CHEOPS (CHaracterizing ExOPlanets Satellite)
CHEOPS is a minisatellite, the first mission in ESA's small Science Program, selected in October 2012; the project was adopted for implementation in February 2014. The SPC (Science Program Committee) of ESA decides on the program content, which is structured along the Cosmic Vision Plan since 2007.
The objective is to target nearby, bright stars already known to have planets orbiting around them. Through high-precision monitoring of the star's brightness, scientists will search for the telltale signs of a ‘transit' as a planet passes briefly across its face. This will allow an accurate measurement of the radius of the planet. For those planets with a known mass, the density will be revealed, providing an indication of the internal structure. 1)
Figure 1: Artist's impression of a planet transiting a star (image credit: ESA/ATG medialab)
The CHEOPS mission is devoted to the fine characterization of exoplanets orbiting around bright stars by using transit photometric measurements. The mission is envisaged as a partnership between the ESA Science Program and Switzerland, through the Swiss Space Office (SSO). The University of Bern leads a consortium of 11 ESA Member States contributing to the mission and represented in the CHEOPS Science Team. ESA is the mission architect responsible for overall mission definition and procurement of the spacecraft and launch. ESA is also responsible for the early operations phase that will be executed by the spacecraft contractor, Airbus Defence and Space – Spain. The science instrument is led by the University of Bern, with important contributions from Austria, Belgium, Germany and Italy. 2) 3) 4) 5)
CHEOPS is considered as a test case for implementing a S-Mission (Small Mission) in the ESA Science Program, and it's being implemented according to the following requirements:
- S-class missions must be science-driven and selected through an open call for missions (bottom-up process)
- The mission implementation cycle, from the call for proposals to launch, must be drastically shorter than for medium (M) and large (L) class missions
- Missions must be cost-capped, possibly with a proportionally larger Member States involvement than for M or L missions.
Table 1: High-level CHEOPS mission requirements
Project organization & responsibilities: The CHEOPS Mission Consortium (CMC), led by the University of Bern (CH), is responsible for the following mission elements: 6)
• Development, procurement, qualification and calibration of the instrument
• Provision of AIT services in support to the satellite activities
• In-orbit monitoring, calibration and evaluation of the instrument performance
• Provision of the CHEOPS ground segment, including:
- The Ground station(s)
- The MOC (Mission Operations Center)
- The SOC (Science Operations Center)
ESA will be in charge of the overall mission architecture and is responsible for the following activities:
• Procurement of the CHEOPS platform accommodating the instrument assembly
• Procurement of the instrument detector CCD
• Integration, test and qualification of the complete satellite
• Procurement of the mission control system, satellite and instrument simulator and any required flight dynamics system
• Procurement of the launch opportunity and launch operations
• LEOP and in-orbit commissioning.
Table 2: Responsibilities for the CHEOPS mission
Technology readiness requirements:
The Call for S1 mission requested the mission concept to be compatible with existing platforms, with minimum modification. All platform equipment must be off-the-shelf, flight qualified (TRL ≥ in ISO scale), and preferably flight proven (TRL ≥ 9). Modifications to existing products must be compatible with the development schedule (less than ~ 3 years for the platform) with satisfactory margins. Limited adaptation of the platform mechanical design and accommodation is an illustrative example of allowable modifications: although such modification may require the requalification of the platform structure, it can be achieved in less than 2 years with low schedule risks, and is therefore acceptable.
The science payload was requested to rely on available technologies (TRL > 5-6 in ISO scale), without ruling out new instrument developments, provided they can be safely achieved in less than three years (e.g. using some heritage or building on previous developments).
With the above approach, the major development risks are on the payload side. One could have equally requested a high TRL for the science instrument, as for the platform equipment. Such an approach was deliberately not followed for maximizing the "science accessibility" of the Call and the competition at mission level. Indeed, a higher payload TRL (TRL ≥ 7) would imply designing the mission around an existing instrument that would have been recently developed for a previous mission. As a matter of fact, most of the shortlisted missions, including CHEOPS, could not have been proposed under such condition.
Industrial implementation through ESA:
The approach retained for the industrial development of S-missions differs substantially from that for L and M missions, while obeying all the Agency rules. L or M missions are defined and implemented in three major phases:
1) The preparation phase: This phase starts by a Phase 0 performed by ESA, then includes a detailed study phase (Phases A and B1) where the mission definition and requirements are progressively matured and finally frozen, and where the mission critical technologies are developed in parallel aiming at TRL ≥ 6 (ISO scale) prior to starting the spacecraft development. The study phase is achieved with parallel industrial contracts, awarded in competition in two steps, for preserving the industrial competition for the spacecraft development. At the end of the preparation phase, the SPC in principle has available all critical elements for adopting the mission (i.e. deciding
2) The implementation phase: Following the mission adoption by the SPC, the Agency initiates the implementation phase by first selecting the Prime contractor for the space segment development, and then the subsystem contractors and the equipment suppliers. Most of the industrial contractors are known at the end of phase B2, which is closed by the system PDR (Preliminary Design Review). The selection of all industrial contractors is made through ESA "procurement best practices", i.e. in open competition with due consideration to specific procurement constraints, such as the geographical distribution of contracts. Following successful development and verification, the space segment is launched and the implementation phase ends with in-orbit commissioning.
3) The operational phase: The operational phase covers the nominal lifetime in-orbit plus possible extended time. The operations include: the control of the spacecraft, including communications; the execution of the required science observations (science operations); the data processing prior to delivery to the science community; and the data archiving.
For CHEOPS, the ESA Industrial Policy Committee approved the following approach, which is being implemented:
• A single Invitation to Tender was issued, covering both the study phase and the implementation phase for the platform. Obviously, this is possible only if 1) no technology developments are required and 2) the space segment requirements are mature enough at the end of Phase 0. Both conditions should be met by S-missions for enabling a fast implementation, and both were actually met by CHEOPS. A ceiling price was required from the bidders for the implementation phase.
• Parallel contracts were awarded for a "study and final contractor selection phase" limited to ~8 months, which was used for maturing the instrument definition and interfaces, and clearing critical contractual aspects. The parallel contracts were awarded to EADS CASA ESPACIO (ECE, Spain) and SSTL (UK). At the end of the study phase, the System Requirement Review was held with both competing contractors. The two shortlisted contractors were also required to convert their initial proposal to a firm fixed price proposal covering any requirement update resulting from the study phase, and providing their "best and final offer" prior to selection. ECE was finally selected by ESA for the platform development.
• While the platform contractor selection was made following ESA standard best practices, the platform equipment level was not subject to the same selection process. Indeed, since the concept is asking a fast development schedule and is therefore based on re-using existing "off-the-shelf" platforms with minimum modifications, it makes little sense to re-open the platform equipment selection through extensive ESA procurement best practices. The two competing platform contractors were requested to make best use of their respective heritage for meeting the high level requirements of S-missions, namely: a fast development schedule; mastered development risks; and cost ceiling. The faster selection of equipment suppliers, mostly through direct negotiations, also enabled to lower ESA internal costs.
The Science Program requirements regarding geographical return balance were taken into account by strongly constraining the over-returned countries (France and Germany in this case) in the platform Invitation to Tenders de facto excluding them from the platform competition. Therefore, any platform implemented under these conditions would contribute to the geographical return re-balance of the program – although modestly, since the CHEOPS contract value is only 6-7% of a typical Prime industrial contract for a M-class mission. In a way, the full CHEOPS contract is comparable in value to a subsystem of an M-mission spacecraft, and it is not excluded by ESA rules to make use of a simplified procurement scheme if, for some reason, the subsystem is required to be recurring from a previous mission.
Figure 2: Organogram of the CHEOPS project (image credit: CHEOPS Team)
The discovery of planets around other stars (exoplanets) has opened one of the most interesting and exciting field in modern astronomy. After nearly two decades mainly dedicated to a census of exoplanets, emphasis is now shifting on the physical and chemical characterization of exoplanets and their systems. CHEOPS will be the first mission dedicated to search for transits of exoplanets by means of ultrahigh precision photometry on bright stars already known to host planets. The main science objective of the CHEOPS mission will be to study the structure of exoplanets smaller than Saturn orbiting bright stars and to measure the bulk density of super-Earths and Neptunes orbiting bright stars (Ref. 4).
