ERS-2 Ground Segment and Operations Evolution

Transcription

1 SpaceOps 2008 Conference (Hosted and organized by ESA and EUMETSAT in association with AIAA) AIAA ERS-2 Ground Segment and Operations Evolution D. Milligan 1, J.B. Gratadour 3, P.P. Emanuelli 2, F.J. Diekmann 1, D. Mesples 3 European Space Operations Centre, Robert Bosch Str. 5, Darmstadt, Germany J. Lerch 3, V. Bozzi 3 VEGA,Europaplatz 5, 64293Darmstadt, Germany and W. Lengert 4 ESRIN Via Galileo Galilei, Casella Postale 64, Frascati, Italy The ERS-2 satellite was launched in 1995, to provide continuity of service to the ERS-1 mission. Today, despite several major failures, ERS-2 is still operational and providing valuable data for scientific and operational purposes. This has been achieved despite the failure of all onboard science data storage and several gyros, through the use of innovative workarounds. Satellite performance in general has remained robust with high reliability despite the failures, whilst the demand for the ERS-2 products from the user community has remained high, and in some areas significantly increased. The ERS-2 ground segment was developed in the late 80s and early 90s. Since this time ground segment design and operational techniques have evolved considerably at ESOC, whilst some ERS-2 legacy systems have become harder to maintain and operate as they become outdated and obsolete. This paper presents recent updates to the ERS-2 ground segment and to the satellite operations that have boosted the return from the mission, including, the development and implementation of an ERS-2 / Envisat tandem mission requiring ERS-2 orbital change; the development and implementation of fast replanning services as a trial for GMES; the migration of certain ground segment elements, including the Flight Operations Plan; and the merging of Envisat and ERS-2 on-call engineering support teams. Acronyms BOL = Beginning of Life ERS = European Remote-Sensing satellite AMI = Active Microwave Instrument RA = Radar Altimeter ATSR = Along Track Scanning Radiometer GMES = Global Monitoring for Environment and Security ESOC = European Space Operations Centre ESRIN = ESA centre for Earth Observation ESA = European Space Agency FOP = Flight Operations Plan UTC = Co-ordinated Universal Time SPACON = SPAcecraft CONtroller MPS = Mission Planning System GOME = Global Ozone Monitoring Experiment 1 Spacecraft Operations Manager, Mission Operations Department 2 Head Earth Observation Division, Mission Operations Department 3 Spacecraft Operations Engineer, Mission Operations Department 4 Mission Manager, Earth Observation Department 1 Copyright 2008 by European Space Agency. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

