Earth Planets Space, 65, 11891200, 2013
The Swarm Satellite Constellation Application and Research Facility (SCARF) and Swarm data products
Nils Olsen1, Eigil Friis-Christensen1, Rune Floberghagen2, Patrick Alken3,4, Ciaran D. Beggan5, Arnaud Chulliat3, Eelco Doornbos6, Joao Teixeira da Encarnaao6, Brian Hamilton5, Gauthier Hulot3, Jose van den IJssel6,
Alexey Kuvshinov7, Vincent Lesur8, Hermann Lhr8, Susan Macmillan5, Stefan Maus4, Max Noja8, Poul Erik H. Olsen1, Jaeheung Park8, Gernot Plank9, Christoph Pthe7, Jan Rauberg8,
Patricia Ritter8, Martin Rother8, Terence J. Sabaka10, Reyko Schachtschneider8, Olivier Sirol3, Claudia Stolle1,8, Erwan Thbault3, Alan W. P. Thomson5,
Lars Tffner-Clausen1, Jakub Velmsk11, Pierre Vigneron3, and Pieter N. Visser6
1DTU Space, Technical University of Denmark, Elektrovej, DK-2800 Kgs. Lyngby, Denmark
2Directorate of Earth Observation Programmes, ESRIN, via Galileo Galilei, 2, 00044 Frascati, Italy
3Institut de Physique du Globe de Paris, France
4National Geophysical Data Center, NOAA, USA
5British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, Scotland
6Delft University of Technology, The Netherlands7ETH Zurich, Switzerland
8Helmholtz-Zentrum Potsdam, Deutsches GeoForschungsZentrum, 14473 Potsdam, Germany
9ESTEC, The Netherlands
10Planetary Geodynamics Laboratory, GSFC/NASA, Greenbelt, MD 20771, USA
11Department of Geophysics, Faculty of Mathematics and Physics, Charles University in Prague, Czech Republic
(Received March 30, 2013; Revised June 30, 2013; Accepted July 1, 2013; Online published November 22, 2013)
Swarm, a three-satellite constellation to study the dynamics of the Earths magnetic eld 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 eld and its temporal evolution, in order to gain new insights into the Earth system by improving our understanding of the Earths interior and environment. In order to derive advanced models of the geomagnetic eld (and other higher-level data products) it is necessary to take explicit advantage of the constellation aspect of Swarm. The Swarm SCARF (Satellite Constellation Application and Research Facility) 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.
Key words: Earths magnetic eld, core eld, lithosphere, ionosphere, magnetosphere, electromagnetic induction, comprehensive inversion, Swarm satellites.
1. Introduction
Swarm, a constellation mission comprising three identical satellites to study the dynamics of the Earths magnetic eld and its interactions with the Earth system (Friis-Christensen et al., 2006, 2008) is expected to be launched in late 2013. The objective of the Swarm mission is to provide the best ever survey of the geomagnetic eld and its temporal evolution, in order to gain new insights into the Earth system by improving our understanding of the Earths interior and environment.
Each of the three Swarm satellites will make high-precision and high-resolution measurements of the strength, direction and variation of the magnetic eld, complemented
Copyright c
[circlecopyrt] The Society of Geomagnetism and Earth, Planetary and Space Sci
ences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB.
doi:10.5047/eps.2013.07.001
by precise navigation, accelerometer, plasma and electric eld measurements. These observations will be provided as Level-1b data, which are the calibrated and formatted time series of e.g. the magnetic eld measurements taken by each of the three Swarm satellites. These Level-1b data, as well as the higher-Level Swarm data products described in this paper, will be distributed by ESRIN (Frascati/I).
Swarm will simultaneously obtain a space-time characterisation of both the internal eld sources in the Earth and the ionospheric-magnetospheric current systems. The research objectives assigned to the mission are: (a) studies of core dynamics, geodynamo processes, and core-mantle interaction; (b) mapping of the lithospheric magnetisation and its geological interpretation; (c) determination of the 3-D electrical conductivity of the mantle; and (d) investigation of electric currents owing in the magnetosphere and ionosphere.
A challenging part, however, is the separation of the var-
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ious sources (in the core, lithosphere, ionosphere, magnetosphere etc.) which in total yields the measured magnetic eld. A constellation consisting of several satellites, like Swarm, opens new possibilities for exploring the geomagnetic eld from space beyond those achievable with single-satellites. At rst glance one would expect that using simultaneous data from N satellites results in a reduction of the error of geomagnetic eld models by N, since the amount of data is increased by N compared to one single satellite. This error reduction by N of course only holds if the data are statistically independent, which is highly idealistic and unrealistic since the main limiting factor for improved eld modelling is not the measurement error but the dynamic behaviour of external sources. Treating data from a constellation of N satellites in a single-satellite approach thus typically results in an improvement of the model error by less than N. However, if explicit advantage is taken of the constellation, there is some potential for model improvement better than N. A constellation of three satellites can do more than three single satellites, and therefore a (smart) combination of data from all three satellites, and of the various instruments, allows for taking full advantage of the Swarm constellation. Analysis of the Swarm data will greatly improve existing and provide new models of the near-Earth magnetic eld of high resolution and authenticity compared to what is possible with single-satellite missions like rsted (Olsen, 2007) and CHAMP (Reigber et al., 2005).