CHEOPS will provide the unique capability of determining accurate radii for a subset of those planets for which the mass has already been estimated from ground-based spectroscopic surveys, providing on-the-fly characterization for exoplanets located almost everywhere in the sky. It will also provide accurate radii for new planets discovered by the next generation of ground-based transits surveys (Neptune-size and smaller). By unveiling transiting exoplanets with high potential for in-depth characterization, CHEOPS will provide "golden targets" for future missions, such as the recently selected M3-Plato mission, and for future instruments suited to the spectroscopic characterization of exo-planetary atmospheres.
CHEOPS is built to achieve a photometric precision similar to the Kepler mission of NASA while observing much brighter stars located almost anywhere on the sky. The CHEOPS target list will mainly consist of stars with small exoplanets previously detected by accurate Doppler surveys and known transiting Neptune-size planets detected by ground-based photometric surveys. CHEOPS is also currently envisioned following-up a significant number of targets from the planned TESS (Transiting Exoplanet Survey Satellite) mission of NASA.
In comparison with random searches such as those carried out by CoRoT and Kepler, CHEOPS will be incredibly more efficient. Knowing where to look and at what time to observe is priceless information particularly to detect (and characterize) long-period transiting systems (up to 50 days of revolution period).
Target observability and sky coverage: The key to precise photometric transit measurements is to keep the noise in the measurements to an absolute minimum. Besides the noise associated with the detector itself, stray light is the main source of noise. This stray light is minimized through the design of the telescope itself, the orbit chosen, and by limiting the directions in which the telescope points in order to avoid solar light reflected by the Earth and/or the Moon reaching the detector. With the current design of the CHEOPS telescope, in order to meet the photometric requirements mentioned above, the following conditions have to be met:
- the angle between the line-of-sight and the sun must be larger than 120º
- the angle between the line-of-sight and any illuminated part of the Earth must be larger than 35º. In addition, the target must have a projected altitude from the surface of the Earth equal or higher than 100 km
- the angle between the line-of-sight and the Moon must be larger than 5º.
The prime mission goals of CHEOPS are:
1) Performing first-step characterizations of super-Earths: Measuring the radius, density, and inferring the presence or absence of a significant atmospheric envelope for super-Earths in a wide range of environmental conditions. This will be achieved by searching for shallow transits on bright stars (6 < V < 9 mag) already known to host planets in this mass range with revolution periods up to 50 days.
2) Obtaining new insights into the physics and formation processes of Neptunes: Measuring accurate radii for Neptunes, determining precise densities, deriving minimum values of their gas mass fractions, and inferring possible evolution paths. This will be achieved by characterizing the transit light curves of tens of Neptunes with revolution periods up to 13 days, previously detected in transit across stars brighter than the 12th magnitude in the V band.
3) Building a collection of "golden targets" for exo-planetology: The small mass exoplanets transiting bright stars observed by CHEOPS will be the best suited targets for in-depth atmospheric characterization by spectroscopic facilities contemporaneous or subsequent to the mission (e.g., the James Webb Space Telescope).
Figure 3: Illustration of the CHEOPS mission (image credit: Airbus DS, Ref. 9)
The Spacecraft platform has undergone a successful SRR (System Requirements Review) in March 2014 leading to the selection of the platform contractor by end of April 2014, with a scheduled PDR (Preliminary Design Review) for September 2014, and a fast implementation phase with a readiness for launch in 2018. 7)
The short platform development schedule (~2.5 years) precludes any new technology development, requiring preferably TRL 6 (qualified items) for all elements of the platform. Therefore, as specified in the ITT (Invitation To Tender), the proposed CHEOPS platform had to be an adaptation of an existing product, flight qualified and with heritage for all aspects including equipment qualification, design, development and verification approach (Ref. 4).
From the mission analysis and to satisfy the science requirements, the driving design requirements for the platform design are:
• Sun-synchronous orbit with a local time in the ascending mode (LTAN) to be at 6:00 or at 18:00 hours (decision open in order to increase possibility to find a co-passenger for launch) with an altitude in the range between 650 and 800 km, enabling to have the sun permanently on the backside of the spacecraft and minimizing Earth stray-light.
• A 3-axis stabilized spacecraft, keeping the telescope line of sight fixed in inertial reference frame with an APE (Absolute Performance Error) better than 8 arcsec for pointing accuracy (for 68% of the observation time), slowly rotating around the telescope line of sight for maintaining the focal plane radiator oriented towards cold space (i.e., at zenith of the subsatellite point), enabling a passive cooling of the CIS (Contact Image Sensor) detector to 233 K.
• Instrument to avoid pointing at the sun once cover is opened and provision of a fixed sunshield to protect the instrument radiators from sun illumination and providing it with a stable thermal environment.
• Required compatibility with a variety of different launcher vehicles and thus an envelope launch environment requirements have been defined, presently covering baseline launchers: VEGA and Soyuz, and optional launchers: Rockot, PSLV, Dnepr and Falcon-9. (It is planned to select the Launcher Vehicle at the latest for the mission level Critical Design Review).
• Mass of the spacecraft is approximately 290 kg to be compatible with a shared launch as co-passenger.
• Compatibility with several Ground Station coverage scenarios, e.g. using Redu, Harwell, Malindi and/or Kiruna, and allowing for 1.2 Gbit/day downlink in S-band.
• Deorbiting of the spacecraft within 25 years at the end of the mission, sized for a worst case 800 km orbit, to comply with ESA orbit debris mitigation requirements.
The project development is organized on two main phases:
• Phase 1 (A/B1) – Competitive definition phase to consolidate all CHEOPS mission requirements, provide evidence that the proposed spacecraft design meets those technical, quality and programmatic requirements and the budget constraints. This phase is ending with the SRR.
• Phase 2 (B2/C/D/E1) – Implementation phase encompassing activities to freeze the spacecraft design until Launch, LEOP and In-Orbit Commissioning.
And, as a key element in the CHEOPS implementation approach, ESA released a single ITT (Invitation To Tender) for the satellite procurement covering both phases calling for a full implementation proposal, with a corresponding ceiling price, to be converted in a firm-fixed price at the end of Phase 1.
• In July 2014, ESA announced that the CHEOPS project had entered the implementation phase (meaning that the spacecraft has gone from selection to implementation in less than 18 months). Capped to an ESA cost of €50 million, CHEOPS is being developed in collaboration with the Swiss Space Office (SSO), a division of the Swiss State Secretariat for Education, Research and Innovation (SERI), and the University of Bern, Switzerland. The Swiss organizations lead the consortium of 11 ESA Member States contributing to the mission and represented in the CHEOPS Science Team. The spacecraft will be built by Airbus Defence and Space, Spain. 8)
- The science instrument is led by the University of Bern, with important contributions from Italy, Germany, Austria, and Belgium. Other contributions to the science instrument in the form of hardware, or in the science operations and exploitation, are provided by the United Kingdom, France, Hungary, Portugal and Sweden. The MOC (Mission Operations Center) is under the responsibility of Spain, while the SOC (Science Operations Center) is located at the University of Geneva, Switzerland.
- The Prime contractor for CHEOPS is Airbus Defence and Space, Spain (note: former EADS CASA Espacio S.L. Is now part of Airbus Defence and Space).
Figure 4: ECE-CASA spacecraft design (image credit: ESA, CHEOPS Team)
CHEOPS platform: 9)
The CHEOPS spacecraft is based on the Airbus Defence and Space AstroBus family of low cost satellite platforms (following on from e.g. Spot- 6 & -7, KazEOSat-1), and the ninth for an ESA program following on from Sentinel-5 Precursor and the MetOp Second Generation satellites.
The satellite is composed by a prismatic body platform, where the Optical Instrument is mounted on its top surface, plus three body-mounted solar array panels (Figure 4). There is a fixed sunshield in the central solar panel, to provide the required shadow to Instrument radiators.