2 MARISS = European MARitime Security Services MWR = Microwave Radiometer PEP = Preferred Exploitation Plan PRARE = Precise Rate and Ranging Equipment LRR = Laser Retro Reflector IDHT = Instrument Data Handling and Transmission MGM = Mono Gyro Mode ZGM = Zero Gyro Mode DES = Digital Earth Sensor DSS = Digital Sun Sensor TTC = Tracking, Telemetry and Command HKTM = House Keeping Telemetry InSAR = Interferometric Synthetic Aperture Radar I. Introduction and Background RS-2 was launched on 21 st April 1995 as a continuation of the first European Remote Sensing satellite, ERS-1, E which was launched in July ERS-1 was developed during the 1980s with the objective of measuring the Earth's atmosphere and surface properties, both with a high degree of accuracy and on a global scale. The primary scientific reason behind acquiring such data was to increase our understanding of the Earth system (Ocean, Land, Ice and Atmosphere), in order to deepen our knowledge of the climate and improve global climate modelling. The ERS- 1 data allowed other improvements over the then state-of-the-art including: improved weather and sea-state forecasting and 'nowcasting'; a greater knowledge of the structure of the sea-floor; detailed measurements of the Earth's movements following seismic events; measurements of ice coverage; and the monitoring of pollution, dynamic coastal processes, and changes in land use. In addition, the ERS spacecraft have provided the most accurate sea surface temperature measurements on a global scale (to an accuracy of 0.1K and improving, with improved archive reprocessing algorithms). In order to ensure the continuity of ERS-1 measurements, ERS-2 s development was started in the late 1980s and the satellite was launched on 21 April Although it is essentially the same as ERS-1, the satellite includes a number of enhancements, including an extra payload instrument that measures the chemical composition of the atmosphere (GOME). ERS-2 was launched from Europe's Spaceport in French Guiana on 21 April 1995 by an Ariane 4 rocket and was injected directly into its Sun-synchronous polar orbit at an altitude of approximately 780 km. ERS-2 is a 2.5 tonne three-axis stabilised, Earth-pointing satellite, orbiting Earth with an inclination of in a period of 100 minutes, which gives it a high visibility of the Earth s surface as the planet rotates beneath the satellite's orbit. The repeat cycle can be chosen by relatively minor adjustments of the orbital inclination to modify the altitude allow the orbital track pattern on the Earth's surface to repeat itself exactly after a certain number of days, for example 3, 35 or 168, with 35 chosen for ERS-2. The current orbital characteristics are given below: - semi-major axis= km, - inclination= deg, - mean-local solar time= 10:30 A.M. (at the descending node) - repeat cycle of 35 days with 501 orbits (14 11 / 35 orbits/day). A. The ERS-2 Satellite and Ground Segment The ERS-2 spacecraft platform is based on France's SPOT (Système Probatoire d'observation de la Terre) platform, modified according to ERS power, energy-storage and attitude requirements. The solar array consists of two 5.8 x 2.4m wings, on which are mounted solar cells providing an average power of about 2.5 kilowatts at BOL. The solar array is continuously rotated to align the SA normal towards the sun. Four nickel-cadmium (NiCd) batteries are sized to allow payload operations during non-sunlit periods. Attitude measurement is performed by the attitude measurement subassembly, which is composed of a core of six single-axis gyroscopes (three redundant), two Digital Earth Sensors (DES), one being redundant, and two Digital Sun Sensors (DSS), one being redundant. The DES is used to obtain pitch and roll information from detecting the infrared Earth horizon boundary, and the DSS are aligned to point at the Sun as the satellite crosses the day/night boundary, and are used for yaw 2

3 measurements. In the nominal mode, attitude is controlled by reaction wheels and magnetic torquer bars are used for continuous wheel de-saturation, and the satellite Z face, which carries the instruments, is pointed toward the Earth, with a yaw steering offset applied (Yaw Steering Mode). Other AOCS modes used routinely are the orbit manoeuvre modes to maintain the satellite ground track. Fine Control Manoeuvres (FCMs) use the hydrazine thruster system to impart small in-plane delta-vs to control the E-W drift of the satellite close to the equator. Orbit Control Manoeuvres (OCMs) are used to correct the orbital inclination. This mode uses the thrusters to slew the satellite +/- 90deg from the nominal attitude and then impart a delta-v, to correct orbital inclination drift. The ERS-2 platform also includes thruster based acquisition modes that can be activated as part of the on-board FDIR in case of anomalies. In case of major (or multiple) failures, a satellite survival mode, known as Safe Mode, can be entered, which uses independent AOCS software and modes and, as far as possible, also independent hardware. The ERS-2 payload, except for PRARE, is mounted in the payload module, which can be seen as the top two thirds of the satellite as shown on Figure 2 (left). The payload module contains the following main elements: - AMI Active Microwave Instrument. This instrument consists of a Synthetic Aperture Radar (SAR) and a wind scatterometer (both in the C-band). - RA Radar Altimeter: This instrument takes precise measurements of the distance from the ocean surface and of wave heights. - ATSR-2 Along-Track Scanning Radiometer. Operating in the infrared and visible ranges, this instrument measures sea surface temperatures and the vegetation cover of land surfaces. - GOME Global Ozone Monitoring Experiment. This is an absorption spectrometer which measures the presence of ozone, trace gases and aerosols in the stratosphere and troposphere. - MWR Microwave Radiometer: supplies data on atmospheric humidity. - PRARE Precise Range and Range Rate Equipment. This was used for Precise Orbit Determination. - LRR Laser Retro Reflector. This is a passive instrument that can be used by ground-based laser stations to determine satellite position. - IDHT Instrument Data Handling and Transmission: This was used for temporary on-board data storage by means of two 6.5 GBit tape recorders, equivalent to the data volume acquired in one orbit. Transmission of data is at 105 Mbit/s (high rate transmission of SAR imaging data in real time), or kbit/s in low rate mode. Figure 1. Launch (left) and artist impression in orbit (right) of ERS-2 The ERS-2 flight operations are performed from ESOC, Darmstadt. S-band TTC passes are taken routinely through Kiruna and Svalbard, with Svalbard being used to fill two of the blind orbits from Kiruna, such that 12 of the 14 orbits per day contain a TTC pass. X-band instrument data is also downlinked via Kiruna, and a network of 3

