Swarm – Mission & Orbit Design

Image: ESA
Image: ESA
Image: ESA
Image: ESA

The Swarm mission consists of three identical satellites that conduct science operations from Low Earth Orbit making high-precision and high-resolution measurements of the strength of the magnetic field and the magnetic field vector with high spatial and temporal resolution to examine variations of the field.

All three spacecraft are launched in November 2013 atop a Rockot launch vehicle with a Briz-KM upper stage blasting off from Site 133/3 at the Plesetsk Cosmodrome, Russia.

Rockot is based on the UR-100N Intercontinental Ballistic Missile that was developed in the 1970s and re-purposed after the end of the Cold War to serve as orbital launch vehicle. The vehicle is operated by Eurockot that is owned by Astrium and Khrunichev.

Rockot is a two-stage liquid-fueled launch vehicle, equipped with a Briz-KM Upper Stage, that is capable of delivering payloads of up to 2,140 Kilograms to Low Earth Orbit and 1,200 Kilograms of Payloads to Sun-Synchronous Orbit.

The vehicle stands 29.15 meters tall and has a main diameter of 2.5 meters. Rockot has a launch mass of 107,000 Kilograms and uses Unsymmetrical Dimethylhydrazine and Nitrogen Tetroxide as Propellants.

Rockot’s first stage is 17.2 meters long and holds 72,000 Kilograms of propellants that are consumed by three RD-0233 engines and a single RD-0234 for a total liftoff thrust of 190,700 Kilograms. The first stage separates from the second stage in hot-staging mode.

Stage 2 is 3.9 meters long and has a fuel load of 10,700 Kilograms. It is powered by a single main engine, called RD-0235 that delivers 24,500 Kilograms of thrust.

Vehicle steering is accomplished by gimbaling the four nozzles of the RD-2036 vernier engine that operates at a thrust of 1,600 Kilograms. The second stage main engine burns for 183 seconds while the small vernier operates 17 seconds longer due to hot staging.

Photo: Eurockot
Photo: Eurockot

As the launch countdown hits zero, Rockot begins its launch sequence with the reconfiguration of the inertial navigation system of the launcher. Liftoff from Site 133/3 occurs at T+14 seconds as the powerful RD-0233/0234 engines on the first stage ignite – boosting the Rockot out of its Transport and Launch Container to start its climb. Following liftoff, the rocket performs a short vertical ascent before starting to pitch over and align itself with its launch trajectory, swinging to the north, headed for a high inclination orbit.

About 50 seconds after liftoff, Rockot experiences Maximum Dynamic Pressure as it climbs uphill and races downrange. At T+2 minutes and 15 seconds, the first stage of the vehicle shuts down, The RD-0236 vernier of the second stage will be firing at that point as part of the hot-staging process. At T+2:16, the separation system is fired and the spend first stage if pushed away by the exhaust of the vernier engine. Four small solid-fueled retrorockets on the first stage are fired to move it to a safe distance to the second stage that ignites its RD-0235 main engine.

Image: ESA
Image: ESA

With the second stage firing, Rockot continues powered ascent and quickly departs the dense atmosphere. At T+3 minutes and 4 seconds, the Rockot jettisons its payload fairing that is 7.8 meters long and 2.5 meters in diameter. It protects the satellites before launch and during the initial portion of the ascent when aerodynamic forces could damage the spacecraft. The second stage boosts the stack into a sub-orbital trajectory, shutting down after a 183-second main engine burn. The verniers continue firing for a short moment to achieve their full burn of 200 seconds.

At T+5 minutes and 19 seconds, the Orbital Unit consisting of the Briz-KM Upper Stage and the Swarm Satellites is separated from the Booster into a sub-orbital trajectory. Briz-KM 2.6 meters long and 2.5 meters in diameter with a launch mass of 12,200kg. The upper stage is equipped with a main propulsion system, vernier thrusters and an attitude control system. The main propulsion system features a single S5.98 engine that provides 2,000 Kilograms of thrust and can be ignited up to eight times to support complex mission designs.

For Swarm, Briz-KM will perform two burns, the first just after separation from Rockot to achieve an elliptical Parking Orbit with the apogee matching the target altitude. Another burn around apogee serves as circularization maneuver and delivers the stack to its planned insertion orbit.

