CHAMP, GRACE, GOCE and Swarm Thermosphere Density Data with Improved Aerodynamic and Geometry Modelling
TU Delft, Netherlands, The
Since 2000, accelerometers on board of the CHAMP, GRACE, GOCE and Swarm satellites have provided high-resolution thermosphere density data, improving knowledge on atmospheric dynamics and coupling processes in the thermosphere-ionosphere layer.
Most of the research has focused on relative changes in density. Scale differences between datasets and models have been largely neglected or removed using ad hoc scale factors. The origin of these variations arises from errors in the aerodynamic modelling, specifically in the modelling of the satellite outer surface geometry and of the gas-surface interactions. Therefore, in order to further improve density datasets and models that rely on these datasets, and in order to make them align with each other in terms of the absolute scale of the density, it is first required to enhance the geometry modelling. Once accurate geometric models of the satellites are available, it will be possible to enhance the characterization of the gas-surface interactions, and to enhance the satellite aerodynamic modelling.
This presentation offers an accurate approach for determining aerodynamic forces and torques and improved density data for CHAMP, GRACE, GOCE and Swarm. Through detailed high fidelity 3-D CAD models and Direct Simulation Monte Carlo computations, flow shadowing and complex concave geometries can be investigated. This was not possible with previous closed-form solutions, especially because of the low fidelity geometries and the incapability to introduce shadowing effects. This inaccurate geometry and aerodynamic modelling turned out to have relevant influence on derived densities, particularly for satellites with complex elongated shapes and protruding instruments, beams and antennae.
Once the geometry and aerodynamic modelling have been enhanced with the proposed approach, the accelerometer data can be reprocessed leading to higher fidelity density estimates. An overview of achieved improvements and dataset comparisons will be provided together with an introduction to the next gas-surface interactions research phase.
GPS-derived Density Data for the Swarm Satellites During the Declining Phase of the Solar Cycle
Delft University of Technology, Netherlands, The
After the detection of many anomalies in the accelerometer data, the development and production of GPS-derived acceleration and thermosphere density products for the three Swarm satellites has been intensified. In order to convert the range and phase information in the Swarm GPS measurements into accelerations, a precise orbit determination approach needs to be used, in which gravitational accelerations are modelled with a very high fidelity, but in which the non-gravitational accelerations are part of the parameters to be estimated. After initial tests with both batch least-squares and Kalman filter orbit determination approaches, a Kalman filter approach was tuned and selected for computing the acceleration data. The resulting GPS-derived accelerations currently serve as a baseline for the correction of Swarm C along-track accelerometer data. In addition, the GPS-derived accelerations for all three Swarm satellites are converted directly to thermosphere neutral density data. This GPS-derived density data can serve as a replacement for the originally planned accelerometer thermosphere density products, albeit at a much lower temporal resolution than the accelerometers would have been able to deliver. The accuracy at which accelerations, and subsequently densities, can be derived from the GPS range and phase observations depends on the parameterisation in the orbit determination process. In principle, a higher accuracy can be traded off against a lower temporal resolution. An additional source of error is the modelling of radiation pressure accelerations, which need to be removed from the estimated signal to arrive at the aerodynamic accelerations, which are used as a source to determine density. We have assessed the impact of the declining solar activity level on the currently available acceleration and density data, as well as the impact of various scenarios for the future evolution of the Swarm orbits. Most of the currently available data contains a significant signal well above 2 cycles per orbit revolution at high solar activity. At low solar activity this maximum significant frequency is reduced. A complicating factor is that it would not be very useful for the interpretation of the data to estimate the accelerations and densities above 1 but below 2 cycles per revolution. Currently, the Swarm satellites are still in relatively high orbits, while solar activity is getting lower. Our conclusion is that with the current level of error sources, and keeping the orbits at the current altitude or higher, as proposed in some scenarios, will make it very difficult to resolve latitudinal density variability at solar minimum, using Swarm GPS data.
