Figure 1. Overview of the Cluster Ground Segment
The Cluster mission is scheduled to be launched on the first Ariane-5 flight, which will put the four identical spin-stabilised satellites into Geostationary Transfer Orbit. Mission operations, for the baseline duration of 2.5 years, will be carried out from ESOC, in Darmstadt (D).
The ground-segment design and mission operations concepts for the Cluster mission have been defined according to the basic mission requirements, to allow the transfer of the four spacecraft from the initial Geostationary Transfer Orbit (GTO) achieved at separation from the launcher, into the final highly elliptical polar mission orbits. These orbits are such that, in the areas of scientific interest along the orbital paths, the four spacecraft will form a tetrahedral configuration with pre-defined separation distances. These distances will be changed every six months during the mission.
From the mission-operations point of view, the most critical phase of the mission will occur immediately after separation from the Ariane-5 launcher when, within a few minutes, all four spacecraft will be released and time-critical configuration activities have to be commanded from the ground within a few hours. During the routine phases of the mission, the challenge for the Cluster ground segment will be that of supporting and coordinating the simultaneous operations of the four spacecraft with careful utilisation of the available space- and ground-segment resources.
The Cluster ground segment (Fig. 1) consists of a ground stations and communications network, an Operations Control Centre (OCC), a dedicated communications network for data distribution to the scientific community, and the Cluster Science Data System.
Figure 1. Overview of the Cluster Ground Segment
The Ground Station Network (Fig. 2) interfaces to the spacecraft via S-band antennas, which support all telemetry, telecommand and tracking activities. In the initial phases of the mission, from launch until all four spacecraft have reached their science operations orbits, five ESA ground stations - Redu, Odenwald, Kourou, Perth and Malindi - will be used to maximise the ground contact during critical activities. In addition during those phases, NASA's two Deep Space Network (DSN) stations, at Goldstone (USA) and Canberra (Aus), will be available. In particular, their 26 m and 34 m-diameter antennas will provide support during the periods when one or more spacecraft are cruising in a transfer orbit at large distances from the Earth (up to 60 Earth radii). The 15 m antennas of the ESA stations are not sufficient for tracking the weak telemetry signals coming from the Cluster spacecraft at such distances.
Figure 2. The Cluster ground-station network
The routine science operations phase will be supported by two dedicated ESA ground stations, at Redu in Belgium and in the Odenwald in Germany. These stations will be unmanned and remotely controlled from the main computers located in the OCC at ESOC. The DSN stations will be used once per orbit during this routine phase to receive high-speed dumps of science telemetry from the Wide Band Data instrument on each of the four spacecraft.
The OCC will provide the necessary mission control functions for both the spacecraft and payload, commensurate with satellite safety, the scientific requirements, and the overall system resources. It will also conduct all activities relating to mission planning and scheduling of the overall system - ground segment, spacecraft and payload - taking into account the requests received from the Principal Investigators, as well as all system constraints and capabilities.
The core of the OCC facilities is the Cluster Mission Control System (CMCS), which will support all of the mission-control tasks, including: real-time/near-real-time data-processing tasks essential for controlling the mission; acquisition and interim storage of raw scientific data, to be accessible together with raw housekeeping and auxiliary data to the scientists at remote locations, as well as distribution of all data on a hard data medium; payload command request handling and the planning and scheduling of satellite operations.
The Mission Control System is supplemented by the Flight Dynamics System (FDS), which will handle all activities related to orbit and attitude determination and control, and by a number of auxiliary hardware and software tools.
All detailed requests for science payload operations during the routine phase of the mission will be collected by the Joint Science Operations Centre (JSOC), which forms part of the Cluster Science Data System and is located at the premises of the Rutherford Appleton Laboratories in the United Kingdom. The JSOC will transmit to ESOC, over a dedicated data network, the payload operations requests necessary to support the weekly mission planning activities at the OCC.
The science data are to be processed separately at the OCC and temporarily stored in a dedicated Data Disposition System, from which all Principal Investigators and authorised Science Data Centres will be able to recover the data collected during the previous 10 days of the mission. The same computer system will generate, on a daily basis, CD-ROMs on which all scientific, housekeeping and auxiliary data are to be stored and distributed to the users.
The Cluster mission can be broadly divided into three phases:
For the first ten hours of the mission, only two ESA ground stations will be in contact with the spacecraft, which means that a maximum of two spacecraft will be in ground contact at any given time. During this period, the critical initialisation of the reaction control subsystem and the first spin-up manoeuvre from 5 to 15 rpm will be carried out on all four spacecraft. For the rest of the phase, which nominally lasts about three weeks, five ESA stations - Kourou, Odenwald and Redu in addition to Malindi and Perth - and the two Deep Space Network (DSN) stations at Goldstone and Canberra will be dedicated to tracking the Cluster fleet.