In recognition of the large effort needed to extract the various types of scientic information from the complex set of observations a group of institutions and organisations have joined the SMART consortium (Swarm Magnetic and Atmospheric Research Team). The purpose of the consortium is to contribute to the optimal science return from the Swarm mission by a coordinated effort to exploit the constellation aspects of this unique mission. This effort is obviously in accordance with ESAs aim of providing the scientic community with the best possible products from the Swarm mission, and it was decided to establish a Swarm SCARF (Satellite Constellation Application and Research Facility), with the purpose of deriving commonly used scientic models and quantities, the so-called Level-2 products and make them available to the scientic community at large. Advanced Swarm-derived models of the geomagnetic eld and other Level-2 data products are determined from the Level-1b data and auxiliary (i.e. non-Swarm) data and provide the prospect of investigating hitherto undetected features of the Earths interior.
SCARF (sometimes also called Level-2 Processing System, L2PS) comprises in its present form a joint effort between the six European partners: DTU (Lyngby/DK), TU Delft (Delft/NL), BGS (Edinburgh/GB), ETH (Zrich/CH), GFZ (Potsdam/D) and IPGP (Paris/F) with contributions from CUP (Prague/CZ), NOAA (Boulder/USA) and GSFC/NASA (Greenbelt/USA). The team behind SCARF has designed and implemented algorithms to derive advanced models of the geomagnetic eld describing sources in the core, lithosphere, ionosphere and magnetosphere, models of the electrical conductivity of Earths mantle and time series of thermospheric wind and density at
the positions of the Swarm satellites. These models, which are state-of-the-art implementations of current knowledge, are intended to facilitate and increase the use of the Swarm data by a much wider community than the one represented in the SMART consortium itself.
The work performed by SCARF is a major extension on the End-To-End mission simulation that has been performed during Phase A of the Swarm mission, the results of which have been published in a special issue (Vol. 58 No. 4, 2006) of Earth, Planets and Space (cf. Olsen et al., 2006 for an overview).
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 and distributed by ESA through the PDGS (Payload Data Ground Segment) at ESRIN.
The content of the paper is as follows: Section 2 summarizes the various Level-1b data, with emphasis on the 1 Hz time series of the magnetic eld observations. Section 3 describes the various processing chains and resulting Level-2 data products. All processing chains have been tested using synthetic data from a full mission simulation; the creation of this synthetic data set is described in Section 4. Data processing time-line and data availability are discussed in Section 5.
2. Swarm Level-1b Products
The Level-1b products of the Swarm mission contain time-series of quality-screened, calibrated, and corrected measurements given in physical, SI units in geo-localized reference frames. Level-1b products are provided individually for each of the three satellites Swarm A, Swarm B, and Swarm C on a daily basis, i.e. each product contains all available data of that day from 00:00 until (but not including) 24:00 UT time. An example illustrating the lenaming convention of Swarm Level-1b 1 Hz magnetic product MAGx LR from 23. February 2013 is given in Fig. 1.
Table 1 lists the various Level-1b data products. Ephemeris Products are data obtained by the GPS receiver, the accelerometer and the star tracker; the Magnetic Products contain measurements taken by the two magnetometers (augmented with position and attitude measured by the star tracker), while the Plasma Products contain data obtained by the Thermal Ion Imager and the Langmuir probe. Most Level-1b products are provided in the Common Data Format (CDF, see Goucher and Mathews (1994) and http://cdf.gsfc.nasa.gov/) which is a le format optimized for storing time series, although data from the GPS receiver are provided in the RINEX and SP3 formats as is typical for these kind of data.
Probably the most important Level-1b product, MAGx LR(where x is a placeholder for A, B, or C indicating the satellite), contains 1 Hz time series with magnetic eld observations from each of the three satellites. The content of the product is described in more detail in Table 2. In addition to time and position it contains the magnetic scalar intensity F and the three components of the magnetic vector BNEC = (BN, BE, BC) in the North-East-Center (NEC) lo
cal Cartesian coordinate frame (where BN is the component
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Fig. 1. Example illustrating the lename of the Swarm-A Level-1b low-rate magnetic product MAGx LR from 23. February 2013.