Horizontal and Vertical Wind Measurements from GOCE Angular Accelerations
Delft University of Technology, Netherlands, The
Because of the highly accurate accelerometers, the GOCE mission has proven to be a unique source of thermosphere neutral density and cross-wind data. In the current methods, in which only the horizontal linear accelerations are used, the vertical winds cannot be obtained. In the algorithm proposed in this paper, angular accelerations derived from the individual gradiometer accelerations are used to obtain the vertical wind speeds as well. To do so, the measured angular rate and acceleration are combined to find a measurement of the torque acting on the spacecraft. This measurement is then corrected for modeled control torque applied by the magnetic torquers, aerodynamic torque, gravity gradient torque, solar radiation pressure torque, the torque caused by the misalignment of the thrust with respect to the center of gravity, and magnetic torque caused by the operation of several different subsystems of the spacecraft bus. Since the proper documentation of the magnetic properties of the payload were not available, a least squares estimate is made of one hard- and one soft-magnetic dipole pertaining to the payload, on an aerodynamically quiet day. The model for aerodynamic torque uses moment coefficients from Monte-Carlo Test Particle software ANGARA. Finally the neutral density, horizontal cross-wind, and vertical wind are obtained from an iterative process, in which the residual forces and torques are minimized. It is found that, like horizontal wind, the vertical wind responds strongly to geomagnetic storms. This response is observed over the whole latitude range, and shows seasonal variations.
Long-term Variations of the Upper Atmosphere Parameters on Rome Ionosonde Observations and their Interpretation
1INGV, Italy; 2IZMIRAN, Russian Federation
A new self-consistent approach to the analysis of thermospheric and ionospheric long-term trends has been applied to Rome ionosonde summer noontime observations for the (1957-2015) period. This approach includes: i) a method to extract foF2, hmF2, foF1 long-term trends; ii) a method to retrieve thermospheric neutral composition (O, O2, and N2), exospheric temperature Tex, and the total solar EUV flux with λ<1050 Å from routine foF1 ionosonde data. The method was tested using CHAMP/STAR neutral gas density measurements.; iii) a combined analysis of the ionospheric and thermospheric parameter long-term variations using the theory of ionospheric F-layer formation.
In accordance with the geomagnetic control concept daytime 11-year smoothed (δfoF2)11y and (δfoF1)11y manifest mainly anti-phase with Ap11y variations. Periods of increasing geomagnetic activity correspond to negative trends in (δfoF2)11y and (δfoF1)11y and vice versa. On the contrary, (δhmF2)11y demonstrate in-phase with Ap11y variations. The retrieved neutral gas density r, atomic oxygen [O], and exospheric temperature Tex, from monthly median foF1 noontime observations for the period of ~ 5 solar cycles (1957-2015), exhibit very small (< 1% per decade) and insignificant linear trends. This contradicts the results obtained on satellite drag measurements and those derived from ground-based incoherent scatter radars. The retrieved thermospheric parameter long-term variations were shown to be controlled only by solar and geomagnetic activity. Atomic oxygen, [O] and [O]/[N2] ratio control foF1 and foF2 while neutral temperature, Tex controls hmF2 long-term variations. Noontime foF2 and foF1 long-term variations demonstrate a negative trend over the (1962-2010) period which should be attributed to atomic oxygen decrease after ~1990. vV
Molecular Ions and Hot Oxygen Atoms in the Topside Ionosphere and their Possible Effects on Satellites in Low-Earth Orbits
1University of Calgary, Canada; 2University of British Columbia, Canada
The imaging and rapid-scanning ion mass spectrometer (IRM) onboard the CASSIOPE Enhanced Polar Outflow Probe (e-POP) frequently observes enhanced densities of NO+ and other molecular ions, especially during active times. The dissociative recombination of molecular NO+ and O2+ ions in and above the F region ionosphere is believed to be a significant source of hot oxygen atoms in the exosphere. In this study we present the observed characteristics of these molecular ions on e-POP. We use the Boltzmann equation to model the energy distribution of the product oxygen atoms from dissociative recombination and examine the effect of the energetic oxygen on the orbital decay of low-earth orbiting satellites.
