Flight Operations of the Swarm Constellation Frank-Jürgen Diekmann for the SWARM Flight Control Team Earth Observation Missions Division ESA/ESOC 19/06/2014

Mission Phases Supported By Swarm FOS Development Phases Ground Segment definition, development & procurement, support to spacecraft planning Testing & validation, tracking campaigns Team training LEOP : Launch and Early Orbit Phase Take over control after separation of 3 satellites in parallel Switch each satellite into a fully operational configuration Functional check-out Commissioning Platform and payload commissioning, calibration and validation Orbit injection phase and acquisition of final constellation Ground segment commissioning Routine Continuous monitoring & control, anomaly handling Orbit / Attitude and Constellation maintenance Ground Segment maintenance and evolution End-of-Life Passivation Deorbiting (passive re-entry) The support to the various mission phases start already far before launch at the initial phases of a new mission. The time is needed for …. 1… 2… The LEOP was a particular challenging phase, since the flight control team had to take over three satellites separating from the launcher in parallel. All activities had to follow a strict timeline in order to eventually command them into a fully operational configuration. Only then the first functional check-outs could follow. 3. The commissioning phase followed then after the three day LEOP phase and was finished with the last orbit injection manoeuvres for the three satellites on 17. April this year.

Flight Operations Segment (FOS) High Level Overview Satellite TM/TC Database Simulator Communication Network Flight Dynamics System FOCC (ESOC) Kiruna PDGS : Payload Data Ground Segment, ESRIN FOS Subsystems: Ground Stations and Communication Network Mission Control System Flight Dynamics System Satellite TM/TC Database External Server (sftp) and EDDS Svalbard Realtime and recorded telemetry Commanding/TC External File Server Kiruna / Svalbard / Troll / Perth (LEOP) PDGS Recorded telemetry to PDGS Mission Planning System PDGS EDDS PLSO The FOS consist of a number of elements which are coloured in this diagram. The actual MCS consist of … The orange boxes are part of the MCS, but separate teams and functions within ESOC ; FDS, SDO, SWS, hardware, … A Ground Station and Communications Network performing telemetry, telecommand and tracking operations. For SWARM both TT&C and onboard stored TM dumps will be done via S- Band. The S-band ground station used throughout all mission phases will be the ESA Kiruna and the KSAT operated station at Svalbard terminals (complemented by other stations during LEOP). A general purpose communications network provides support services for exchanging data with external systems, like the PDGS, scientific community, industry. The PLSO at ESTEC provides technical support for the onboard systems. ODAD is Online Data Access and Distribution. The FOS sends the CPF (Consolidate Predicted Files) files to the ILRS (laser ranging service) for distribution, in fact Flight Dynamics sends them via email daily. PLSO : Post Launch Support Office, ESTEC PLSO Mission Prime Scientific Community Support Teams To SLR Industry Support

Mission Control System and Mission Planning Multi-domain capabilities of the Mission Control System (MCS) Manual commanding (TC) capabilities (to several Swarm satellites in parallel during LEOP) Processing of real-time telemetry (TM) (VC-0 and VC-1) Processing of mass memory retrieved telemetry (VC-4) Monitoring and retrieval functions from real time servers Interface with Flight Dynamics System Interfaces to ground stations via ground comms network for transmission of TM, TC, tracking data, etc. Mission Planning and related interfaces, main tasks : Command generation for station handling (automated pass operations) Command generation for onboard data handling (time tagged commands in MTL) Data of the full mission will be archived on the Swarm prime and backup servers. Retrieval and provision of telemetry from long term archive The main challenge is the multi-satellite system of Swarm, which required the development of a multi-domain functionality of the MCS.l Operated by small Flight Control Team

Mission Data Distribution Recorded telemetry (TM) files are retrieved from the ground stations by the Generic File Transfer System (GFTS) and transferred to the prime and backup servers at ESOC. At Kiruna and Svalbard stations two sets of files are created : one for the FOS (has a special header) and one with files containing frames only for the PDGS At ESOC the telemetry files are automatically replayed (a software tool runs permanently to populate the FOS control system with VC2 and VC4 data) Telemetry retrieval based on some new ESOC infrastructure systems : EDDS (EGOS Data Distribution System) server per domain DARC (TM parameter) and PARC (packet) archives EDDS provides a user friendly client (EDDS MMI) to access standard data product files containing Telemetry Packets (Housekeeping and Science TM) Telemetry Parameters (Housekeeping and partially Science TM) Telecommand History External users can access the mission data through ESOC external servers that run the external parts of the EDDS system. The access is done via the EDDS web page. Requests are received by the EDDS server and the data is made available to the external clients either via EDDS client application or in the dedicated sftp account on the same external server. In addition, files are dropped on the external server by external users and made available to the SMCS via GFTS and vice versa. GFTS : Generic File Transfer System (prime node on smcmcb, 2 b/u on swbmca and swbmcb) it polls from internal and external servers for destinations FARC, Swarm Servers, External Servers FOS is distributing 193 files per day to external recipients (number of users and files unusual high for a FOS)