The three body-mounted solar array panels have at 67 degree deviation between them, in order to have best illumination in any mission attitude. The bottom surface is dedicated to the launcher interface, for which a 937 standard clamp-band system is proposed. The propulsion module thrusters and S-band –Z antenna are in bottom panel with the minimum protrusion downwards from separation plane. The top surface of the platform prismatic structure is dedicated to the accommodation of the Instrument elements, the Optical Telescope Assembly and Baffle and Cover Assembly.
Two Instrument electronic units are accommodated inside the satellite platform, Sensor Electronics Module and the Back-End Electronics.
The structure design is made of sandwiches with metallic skins for top-bottom-side panels to benefit of the strong SEOSAT/Ingenio heritage. The panels accommodate the platform and instrument equipment. The prismatic hexagonal body allows the side panels to be deployed using special removable hinges for units' assembly and electrical and functional integration campaigns.
The thermal control design driver is to achieve a stable payload thermal environment in orbit while minimizing the perturbations on instrument radiator. For such purpose, a sunshield prevents the sun to illuminate instruments radiator under any observation condition. Besides, passive thermal control means are MLI (Multi-Layered Insulation) to insulate from cold space and external fluxes, use of massive parts to improve thermal stability in time, high conductance link between the sensitive parts and their dedicated radiator, radiative surfaces to reject heat and cool down the sensitive parts and conductive thermal insulation. Additionally, heating lines are controlled by the instrument electronics in operating mode, while the survival mode heating lines are operated by the platform. 10)
The platform thermal architecture is based on the AS250 avionics architecture adapted to the CHEOPS mission specific needs and observation constraints, whose main constituents are:
- MLIs to provide insulation from the external environment
- Radiator foils to allow rejection of the heat dissipated by electronic units
- Black coatings to increase the radiative coupling inside the platform
- Interface fillers to increase the thermal contact with the mounting panel
- Thermal washers to conductively decouple from the mounting panel
- Active thermal control (foil heaters and temperature sensors) to maintain all the equipment above their minimum design temperature during cold phases.
The electrical architecture for CHEOPS (Figure 5) is fully recurrent of the AS250 electrical architecture with the corresponding tuning for adaptation to CHEOPS needs, namely:
- A compact S-band communication transceiver (TRCV)
- New models of AOCS sensors and actuators sized to CHEOPS needs: reaction wheels and magnetorquers
- A 30 kg propulsion module.
Figure 5: CHEOPS electrical architecture (image credit: Airbus DS)
The CHEOPS avionics architecture is built around a core DHS (Data Handling Subsystem) made of the OBC (On-Board Computer), the RIU (Remote Interface Unit), and a set of two redundant 1553 buses, one dedicated to the platform including the power management unit and star tracker, the other dedicated to the instrument.
Analog/discrete interfaces are handled through the RIU that is in charge of acquiring the housekeeping information of the spacecraft. It is also in charge of managing through dedicated boards the propulsion module items (valves, pressure transducer, cat-bed heaters), the AOCS components (magnetometers, magnetorquers and reaction wheels) and to control through high power commands the configuration of the satellite units.
The S-band transceiver and the reaction wheels interface with the RIU through an UART link and further with the OBC through the 1553 RIU interface. These UART interfaces are used for commanding and housekeeping/monitoring telemetry acquisition.
The synchronization between the spacecraft equipment is provided by means of synchronization lines distribution generated by the OBC and distributed via the RIU towards the client units (instrument, star tracker).
Figure 6: Satellite units' layout in the hexagonal body surfaces (image credit: Airbus DS)
The PCDU (Power Conditioning and Distribution Unit) handles the switching and protection of the power lines for all the satellite units. Most units are protected by a LCL (Latch-up Current Limiter) against any over-current anomaly. Vital units, transceiver and the OBC, that allow recovery and controllability of the satellite in case of critical anomaly, are powered through FCLs (Fault Current Limiters) that maintain powering of the protected unit even after an anomaly.
The non-regulated primary power bus is directly connected to the battery. Its voltage is thus fully dependent of the battery state of charge and of the instantaneous power consumption. It ranges from 22 V to a maximum voltage of 34 V. Secondary power distribution is ensured either within the unit for elaborated units [STR (Star Tracker)], or distributed by the RIU for the most elementary ones (AOCS sensors/actuators).
The Solar Array (SA) accommodation to the primary power bus is performed by DET (Direct Energy Transfer) and is implemented by switches in parallel with SA sections (shunt regulation). Excess of SA power is dissipated within the PCDU.
AOCS (Attitude and Orbit Control Subsystem): The CHEOPS AOCS is mostly inherited from the AS250 avionics product line with some adaptations. The AOCS design for CHEOPS has shown that a GPS receiver is not needed, as opposed to the AS250 baseline. Although this decision leads to a lack of an on-board accurate state vector measurement, a two-steps thorough analysis proved the feasibility of the solution despite this modification. First, it was confirmed that science pointing requirements are not degraded by this issue (regarding parallax effects), and second the analysis showed that degradation of performances of on-board AOCS functions using on-board propagated state vector are acceptable (roll guidance, magnetic model and orbit control functions). In order to avoid long term drifting of the onboard propagator, a periodic state vector update from the ground is used, based on Doppler measurements. Another change with respect to the AS250 architecture is the removal of a CSS (Coarse Sun Sensor). Since CHEOPS features a dusk-dawn orbit, the B-dot law used in Safe Mode ensures the convergence of solar arrays towards Sun direction by design, without further reorientation of the axis perpendicular to the orbit as it is the case in a barbecue configuration. This fact renders the CSS unnecessary for this mission. Additionally, the design incorporates payload-based data of the direction of the target star centroid, which is used by the AOCS to estimate and compensate for thermoelastic deformations between the star tracker and the instrument. This approach allows improving the pointing performance of the satellite, reaching a value of a few arcseconds. 11)
Figure 7: CHEOPS satellite (left)) and AOCS architecture (right), image credit: Airbus DS
The AOCS must ensure an accurate and stable pointing during the observation, with an APE (Attitude Performance Error) of < 4 arcseconds (8 arcseconds in case of optical head failure) at a 68% confidence level. The inertial direction of observation of the star can be selected by the ground segment with the following constraints:
- The Sun must not be inside a cone around the line of sight of the telescope (Sun exclusion angle) of at most 120º
- The target shall not be occulted by the Earth, taking a slightly enhanced Earth radius by 100 km to avoid atmospheric glow
- Additionally, an Earth stray light exclusion angle of 35º and a Moon exclusion angle of 5º are also applicable.
As shown in Figure 7, the X axis of the spacecraft is nominally aligned with the instrument LoS (Line of Sight). This means that there is a degree of freedom around roll axis during observations, since the rotation of the image around the star center is irrelevant for the scientific purposes, because an integration of the incoming light is performed, which is independent of the roll angle. This degree of freedom is used to keep the instrument radiators away from Nadir direction for thermal stability purposes. This is a constraint in the body Z axis which unambiguously defines the reference quaternion to be computed by the guidance law and followed by the control algorithms.
Another important mission constraint is related to the need to avoid the sunlight intrusion in the payload CCD. Indeed, in order to ensure this occurrence immediately launch, a cover is included at the aperture of the instrument which is released after initial stabilization phase. The three-axes attitude control during nominal phases of the mission guarantee by design the compliance of this constraint. For the Safe Mode, an extensive analysis has been conducted to characterize the probability of such event, which proves to be sufficiently low and only possible under very special initial conditions.
Regarding orbit control, the mission needs ΔV maneuvers to correct launcher injection errors, to conduct collision avoidance maneuvers and to carry out a deorbiting campaign at the end of the satellite lifetime. Nevertheless, due to the mission features, orbit control to compensate for disturbance forces is not needed during nominal life, which simplifies the operations on-ground and reduces the propellant budget.
Figure 7 depicts the final architecture implemented for CHEOPS. One compact and accurate multi-head STR (Star Tracker ) is used as attitude sensor for nominal operations. Two optical heads are mounted in a direction close to the instrument line of sight and used in hot redundancy. Two electronic units are embarked also in cold redundancy, each one of them cross-strapped to the two optical heads of the star sensor. The optical heads are accommodated on the payload panel to optimize performance because of lower thermoelastic deformations, and their orientation is chosen to ensure their availability for all mission phases.