Table 1. Swarm Level-1b products. The x in the product name is a placeholder for A, B, or C indicating the satellite. Ephemeris Products
Type Descriptive Name Format Contents
GPSx RO RINEX Observations RINEX 3.0 Processed and corrected data from the GPS receiver, 0.1 Hz sampling
GPSx RN RINEX Navigation data RINEX 3.0 GPS navigation data from the GPS receiver
GPSxNAV Navigational Solution SP3 Position and velocity from on-board GPS receiver navigational solution,1 Hz sampling
MODx SC Medium-Precision OrbitDetermination
SP3 Position and velocity from automated, medium-precision orbit determination based on GPSx RO data, 1 Hz sampling
STRxATT Attitude CDF Processed and corrected attitude information of the spacecraft frame, from the Star Tracker, 1 Hz sampling
ACCx PR Non-gravitational Accelerations
CDF Pre-processed linear and angular, non-gravitational acceleration measurements from the accelerometer (Fully calibrated and corrected linear, non-gravitational accelerations are provided as Level-2 product ACCxPOD 2 ), 1 Hz sampling
Magnetic Products
Type Descriptive Name Format Contents
MAGx LR Low Rate (1 Hz) magnetic data
CDF Calibrated and corrected magnetic vector and scalar measurements, interpolated to UTC seconds and provided in VFM sensor as well as in NEC frame
MAGx HR High Rate (50 Hz) magnetic data
CDF Calibrated and corrected magnetic vector measurements, provided inVFM sensor as well as in NEC frame
MAGx CA Magnetic calibration data CDF Magnetic scalar calibration data for long-term VFM analysis
Plasma Products
Type Descriptive Name Format Contents
EFIx PL Plasma data CDF Processed and corrected plasma measurements from the EFI instruments including electric eld vector, plasma density, plasma electron and ion temperatures, 2 Hz sampling
LP x CA Langmuir probe calibration data
CDF Offset determination mode data and corresponding estimated calibration parameters
TIIx CA Thermal Ion Imager calibration data
CDF Daily calibration parameters used for monitoring and optimisation of the TII instrument performance
towards geographic North, BE is the component towards geographic East, and BC is the component towards the center of the Earth). In addition, the magnetic vector BVFM
in the instrument frame of the Vector Field Magnetometer (VFM) is given, where
BNEC = Rq R3 BVFM (1)
with
R3 =
cos sin 0
sin cos 0 0 0 1
cos 0 sin 0 1 0
sin 0 cos
1 0 00 cos sin
0 sin cos
(2)
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Table 2. Content of Swarm Level-1b Product MAGx LR.
Element Contents Units
Timestamp Time of observation CDF EPOCHSyncStatus Time reference information Latitude Latitude of observation in the International Terrestrial Reference Frame (ITRF) degree Longitude Longitude of observation in the ITRF degree Radius Radius of observation in ITRF mF Magnetic eld intensity nTdF AOCS Magnetic stray eld correction intensity related to attitude control magneto-torquers nTdF other Magnetic stray eld correction intensity of all other sources nTF error Error estimate on magnetic eld intensity nTB VFM Magnetic eld vector in the instrument frame of the VFM magnetometer nTB NEC Magnetic eld vector in the NEC (North, East, Center) frame nTdB AOCS Magnetic stray eld correction vector related to attitude control magneto-torquers
(in VFM frame)
nT
dB other Magnetic stray eld correction vector of all other sources (in VFM frame) nT B error Error estimates on magnetic eld vector BVFM (in VFM frame) nT
q NEC CRF Rotation from NEC frame to STR Common Reference Frame (CRF), quaternion (NEC CRF)
Att error Error estimate on attitude information degree/103 Flags F Flags characterizing the magnetic eld intensity measurement: spikes or gap in data, discrepancy between ASM and VFM, etc.
Flags B Flags characterizing the magnetic eld vector measurement BVFM and BNEC: spikes or gap in data, discrepancy between ASM and VFM, etc.
Flags q Flags characterizing the attitude information: identication of active heads, blinding, etc. Flags Platform Flags characterizing the spacecraft platform information: thruster activation, lack of telemetry,etc.
ASM Freq Dev ASM reference frequency calibration data deviation for ASM stability assessment Hz1/2
as the matrix describing the rotation of the magnetic eld from the VFM system to the Common Reference Frame (CRF) of the star tracker (, and are the Euler angles describing this rotation; they are provided in the Level-2 data product MSW EUL 2 ) and
Rq =
(3)
as the rotation matrix from the CRF to the NEC coordinate frame, described by the quaternions qNECCRF =
[q1 q2 q3 q4]T. Finally, the magnetic eld corrections (dF AOCS, dF other, dB AOCS, dB other) that have been applied to the data, error estimates (F error, B error, Att error) and ags characterizing the status of the various instruments and the spacecraft are also provided.
The quality of Level-1b magnetic and plasma products MAGx LR and EFIx PL can be inspected using quick-look products, essentially comprising various daily and mission-to-date plots designed to reveal a range of possible measurement problems (Beggan et al., 2013).
The positions provided in the Level-1b data are generated automatically as part of the Level-1b processing as Medium Precise Orbits (MOD) with an expected accuracy not exceeding a few meters. In case higher precision of the position is needed (or for periods where the MOD automatic calculation yields less optimal results) it is recommended to use the positions provided by the Precise Orbit Determination (POD) chain (Level-2 product
SP3xCOM 2 ) discussed in Section 3.2.
3. Swarm Level-2 Data Processing and Products
Depending on the complexity of the processing, there are two types of Level-2 products, which are called Cat-1 and Cat-2 products, respectively. Cat-1 data processing involves complex algorithms to derive Level-2 products describing specic sources of the Earths magnetic eld like the lithospheric eld or time series of the large-scale magnetospheric signal. Cat-1 products are derived by SCARF since scientic expertise is required during processing. In contrast, processing of Cat-2 products is less demanding, and therefore these products are derived by ESA on a daily basis in ESAs Swarm PDGS using algorithms designed by SCARF. The processing runs automatically, leading to product release with minimum delay; Cat-2 products are tested for their near real time capability with processing delays of less than 1 hour. Cat-2 products are therefore suitable e.g. for space weather applications (Stolle et al., 2013).