Mission Database And Flight Procedures The Mission Database is the backbone for all operational activities. Telecommands 2966 Sequences 2786 Alpha Numeric Displays 629 Graphic Displays 228 Packets 2185 Telemetry Parameters (total) 47409 Parameters (in Packets) 29398 Out Of Limit Checks 1675 All spacecraft commanding is based on Flight Operations Procedures (FOP) 617 Nominal Procedures 116 Contingency Procedures 32 specific LEOP Procedures and Timelines All procedures were validated and tested on the simulator Procedures are constantly reviewed and updated, when necessary Complexity of the mission demonstrated by a few numbers : The ODB contains the definitions of all defined TC, TM, command sequences, ….

Provision of VC-4 TM files to the PDGS from Kiruna and Svalbard Kiruna, Svalbard, Troll, ESRANGE : TC and ranging, TM low and high bit rate Provision of VC-4 TM files to the PDGS from Kiruna and Svalbard Interface with the Mission Control Center for 2 passes per day and per satellite Interface with Flight Dynamics for ranging data provision and reception of antenna pointing elements 15 m antenna, ESA station in Kiruna, Sweden 10 ESA ground stations, 5 augmented stations, 11 cooperated network stations ESA UNCLASSIFIED – For Official Use

Swarm Ground Station Passes Usage of the ground stations until May 2014, in number of Swarm passes: Number of passes per day for routine phase : 2 passes per satellite is working very well and will be continued until further notice. Only 5 passes failed completely until now (0.3%). 2 passes gives sufficient margin to dump all stored data on the onboard memory (MMU) during one day.

Flight Dynamics System (FDS) Support The three Swarm satellites are in near circular, nun-Sun synchronous, near polar inclination orbits, with Swarm-A and Swarm-C flying in constellation. Swarm orbit maintenance is provided by the ESOC FDS Team. FDS consists of 4 main sub-systems and an independent test and validation function: Attitude monitoring AOCS TM monitoring, performance of sensors and actuators, inter-comparison of STRs and STR and CESS/FGM, fuel bookkeeping, GPSR data extraction from TM Orbit Determination Determination and prediction of orbit, based on tracking data in LEOP and GPSR data extracted from TM thereafter. Generation of orbital products. Manoeuvre Optimization Calculation of manoeuvres required to meet orbital targets based on latest orbit determination; minimization of fuel usage, respecting constraints. Command Generation Generation of command parameters to be passed to FCT; generation of reports and mission planning inputs. Test and Validation Software and System validation operational internal and external interface validation One of the most important elements of the Swarm FOS is the FDS, which provides support for attitude and orbit maintenance via a small, dedicated team. They did a tremendous job in optimising the Swarm orbit injection phase during the commissioning phase.

Commissioning : Orbit Injection Phase 10 Manoeuvre batches needed for orbit acquisition phase (1-3 days each) Injection Orbit : 6880 km (semi-major axis at ascending node (ANX)) Current altitudes (semi-major axis at ANX) Swarm-A : 6849 km (h=468 km) Swarm-B : 6897 km (h=516 km) Swarm-C : 6849 km (h=468 km) (eccentricity : 0.0004) SWB : orbit raised by ca. 17km, the others were lowered by ca. 31 km. h(altitude) = a(SMA)*(1-e) - 6378 km, e=0.0004 The SMA varies because of the gravitational effect of the Earths equatorial bulge. This effect is dependent on the height of the orbit obviously and is much stronger for lower orbits than higher ones. The altitude will be affected by the height of the Geoid at different latitudes but this does not cancel out the variation in the SMA. This effect is higher at higher latitudes. A variation of up to 20 km is possible. The injection orbit height at the injection point was actually 6870km. An important constraint for the mission was a difference in inclination for the upper and the two lower satellites, in order to allow a slow drift of the RAAN. This was achieved via small out-of-plane thrusts together with the orbit altitude changes, which allowed saving a significant amount of fuel. Current inclinations: Swarm-A : 87.35˚ Swarm-B : 87.75˚ Swarm-C : 87.35˚

Orbital Parameters of Swarm-A and -C Current Right Ascension of Ascending Node, Difference SWA-SWC : ca. 1.4º Current Semi Major Axis, Difference SWA-SWC RAAN : crosstrack difference at equator (ANX/DNX), constant for the two lower satellites at 1.4 deg. Altitude difference between SWA and SWC kept rather constant at +/- 10m VFM/ASM Calibrations Slews on 13.5. for SWA and SWC, constellation manoeuvre for SWA on 12.6.