At T+5 minutes and 25 seconds, the Briz-KM Upper Stage ignites on its first engine burn to boost the stack into an elliptical Parking Orbit. The S5.98 main engine burns for 9 minutes and 16 seconds before shutting down at T+14:41. At that point, the stack begins a 61-minute coast phase.

This coast phase allows the vehicle to reach the proper position for the second main engine burn that begins at T+1 hour 15 minutes and 54 seconds and is just 16 seconds in duration.

Image: ESA
Image: ESA

The Swarm mission is targeting a circular orbit of 490 Kilometers at an inclination of 87.6 degrees. Following Briz-KM Shutdown, the stack enters another short coast period during which the Upper Stage completes re-orientation maneuvers for the separation of the three spacecraft. Spacecraft separation is planned to occur at T+1:31:32. Using a special payload adapter, all three satellites are released simultaneously into very similar orbits. The separation system ensures that no collision takes place between the satellites by sending the spacecraft into slightly different trajectories.

Swarm A and B are set to be acquired by the Kiruna Ground Station, Sweden just 26 seconds after separation to confirm that the satellites are alive and working. Swarm C makes its first contact via the Svalbard Ground Station in Norway at T+1:35:28.

Mission Controllers at ESA’s Control Center watch over the spacecraft as they go through initial orbital operations and perform health polls as part of the Launch and Early Operations Phase that sets the Swarm spacecraft up for commissioning.

Orbit Design

Image: ESA
Image: ESA

In order to accomplish the mission’s science goal of measuring Earth’s magnetic field on small and large scales at high resolution, both spatial and temporal, the orbit of the Swarm mission had to be designed very carefully. For a precise determination of large-scale magnetospheric field properties which is required for better separation of core and lithospheric field components as well as induction studies, the Swarm satellites should fly in orbital planes that are separated by three to nine hours local time.

To achieve a high resolution of lithospheric magnetization mapping, the satellites should fly as low as possible, but the Swarm mission also has to achieve a minimum mission duration of four years and three months without any of the satellites decaying. Also, to achieve global coverage, Swarm should target an orbit with an inclination of close to 90 degrees.

The Swarm constellation has been designed to have three satellites flying in slightly different, but precisely maintained orbits. Satellites A and B are in a similar orbital plane at an inclination of 87.35 degrees. The satellite pair operates at an orbital mean altitude of 462 Kilometers. The two satellites have an east-west separation of 1 to 1.4 degrees (=difference in Right Ascension of Ascending Node). Additionally, the two satellites are required to orbit in close formation with a desirable differential delay of 10 seconds, but no closer than two seconds on-track to avoid collisions.

Swarm Orbit Design - Image: ESA/DLR
Swarm Orbit Design – Image: ESA/DLR

Satellite C completes an orbit-raising campaign at the start of the mission to achieve an orbit of 510 Kilometers at an inclination of 87.75 degrees which differs from the A/B inclination. The change in inclination is completed as part of the orbit-raising campaign by firing the thrusters slightly out of plane to achieve a plane change. With Satellite C at a higher altitude and in a different inclination, its Right Ascension of Ascending Node drifts relative to the other satellites.

This constant drift causes the orbital planes of the two segments to start moving apart which causes a difference in local time at which the satellites overfly a given location.

After three years, the Swarm C satellite crosses the plane of the A&B spacecraft at an angle of 90 degrees and four years into the mission, the separation between the segments is about 9 hours local time.

The Swarm orbit design is focused on two main elements throughout the mission – orbit maintenance and constellation maintenance.

Image: ESA
Image: ESA

The three spacecraft are inserted into almost identical orbits at separation. The specially designed separation system ensures that the satellites do not collide after separation and further maneuvers by the spacecraft moves them into their initial constellation. Overall, each Swarm satellite has a delta-v budget of approximately 100 meters per second over the course of the mission. (The delta-v budget is the total amount of change in velocity a satellite can achieve by using its complete propellant load.)

Satellite C starts its mission with the orbit raising and plane change campaign which will consume a large portion of its delta-v budget that will be made up over the course of the mission as the drag at the higher altitude of Satellite C is smaller than in the A/B orbit, reducing the amount of orbit maintenance required for Satellite C.

For the two other satellites, the mission starts with the setup of the 1.4-degree east-west separation and the flight continues with accurate constellation maintenance, keeping the two satellites flying in close formation. This is done non-propulsively by modifying the satellite’s ballistic coefficients as a function of spacecraft orientation along the pitch axis.