4 local x-band ground stations, which in recent years has been considerably expanded (see below). X-band data is forwarded to ESRIN where different levels of products are built and then made available to the user community. Housekeeping Telemetry is also available via x-band and is forwarded to ESOC via ESRIN. B. In-flight Performance After performing flawlessly for several years, by the end of 1998, one of the six gyros of the attitude measurement system failed, and two others showed serious degradation of performance. As the AOCS design required three gyroscopes (Three-Gyro Mode) for satellite attitude control, all the redundancy was used and an additional gyro failure would have meant the end of the mission. This situation led to the development of a new operational mode in the AOCS based on the measurement of a single gyro: the Mono-Gyro Mode (MGM). The MGM development, validation and uplink activities are described in detail by Canela & Bosquillion 2. The MGM was designed to use only one gyro for the nominal mode used for routine operations (YSM). In this scheme the single gyro is used primarily to estimate the rotation rate around the z-axis, with the x and y rates being estimated by the DES (previously the DES was used in the FDIR and correction of drift). In MGM the DES is able to estimate the X & Y attitude continuously, whereas the Z attitude is only known once per orbit when the sun passes through the field of view of the DSS at the day/night boundary. Using the DES and the single gyro, the satellite rates can be continuously measured and compared with the guidance to maintain attitude control. Specific filters and gain changes were necessary and were also included in the MGM design. After a period of development and validation the MGM software was loaded onto the satellite in February Figure 2. ERS-2 configuration (left) and satellite axes (right) Although the MGM worked well, continued degradation of the remaining operational gyro caused concern and development of a Zero Gyro Mode (ZGM) was initiated. The main problem in progressing from MGM to ZGM is that the yaw attitude is only measured once per orbit on-board by the DSS, and as such it is not possible to obtain a Z rate measurement in the same way as is done for the X and Y rates from the DES, which measures the attitude at 1Hz. The innovative solution was to spin-up the X reaction wheel to high speed to obtain some gyroscopic stiffness, and transfer z disturbances into the x-y system. This system was stable enough to keep the satellite running but suffered from degraded yaw steering performance. This mode (called Extra Backup Mode EBM), was uplinked and made operational in January To improve the yaw steering performance, analysis was made of the in-flight disturbances affecting the z-axis. These were characterized and a system developed where a set of coefficients could be routinely uplinked by ground that would alter the reference wheel rates to counteract some of these disturbances and improve the overall yaw 4

5 steering performance. These coefficients are computed by ground software, using as inputs: the yaw steering information as measured by the DSS; and an independent measurement of the yaw pointing from the Doppler residual on the AMI science data, which was devised specially for the ZGM. Two further patches were uplinked in June and November of 2001 which improved the yaw steering performance to better than +/- 3deg. This has been used since to continue the mission, and despite the degraded yaw performance with respect to the original three gyro design, the mission has continued with high demand from the user community. A second major failure affecting the operational character of the mission was the failure of the prime data storage tape recorder in January A redundant recorder was available but that also failed in June of With no onboard data storage, the operations concept had to be changed to enable a reasonable global coverage to be obtained, since instrument data is now only available with the x-band downlink enabled and a local station acquiring data. From June 2003 to now, the number of local ground stations acquiring ERS-2 data has risen steadily, with 15 ground stations available for low rate x-band downlinks at the time of writing. The current network is shown below in Figure 3. As can be seen from the figure, there is now full coverage over high interest areas, such as the North Atlantic, Europe and the poles with close to full coverage in other areas. MEXICO KOUROU SINGAPORE JOHANNESBERG Figure 3. Network of low rate x-band downlink stations for ERS-2 II. Ground Segment and Operations Evolution The in-flight performance evolution described above has altered the way in which some of the routine operations are performed, mainly in the area of ZGM and Mission Planning of the x-band downlink. For ZGM, AOCS coefficients are regularly uplinked to the spacecraft based on inputs from the DSS and the yaw pointing derived from the AMI science data. A subset of x-band downlink stations is equipped to be able to deliver yaw pointing information. This information is processed via ESRIN and is used in conjunction with the DSS Yaw data downlinked in HKTM to fine tune the AOCS coefficients that are uplinked at every ground station pass. For the mission planning, improvements were made to the mission planning software, allowing the FCT to optimise the switching on/off of the x-band downlink over the ground station network. The optimisation takes into account power consumption (mainly battery energy), IDHT temperature and the switching cycle rate on the IDHT. These elements are traded-off to safely maximise return from the instruments. 5