The "Rocket Experiment for Neutral Upwelling 2 (RENU2)” Sounding Rocket
University of New Hampshire, United States of America
Thermospheric upwelling has been known to exist since the earliest days of the space program, when observers noted increased satellite drag associated with solar activity. Scientists quickly attributed the upwelling to Joule heating effects, explaining that increased solar activity results in increased Joule heating, which can couple energy to the ambient neutral gases to cause the upwelling. Observations by the CHAMP satellite, however, have shown that neutral upwelling often occurs on much smaller scales and is highly correlated with small-scale field-aligned currents in the vicinity of the cusp
region. Several theories have since been put forward that seek to explain this phenomenon. Motivated by these competing theories and outfitted with a comprehensive suite of instruments, the RENU2 sounding rocket was launched into a Poleward Moving Auroral Form (PMAF) in the cusp region on December 13, 2015. In this highly successful mission, instruments on the payload did, in fact, record neutral atomic oxygen above the payload at 350 km as it passed through the PMAF. In addition, signatures of N2+ ions also appeared above the PMAF, evidence of so-called “sunlit aurora”. In this presentation, initial results will be presented from this mission and discussed in the context described above.
Impact of Swarm GPS Receiver Modifications on Baseline Determination
Delft University of Technology, Netherlands, The
The European Space Agency (ESA) Swarm mission is a satellite constellation launched on 22 November 2013 aiming at observing the Earth geomagnetic field and its temporal variations. The constellation consists of two satellites flying in pendulum formation in low earth polar orbits and one satellite flying separately in a higher polar orbit. The three identical Swarm satellites are equipped with high-precision 8-channel dual-frequency Global Positioning System (GPS) receivers, which make the Swarm constellation a good test bed for baseline determination. High-precision baseline determination between low earth orbiting satellites is relevant for e.g. Interferometric Synthetic Aperture Radar (InSAR) research, proximity operations between spacecraft, and possibly gravity field determination.
For Swarm, special attention has to be paid to several aspects regarding the baseline determination. These aspects include the synchronization of the GPS observations collected by the GPS receivers on the different Swarm satellites, the determination of in-flight frequency-dependent Phase Center Variation (PCV) and Code Residual Variation (CRV) antenna patterns, and half-cycle carrier-phase ambiguity resolution. In addition, a number of GPS receiver modifications were made in the October 2014 to August 2016 time frame, such as changes in the Field-of-View (FoV), tracking loop bandwidth, and Receiver Independent Exchange Format (RINEX) converter updates. Moreover, the on-board GPS receiver performance is greatly influenced by the seasonal ionospheric scintillation, which is caused by irregular plasma bubbles that mostly occur at equatorial and polar areas.
The impact of the factors mentioned above is assessed for baseline determination of the pendulum formation flying Swarm-A and -C satellites. They fly at altitudes lower than Swarm-B and their baseline length varies between 30 and 180 km. The assessment is done for four different periods: August 2014, November 2014, August 2016 and November 2016 - are implemented, respectively. These four periods have been selected to especially study the impact of different levels of ionospheric scintillations (normally low in August and high in November) and GPS receiver modifications.
The assessment includes a consistency check between so-called kinematic and reduced-dynamic baseline solutions, a validation of the associated absolute orbit solutions by comparison with Satellite Laser Ranging (SLR) observations, overlap analyses between consecutive baseline solutions, success rate of ambiguity fixing, and analysis of observation residuals. First results indicate the usefulness and importance of the GPS receiver modifications and RINEX converter updates. It is found that the GPS receiver modifications significantly reduce the impact of ionospheric scintillations and improve the baseline determination. Especially, a larger carrier phase tracking loop bandwidth is found to be the most beneficial factor for baseline determination.