A cluster of four reaction wheels is used in hot redundancy for three axes attitude control in normal mode, and three internally redundant magnetorquers are used for wheels offloading. These two groups of actuators are also used in Safe Mode, where the magnetorquers also provide attitude control. Two magnetometers are used for magnetic field measurement in Safe Mode.
The propulsion subsystem simple architecture reduces overall mass, propellant and cost (including design and integration). Uniquely used for orbit corrections, it is based on four thrusters aligned in the same direction and one tank of mono propellant. As the propulsion subsystem is never used for Safe Mode neither for normal operations outside orbit corrections, it allows decreasing the required propellant mass. Furthermore, this architecture has been extensively used in other projects with excellent results.
RF communications: The communications subsystem has been designed to meet the uplink and downlink requirements considering that the baseline nominal ground station is located at Torrejon and the backup station is Vilspa-1. The use of Kiruna or Svalbard stations is a possible option to increase LEOP and/or decommissioning coverage. The CHEOPS proposed communications subsystem is based on the following architecture. 12)
Figure 8: Topology of the Cheops communications subsystem (image credit: CHEOPS Team)
This subsystem is composed by two transceivers (nominal and redundant), two couplers and four patch antennas (two for transmission and two for reception). The transceivers include a transmitter, in cold redundancy with the one in the other chain, a receiver, in hot redundancy with the one in the other chain, and adequate filtering to ensure self-compatibility and avoid interferences.
The architecture considers antennas with the same circular polarization (RHCP) for the uplink. In order to avoid signal cancelling, and discriminate the two different paths, there is a privileged path with no attenuation, and an attenuated path with 14 dB attenuation. Therefore, in case of signal combination, no destructive results will appear.
For the downlink, different circular polarization (RHCP and LHCP) will be used for each antenna. Due to CHEOPS mission attitude, there will be a change from one antenna (one polarization) to the other antenna (other polarization) during one pass. Therefore, it requires the ground station capability to simultaneously receive both polarizations and be able to select the best signal.
Table 3: Uplink specification
Table 4: Downlink specification
Figure 9: Antenna location in spacecraft (image credit: CHEOPS Team)
Figure 10: COM DEV STC-MS03 transceiver (image credit: COM DEV)
Table 5: CHEOPS key features (Ref. 7)
• May 3, 2018: Members of the CHEOPS consortium could be proud of their achievement as the science instrument of the upcoming exoplanet mission left Bern on its journey to Madrid last month. 13)
- The science instrument and its tailor-made handling equipment left Switzerland by truck in six containers, designed to provide protection from shock, moisture and dust, on 10 April 2018. Its safe arrival in Spain the following day marked a key milestone for the CHEOPS project and enabled Airbus Defence and Space Spain, the prime contractor that has designed the spacecraft, to integrate the science instrument and the spacecraft platform and begin test activities.
- The science instrument, including among other elements the telescope and CCD (Charge Coupled Detector) cannot operate without the satellite platform, which comprises solar panels, thrusters, radio transmitters and reaction wheels for providing power, propulsion, communications and attitude control.
- The shipment of the science instrument follows an intense period of qualification and calibration activities at the University of Bern, with the instrument team racing against the clock to deliver the instrument to the spacecraft prime contractor so that CHEOPS can be ready for launch by the end of this year. The key objective of the calibration campaign was to collect the data needed to convert the signals measured by the instrument into values that can be used to determine astrophysically meaningful quantities.
- CHEOPS will make observations of exoplanet-hosting stars to measure small changes in their brightness due to the transit of a planet across the star disc.
- The light from point-like stars will be collected by the telescope and directed onto multiple pixels of the detector to form an image that astronomers call the PSF (Point Spread Function). Knowledge of the shape of the PSF is needed to make sure that the instrument is used in an optimal way. It is essential that any changes in the measured signal caused by transiting planets can be distinguished from changes caused by environmental effects such as variations in the temperature of the instrument or electromagnetic disturbances.
- The calibration campaign conducted in Bern over the past three months involved such sensitive measurements that even the body heat of people in the cleanroom housing the reference sources had to be taken into account. Reassuringly, it was determined that the thermal stability of the telescope was satisfactory and even exceeded the design requirements.
- A number of calibration activities focused on the electronics of the CHEOPS detector. As photons from CHEOPS target stars strike the silicon surface of the CCD, they will be converted into electrons. The resulting charge will be turned into a voltage that must be sampled and digitized. The response of the electronics in this process is unique to CHEOPS and must be calibrated and optimized.
- As a result of the tests completed in Bern, astronomers believe that the CHEOPS instrument can achieve the scientific objectives for which it has been built, and it is with hope and excitement that they can look forward to the new science that the satellite will enable once operating in space.
- Once the final integration checks have been made in Madrid and the last calibration data have been analyzed, most members of the instrument team in Bern will be able to consider their job done. After much hard work, they will be looking forward to the launch eagerly.
- Others will continue work on the remaining elements of the mission, starting with the team at Airbus Defence and Space Spain. The upcoming activities will provide an opportunity to train the future operations team and will include system validation tests to ensure smooth end-to-end operations involving the integrated spacecraft, the mission operations center in Torrejón, Spain, and the science operations center in Geneva, Switzerland. These tests will be followed by thermal tests in France, vibration tests in Switzerland and acoustic tests at ESTEC, ESA's technical center in The Netherlands.
Figure 11: The CHEOPS spacecraft during baffle cover alignment at Airbus Defence and Space Spain (image credit: Airbus Defence and Space Spain)
• March 29, 2018: A colorful design capturing the essence of ESA's CHEOPS mission, which will measure the size of planets as they cross in front of their parent stars, has been selected for the rocket carrying the satellite into space. 14)
- More than 300 were submitted to the competition that offered graphic designers and artists the unique opportunity to feature their work on the rocket that will launch CHEOPS into Earth orbit. -The design will be placed, together with ESA's and other logos, on the Soyuz rocket's fairing, the tough outer shell that protects the satellite during launch and as it passes through the atmosphere into space.
Figure 12: The sticker design was created by Denis Vrenko of Celje, Slovenia, a 25 year-old graphic designer and final-year architecture student at the University of Ljubljana (image credit: ESA/Denis Vrenko)
• December 18, 2017: With the flight models of the CHEOPS spacecraft and telescope maturing, crucial tests can now be run with real hardware in place of simulations. The latest such tests involved operating the payload with the Mission Control System to test and validate real operational command sequences. 15)
- In the field of space engineering, not everything that is essential is glamourous. When a missing minus sign in a single line of software could have irreparable consequences, system engineers need to be meticulous. A determined team of CHEOPS engineers has been hard at work to ensure that a complex ensemble of subsystems work together to deliver precious science data.
- In September, another step on the path towards launch was taken when the CHEOPS spacecraft platform began relaying instructions to a model of the scientific instrument at Airbus Defence and Space - Spain in Madrid. While the flight model of the instrument was under construction over a thousand kilometers away in Bern, tests aimed at preparing for its integration with the spacecraft platform were conducted using an engineering model of the detector and electronics hardware.
- The aim of these tests was to ensure that all procedures requiring communication between the spacecraft platform and the telescope instrument will execute flawlessly. These procedures include capturing images, generating housekeeping data, such as the so-called 'heartbeat report' that lets operators know that the instrument is still operational, and executing an attitude control loop to keep CHEOPS locked onto a target star.
- For accurately measuring the radii of transiting exoplanets, the instrument is designed to have a pointing error of less than 8 arcseconds of jitter during a 48-hour observation, equivalent to an Olympic archer keeping a steady aim at the bullseye from a distance of over 3 km. The attitude of the spacecraft will routinely be sensed by two star trackers; however, during long observations, the attitude of the orbiting spacecraft relative to the Sun will change, resulting in variations in how the structure is heated. This in turn will cause small thermoelastic deformations of the telescope structure inducing a varying offset between the line of sight of the telescope and the attitude sensed by the star trackers. To correct for this subtle effect, the onboard AOCS (Attitude and Orbit Control System) uses the centroid of the target star as provided by the instrument and, if the centroid position moves, it instructs the spacecraft to correct the pointing drift with fine adjustments that keep the star centered in the image.