6 A. ERS-2 Envisat Tandem InSAR Mission In the latter half of 2007 a proposal was made that would enable a new type of product to be derived by combining ERS-2 AMI and Envisat ASAR instruments to create SAR Interferometry (InSAR) images. Although ERS-2 and ERS-1 had previously operated in a tandem mode, ERS-2 and Envisat were not designed to be able to perform interferometry, since the design of the two SAR systems were different. Analysis showed however, that interferometry between the two satellites was feasible, but only if an orbital offset was introduced (see Figure 4). Since the launch of Envisat in March 2002, Envisat and ERS-2 have been flying in similar orbits with Envisat leading ERS-2 by 30 minutes (i.e. the orbital parameters for Envisat are similar to those of ERS-2 with a 10:00AM mean local solar time at descending node crossing instead of 10:30). Since it is not possible to introduce an orbital offset across the entire orbit, some pre-selection of high interest areas is required. After some discussions it was decided to focus on northern latitudes close to the northpole. This new technique enables SAR interferometry images to be obtained of glaciers and ice in northern Polar Regions, thus contributing to the measurements being made for International Polar Year. Since there is a 30 minute difference between the fly-over times of ERS-2 and Envisat, an interferometry image would detect any small, cm scale, changes in terrain. This would make possible measurements of fast-moving glaciers from space. This technique is also suitable for providing unique data for the generation of very high accuracy low relief Digital Elevation Models. The implementation of the orbital offset can be understood schematically from Figure 5. Figure 4. The introduction of an orbital offset between ERS-2 and Envisat to enable SAR interferometry The solution chosen after trade-off studies were performed, was to increase the inclination of the ERS-2 orbit slightly, introducing a 2.1km perpendicular offset, (2km ground-track), such that inter-satellite interferometry could be targeted over high northern latitudes. To achieve this, ERS-2 executed a series of inclination changing manoeuvres. Although manoeuvres were executed to initiate the orbital offset, the net cost in fuel for the interferometry campaign was close to zero. This can be understood with reference to the nominal orbit maintenance strategy. The orbit maintenance strategy of ERS-2 is similar to that on Envisat described in detail in 3,4, with the main difference being that inclination changing manoeuvres are performed in sunlight around the descending node on ERS-2. A short summary of the orbit maintenance follows, for a more detailed account see 3,4. The orbital ground track of ERS-2 (and of Envisat) has to be maintained to within +/- 1km of the reference orbit ground track. The orbit is subject to two main disturbance types that require manoeuvres to counteract: air drag; and the gravitational effect of the sun and the moon. Air drag leads to a loss in altitude, reducing the orbital period causing the orbit to drift away from the reference orbit. This is manifested mainly in equatorial regions first, as an easterly drift at ascending node crossing. The gravitational effect of the sun tends to decrease the inclination (i.e. push the inclination towards the poles). Routine orbit maintenance to counteract these disturbances is of two main types; in-plane FCMs and out-of-plane OCMs. FCMs are small in plane manoeuvres that are used to boost altitude. Depending upon air drag levels these are executed on the order of once per month. To counteract the inclination Figure 5. The ERS-2 and Envisat orbits when 6 configured for inter-satellite SAR interferometry