Table 3 gives an overview of Level-2 products, and Fig. 2 shows a generic processing ow chart of a typical Level-2 chain of SCARF. Input Level-1b data (shown in green) and auxiliary data (yellow) are used in the various processing blocks (orange) to produce Level-2 products (magenta). In a nal step all Level-2 products are validated by inter-product comparison as well as comparison with independent data; details of this validation process are given in Beggan et al. (2013).3.1 Level-2 products related to main magnetic sources
Figure 3 shows the various chains of SCARF that result in Level-2 magnetic products. Most of the chains use 1-Hz
1 2q22 2q23 2(q1q2 + q3q4) 2(q1q3 q2q4)
2(q1q2 q3q4) 1 2q21 2q23 2(q2q3 + q1q4)
2(q1q3 + q2q4) 2(q1q4 q2q3) 1 2q21 2q22
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Table 3. The Swarm Level-2 products.
Science Objective Name Format Description
(needed for Level-1b processing)
MSW EUL 2 ASCII Time series of Euler angles describing transformation from STR-CRF toVFM frame for all three Swarm satellites (3 3 Euler angles)
Core eld MCO SHA 2 ASCII Spherical harmonic model of the core eld and its temporal variation
Lithospheric eld MLI SHA 2 ASCII Spherical harmonic model of the lithospheric eld
Electrical conductivity of MIN 1DM 2 ASCII 1D model of mantle conductivity the mantle MIN 3DM 2 ASCII 3D model of mantle conductivity
MCR 1DM 2 ASCII 1D C-responses
MCR 3DM 2 ASCII 3D C-response maps
External current systems MMA SHA 2 CDF Spherical harmonic model of the large-scale magnetospheric eld and itsEarth-induced counterpart
MIO SHA 2 ASCII Spherical harmonic model of the daily geomagnetic variation at middle latitudes (Sq) and low latitudes (EEJ)
Precise Orbit Determination (POD)
SP3xCOM 2 SP3 Time series of position and velocity of the center of mass for satellite x(x = A, B or C)
ACCxCAL 2 CDF Accelerometer calibration parameters for satellite x
ACCxPOD 2 CDF Time series of non-gravitational accelerations estimated for satellite x
Magnetic Forcing of the Upper Atmosphere
ACCx AE 2 CDF Time series of calibrated and pre-processed accelerometer observations and of aerodynamic accelerations for satellite x
DNSxWND 2 CDF Time series of neutral thermospheric density and wind speed for satellite x
Earth environment and IBIxTMS 2F CDF Ionospheric bubble index for satellite xSpace-Weather TECxTMS 2F CDF Time series of the ionospheric total electron content for satellite x(Cat-2 products) FAC TMS 2F CDF Time series of eld-aligned currents determined from combination of
Swarm A and Swarm B
FACxTMS 2F CDF Time series of eld-aligned currents (single-satellite solution) for satellite x
EEFxTMS 2F CDF Equatorial Electric Field for satellite x
magnetic eld data from each of the three Swarm satellites (Level-1b product MAGx LR) as input (exceptions are the chains that derive models of the electrical conductivity of the Earths mantle which rely on time series of magnetospheric and induced elds as provided as Level-2 data). Models of the core, lithospheric, non-polar ionospheric and large-scale magnetospheric elds are derived using two independent chain branches: in the Comprehensive Inversion (CI) chain these sources are co-estimated in one huge inversion process; details are given in Sabaka et al. (2013). In the Dedicated Inversion (DI) chains the various sources are determined in a sequential approach after removing models describing the other sources; there are dedicated chains for the core eld (Rother et al., 2013), for the lithospheric eld (Thbault et al., 2013), for deriving the daily ionospheric variation at non-polar latitudes (Chulliat et al., 2013) and for determining time-series of the large-scale magnetospheric contributions (Hamilton, 2013).
Processing of Swarm Level-1b products in order to derive Level-2 products requires auxiliary data and models, in particular indices describing the state of the Earths environment. These auxiliary data (shown in yellow in Figs. 3 to 5) are: the 3-hour Kp index describing global geomagnetic activity (auxiliary data product AUX KP 2 ), hourly values of the Dst-index monitoring the strength of the magnetospheric ring-current (AUX DST 2 ), daily F10.7 values
of solar radio ux (AUX F10 2 ) and parameters of the Interplanetary Magnetic Field (AUX IMF 2 ). In addition, checked and corrected hourly mean values from the global network of ground magnetic observatories are provided as auxiliary data product AUX OBS 2 ; details of the data checking procedure are given in Macmillan and Olsen
(2013). Auxiliary data are used in the selection of Swarm data for deriving Level-2 products. In addition, the Comprehensive Inversion chain and the Dedicated Ionospheric chain also use magnetic observatory data in combination with the Swarm magnetic observations. Finally, observa-tory hourly means are also used in the validation of the magnetic Level-2 products (Beggan et al., 2013).