- Testing this control loop involved providing a set of simulated images to the control system and monitoring the reaction of the control algorithms carefully. As every maths student or anyone who has ever had to write a computer program knows, in a long sequence of calculations, errors can easily result from misplaced positive or negative signs, and, since the AOCS control loop involves a series of sub-units using particular sign conventions for positive and negative changes, dedicated tests were required to verify that the correct sign conventions have been applied throughout the control chain.
• December 6, 2017: The scientific instrument, the Telescope, at the heart of the CHEOPS mission has been fully assembled, marking an important milestone in the development of this small, fast-track exoplanet-characterizing mission. 16)
- The FPM (Focal Plane Module), which houses the CCD detector array and the FEE (Front-End Electronics), was secured in place on the telescope assembly at the University of Bern this month (Figure 13), completing the construction of the ultra-high-precision photometer that will be used to measure, with high accuracy, the radii of planets orbiting nearby stars. This instrument will now be calibrated at Bern and later integrated with the spacecraft platform, which is currently undergoing final testing in Madrid at the premises of the prime contractor, Airbus Defence and Space–Spain (ASE).
- The design and development of the CHEOPS instrument is the result of a pan-European endeavour that has drawn together experts from across the continent. The instrument structure was designed and manufactured in Switzerland, the optics came from Italy, the baffle from Belgium, the data processing unit and flight software from Austria, the radiators from Hungary, and the FPM, which was assembled in Germany, uses a CCD detector manufactured in the United Kingdom.
- It has taken over four years of effort to get this far. The CHEOPS instrument consortium started work in mid-2013 and the manufacturing phase of the flight model began at the end of 2015. An intense period followed, during which the subsystems were manufactured and tested prior to shipment to the University of Bern for final integration.
- Although the flight model of the FPM was integrated this month, preparations for its optical alignment have been taking place using a fully representative engineering model of the focal plane and the flight telescope, which became available last May. Alignment procedures have been rehearsed carefully and a test campaign under low-temperature, vacuum conditions has been carried out to ensure that the alignment performed in ambient conditions will give the expected result in orbit. To do this, the instrument was placed inside a vacuum chamber and illuminated from the outside, while the detector and the telescope tube were cooled to their operational temperatures of -40 °C and -10 °C, respectively. By accurately measuring how the shape and size of a star-like image changed as the instrument went from ambient conditions to cold operating conditions, it was possible to evaluate the accuracy of the in-orbit alignment.
- The precise optical alignment must also withstand the mechanical loads that will be experienced at launch. To verify this, the FPM engineering model integrated with the flight telescope was equipped with accelerometers and subjected to vibrational testing. The objectives of the vibration test were to verify the structural integrity of the instrument, to measure the structural response to sinusoidal and random excitation, and to determine the resonant frequencies of the instrument in the out-of-plane and in-plane directions between 5 Hertz and 2000 Hertz. Reassuringly, the instrument met all qualification criteria and, in particular, the alignment was confirmed to be preserved.
Figure 13: The bronze-colored FPM, which houses the CCD detector array and the Front-End Electronics, is seen here mounted on the flat, black optical bench at the rear of the downward-pointing telescope tube (not seen). The FPM is the detector system that captures the image from the telescope (image credit: University of Bern)
- Once in orbit, CHEOPS will target bright stars that are known to host exoplanets. As the spacecraft changes attitude to point at each target star, sunlight will fall onto the instrument at different angles causing variations in the amount of heating it experiences. In September, thermal testing was performed to ensure that the temperature changes experienced by the CCD detector and the FEE will be within acceptable limits. The tests involved equipping the thermal vacuum chamber at the University of Bern with dedicated heaters to simulate parasitic heat flux from the Sun, and mounting a liquid nitrogen shroud in front of the instrument radiators to simulate the radiative sink temperature expected in space. The tests covered the extremes of the expected operational temperature range and investigated six different thermal balance cases, which were used to consolidate the mathematical thermal model of the instrument. The test results show that all units are able to stay within the allocated operational temperature ranges. Moreover, they provide a high level of confidence that, for all the available pointing attitudes, the stringent thermal stability requirements for the CCD and the FEE (respectively 10 and 50 mK root mean square variation) will be met.
- The instrument team is now entering the final stages of the verification campaign. The EMC (ElectroMagnetic Compatibility) acceptance test will be performed. After this, the final vibration test at instrument level will be made to check workmanship, followed by a measurement to confirm that the alignment of the flight model FPM to the telescope has been maintained.
- The final activity to be completed at the University of Bern will be the Instrument Calibration Campaign. Calibration is essential for characterizing the detector and the flight electronics in terms of photo response non-uniformity, system gain, read-out noise, as well as the shape and variation of the point spread function over the field of view, and for verifying compliance with the instrument performance requirements. This information will be carefully reviewed and used in the processing of the scientific data that CHEOPS will collect once operational.
- With calibration completed, the instrument will be shipped to the spacecraft prime contractor—Airbus Defence and Space–Spain (ASE)—for integration with the spacecraft platform, so that the overall satellite verification can be completed by the end of 2018.
- The delivery of the instrument to the prime contractor will represent a key milestone for the CHEOPS project and is the result of a significant effort in a large number of European institutions.
• July 12, 2017: While the scientific payload is being assembled in Switzerland, work in Spain has focused on preparing the spacecraft platform that will carry the payload and provide power, propulsion, communications, and thermal control. 17)
- After mechanical installation and electrical testing of the individual hardware components, it was important to find out whether the ensemble of hardware units on the platform flight model would behave as required when operated by the attitude and orbital control software. This was successfully demonstrated during the first integrated system tests, when the software control modes were executed using a closed-loop, locally connected set-up. The next step was to validate the remote operation of the platform by issuing commands from a distant location.
- In June, the 1.5 m x 1.4 m x 1.5 m spacecraft platform was put through its paces to test the systems responsible for receiving and executing commands from mission control and transmitting science data back to Earth. The tests involved validating the operational procedures that will allow the spacecraft to be controlled in orbit. For example, by operating the propulsion module to maneuver the spacecraft.
Figure 14: Top view of the CHEOPS spacecraft platform (image credit: Airbus DS, Spain)
• May 4, 2017: A major milestone for the CHEOPS mission was passed on 28 April 2017, when the telescope flight model was delivered to the University of Bern by Leonardo-Finmeccanica, on behalf of the Italian Space Agency (ASI) and the Italian National Institute for Astrophysics (INAF). 18)
- CHEOPS will perform ultra-high precision photometry on bright stars already known to host exoplanets with typical sizes ranging from Neptune down to that of Earth. It will allow accurate sizes of these exoplanets to be measured for the first time by monitoring how much dimming they cause as they pass between their host stars and the telescope. The CHEOPS telescope has been carefully designed, manufactured, and tested with this scientific objective in mind, and its delivery marks the end of 4 years of intense activity by a multinational team.
Figure 15: The CHEOPS telescope tube and optical bench (image credit: Leonardo-Finmeccanica)
- The delivered hardware consists of the TTA (Telescope Tube Assembly), integrated with the telescope mirrors and back-end optics that will focus starlight onto the CCD detector. The TTA not only supports the optical elements, but is also the structural core of the payload onto which various subsystems – such as the star trackers, the instrument radiator, the baffle, and the focal plane module – will be mounted.
- A key requirement of the mission is that thermal expansion and contraction of the support structure does not cause misalignment of the optical elements resulting in distortions of the images and inconsistent flux measurements. This requirement has been met by manufacturing the telescope structure from carbon-fibre reinforced plastics (CFRP) to achieve a strong, light, and thermally stable design. Since the thermoelastic properties of a CFRP laminate depend on the orientations of the individual laminae, Swiss company Almatech, which was responsible for the TTA structure, designed it down to the lamina level. Finite-element modelling using a 450 000-element model of the entire structure was combined with test sample measurements to select the orientations and stacking sequence of the laminae. The thermal stability of the manufactured structure was then verified with a dedicated test in space-like thermal and vacuum conditions using highly-accurate interferometric measurements.