Swarm-A and Swarm-C Separation Maintenance Results of a recent manoeuvre modelling: Swarm-A orbit lowering by 13m and 7.5 mm along-track thruster firing, Results for different drag coefficients Because of these very small altitude variations both satellites were very slowly drifting apart in the last months, from about 11sec to ca. 13.5 sec in June. It was therefore planned to lower the orbit of the trailing satellite SWA which would increase it’s velocity and thereby revert the drift. SWA was chosen, because we could thrust in flight direction and thus avoid a slew manoeuvre. The result of such a manoeuvre always depends on the atmospheric drag which have been modelled for various drag coefficients. In case the drag would be not as high as expected, the velocity of SWA would increase too much and the in-orbit time difference become too small after a few months. This would then require a further drift stop manoeuvre. Implemented finally was a manoeuvre of 8 mm/sec causing an orbit lowering of 14 m, which will have the effect modelled in the plot : reduction of time difference to ca. 8 seconds sometimes end August, before it then reverts again to higher values. Manoeuvre actually executed on 12.06.14 : Expected Swarm-A orbit lowering by 14m and 8mm/sec along-track thruster firing (37 sec)

Fuel Consumption Since Launch Manoeuvre performance nominal, marginally less efficient than expected (slightly more fuel consumed on attitude control during the burns, ~15% vs ~13% expectation) Consumption mostly in-line with the Mission Analysis assumption Fuel remaining after Orbit Acquisition (status : 1. June 2014) : SW-A from 105.5 kg, 66.5 kg remaining after the OIP (39.0 kg used) SW-B from 105.2 kg, 67.1 kg remaining after the OIP (38.1 kg used) SW-C from 102.8 kg, 64.2 kg remaining after the OIP (38.6 kg used) This is less than what was assumed for pre-launch fuel budget : SWARM Fuel Load Swarm FM1 Swarm FM2 Swarm FM3 Mission Phases: Attitude Acquisition 0.32 0.32 0.32 Orbit Acquisition 50.93 50.93 62.00 Orbit Maintenance 13.13 13.13 0.76 AOCS 18.04 18.04 19.34 Propellant Margin 21.17 21.17 21.17 Residuals and Leakage 2.40 2.40 2.40 Total Fuel Loaded 106 106 106 In general all manoeuvres performed so far performed quite well, with slightly less efficiency than expected. Since the altitude change manoeuvres were smaller than originally anticipated, the consumption was consequently less as well. Less fuel consumption as expected due to different orbit strategy (SWB less high and SWA and SWC higher than planned in final orbit)

Swarm-A and Swarm-C Orbit Evolution The long term evolution plot showing the altitude (SMA minus 6378 km). The variation is almost certainly due to SMA variation only, and is visible because the plot has a point once per day, and the location of this point varies from day to day in the orbit, so we see the 20km variation in the SMA over a longer time period. Depending on different atmospheric density cases as defined in the Marshall Space Flight Centre Report (50% medium and 96% high Solar Flux and Ap Geomagnetic Index predictions)the orbit evolution of the lower satellites will allow operations until 2019 in the absolute worst case and until a time, we are all retired.

Space Debris Office Support During all Swarm mission phases the ESOC Space Debris Office provides in-orbit collision warnings to the Swarm FCT. Assumed, the orbits of the debris and target objects are known with sufficient accuracy, then for initial assessments the information provided by the USSTRATCOM catalogue is sufficient to predict all close fly-bys (conjunctions) of a target satellite with any of the catalogued objects. The provided service also covers processing of externally provided information, such as Close Approach Warnings and Conjunction Summary Messages from the US Joint Space Command (JSpOC). The collision risk is determined as a function of the object sizes, the predicted miss distance, the fly-by geometry and the orbit uncertainties of the two objects involved. If a chaser object is predicted to approach closer than a predefined limit or exceeds a risk threshold, a warning email is distributed. In a meeting with the involved teams it is then decided, whether to execute a debris avoidance manoeuvre. A collision avoidance manoeuvre might have a non negligible impact in the fuel budget and mission lifetime. Up to now, several collision warnings for Swarm have been received, but no manoeuvre needed to be scheduled yet. One danger which potentially could bring the mission to an unwanted end is space debris.

Outlook The team is well experienced with the overall mission. Cross-training in various areas has already started to maintain a high degree of proficiency and competence. Future in-orbit subsystem tests will be supported to further improve the mission performance. The constellation of the lower satellite pair will be maintained in a way to minimize fuel consumption Further tests and procedure optimizations are under discussion to better handle on-board events like SEU/SEFI, data gaps, etc. Following successful LEOP and Commissioning phases, we are looking forward to a long and hopefully trouble-free routine phase.