After the collection of the tracking measurements from the ground stations and acquisition of the desired spacecraft attitudes, a sequence of manoeuvres will be initiated to reach the pre-defined, highly elliptical (28 000 km perigee, 125 000 km apogee) orbits with an inclination of close to 90 degrees and the line of apsides close to the equatorial plane. The spacecraft will be manoeuvred in pairs to minimise operational complication, particularly in case of contingencies. The nominal transfer scenario foresees six orbit manoeuvres for each spacecraft. A number of attitude and spin-correction manoeuvres will also be carried out during this phase. Most of the activities will be performed in real-time contact with one of the ground stations, although for some orbit manoeuvres ground coverage cannot be guaranteed.
Ground coverage during the commissioning phase will be ensured by the Redu and Odenwald ground stations; the DSN ground stations used during the previous mission phases will also be available, but only for a few hours per orbit. Spacecraft and payload operations will typically be conducted in real time. Experts and scientists for all payload instruments will support the commissioning activities from the OCC during the entire phase. They will be located with their ground-support equipment in a dedicated area of the OCC and linked to the operations control rooms via voice and data links.
Routine mission operations are based on the use of a single control centre in conjunction with two dedicated ESA ground stations (Redu and Odenwald). All payload operations will be pre-planned and executed according to an agreed plan, which is generated and modified in a periodic mission planning exercise. Real-time operations with the spacecraft will normally be limited to the acquisition of telemetry data recorded during the long non-coverage periods and dumped to the ground station during the real-time contacts, and the uplinking to the on-board memory of time-tagged commands for the execution of all pre-planned spacecraft and payload operations.
With the four spacecraft in the nominal operations orbit, the ground coverage is about 50% of the total orbital period of about 57 hours, divided into a long-visibility period (pass) of up to 20 hours and one or two short passes of a few hours each every orbit. Typically, one station will track one spacecraft for half of the available geometrical visibility time, and a second spacecraft for the rest of the pass. The second ground station will be similarly assigned to the second pair of spacecraft, resulting in an average ground coverage for each spacecraft of about 25% of the mission time.
The baseline approach for science operations is to activate the payload instruments and acquire science data only in specific areas of the orbit, when the most interesting regions of the magnetosphere are crossed. If the spacecraft is in real-time contact with the ground upon entering one of the regions of scientific interest, then the science data are to be directly downlinked to the station; otherwise all data are to be stored in one of two on-board recorders (each with a capacity of 2.25 Gbit).
In addition to the above, the Wide Band Data (WBD) instrument will be transmitting data for short periods each orbit to a DSN ground station, the only constraints being availability of the DSN station and non-simultaneity of coverage from an ESA station.
During the science mission phase, it is planned to vary the distance between the four spacecraft once every six months, to allow scientific investigation of the plasma structures over different scales. This will be done by means of a series of orbit manoeuvres conducted over a period of several weeks.
The long-term planning is based on a planning period of 6 months, generally the period in which the spacecraft separation distances remain unchanged. This activity involves the Project Scientist and all Principal Investigators, who define the areas of scientific interest and the related desired data acquisition modes for each orbit in the planning period. The plan is then analysed by ESOC for feasibility from the point of view of spacecraft and ground-segment resources availability and operability. The finalised version of the baseline plan is then passed to the Joint Science Operations Centre (JSOC) to form the basis for the detailed short-term planning cycles.
The short-term planning works on planning periods of three orbits (about 1 week) starting eight weeks before the start of each planning period, when JSOC submits electronically to ESOC a complete set of operations requests for the payloads of all four spacecraft. At ESOC this request is analysed via software mission-planning tools, which check the correctness of the request and its feasibility in terms of available spacecraft resources (power and data storage) and ground-segment resources (data storage and link capacity between the station and the OCC). The results of this activity are then reported back to JSOC.
Further iterations of the request for science operations related to a specific planning period are allowed until the week before the start of the planning period. At this point all requests are frozen and the final implementation cycle begins at ESOC. Only minor change requests involving operations which have no impact on space- or ground-resource utilisation are allowed in this phase.
The final implementation of the science operations works on a cycle of nominally three days, in which the finalised plan is translated into time-tagged operations schedules for the four spacecraft and the two ground stations. The schedules are then released to the relevant control computers. Shortly before the beginning of a visibility period, the ground-station operations schedule is activated and the station operations are performed completely automatically. The time-tagged operations schedule is then uplinked to each spacecraft during the visibility period; in general, a period of 48 hours into the future will be covered by each up-link of time-tagged commands to guarantee continuity to the science operations.
Figure 3. The ESOC Main Control Room, configured for the Cluster mission
For ESOC, the Cluster mission presents the unique challenge of the simultaneous launch and control of four identical scientific satellites. In order to achieve this a very modern design approach has been adopted for the ground segment, based on distributed architectures and local and wide-area networks for data processing and distribution. The operations concept, based on a single mission control team, remotely controlled unmanned ground stations, and the concentratio nof real-time operations responsibilities into two control positions, has allowed costs to be reduced to a level within the envelope of a traditional single-spacecraft mission.
ESA Bulletin Nr. 84.