Impact of GPS Receiver Tracking Loop Modifications on Precise Swarm Orbits
Delft University of Technology, Netherlands, The
The European Space Agency (ESA) Swarm mission was launched on 22 November 2013 to study the dynamics of the Earth’s magnetic field and its interaction with the Earth system. The mission consists of three identical satellites flying in near polar orbits. Two satellites are flying almost side-by-side at an initial altitude of 480 km, while the third satellite was placed in a higher orbit at about 530 km altitude. The Swarm satellites are equipped with high-precision 8-channel, dual-frequency GPS receivers, which are used to compute Precise Science Orbits (PSOs). These PSOs nominally consist of a reduced-dynamic orbit for the geolocation of the onboard scientific instrument observations with highest accuracy, and a kinematic orbit for gravity field determination purposes. Independent Satellite Laser Ranging (SLR) validation shows that the reduced-dynamic Swarm PSOs have an accuracy of better than 2 cm, while the kinematic orbits have a slightly reduced accuracy of about 4–5 cm.
Despite this good performance, the Swarm GPS receivers are shown to be susceptible to ionospheric scintillation. Generally, ionospheric scintillation is most intense in the equatorial and polar regions. When flying over these areas, the Swarm GPS receivers show a slightly degraded performance, resulting in occasional tracking losses, larger GPS carrier phase residuals and a reduced consistency between the kinematic and reduced-dynamic PSOs. For gravity fields determined from Swarm GPS data this can lead to severe systematic errors along the geomagnetic equator. In order to try to make the Swarm GPS receivers more robust for ionospheric scintillation, the GPS tracking loops have meanwhile been adjusted several times. The bandwidth of the L1 carrier loop has been increased from 10 to 15 Hz, while the L2 carrier loop bandwidth was increased from its original value of 0.25 Hz, to, respectively, 0.5 Hz, 0.75 Hz and 1.0 Hz.
To assess which of these settings is optimal, an extensive analysis has been conducted. Because the different tracking loop modifications were first implemented on Swarm-C only, their impact can be assessed by a comparison with the close flying Swarm-A satellite. The assessment includes an analysis of the amount of collected GPS observations and their residual errors, the consistency between the kinematic and reduced-dynamic orbit solutions, as well as SLR validation. This analysis is performed using data collected during different seasons, to take differences in ionospheric scintillation conditions into account. Other low flying satellites with similar GPS receivers, like e.g. the Sentinels, might also benefit from this analysis.
GPS-based Kinematic Orbit Determination of Swarm Satellites
1Institut für Erdmessung (IfE), Leibniz Universität Hannover, Germany; 2Institut für Geodäsie und Geoinformation (IGG), Rheinische Friedrich-Wilhelms Universität Bonn, Germany
The Swarm mission launched on November 22, 2013 is ESA's first constellation of satellites to study the dynamics of the Earth's magnetic field and its interaction with the Earth system. This mission consists of three identical satellites in near-polar orbits, Swarm A and C flying almost side-by-side at an initial altitude of 460 km, Swarm B flying in a higher orbit of about 530 km. Each satellite is equipped with a high precision 8-channels dual-frequency GPS receiver from RUAG Space for precise orbit determination, which is essential in order to take full advantage of the data information provided by this constellation, e.g. for the recovery of gravity field from the kinematic orbits.
In this contribution, we will analyse first the performance of the Swarm on-board receivers, especially under the influence of ionospheric scintillation. After analysing and sophisticated preprocessing of the observations, kinematic orbits for Swarm satellites are generated with a MATLAB-based Precise Point Positioning software which is developed of IfE Hannover using the least-squares adjustment method. The generated kinematic orbits are compared with the reduced-dynamic orbits provided by ESA Swarm Level 2 Product. The root mean square (rms) of the position residuals for Swarm satellites in along, cross and radial track are around 1.5, 1.5 and 2 cm, respectively. Next the impact on gravity field solutions is investigated by IGG Bonn. Finally, we will show first results from the kinematic baselines between Swarm satellites and compare the baseline components with differences between their kinematic orbits, respectively.