In addition to auxiliary data, the processing also requires auxiliary models like the IGRF (AUX IGR 2 ) or more advanced models of the core eld (AUX COR 2 ) and the lithospheric eld (AUX LIT 2 ). Models of the electrical conductivity of the Earths mantle (AUX MCM 2 ) and of the surface conductance of oceans and sediments (AUX OCM 2 ) are used to account for secondary, Earth-induced contributions connected to the temporal variations of magnetospheric and ionospheric origin. Finally, a model of the magnetic signature of ocean tides (AUX MTI 2 ) is provided.
In the following we briey discuss the various Level-2 products.
Spherical Harmonic Models of the core, lithospheric, ionospheric and magnetospheric eld. Models of the core eld and its time changes are provided as spherical harmonic expansion coefcients in the Level-2 product MCO SHA 2 (where M indicates that the product describes a Magnetic source, CO stands for COre eld, SHA denotes that the model is given as an expansion of a Spherical Harmonic Analysis, and 2 refers to the fact that this is a Level-2 data product. The last character, in this example , indicates the generic form of the Level-2 product; other values are C if the product is derived in the Comprehensive Inversion chain or D if the product is
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Fig. 2. Data ow in a typical Level-2 processing chain.
Fig. 3. Level-2 processing chains which result in magnetic Level-2 products.
derived in one of the Dedicated Inversion chains.)The magnetic eld vector, BNEC = V , is described
using a magnetic scalar potential V which is expanded into series of spherical harmonics. For the potential describing internal sources (in the core and lithosphere) this expansion reads
V = a
Nmax
n=Nminn
m=0
gmn cos m + hmn sin m
are the Gauss coefcients describing internal sources, and Nmin, Nmax are the minimum, resp. maximum, degree and order of the spherical harmonic expansion.
For the core eld models these are chosen to be Nmin =
1, Nmax = 18 and the time dependence of the Gauss co
efcients {gmn(t), hmn(t)} is parametrized using B-splines;
however, the nal product MCO SHA 2 contains a series of snapshot models (corresponding to order 6 splines and 6 months separation of the spline knots). Details of the data format, and how to transform back from the snapshot representation to the original spline representation, are given in the Level-2 Product Denition Document (Swarm Level 2 Processing System Consortium, 2013).
Core eld model version MCO SHA 2C is derived in the Comprehensive Inversion chain (see Sabaka et al. (2013)
n+1 Pmn (cos ) (4)
where a = 6371.2 km is a reference radius, (r, , )
are geographic coordinates, Pmn are the associated Schmidt semi-normalized Legendre functions,
gmn, hmn
a r
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for details), while model version MCO SHA 2D is derived in the Dedicated Inversion chain (Rother et al., 2013).
A proper determination of the Euler angles , , describing the rotation between the instrument frames of the vector magnetometer and star tracker (see Eq. (2)) is only possible in-orbit. A pre-ight determination made on ground is limited e.g. by atmospheric turbulence to an accuracy of, say, 20 arc-seconds since global data coverage is required in order to obtain Euler angles within a few arc-seconds. This is only possible in-orbit. A determination of the Euler angles by co-estimation with all major contributions to the near-Earth magnetic eld is made in the Comprehensive Inversion chain (Sabaka et al., 2013), and an independent determination is made in the Dedicated Core chain (Rother et al., 2013). The resulting Euler angles are provided as Level-2 data product MSW EUL 2(MSW EUL 2C and MSW EUL 2F) and will be used in the re-processing of Level-1b data by the PDGS.
Spherical Harmonic Models of the lithospheric eld are provided in the Level-2 product MLI SHA 2 (where LI stands for LIthospheric eld). Similar to the core eld products, model version MLI SHA 2C is derived in the CI chain, while model MLI SHA 2D is determined using the Revised Spherical Harmonic Cap method, as described in Thbault et al. (2013). The minimum, resp. maximum, spherical harmonic degree and order is Nmin = 16 and
Nmax = 150.
A model of the non-polar daily geomagnetic variation caused by ionospheric currents, including their variability with season and solar ux, is given in the Level-2 product MIO SHA 2 , where IO stands for IOnospheric eld.Details of the dedicated chain leading to product version MIO SHA 2D are given in Chulliat et al. (2013). Product version MIO SHA 2C is derived in the CI chain.
Finally, time-series of spherical harmonic expansion coefcients of the large-scale magnetospheric eld and its Earth-induced counterpart are provided in the Level-2 product of generic name MMA SHA 2 . As part of the CI chain time series of the magnetospheric and induced expansion coefcients are provided with a sampling rate of 90 minutes (corresponding approximately to the orbital period of the satellites) for degree n = 1 and order m = 0, and
with a sampling rate of 6 hours for degrees up to n = 3
and order m = 0, 1 for the magnetospheric eld and up to
n = m = 5 for the induced eld. The name of the resulting
product is MMA SHA 2C. The dedicated chain (Hamilton, 2013) for deriving a related product called MMA SHA 2Fcontains time series of magnetospheric and induced elds for degree n = 1 and order m = 0, 1 with sampling rate
of 90 minutes. (The last character F in the product name indicates that this is a fast-track product which is provided without an independent regular validation as is the case for most other Level-2 products).