Analysis has shown that variations in the pitch and yaw orientation of a satellite orbiting at altitudes with sufficient drag can be used to modify the ballistic coefficient of a satellite which can be used to control the relative drift of satellites within a constellation. For Swarm, the attitude control system provides an accuracy of one degree which leads to an accuracy of ballistic coefficient control of about 10% which is enough to use it to control the two-satellite constellation. The yaw angle is kept constant at 10 degrees and Swarm A and B use pitch angle adjustments to keep flying in formation.

Orbital Maintenance of Satellite A&B - Image: ESA
Orbital Maintenance of Satellite A&B – Image: ESA
Orbit Evolution (left) & Difference in Local Time over nominal Mission - Image: NASA
Orbit Evolution (left) & Difference in Local Time over nominal Mission – Image: NASA

Because Satellites A and B fly 50 Kilometers below Swarm C, those satellites will experience higher drag in the uppermost layers of the atmosphere. Without orbit maintenance, the two satellites would have a lifetime of about two years before re-entering. To reach the minimum mission duration of 4 years and 3 months, an orbit maintenance plan was developed for the two lower-orbiting satellites involving small engine burns every two weeks to keep the spacecraft orbiting at a constant altitude of 462 Kilometers. This maintenance is performed for the first two to three years of the mission.

After that, the two Satellites are allowed to drop in altitude. This built-in altitude reduction allows the spacecraft to perform higher-resolution measurements of the magnetic field which is also desired by scientists. Within one year and three months, the spacecraft reach the minimum End of Life altitude of 350 Kilometers. Should propellants remain at the end of nominal orbit maintenance, mission planners could decide to extend the 462-Kilometer science phase or insert a segment of orbit maintenance at a lower altitude for scientific measurements.

Once propellants are depleted on satellites A and B, they will start decaying for re-entry within half a year of crossing the minimum EOL altitude.

Swarm C on the other hand does not need these frequent orbit maintenance maneuvers because it experiences smaller drag at its higher altitude. However, the satellite has to keep up with the other two satellites because it is required to remain in an orbit that is at least 50 Kilometers above that of the A & B satellites at all times throughout the mission. To make sure that requirement is fulfilled, Swam C begins its flight with a segment of orbital maintenance, keeping its altitude constant at 510 Kilometers for about 1.5 years. Afterwards, the satellite is allowed to start a very slow decay that will take about 5.5 years, much longer than that of the two other satellites.

Mission Operations

The three Swarm satellites complete a mission of at least four years and three months in orbit including a primary science mission of four years. Actual mission duration and possible extensions depend on spacecraft performance and propellant usage.

Image: ESA
Image: ESA

Starting at launch, the Swarm mission enters its Launch and Early Orbit Phase which covers the first three days. This phase includes the initial acquisition of the satellites after separation from the Upper Stage and the first health checks and activations performed on the satellites. LEOP uses three tracking stations to maximize commanding capability and telemetry relay.

LEOP is used to put the spacecraft in a safe configuration, ensuring that all satellites are receiving power from their solar panels and verifying that all Swarm vehicles can respond to ground commands.

After three days, LEOP transitions to the Satellite Commissioning Phase that covers the first three months on orbit. During commissioning, the satellites and payloads are fully activated and checked out. Boom deployment occurs shortly after insertion. Also, the orbital maneuvering to position the three satellites in their planned constellation is performed. Instrument calibrations get underway during commissioning as well.

At the end of the commissioning phase, all three Swarm satellites are flying in their expected orbits and are properly configured to start science data acquisition. The primary science phase of the flight is four years in duration and is divided into three segments based on the orbital altitude of the Swarm satellites.

The first 1.5 years dedicated to Lithospheric studies while the next 2.5 years feature Core Field and Induction measurements. Starting at the nominal EOL point, the mission enters another period of lithospheric studies that are performed as long as the two low-orbiting satellites can stay functional in orbit. The flight of the Swarm C satellite lasts several years longer than that of the A and B spacecraft allowing additional science data to be collected as this spacecraft’s orbital altitude decreases over the years. Measurements of the particle environment in the upper atmosphere are performed continuously throughout the mission and at all altitudes the satellites are orbiting at.