- After the telescope structure was delivered to Leonardo-Finmeccanica in Florence, the integration of the mirrors and back-end optics – also manufactured by Leonardo-Finmeccanica – took place. The high-precision optics were then aligned using interferometric techniques, after which the optical system was characterized under simulated in-orbit conditions in a thermal-vacuum chamber. In particular, the precise position of the focal plane was established so that the CCD detector can be placed accurately when the focal plane module is mounted.
- To reach the low noise levels required to achieve the science goals of the mission, it is important that the only light arriving at the CHEOPS detector should be that from the star itself, and that light reflected by surfaces within the instrument is kept to a minimum. Both the baffle and the field stop in the back-end optics have been designed to reject this stray light. However, stray light effects can also result from contamination of the optics, and therefore very stringent cleanliness requirements were applied during integration and alignment of the optics. The CHEOPS telescope baffle assembly has a cover to guard against contamination – this is kept closed for almost all further integration activities and will be opened once CHEOPS is in orbit.
- In parallel with the telescope activities, in March, team members from the University of Bern performed electromagnetic compatibility (EMC) testing of an electrical qualification model of the instrument at the EMC-Testcenter AG in Regensdorf. These tests were carried out to confirm that the electronic subsystems of the instrument are compatible with the electromagnetic environment during launch, and to verify that spacecraft operations and science data are not adversely affected by electromagnetic emission from onboard electronics such as the onboard antennae and payload heaters. The test results showed that the instrument complies with the EMC requirements. Integration of the electronic components in the payload can now proceed with confidence.
- Much effort has gone into ensuring that CHEOPS will have a steady gaze, free from electromagnetic interference, thermally-induced artefacts, and stray light, as it stares at the pale shadows of distant exoplanets. The optical and structural core of the payload has completed its journey from Florence, over the Alps, to Bern and a new phase of integration work can now begin.
• January 30, 2017: The CHEOPS spacecraft underwent important testing last year to be ready for launch by the end of 2018. CHEOPS will operate from a low orbit circling Earth, taking its power from the Sun. As such, an important focus of the prelaunch testing is qualifying the satellite's solar arrays and their cells. 19)
- The image of Figure 16 shows part of the 12 solar cell assemblies in the Vacuum Solar Cell Illumination Facility at ESA/ESTEC in the Netherlands. The cells were heated to high temperatures to reflect what the satellite will experience once in space. In fact, the actual temperatures were scaled in order to accelerate the ageing effects experienced in flight, to represent a 3.5 year mission in just a few months. - The cells spent 2000 hours at 140ºC, 2000 hours at 160ºC and 2090 hours at 175ºC. After the tests, the cells' maximum power and short circuit current had degraded by less than 2%, clearly below the acceptance criterion of 3%.
- As a result of these tests, the CHEOPS solar arrays and their elements are now ready for the mission. Once in space, Cheops will measure the density of exoplanets with sizes or masses in the super-Earth to Neptune range. Its data will set new constraints on the structure of planets in this mass range, and therefore also on their formation and evolution.
Figure 16: Photo of the solar cell assemblies in the Vacuum Solar Cell Illumination Facility at ESA/ESTEC (image credit: ESA, C. Carreau)
• December 16, 2016: Over the last 10 months, the CHEOPS solar cell assemblies have been put through their paces in the test facilities at industry and in the Solar Generator Laboratory of the Power Systems Division (TEC-EP) at ESTEC (European Space Research and Technology Centre) as part of the qualification campaign. In parallel, the flight solar arrays have been manufactured and delivered. 20)
- As with many ESA missions, the Sun is the sole source of power for CHEOPS and for this reason the solar arrays and their constituent solar cells play a critical role in the operation of the satellite. The CHEOPS solar arrays cover a total geometrical area of about 2.7 m2 with an effective active area of just over 2.1 m2. The 3 fixed panels are body mounted with an angle of 67 degrees between them to provide the best possible illumination by the Sun at a given spacecraft attitude.
- The individual solar cell assemblies comprise AzurSpace 3G30 Triple Junction GaAs/Ge cells, and are provided by Leonardo-Finmeccanica. The 3G30 is a new generation of more efficient cells, which were initially qualified with respect to lifetime requirements only for geostationary orbit missions in the ESA MTG (Meteosat Third Generation) Program.
- Due to stringent thermal stability requirements at instrument level, the satellite will roll around the line-of sight to ensure that instrument radiators are always pointing towards cold space. This, combined with the attitude range, means that the solar cells will be subjected to a range of temperatures and thermal variations that are much larger than those for which the cells were previously qualified, and this is the reason for this 'delta-qualification' that was carried out at ESTEC.
Figure 17: Left: Schematic of the CHEOPS spacecraft, right: CHEOS solar panels (image credit: Airbus DS, Spain)
• May 2016: A little more than 3 years after the mission selection, the CHEOPS project is progressing as planned. 21)
• May 2016: After successful PRR and SRR in 2013, PDR in July 2014, a complete instrument STM (Structural and Thermal Model) has been built and successfully tested at instrument and spacecraft level, including a challenging stability test with the flight model of the telescope structure. The instrument EM (Electrical Model) has been tested and provided to Airbus DS beginning of April 2016 and is being tested with the spacecraft EFM (Electrical Functional Model). This provides confidence that the design performs successfully (Ref. 27).
- The system CDR is expected to be closed by mid-2016 including the instrument CDR. Instrument flight model is under preparation with several flight elements already manufactured and tested. The delivery to the platform contractor is expected mid-2017 for a launch opportunity within 2018.
• March 31. 2016: Thousands of children across Europe have taken part in a competition to submit drawings that will be miniaturized and sent into space onboard ESA's Cheops astronomy satellite. Out of the many excellent entries featuring a variety of cosmic settings, a total of 3000 were selected. These will now be scanned and shrunk by a factor of 1000 to be engraved on two metal plaques that will be attached to the satellite. 22) 0
- The competition was coordinated by the University of Bern, Switzerland, the lead institution, and run in collaboration with ESA and mission partner institutions in the Cheops countries: Austria, Belgium, France, Germany, Hungary, Italy, Portugal, Spain, Sweden, Switzerland and the UK.
- The competition ran between May and October 2015 and was aimed at children between the ages of 8 and 14 from ESA member and cooperating states. Around 1700 drawings were sent directly to ESA, while the partner institutions across Europe received thousands more.
Figure 18: Cheops competition drawing entries featuring a variety of cosmic settings (image credit: ESA and University of Bern)
• Nov. 11, 2015: A test model of ESA's exoplanet-watching Cheops satellite is being placed into an acoustic chamber in Europe's largest spacecraft testing center, helping to ensure the flight version can endure the extreme conditions of a rocket launch. 23)
- The "Characterizing ExOPlanet Satellite" is ESA's first small science mission. Selected in October 2012, it will track the crossings of known planets across the face of their parent stars, to make detailed deductions of their size and composition. The telescope will detect tiny shifts in stellar brightness with ultra-high precision.
- Once the tests are completed, this ‘structural qualification model' will be reconfigured as the actual satellite, helping to meet a tight development schedule that is aiming for launch readiness at the end of 2017 and a shared launch opportunity in the first half of 2018.
Figure 19: Cheops is seen here being moved into ESA's Large European Acoustic Facility, capable of subjecting satellites to the same noise as a rocket produces as it takes off and flies through the atmosphere (image credit: ESA, C. Carreau)
• Sept. 30, 2015: The most stable source of light in the world: In order to detect planets similar to planet Earth, the satellite must therefore be able to measure the luminosity of a star with exceptional stability (0.002%). The CHEOPS detectors must be tested using a source of light whose stability is ten times superior than that demanded by the satellite itself. Since no existing light source could guarantee this level of stability, UNIGE (University of Geneva) engineers and technicians and the Swiss PlanetS NCCR (National Center for Competence and Research) designed a brand-new instrument, which produces the most stable light source in the world. Unlike other procedures, which stabilize light at its source, the system developed in Geneva modifies the intensity of a beam of light. By activating a "mobile finger", which more or less obscures the beam of light, the intensity and finally the stability of the light can be modulated. - The team of the University of Geneva has just filed for a European patent of their most stable light source. 24)
- "We have presented our instrument to the American leaders of the TESS mission, a satellite researching exoplanets, and they were so enthusiastic that they ordered one from us," said François Wildi, engineer in UNIGE's Department of Astronomy, and member of PlanetS.
