Rune Floberghagen

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.

VII: CLOSING SESSION: GOCE: ESA's First Earth Explorer Core Mission

This paper introduces the first ESA Core Earth Explorer mission, GOCE, in the context of ESAs Living Planet programme. GOCE will measure highly accurate, high spatial resolution differential accelerations in three dimensions along a well characterised orbit: the mission is planned for launch in early 2006. The mission objectives are to obtain gravity gradient data such that new global and regional models of the static Earths gravity field and of the geoid can be deduced at length scales down to 100 km. These products will have broad application in the fields of geodesy, oceanography, solid-earth physics and glaciology.

The Swarm Initial Field Model for the 2014 geomagnetic field

Data from the first year of ESAs Swarm constellation mission are used to derive the Swarm Initial Field Model (SIFM), a new model of the Earths magnetic field and its time variation. In addition to the conventional magnetic field observations provided by each of the three Swarm satellites, explicit advantage is taken of the constellation aspect by including east-west magnetic intensity gradient information from the lower satellite pair. Along-track differences in magnetic intensity provide further information concerning the north-south gradient. The SIFM static field shows excellent agreement (up to at least degree 60) with recent field models derived from CHAMP data, providing an initial validation of the quality of the Swarm magnetic measurements. Use of gradient data improves the determination of both the static field and its secular variation, with the mean misfit for east-west intensity differences between the lower satellite pair being only 0.12 nT.

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.

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.

A method to derive maps of ionospheric conductances, currents, and convection from the Swarm multisatellite mission

The European Space Agency (ESA) Swarm spacecraft mission is the first multisatellite ionospheric mission with two low-orbiting spacecraft that are flying in parallel at a distance of ~100–140 km, thus allowing derivation of spatial gradients of ionospheric parameters not only along the orbits but also in the direction perpendicular to them. A third satellite with a higher orbit regularly crosses the paths of the lower spacecraft. Using the Swarm magnetic and electric field instruments, we present a novel technique that allows derivation of two-dimensional (2-D) maps of ionospheric conductances, currents, and electric field in the area between the trajectories of the two lower spacecraft, and even to some extent outside of it. This technique is based on Spherical Elementary Current Systems. We present test cases of modeled situations from which we calculate virtual Swarm data and show that the technique is able to reconstruct the model electric field, horizontal currents, and conductances with a very good accuracy. Larger errors arise for the reconstruction of the 2-D field-aligned currents (FAC), especially in the area outside of the spacecraft orbits. However, even in this case the general pattern of FAC is recovered, and the magnitudes are valid in an integrated sense. Finally, using an MHD model run, we show how our technique allows estimation of the ionosphere-magnetosphere coupling parameter K, if conjugate observations of the magnetospheric magnetic and electric field are available. In the case of a magnetospheric multisatellite mission (e.g., the ESA Cluster mission) several K estimates at nearby points can be generated.

More Than 50 Years of Progress in Satellite Gravimetry

“Whats up?” is a question that is answered by the gravity field. Gravity not only determines what is up and down but also reflects the Earths mass distribution and its changes with time.

Evolution of Flight Operations for ESA's Gravity Mission GOCE

ESA’s Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) is operated in an extremely low Earth orbit at 260 km altitude. The spacecraft has an aerodynamic shape and employs drag-free control with an ion propulsion system to counteract the atmospheric drag. GOCE’s extraordinary mission profile and spacecraft design led to many peculiarities for flight operations. The experience gained after launch has resulted in a significant evolution of the flight operations approach. Changes include a revised approach for orbit control manoeuvres, a new altitude selection owing to the low level of solar activity, and the decision to not interrupt science operations in eclipse season. Flight operations in 2010 were particularly challenging due to severe anomalies in the on-board data handling subsystem. Most notably, the spacecraft had to be operated “in the blind” with little to no status information for a period of about 1.5 months. This paper presents the experience gained when operating GOCE, describing the evolution of the flight operations approach and ground segment.