Level-2 data product MMA SHA 2C of the large-scale magnetospheric eld and its Earth-induced counterpart is used to determine models of electrical conductivity of the mantle, regarding both its 1-D structure (which means that conductivity is assumed to only vary with depth, resulting in Level-2 product MIN 1DM 2 , see Pthe and Kuvshinov (2013a) for details) and lateral
variations of conductivity (3-D models, Level-2 product MIN 3DM 2 ). The latter is derived using two independent chains, working in the frequency domain, leading to product version MIN 3DM 2a (Pthe and Kuvshinov, 2013b), or in the time domain, leading to product version MIN 3DM 2b (Velmsk, 2013). Electromagnetic transfer functions (C-responses) are also provided (Level-2 products MCR 1DM 2 and MCR 3DM 2 ).3.2 Level-2 products related to acceleration, orbit determination and thermospheric wind and density
Precise reduced-dynamic and kinematic orbit solutions for the three Swarm satellites are provided in the Level-2 products SP3xCOM 2 and SP3xKIN 2 . As part of this POD processing chain also the accelerometer calibration parameters (product ACCxCAL 2 ) as well as non-gravitational accelerations (ACCxPOD 2 ) are determined. Time series of thermospheric winds and density at the location of each of the three Swarm satellites are provided in the Level-2 product DNSxWND 2 . Details of the processing scheme are given in Visser et al. (2013). The data ow of the two chains that result in these Level-2 products is shown in Fig. 4.3.3 Level-2 products related to the Earth environment and space weather (Cat-2 products)
Figure 5 shows the processing chains that result in the Cat-2 Level-2 products (listed in the bottom part of Table 3). All Cat-2 products are provided as daily CDF les (similar to most of the Level-1b products) since they all contain time-series of a certain geophysical quantity.
Time series of an Ionospheric Bubble Index (IBI), derived using magnetic and plasma observations from each of the three satellites, are provided in IBIxTMS 2F. Details of the processing can be found in Park et al. (2013). Time series of the ionospheric and plasmaspheric Total Electron Content (TEC) as determined by each of the three satellites are provided in the product TECxTMS 2F. The implemented algorithm for TEC determination is identical to that described by Noja et al. (2013). The processing schemes resulting in time series of Field-Aligned Currents (FAC) as provided in FACxTMS 2F (single satellite solution), resp. FAC TMS 2F (obtained by combining data from Swarm A and B) are described in Ritter et al. (2013).
Dayside Eastward Equatorial Electric Field (EEF) values are derived for each equatorial crossing of each satellite (x = A, B, or C) and are provided in the product
EEFxTMS 2F. More details on that chain are given in Alken et al. (2013).
4. Development and Test of the Swarm SCARF
The various Cat-1 chains of SCARF have been tested using synthetic Level-1b data from a full 4.5 year long simulation of the Swarm mission. Since it is very difcult, if not impossible, to verify product requirements in-orbit with real data (because the reference models, the true world, which are needed to compare the estimated models with, are not known) the performance of each chain has been investigated using synthetic data. The quality of the estimated magnetic Level-2 products is assessed against the performance requirements as listed in Table 4 using the same criteria as in the similar study of Olsen et al. (2006):
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Fig. 4. Level-2 processing chains that result in precise orbits and thermospheric Level-2 products.
Fig. 5. Level-2 processing chains related to the Earth environment and space weather (Cat-2 products).
Fig. 6. Left: Local Time, Local Time difference between the upper satellite and the lower pair. Right: satellite altitude vs. time.
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Difference in spectra, degree error, and accumu
lated error. The Mauersberger-Lowes spectrum (degree variance)
Rn(r) = (n + 1)
a r
( gmn)2 + ( hmn)2
2n+4n
m=0
(5)
of the differences between the true (i.e. input) and the estimated model coefcients, gmn, hmn, in combination with the spectrum of the input model, has been used to evaluate an estimated model. Degree error is dened as Rn, and accumulated error at degree N is dened as
Nn=nmin Rn.
Degree correlation
n =
n
m=0
gmn,egmn,t + hmn,ehmn,t
n
m=0
(gmn,e)2 + (hmn,e)2
n
m=0
(gmn,t)2 + (hmn,t)2
(6)
(Langel and Hinze, 1998, eq. (4.23)), where gmn,e and hmn,e are from the estimated model and gmn,t and hmn,t are from the input model, has also been used to evaluate a spherical harmonic model. Models are considered compatible up to that degree n where n drops below0.7.
Sensitivity matrix is the relative error of each coef
cient in a degree versus order matrix:
S(n, m) =
100
1
2n+1
hmn,ehmn,t
n m=0[(gmn,t ) 2
+(hmn,t )2], for m < 0
100
1
2n+1
gmn,egmn,t
n m=0[(gmn,t ) 2
+(hmn,t )2], for m 0.
(7)
S(n, m) represents the percentage degree-normalized error in a recovered coefcient of degree n and order m.
Global maps of eld differences (for instance of Br)
between the input and the estimated model are used to nd geographically conned deciencies in the estimated models, for instance in connection with the polar gaps.