- Having proved themselves in the laboratory setting, thanks to the stable light source, the CHEOPS detectors will then be tested – using the same source, in space conditions, in a University of Bern simulation tank whose temperature variations reflect those that the satellite will encounter in space.
• Sept. 28, 2015: Over the Summer, Airbus Defence and Space (ADS- Spain) completed the manufacturing and integration of the spacecraft platform Structural Qualification Model. In parallel, the Instrument Consortium, led by the University of Bern, completed the test campaign of the instrument Structural and Thermal Model and delivered the model to ADS-Spain. 25)
- The availability of both platform and instrument units allows the integration of the spacecraft-level Structural Model, for which the environmental test campaign is planned to commence at the beginning of October. The first test activities will be mass property measurements carried out at ADS-Spain, followed by vibration tests at the Zürich premises of RUAG, Switzerland.
Figure 20: Photos of CHEOPS instrument Structural and Thermal Model and (right) spacecraft Structural Model (image credit: ADS-Spain)
• As of mid-June 2015, the Instrument Consortium is preparing for testing of the instrument structural and thermal model (I-STM), which will take place in Bern and run from mid-July to the end of August. At spacecraft level, tests will start on the structural qualification model at the end of September (Ref. 26).
Launch: The launch readiness of CHEOPS minisatellite is scheduled for the end of 2018 with a launch followed on a Soyuz vehicle from Kourou. 26)
Orbit: The baseline orbit satisfying the science requirements is a 6:00 hr (baseline) or 18:00 hr (backup) sun-synchronous orbit (dawn/dusk orbit) with an altitude of 700 km, inclination of ~98º, enabling to have the Sun permanently on the backside of the satellite and minimizing Earth straylight.
Figure 21: CHEOPS nominal attitude definition (image credit: Airbus DS, ESA)
Sensor complement: (Telescope)
The main science goals of the CHEOPS mission will be to study the structure of exoplanets with radii typically ranging from 1 to 6 Earth radii orbiting bright stars. With an accurate knowledge of masses and radii for an unprecedented sample of planets, CHEOPS will set new constraints on the structure and hence on the formation and evolution of planets in this mass range. To reach its goals CHEOPS will measure photometric signals with a precision of 20 ppm in 6 hours of integration time for a 9th magnitude star and a precision of 85 ppm in 3 hours integration time for Neptune-size planets orbiting 12th magnitude star. This corresponds to a signal to noise of 5 for a transit of an Earth-sized planet orbiting a solar-sized star (0.9 solar radii). This precision will be achieved by using a single frame-transfer backside illuminated CCD detector cool down at 233 K and stabilized within ~10 mK (Ref. 6). 27)
Figure 22: The CHEOPS telescope and baffle (image credit: CHEOPS instrument consortium)
The CHEOPS mission payload consists of a single instrument, a space telescope of 30 cm clear aperture, which has a single frame-transfer back-side illuminated CCD detector in a FPA (Focal Plane Assembly) of a 32 cm diameter on-axis telescope. The telescope feeds a re-imaging optic which supports the stray light suppression concept by providing a position for a field stop and reducing the impact of scattering from the baffling system reaching the detector directly. The optical configuration consists of a Ritchey-Chrétien telescope with additional lenses to provide a de-focussed image of the target star with a PSF (Point Spread Function) covering an area of ~765 cm2. The main design drivers are related to the compactness of the optical system and to the capability to reject the stray light. 28)
The optical design allows incorporation of a central baffle "tower" on which the field stop is mounted. Scattering of reflected Earthshine from the baffle onto the primary mirror is the largest source of stray light in the current baffle design. By increasing the length of the baffle, both scattering from the spider and scattering from the baffle to the secondary mirror are minimized. Combined with an inner and outer baffling system the major stray light requirement (reaching < 1 ph/px/s) can be met. A comparison of the point source transmission functions of CoRoT, the Cassini orbiter narrow angle camera, and our preliminary design is made in Figure 23.
Figure 23: Comparison of CoRoT, Cassini NFC and CHEOPS preliminary optical design PST (Point Source Transmission) functions. The CHEOPS baffle is optimized for angles> 35º (image credit: CHEOPS instrument consortium)
The entrance pupil is located at the primary mirror and has a diameter of 320 mm. The central obstruction has a diameter of 68 mm (equal to the secondary mirror diameter), giving a relative central obstruction diameter of 0.2125. The effective collecting area is 767.93 cm2 (about 8.64% more than the required one corresponding to an unobstructed telescope having a diameter of 300 mm). This redundancy has been maintained to provide for possible lack of throughput efficiencies.
The telescope tube assembly is passively cooled and thermally controlled with on-board heaters. In the baseline design, the distance between the primary mirror and the secondary mirror is 300 mm, which makes it a very compact instrument maximizing launch fairing compatibility. The telescope effective focal length is 1600 mm, giving a telescope focal ratio F/5. The focal plane has a diameter of 11.23 mm corresponding to a FOV (Field of View) of 0.4º. At this location, a focal plane mask of the same size is envisaged for the attenuation of the stray light background.
An estimation of the background contribution due to direct illumination of the focal plane by the sky has been computed. The result is that this background is negligible with respect to the other background sources. As a consequence, no internal baffling for focal plane shielding is required.
Figure 24: A cutaway view of the CHEOPS flight instrument and its various components (CHEOPS instrument consortium)
The BEO (Back-End Optics) re-images the telescope focal plane on the detector and provides an intermediate pupil, at which location a mask is placed for the stray light rejection. The BEO is basically composed by three elements: a collimator which forms an intermediate pupil, a flat fold mirror which is inserted to minimize the envelope of the whole optical system, and a camera that re-images the focal plane at the required plate scale. The BEO has been optimized on a FOV having diameters 0.32º. The detector area of interest is 200 x 200 pixels and its location will set inside the optimized FOV. During the optimization, all the wavelengths in the range 400-1100 nm have been associated the same weight, that is, the efficiency of the system has been assumed to be the same at all wavelengths. The current design of the collimator and the camera is based on two spaced achromatic doublets. The goal is to maintain the system as simple as possible, and in compatibility with the performances.
The CIS (CHEOPS Instrument System) is composed of four main units:
1) The BCA (Baffle and Cover Assembly) minimizes the stray-light and includes a protective cover and release mechanism.
2) The OTA (Optical Telescope Assembly) includes the structure carrying the telescope, the BEO (Back End Optics), the FPM (Focal Plane Module), and the radiators. In order to minimize the impact of thermoelastic deformations on the instrument pointing, the optical heads of the platform star trackers will be mounted on the OTA, in proximity of the isostatic mounts of the instrument.
3) The SEM (Sensor Electronics Module)
4) The BEE (Back End Electronics).
Table 6: The CIS (CHEOPS Instrument System) main specifications
Figure 25: CHEOPS optical design: Telescope (left) and Back-End-Optics (right), image credit: INAF
Figure 26: CAM/CAD pictures of the Baffle and Cover Assembly (BCA) and Optical Telescope Assembly (OTA), image credit: University of Bern, ESA)
Defocused PSF (Point Spread Function): The CIS optical design is intended to produce a relatively wide point spread function at the detector plane. The width of the PSF is a trade-off between reducing the noise in the stellar image (pushing to large PSFs) and the increased susceptibility to straylight which a larger stellar image generates.
The theoretical PSF shape is a top-hat cylinder having diameter of about 30 arcsec (i.e., 30 pixels). A figure of merit to evaluate the performance has been derived by considering the optical PSFs, the jitter effect and the flat field performances. The result of the simulations has pointed out that a defocused PSF gives sufficient performance to meet the requirements.