7 drift, out-of-plane inclination changing OCM manoeuvres are executed (see Figure 5). These are relatively infrequent (1-2 per year) but account for the vast majority of fuel use. With reference to Figure 5 it can be seen that the orbital offset required for interferometry requires extra manoeuvres of the OCM type (i.e. the orbital inclination is increased beyond what it is normally, against the Figure 6. Period of high latitude deadband excursion for inter-satellite interferometry perturbing force direction). Even though extra manoeuvres were required to initiate the orbital offset for interferometry, it can be seen that the perturbing gravitational force will eventually restore the orbit to its nominal ground track. In this way, there is no fuel required to restore the orbit after the inter-satellite interferometry campaign ends. This is the reason that the campaign could be executed for effectively no fuel cost, since the extra manoeuvres are simply manoeuvres that would have had to be executed at a later date in any case. The ERS-2 measured and predicted ground track at its northern most point during the campaign is shown above in Figure 6. The period of inter-satellite interferometry can be clearly seen as a negative excursion on the deadband. The general trend due to the perturbing gravitational force can be seen as a gradual positive deflection, with the manoeuvres shown as a sharp negative drop. The period of inter-satellite interferometry lasted for three repeat cycles, from September 2007 to mid February During this time AMI images were compared to ASAR images. The offset proved successful and an example interferometric image is shown below in figure 7. The fringes detail cm scale changes in terrain that have occurred in the time between the Envisat and ERS-2 image acquisitions (30 minutes). Such measurements are used to help understand the dynamics of fast moving glaciers. 7

8 Figure 7. Example InSAR image constructed from ERS-2 and Envisat SAR images. Image shows the Kangerdlugssuaq Glacier in Greenland. Courtesy of E. Rignot, NASA-JPL. B. Fast re-planning as a trial GMES service 1. Routine planning by PEP Routine mission planning at ESOC starts with the reception of the weekly PEP (Preferred Exploitation Plan) from ESRIN, received typically mid-week. The weekly PEP contains one week of AMI image requests, starting on the Friday of the following week. The other AMI modes, and all other instruments, are scheduled by ESOC based on global zones and on the seasonal illumination. Nominally on Monday, the schedule for Tuesday to Thursday is planned. After this, additional 24 hour periods are added to the schedule each day of the working week. Three times per day the Spacecraft Controller (SPACON) uplinks commands from the schedule to the satellite to keep the onboard queue topped up. This process ensures that at any time between 12 and 24 hours worth of MPS scheduled payload commands are in the on-board time-tagged-queue. 2. Mid term re- planning updates by Delta-PEP If ESRIN receives AMI image requests for a time which is already covered by a weekly PEP delivered to ESOC, then a Delta PEP containing one or more image requests can be sent to ESOC. Delta PEPs can also cancel image requests for various reasons, like ground station unavailability or to allow the addition of another image in the same orbit, subject to the AMI image duration constraints (maximum 2 minutes in eclipse, 12 minutes per orbit, 10 minutes consecutive). The Delta PEP can contain image requests that are up to 14 days in the future, if it is issued on the same day as a weekly PEP and the image is scheduled for the last day of the weekly PEP. Delta PEP requests a few days in the future can be fulfilled easily by generating or regenerating the schedule for that day. 8

9 3. Short term re-planning by Emergency-PEP (Fast Replanning) Delta PEPs that contain image requests less than 36 hours in the future are called Emergency PEPs (EPEP) and are undertaken by ERS-2 for high priority customers like the International "Space and Major Disasters" Charter 6 or for European Maritime Security Services (MARISS) 5 by following the fast re-planning procedure developed in August Such requests are approved by the Mission Manager in ESRIN, and are executed if sufficient time for rescheduling and uplink remains, and there are no conflicts with high priority activities. Figure 8. Overview of the planning cycles over two weeks, with examples for DPEPs with 13 and 2 day notice EPEPs received before noon of working days at ESOC are processed immediately. When an EPEP is received, an MPS schedule for the orbit containing the new image will either be already in the control room awaiting uplink or onboard the satellite. Therefore, when an image request is received via EPEP, and nothing is on board yet for the time of its execution, the Mission Planner instructs the SPACON to disregard the existing schedule and suspend uplink of further payload commands until the new schedule has been generated and sent to the control room. If commands are already on board, the only option is for the engineer to delete all commands in the on-board payload queue and uplink the new schedule containing the new image and associated high rate X-band downlink commands. The overall planning concept is illustrated in Figure 8. An example of the fast re-planning performance obtained can be seen from a request received by ESOC in October as part of a trial GMES service (ERS-2 and Envisat are providing GMES services until the launch of the Sentinels 7 ). On 24 th of October 2007, ESOC received an image request for GMES MARISS 5 trials at noon (11 UTC), the commands were uplinked 6 hours later and the image was successfully acquired after 11 hours, at 21:57 UTC. This fast replanning is combined with Near Real Time (NRT) processing to achieve a performance already within the GMES timeliness requirements for Sentinel-1, which will be the Sentinel for day/night all weather radar services 7. An example fast replanning timeline is shown in Figure 9. Figure 9 Example timeline of a fast re-planning. 9