Finally, the quality of time series (like those of the
magnetospheric and induced spherical harmonic expansion coefcient) is assessed for various target periods using Squared Coherency coh2: If F() and G() are the Fourier transform of the two time series f (t) and g(t) and F() is the complex conjugate of
F() then squared coherency at frequency is dened as
coh2 =
F()G() G()F() F()F() G()G()
(8)
with F()G() and G()F() as the cross-
spectra and F()F() and G()G() as the
auto-spectra of f (t), g(t) (e.g. eq. (20) of Olsen, 1998).
At the beginning of the SCARF activity the launch of the Swarm satellites was still scheduled for 2010. In order to have similar ambient conditions, but access to actual input values to parametrize e.g. atmospheric drag or Earth rotation variations, we simulated a launch on July 1, 1998, 00:00 UT, which is approximately one solar cycle (11 years) before the anticipated launch in 2010.
The performed simulation is described in more detail in Olsen et al. (2007), which is an extension of the work of Olsen et al. (2006). In a rst step we calculated synthetic orbits. We assumed all three satellites to be in circular near-polar orbits with injection altitude of h0 = 450 km altitude
and orbital inclination i = 87.4 for the lower pair (Swarm
satellites A and B) and of h0 = 530 km altitude and orbital
inclination i = 88 for the third satellite Swarm C. The
two lower satellites were assumed separated in longitude by1.4. They are not exactly side-by-side (which would imply collision risk near the poles) but are shifted along-track by a time lag between 2 and 10 seconds. This simulates the requirement that The maximum time difference between Swarm A and Swarm B when crossing the equator shall be 10 seconds. The chosen orbital conguration is similar (though not identical) to the one that is presently foreseen for Swarm (h0 = 460 km and i = 87.35 for Swarm A and
B; h0 = 530 km and i = 87.95 for Swarm C, and a Local
Time of the Ascending Node of about 14:30).
In order to get a lifetime of 4.5 years duration for the lower pair we included four orbit manoeuvressimilar to the ones of the CHAMP satellite: we increased the altitude of Swarm A and B by 15 km on April 1, 2001 and September 1, 2001, respectively, and by 12 km on November 15, 2001 and April 1, 2002, respectively. The obtained Local Time, the Local Time difference between the upper satellite and the lower pair, and satellite altitude as a function of time are shown in Fig. 6.
Magnetic eld data generation follows mainly the approach described in Olsen et al. (2006) with updates given in Olsen et al. (2007). The various input models have been designed in the following way: The core eld is taken from the GRIMM model (Lesur et al., 2010) for the years 2003 to 2008, but shifted by 5 years (i.e. to 1998.0 to 2003.0) in order to be compatible with the simulation period. The lithospheric input model contains spherical harmonic expansion coefcients up to degree and order 250. Degrees n = 14 and 15 are taken from
model POMME-6.1, degrees n = 16 to 90 are taken
from model MF7, and degrees 91 to 250 are taken from model NGDC-720 (version 3p1) scaled by factor 1.1. See http://geomag.org/models/index.html for more information on these models. The magnetospheric eld contribution is simulated using an hour-by-hour spherical harmonic analysis of world-wide distributed observatory hourly mean values of the years 19972002 in dipole-latitude and magnetic local time. Expansion coefcients of degrees n = 1, . . . , 3 and order m = 0, . . . , 1
have been determined. Secondary, Earth-induced elds are determined (up to n = 15) from those primary coefcients
using the 3D mantle conductivity model, including oceans, discussed in Kuvshinov et al. (2006). The input model describing the ionospheric primary eld is taken from CM4
1198 N. OLSEN et al.: THE SWARM SATELLITE CONSTELLATION APPLICATION AND RESEARCH FACILITY (SCARF)
Table 4. Product requirements for magnetic Level-2 products.
Product Target Requirement Threshold Requirement Core eld (MCO), rst time derivative (secularvariation) at ground, n = 216, averaged over
time
1 nT/yr 3 nT/yr
Lithospheric eld (MLI), accumulated error at ground, n = 16150
40 nT 120 nT
10% globally 10% at magnetic latitudes below 55
Magnetospheric eld (MMA) Squared coherency coh2 > 0.8, though
> 0.95 for n = 1
Squared coherency coh2 > 0.8, though
> 0.75 for n = 1
Ionospheric eld (MIO), average relative error on ground
Mantle conductivity (MIN) 1/2 order of magnitude error, though
1/4 order of magnitude at depths 400 1500 km
1 order of magnitude error, though 1/2 order of magnitude at depths 4001500 km
(Sabaka et al., 2004) while the secondary, induced, eld is calculated from those primary coefcients using the same 3D mantle conductivity model as for the magnetospheric induced eld. Finally, we added synthetic noise based on CHAMP experience. We assumed correlated random noise of standard deviation (0.1 0.07, 0.07) nT for (BN, BE, BC),
in agreement with the Swarm performance requirements.
The magnetic eld vector in the Level-1b CDF les is given both in the NEC coordinate frame and in the VFM frame of the vector magnetometer. In order to transform the synthetic data to the VFM frame we have arbitrarily chosen the (input) Euler angles ( = 1724, =
3488, = 6184) arcsecs for Swarm A, ( = 808, = 434, = 1234) arcsecs for Swarm B and ( =
2222, = 2991, = 3115) arcsecs for Swarm C.