Figure 27: The CHEOPS defocused PSF. The color gradient indicates the relative energy distribution. The axis x, y correspond to the (10 x oversampled) pixels in the detector (image credit: INAF)
PSFs have been generated at the defocused focal plane (about 3.5 mm from the nominal focal plane) with a flat spectral wavelength between 400 and 1100 nm with a spectral sampling of 50 nm. They have been spatially sampled with a sub-pixel size of 1.3 µm corresponding to 1/10th of the nominal detector pixel size. Moving from the center of the FOV to the edge, anisotropy starts to affect PSFs while at the very edge the PSF starts to be dominated by aberrations. As expected, in the PSF the feature due to the telescope central obstruction and to the Poisson spot can be clearly seen.
BCA (Baffle and Cover Assembly): The BCA is the key to the stray light mitigation strategy of the CHEOPS instrument. The external baffle goal is to limit the amount of straylight issued from sources located more than 35º from the optical axis. The baffle design as well as the cover and actuator are of CoRoT heritage. The purpose of the cover is to provide a light tight lid as well as contamination control for the telescope integration prior to launch. The cover release mechanism is based on a spring-loaded hinge and a launch lock mechanism. The launch lock is based on a Frangibolt actuator design. This solution provides reliability and also avoids contamination issues during launch and early orbital phase.
FPM (Focal Plane Module) and SEM (Sensor Electronics Module): The detector selected is an e2v CCD47-20 (13-µm pixel 1 k x 1 k, AIMO). The CCD will be nominally operated at 233 K. The FM (Flight Model) CCD will be characterized prior to installation in the PFM (Proto Flight Model) instrument.
There are two electrical modules for interfacing, controlling and reading-out the CCD detector, the FPM located on the optical bench and the SEM located inside the platform. The FPM (Focal Plane Module) contains the FPA (Focal Plane Assembly) where the CCD is located and the FEE (Front End Electronics) both with two separate interfaces to the radiator for cooling down the units. The SEM as a physically separated module and electrical harness to the FPM/FEE contains a SCU (Sensor Control Unit) and a PCU (Power Conditioning Unit) interfacing the DPU (Data Processing Unit) and the PSU (Power Supply Unit). Both, the DPU and PSU are integrated in the BEE (Back End Electronics) located inside the platform. The electrical subsystem architecture is given Figure 28.
Figure 28: CHEOPS electrical subsystem fault tolerance architecture (image credit: DLR)
The FPM-SEM architecture is mainly driven by the thermal design having 3 different categories reflecting the requirements of thermal control.
1) Focal Plane Assembly (FPA) with CCD and proximity electronics operating at lower than 233K nominally stabilized by heating against a dedicated radiator
2) Front End Electronics (FEE) with analog and CCD low level control electronics operating between 253 – 283 K stabilized by heating against a dedicated radiator
3) SCU & PCU (Sensor Controller Unit and Power Conditioning Unit), including the FPGA – based digital electronics for data handling and controlling the CCD detector by different readout modes at standard temperatures without stabilization needs.
Due to the sensitivity of signals and clocks against cross talk and disturbances the analogue electronics according 1 and 2 is organized in close vicinity.
The gain stability of the analog electronics over several hours is one of the most important design drivers for selecting the EEE (Electrical, Electronic, and Electromechanical) components and designing the electronics in detail. Especially the BIAS and clock voltages have to be very stable because of the significant sensitivity of the CCD against voltage drifts. The challenge is to use available space qualified components which fulfil required thermal drift characteristics of gain and offset parameters but have adequate electrical performance fulfilling the low read-out and quantization noise requirements. Even if a temperature stabilization of dedicated electronics areas is foreseen a favored very low stability (or very low systematic error) of 10ppm over hours requires extended calibration of the electronics e.g. measurement the CCD BIAS voltage drift vs temperature.
The current design philosophy is to maximize fault tolerance of the digital and analog subsystems by avoiding SPF (Single Points of Failure). This has led to a cold redundant design for SEM and BEE. The FPM also is cold redundant to a large degree, with the exception of the CCD, the CCD clock driver and BIAS voltage supply.
The channel is selected by powering-on either the main or redundant electronics chain. The SEM main/redundant channels and BEE main/redundant channels are powered separately, so that an operation of main/main or redundant/redundant is possible if one chain fails. A cross-strapping at the SpaceWire links and secondary power voltages are not foreseen. The data are acquired by the BEE/DPU either from the main or redundant SpaceWire link.
BEE (Back End Electronics): The BEE is composed of the DPU (Data Processing Unit) and the PSU (Power Supply Unit). The main task of the DPU besides the communication with the camera read-out electronics is the compression and packaging of telemetry data. The DPU will also compute centroid the stellar images and transmit it to the spacecraft AOCS thereby allowing more accurate control of the spacecraft pointing (requirement of <8 arcsec).
The RPW (Radio Plasma Wave) DPU hardware is based on the GR712, which contains two LEON3 processors and provides space wire and MIL-1553 interfaces. The DPU carries a mass memory to allow for 3 days operation without ground contact. 3D-Plus provides a FLASH memory in the configuration of 4 Gbit times eight bit. For effective operation of the processor four components are used to provide 32 bit access and EDAC. The present configuration foresees the flash memory for storage of telemetry data. In addition the onboard backup of the application software shall be stored in this area. To increase the reliability, in particular for the backup of the application software, it is recommended to keep more than one copies, located at different pages. Four standard chips are combined to a stack and packed into a common package. The total height of the stack is ~12 mm. Due to this configuration, it is unlikely that one high energetic particle would hit a similar address range on all pages at the same time. The four chips are accessed by individual chip enable, read and write enable signals. The command latch enable and the address latch enable is common for all four chips. The total size of the memory to be used for data will be 32 bit times 4 Gbit (128 Gbit = 16 GByte). It is recommended to organize the mass memory as a ring buffer. This would use all memory cells with the same frequency. In this case approximately ~127 Gbit are available. Since the average telemetry is 1.2 Gbit/day, the same memory cell will be used approximately three times per year. Therefore, the limited life time of the component (~100.000 write cycles) is negligible.
The PSU provides dedicated DC-DC converters for the DPU and the SEM & FPM with high accurate secondary voltages and switches to control up to four instrument operational heaters. The spacecraft interface includes an EMC filter, solid state switches for ON/OFF control and the status monitoring. Presently it is foreseen to have a temperature sensor connected to the analog telemetry provided by the spacecraft.
Ground Segment (GS):
CHEOPS nominal ground station is located at Torrejon (near Madrid, Spain) and the backup station is the Vilspa ESA Station-1. The use of Kiruna or Svalbard stations is a possible option to increase LEOP coverage.
The operational concept of the mission control during the different phases has been optimized in order to have a high autonomy in the space segment so that the ground operations are minimized in nominal behavior.
The CHEOPS mission control comprises two main operational parties:
• MOC (Mission Operations Center): The MOC is a set of facilities used to control and operate the space segment, including the control center, the mission planning and the ground station. It is located at Torrejón de Ardoz (Spain).
• SOC (Science Operations Center): The SOC is a facility used to define the scientific targets, archive the obtained science data. It is located at the University of Geneva (Switzerland).
The spacecraft communication with the ground segment is performed through S-band chain, both for commands and telemetry (science & HK TM). The MOC commands and controls the satellite and analyzes the HK telemetry. The operation starts at the SOC, where the scientists define the payload mission planning with the targets to be observed during an observation period . This planning is added to the platform commands into a complete mission planning that is given to the MOC for a final check. Through the MCS (Mission Control System) and the GS, the MTL (Mission TimeLine) is uploaded to the CHEOPS S/C and the S/C will execute the commands included in the MTL so that the targets are observed during the desired periods of time. Once the data is archived on-board, it is downloaded to the GS during a visibility pass, through the S-band communication subsystem. The data is archived in the MOC and then, transfer to SOC for cataloguing and processing.
Figure 29: CHEOPS operations concept overview (image credit: CHEOPS Team)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).