On the information contents and regularisation of lunar gravity field solutions

Till the present day the recovery of the lunar gravity field from satellite tracking data depends in a crucial way on the level and method of regularisation. With Earth-based tracking only, the spatial data coverage is limited to only slightly more than 50% and the inverse problem remains severely ill-posed. The development of global gravity models suitable for precise orbit modeling as well as geophysical studies therefore requires a significant level of regularisation, limiting the solution power over the far-side where no gravity information is available. Unconstrained solutions, within the framework of global harmonic base functions, are only possible for very low degrees (< 10). Any significant change to this situation is only to be expected when global satellite-to-satellite tracking data of high quality becomes available early in the next decade. Yet, a rigorous analysis of the impact of the chosen method and level of regularisation is lacking. Most gravity models employ a Kaula-type signal smoothness constraint of 15 × 10−5 /l2, which allows a good overall data fit as well as a smooth field over the far-side. Furthermore, a geographical type of constraint has been suggested, where surface accelerations have been introduced in areas of no data coverage. Modern numerical methods, on the other hand, offer direct tools and search mechanisms for the optimal level of regularisation. This paper presents a study of Tikhonov-type regularisation of lunar gravity solutions, with emphasis on the so-called L-curve and quasi-optimality methods for regularisation parameter estimation. Furthermore, new quality measures of lunar gravity solutions are presented, which account for the bias introduced by the regularisation.

Special issue “Swarm science results after 2 years in space”

© The Author(s) 2016. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Swarm is a three-satellite constellation mission launched by the European Space Agency (ESA) on 22 November 2013. It consists of three identical spacecraft, two of which (Swarm Alpha and Swarm Charlie) are flying almost side-by-side in polar orbits at lower altitude (about 470 km in September 2016) with an East-West separation of 1.4◦ in longitude corresponding to 155 km at the equator. The third satellite (Swarm Bravo) is in a slightly higher orbit (about 520 km altitude in September 2016). Each of the three satellites carry a magnetometry package (consisting of absolute scalar magnetometer, fluxgate vector magnetometer, and star imager) for measuring the direction and strength of the magnetic field, and instruments to measure plasma and electric field parameters as well as gravitational acceleration. Time and position are provided by on-board GPS. The configuration of the various instruments on each of the three Swarm spacecraft is shown in Fig. 1. More information about the mission can be found at http://earth.esa.int/ swarm. The 21 articles collected in this special issue were stimulated by the Joint Inter-Association Symposium “JA4 Results from Swarm, Ground Based Data and Earlier Satellite Missions” at the 26th General Assembly of the International Union of Geodesy and Geophysics (IUGG) held in Prague in July 2015. Tøffner-Clausen et al. (2016) report on the advanced calibration of the magnetometry package of the Swarm satellites. Finlay et al. (2016) and Olsen et al. (2016) present models of Earth’s core magnetic field, while Thébault et al. (2016) and Kotsiaros (2016) determine models of the lithospheric field. The importance of high-resolution magnetic field models for studying external magnetic field contributions, in particular during geomagnetic quiet conditions, is discussed by Stolle et al. (2016). Five contributions discuss the magnetic field produced by ionospheric and magnetospheric currents: Chulliat et al. (2016) present a climatological model of the ionospheric currents responsible for geomagnetic daily variations at non-polar latitudes, while the work of Laundal et al. (2016) concentrates on a consistent description of horizontal ionospheric and field-aligned currents in the polar ionosphere, in particular regarding their dependence on solar irradiation that controls ionospheric conductivity. A scheme for estimating the polar ionospheric currents that form the Polar Electrojets on an orbit-byorbit basis is presented by Aakjær et al. (2016), while Tozzi et al. (2016) discuss unmodelled magnetic field contributions in satellite-based magnetic field models. Michelis et al. (2016) present a study of high-latitude magnetic field variations during the St. Patrick’s Day Storm. The same event is investigated by Pignalberi et al. (2016) using Swarm plasma density measurements, and by Cherniak and Zakharenkova (2016) using GPS data from ground and Swarm. Calibration of the electric field instrument of Swarm is presented by Fiori et al. (2016). A combination of electric, magnetic, and TEC observations has been used by Astafyeva et al. (2016) to investigate the magnetic storm of 22–23 June 2015. Aoyama et al. (2016) combine ground magnetic data and Swarm TEC observations to study possible ionospheric effects of the 2015 eruption of a volcano in Chile, Zakharenkova et al. (2016) used GPS and Swarm plasma observations to study equatorial plasma density irregularities in the topside ionosphere, and Xiong et al. (2016) performed a scale analysis of equatorial plasma irregularities. van den IJssel et al. (2016) describe improvements of Swarm GPS antenna settings to enhance high-precision positioning of the spacecraft, and da Encarnação et al. (2016) discuss various attempts to determine monthly Open Access