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Contents

Introduction

Swarm is ESA's first constellation mission for Earth Observation (EO). This mission is operated by ESA's European Space Operations Centre (ESOC), in Germany, via the primary ground station in Kiruna, Sweden. This mission consists of the three identical Swarm satellites (A, B, and C), which were launched on 22 November 2013 into a near-polar orbit.

The final constellation of the mission was achieved on 17 April 2014 (more detailed information). Swarm A and C form the lower pair of satellites flying side by side at an altitude of about 470 km (inclination angle is equal to 87.30°), whereas Swarm B is cruising at higher orbit of about 520 km (inclination angle is equal to 87.75°).

All Swarm satellites are equipped with a set of six identical instruments (more detailed information): Absolute Scalar Magnetometer (ASM), Vector Field Magnetometer (VFM), Star Tracker (STR), Electric Field Instrument (EFI), GPS Receiver (GPSR), and Accelerometer (ACC).

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Swarm Constellation Mission

Swarm is ESA's first constellation of satellites for Earth observation. The three identical satellites have a rather unusual shape: trapezoidal with a long boom that is deployed once they are in orbit. Their design is a result of overcoming a number of challenges in accommodating the instrument package, and, since they are launched together on a single rocket, they have to be compact enough to all fit into the launcher fairing.

Developed on behalf of ESA by an industrial consortium led by EADS Astrium GmbH, each satellite is about 9 m long, including the boom, with the surface at the front only measuring about 1 m2. This is to reduce the effect of air drag and to cut down on the amount of propellant needed to stay at the correct altitude. Below around 500 km, air drag tends to slow satellites down and lower the orbit.

The boom, which accounts for almost half the length of the satellite, trails at the back. This is because the front surface is needed for the electric field instrument so that it can collect and measure the speed and direction of incident ions along the orbital path.

Once the boom was deployed, which happened soon after injection into orbit, the satellite has no moving parts. This ensures that there are no vibrations that could influence the measurements made by the accelerometer, which is fixed at the very centre of the satellite. Likewise, the solar panels are fixed, forming the satellite 'roof'.

Magnetic cleanliness is of paramount importance to the mission, so the sensitive scalar magnetometers are mounted at the end of the boom, far away from any magnetic disturbance that the electrical units on the body may cause. The optical bench holding the vector field magnetometer and the three startrackers is mounted halfway along the boom. Throughout the design and manufacturing phases, magnetic cleanliness has been a priority, with many tests carried out on almost every unit and final testing of the assembled satellite taking place in a special facility.

The development of Swarm, which was carried out through ESA's European Space Research and Technology Centre ESTEC, in the Netherlands, also included extensive testing of the optical bench out in the field. Intercalibration campaigns took place in southern Spain at the German-Spanish Calar Alto Astronomical Observatory. The main purpose was to establish precisely the relative orientation of the vector magnetometer and the three startrackers by taking simultaneous measurements at night.

Swarm lifted off from a Rockot launcher, which is a converted SS-19 intercontinental ballistic missile, from the Plesetsk Cosmodrome in northern Russia. The three satellites were placed on a Breeze-KM upper stage with a tailor-made dispenser for simultaneous separation.

The mission is operated by ESA's European Space Operations Centre ESOC, in Germany, via the primary ground station in Kiruna, Sweden. The critical launch and early orbit phase lasted about three days, during which the booms were deployed and all the satellite units were switched on. This was followed by a three-month commissioning phase to ensure all the instruments are working correctly. To increase daily contact with the satellites during this phase, complementary ground stations in Norway, Antarctica and Australia are also used ESA's Centre for Earth Observation, ESRIN, in Italy, manages the scientific data, with processing and archiving in the UK.

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Science Objectives

According to ESA SP-1279(6) "Swarm - The Earth Magnetic Field and Environment Explorer, Report for Mission Selection", the five research objectives of the Swarm mission are:

  • O1: Studies of core dynamics, geodynamo processes, and core-mantle interaction
  • O2: Mapping of the lithospheric magnetisation and its geological interpretation
  • O3: Determination of the 3-D electrical conductivity of the mantle
  • O4: Investigation of electric currents flowing in the magnetosphere and ionosphere
  • O5: Magnetic Forcing of the Upper Atmosphere

During the ESA Swarm study "Preparation of the Swarm Level 2 Data Processing" advanced models for the various contributions to the Earth's magnetic field have been identified as Level 2 products. In particular, the Comprehensive Inversion (CI) providing mathematical descriptions of the various geomagnetic field contributions is required. In order to derive the best model for each of the various contributions to the geomagnetic field, it is necessary to co-estimate or correct for various other parts to the field. As an example, fields from magnetospheric and ionospheric currents and their Earth-induced counterparts have to be considered when modelling the internal part of the field. If a typical user of magnetic field models relies only on standard models, he/she may not be able to take full advantage of the constellation measurements and thus miss the advantages offered by Swarm. This clearly argues for advanced Swarm Level 2 magnetic field models.

Also the other science objectives, for instance determination of the electrical conductivity of the mantle, require data products with a good description of the time-space structure of the external (magnetospheric) field and its Earth-induced counterpart.

How the products are attributed to the Swarm Science Objectives is described in Table 1 and 2 in Level 2 Product Definitions.

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