10 C. Migration of the FOP from FOPS to MOIS FOPS, the system used to maintain the procedures in the ERS-2 FOP (Flight Operation Plan), was developed before the launch of ERS-1, in FOPS is a text based procedure writing tool with some similarities to html (see figure 10). In 2007, ERS-2 was the only mission still using FOPS at ESOC. In the intervening 16 years, the MOIS system became the ESOC standard, and had evolved with the experience of the flying missions to a state which offered many potential advantages to over FOPS. Expected improvements in employing this tool, included: - Direct access to the operations database (ODB) - Automatic enforcement of operational rules - Consistency of the FOP and the ODB - Configuration control - Release of FOP Figure 10 FOPS based procedure working environment (left) and output format (right) Also to be considered was the positive effect of ERS-2 moving to the ESOC standard system, which would increase synergy with other missions. This facilitates knowledge and cost sharing, and staff mobility between ERS-2 and other ESOC missions. In particular, it would benefit team sharing with Envisat (see below), a project being developed at the time. Such advantages were traded-off with the expected disadvantages, the main disadvantage being the effort involved in moving to a new FOP procedure maintenance tool. Bearing in mind also that even on a mission that has been flying for thirteen years, both new procedures, and procedure updates, are continually required; the outcome of the trade-off favoured migration. The development to "tailor" MOIS for ERS2 so that it could be integrated in the ERS2 ground segment started in January It focused mainly on the importation and utilisation of the ODB and the implementation of ERS2 specific constraints. Fortunately, the structure of the ERS2 database is similar to the one of Envisat; thus it was possible to re-use much of the development performed to tailor MOIS to Envisat. An importer tool was also developed to automatically convert a procedure from the FOPS to the MOIS format. The importer tool was an important addition to what is normally in the MOIS tool suite, in that it created a first draft MOIS version automatically from the FOPS procedure, which contained all the important information (e.g. TC content, TM checks). This version was then checked by the FCT responsible, who would run the MOIS consistency checks and finish organising the procedure in the MOIS format. The first version of MOIS for ERS2 was delivered in March The validation of the MOIS system was performed by the FCT. It involved a test of the functionalities affected by the tailoring and the conversion of a set of FOPS procedures. MOIS tailoring was accepted in July From this time onwards, new procedures and procedure updates have been written in the MOIS system. 10

11 Figure 11 Example MOIS format procedure (working environment and output format are the same) With new procedures and procedure updates written in the MOIS format (see figure 11) the FOP contains procedures of both types. Migration of the complete FOP to the new format entails a significant effort, but has some important benefits, perhaps one of the most important being the positive effect on team training. With a mission that has been flying for thirteen years, the team is made up of members who did not develop the FOP and were not present at launch. A FOP migration necessarily entails a detailed review, and the migration process put in place also included validation of the MOIS format procedures on the simulator, before being made operational. To manage the migration, a system was put in place where each procedure was assigned with a reference number, a responsible engineer and a level of priority for the migration. Procedure migration is then a background engineer task, performed in the absence of high priority operational work. At the time of writing, the FOP migration is on-going. As expected, the migration represents a significant effort, but it has proven valuable: in term of training and in terms of improving existing procedures that have been indirectly affected by the failures and the evolutions on the spacecraft that have occurred since launch. D. Cross training between Envisat and ERS2 Flight Control Teams (FCTs) ERS-2 and Envisat flight operations share many synergies. Both platforms are based on the French SPOT design, with Envisat being an evolved version of the ERS-2 SPOT platform. The payloads present on both missions also share many commonalities, with Envisat flying the next generation of some of the instruments flown on ERS-2 (e.g. AMI / ASAR, RA / RA2, ATSR2 / AATSR). In the ground segment both missions use similar infrastructure (e.g. SCOS 1B). For these reasons, and for increased efficiency of operations with possible further mission extensions for both missions, a system of cross-training was introduced to initiate integration of the FCTs of both missions. On ERS-2, in terms of satellite expertise, the FCT is split into payload and platform specialists, with two team members sharing the platform subsystems and three team members sharing the payload (with one payload team member assigned 50% to Envisat). Since 12 passes are taken per day, out of a possible 14, seven days per week; the on-console monitoring and commanding is performed by spacecraft controllers (SPACONs), who work on a shift rotor providing support 24/7. The SPACONs are shared with Envisat, which can be done without conflicts due to the fact that the orbits of ERS-2 and Envisat are synchronised (i.e. an ERS-2 pass follows 30 minutes after an Envisat one). To ensure full expertise during working hours on the various subsystems, instruments and ground segment elements; a matrix of responsibilities involving the concept of a Prime and Back-up engineer for each element is used. In case of problems outside of working hours, a system of engineering on-call support is in place, where one payload engineer and one platform engineer was on-call for both missions (i.e. four engineers were on-call at any one time). Previously the FCTs of Envisat and ERS-2 were almost entirely separate entities. The goal of the on-call integration was to integrate on-call engineering support, such that there would be one platform on-call and one payload on-call engineer serving both missions. The levels of engineering support were reviewed and a third level of responsibility assigned: 1. Prime Operations Engineer: ESOC expert on subsystem/instrument/ground segment element. Responsible for Procedures / Reporting / Interfaces. 11

