Furthermore, these wind measurements indicate that the Earths atmosphere superrotates. The average superrotation speed amounts to about 22 m s?1 for a solar flux level of F10.7 ? 100 × 10?22 W m?2 Hz?1 but increases to 63 m s?1 for F10.7 ? 190 × 10?22 W m?2 Hz?1. Finally, the wind behavior presented in this study is longitudinally averaged and may differ from wind measurements at a certain longitude.

Dedicated gravity field missions—principles and aims

Abstract Current knowledge of the Earths gravity field and its geoid, as derived from various observing techniques and sources, is incomplete. Within a reasonable time, substantial improvement can only come by exploiting new approaches based on satellite gravity observation methods. For this purpose three satellite missions will be realised, starting with CHAMP in 2000, followed by GRACE in 2002 and GOCE in 2004. Typical for all three missions is their extremely low and (almost) polar orbit, continuous and three-dimensional tracking by GPS and their ability to separate non-gravitational from gravitational signal parts. A further amplification of the gravity signal is achieved by inter-satellite tracking between two low orbiters in the case of GRACE and by gravity gradiometry in the case of GOCE. The rationale of GOCE will be discussed in more detail. The missions have a wide range of applications in solid Earth physics, oceanography, ice research, climatology, geodesy and sea level 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.

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.

Lunar albedo force modeling and its effect on low lunar orbit and gravity field determination

A force model for the lunar albedo effect on low lunar orbiters is developed on the basis of Clementine imagery and absolute albedo measurements. The model, named the Delft Lunar Albedo Model 1 (DLAM-1), is a 15 × 15 spherical harmonics expansion, and is intended for improved force modeling for low satellite orbits as well as to minimise aliasing of non-gravitational force model defects in future lunar gravity solutions. The development of the model from the available lunar albedo data sources is described, followed by a discussion on its calibration using absolute albedo measurements and its correlation with main selenological features. DLAM-1 is next applied in low lunar orbit determination, and results for orbits typical of lunar mapping missions are presented. Finally, the effect of lunar albedo on future gravity solutions is analysed with particular emphasis on gravity mapping from global data sets, i.e. satellite-to-satellite tracking, which is expected to be a core experiment of coming lunar missions. It is shown that albedo-induced orbit perturbations have a magnitude and frequency signature which are non-negligible for precise orbit and gravity modeling. Radial orbit errors for low orbits are in the order of 1–2 m for one week arcs.

CHAMP Gravity Field Recovery with the Energy Balance Approach: First Results

Using the principle of energy conservation has been considered for gravity field determination from satellite observations since the early satellite era, see e.g. O’Keefe (1957), Bjerhammar (1968), Reigber (1969) or Ilk (1983). CHAMP is the first satellite to which the energy balance approach can be usefully applied, now that near-continuous orbit tracking by GPS is available, aided by accelerometry. Simulation studies show the feasibility of the approach. One concern is the sensitivity to velocity errors. As a next step CHAMP’s Rapid Science Orbits (RSO) are used. Their error level is sufficiently low to demonstrate the feasibility of the energy balance approach with real data. The accelerometer data (ACC), used for modeling the non-conservative forces, give rise to further concern.

Gravity field determination with GOCE and GRACE

The Gravity field and steady-state Ocean Circulation Explorer (GOCE) and Gravity Recovery And Climate Experiment (GRACE) are two gravity missions defined by the European Space Agency (ESA) and the National Aeronautics & Space Administration (NASA), respectively. The primary mission objective of GOCE is a high-accuracy, high-resolution determination of the constant part of the gravity field of the Earth, down to wavelengths of less than 200 km, whereas the GRACE mission aims at wavelengths down to 400 km and also focuses on monitoring changes in the Earths gravity field. The foreseen mission duration is equal to 9 months for GOCE and 5 years for GRACE, respectively. With these mission durations, rigorous covariance analyses indicate that the gravity signal to noise ratio reaches 1 at about degree 250 for GOCE and 130 for GRACE, i.e. wavelengths of about 160 and 310 km, respectively. The covariance analyses indicate that GRACE and GOCE perform best in the long-to-medium wavelength (800 – 40,000 km) and medium-to-short wavelength (1500 – 160 km) domains, respectively. The GRACE and GOCE missions can be considered to be both supplementary and complementary. This offers the possibility to verify and calibrate gravity field recovery results, especially in the medium-wavelength domain. Moreover, a combined GOCE/GRACE gravity field solution might be formed, with optimal performance over all wavelengths down to 180 km.

Space Weather opportunities from the Swarm mission including near real time applications

Sophisticated space weather monitoring aims at nowcasting and predicting solar-terrestrial interactions because their effects on the ionosphere and upper atmosphere may seriously impact advanced technology. Operating alert infrastructures rely heavily on ground-based measurements and satellite observations of the solar and interplanetary conditions. New opportunities lie in the implementation of in-situ observations of the ionosphere and upper atmosphere onboard low Earth orbiting (LEO) satellites. The multi-satellite mission Swarm is equipped with several instruments which will observe electromagnetic and atmospheric parameters of the near Earth space environment. Taking advantage of the multi-disciplinary measurements and the mission constellation different Swarm products have been defined or demonstrate great potential for further development of novel space weather products. Examples are satellite based magnetic indices monitoring effects of the magnetospheric ring current or the polar electrojet, polar maps of ionospheric conductance and plasma convection, indicators of energy deposition like Poynting flux, or the prediction of post sunset equatorial plasma irregularities. Providing these products in timely manner will add significant value in monitoring present space weather and helping to predict the evolution of several magnetic and ionospheric events. Swarm will be a demonstrator mission for the valuable application of LEO satellite observations for space weather monitoring tools.

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.