The various input (reference) models are available at ftp.space.dtu.dk/data/magnetic-satellites / Swarm / SCARF/TDS-1/Reference/ while the synthetic 1 Hz Level-1b data product MAGx LR les can be found at ftp.space.dtu.dk / data / magnetic-satellites/Swarm/SCARF/TDS-1/Level1b/Mag/. Further details of the results of the closed-loop modelling tests to check each chain meets the performance requirements can be found in the respective references and papers in this volume.
5. Swarm Processing Time-line and Data Availability
The processing of Swarm data into Level-1b and Level-2 products is generally performed as soon as the appropriate input data are available. As baseline, data from the Swarm satellites themselves are downlinked on a daily basis (a more frequent downlink, in particular for making Swarm data more suitable for space-weather applications, is under consideration), however, the various processing steps may require certain auxiliary data inputs and/or a significant time-span of Swarm input data in order to produce high quality output products. Consequently the Swarm data processing time-line is as follows:
Three days (72 hours) after downlink
Swarm Level-1b data are processed
Next working day
Swarm Level-2 Quick-Look (MAGx QL 2and EFIx QL 2 ), Fast-Track Magnetospheric (MMA SHA 2F) and all Cat-2 data products (which
require up to 2 hours of processing time) are processed
Up to three weeks later
Swarm Level-2 Products regarding Precise Orbit Determination (SP3xCOM 2 ), Accelerometer data (ACCxCAL 2 , ACCxPOD 2 , ACCx AE 2 ), and Thermospheric (Neutral) Density and Winds (DNSxWND 2 ), are processed
Every three months
Swarm Level-2 Fast-Track core eld and Euler angles products (MCO SHA 2F and MSW EUL 2F) are processed
Every year
Every yearplus a few extra times in reduced form during the rst year of the missionthe Swarm Level-2 magnetic models are estimated and evaluated. The estimations are performed in two parallel processing chains:
The Comprehensive Inversion (MSW EUL 2C, MCO SHA 2C, MLI SHA 2C, MMA SHA 2C, MIO SHA 2C) and Mantle Conductivity estimations (MIN 1DM, MIN 3DM, MCR 1DM, MCR 3DM) each with a processing time of one month
The Dedicated Inversions consisting of (in sequence, each step with a processing time of one month)
Core eld inversion (MCO SHA 2D) Lithospheric eld inversion (MLI SHA 2D) Ionospheric eld inversion (MIO SHA 2D). All estimated models are subject to an evaluation and when parallel models are availablecross-comparisons which will be documented in the Swarm Level-2 Validation Products (Myy VAL 2 ) with a processing time of up to one month.
Level-1b and Level-2 data are available at http://earth.esa.int/swarm .
6. Conclusions
The Swarm mission is devoted to provide the best ever absolute measurements of the geomagnetic eld. Its various instruments have been selected in order to optimize the scientic interpretation of the measurements in terms of the various sources of the magnetic eld. In recognition of the large effort needed to extract the various types
N. OLSEN et al.: THE SWARM SATELLITE CONSTELLATION APPLICATION AND RESEARCH FACILITY (SCARF) 1199
of scientic information from the complex set of observations a group of institutions and organisations have joined the SMART consortium (Swarm Magnetic and Atmospheric Research Team). The consortium has decided to contribute to the optimal science return from the mission by supporting the creation of a Swarm SCARF (Satellite Constellation Application and Research Facility), with the purpose of deriving commonly used scientic models and parameters, the so-called Level-2 products and make them available to the scientic community at large.
During the 3-year long development phase of SCARF the various processing chains have been optimized and thoroughly tested, demonstrating that the facility is ready to enter the data exploitation phase and process real Swarm data.It is believed that some of the results of the SCARF exercise may also be of relevance for future Earth Science constellation missions that undoubtedly will be implemented.
Acknowledgments. The Development of Swarm SCARF has been funded by ESA through contract No. 4000102140/10/NL/JA.
List of Acronyms
ASM Absolute Scalar Magnetometer (instrument)
CDF Common Data Format
(Goucher and Mathews, 1994)
CI Comprehensive Inversion
CRF Common Reference Frame (of Star Tracker)
DI Dedicated Inversion
EFI Electric Field Instrument (LP and TII)
FAC Field-Aligned Currents
GPS Global Position System (Receiver)
ITRF International Terrestrial Reference Frame
LP Langmuir Probe (instrument)
NEC North, East, Center coordinate frame
MOD Medium Precise Orbit Determination
POD Precise Orbit Determination
PDGS Payload Data Ground Segment
RINEX Receiver Independent Exchange Format
(Gurtner and Estery, 2007)
SP3 National Geodetic Survey Standard GPS Format
(Hilla, 2007)
STR Star Tracker (instrument)
TEC Total Electron Content of ionosphere
TII Thermal Ion Imager (instrument)
VFM Vector Field Magnetometer (instrument)
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The Society of Geomagnetism and Earth, Planetary and Space Sciences, The Seismological Society of Japan 2013
Abstract
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 (S atellite C onstellation A pplication and R esearch F acility) 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.
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