12 2. Backup Operations Engineer: Good knowledge of subsystem / instrument /ground segment element & procedures. Able to take up duties of the Prime when the Prime is absent. 3. On-call Engineer: Possesses sufficient knowledge and training to run all 'first-line' urgent operations in contingency cases With the above roles defined, the duties of the on-call engineer are then to recover from recurrent well known anomalies with recovery procedures clearly identified and to perform any first-line urgent operations that are required to put the satellite into a safe state. This definition was then used to design a cross training campaign for the engineers of the Envisat and ERS-2 teams, whereby ERS-2 team members are trained on Envisat on-call activities and visa versa. This activity lasted a few months and from mid 2007 the on-call engineering support service for ERS-2 and Envisat has been integrated. In addition to the positive effect of reducing the time spent on-call for an individual team member, the system has also improved the knowledge sharing between the missions to the benefit of both. III. Conclusion Despite the onboard failures of gyros and onboard data storage occurring between 1999 and 2003, user demand for ERS-2 products has remained high, and in some areas significantly increased. Recent updates to the ERS-2 ground segment and to the satellite operations have boosted the return from the mission. The synergy between ERS- 2 and Envisat has been increased in both operations and exploitation for the user community. In particular, the development and implementation of the ERS-2 / Envisat tandem InSAR mission, where ERS-2 s orbit was changed slightly to allow ERS-2 and Envisat to work in concert, has seen the development of an entirely new product. InSAR images obtained between Envisat and ERS-2 detail cm scale movements in terrain occurring in a 30 minute timescale, useful for the study of fast moving glaciers. On the ground segment side, the development and implementation of fast replanning services has returned timeliness performance in line with GMES, with MARISS requests being executed in 11 hours. The migration of certain ground segment elements, including the Flight Operations Plan; and the merging of Envisat and ERS-2 on-call engineering support teams has further enhanced operational robustness and efficiency. The ERS-2 mission was recently extended until Acknowledgments The authors would like to thank experts at ESTEC and the industrial team at Astrium SaS for their support and contribution to the ongoing success of the mission. The authors would also like to thank the members of previous flight control teams who have looked after the satellite since launch. References 1 Francis, C. R. et al, The ERS-2 Spacecraft and its Payload, ESA Bulletin no. 83, August Canela, M, Bosquillon de Frescheville, F, Implementation of a Single Gyroscope Attitude and Orbit Control System on ERS-2, Envisat / ERS-2 Symposium, Gotheburg, Sweden, October Milligan, D, et al Envisat Flight Experience: FDIR and Lifetime Optimisation, The 9th International Conference on Space Operations, Rome,, June 19-23, Milligan, D, et al ENVISAT PLATFORM OPERATIONS: PROVIDING A PLATFORM FOR SCIENCE Envisat Symposium, Montreux, Switzerland, April European MARItime Security Services (MARISS): 6 International Charter "Space and Major Disasters": 7 Bargellini, P, et al The GMES-Sentinels Flight Operations Segment, The 10th International Conference on Space Operations, Heidelberg,,. 12