© ESO, 2011

1. Introduction

The Planck satellite¹ was launched on 14 May 2009, and has been surveying the sky stably and continuously since 13 August 2009. Planck carries a scientific payload consisting of an array of 74 detectors sensitive to a range of frequencies between  ~25 and  ~1000 GHz, which scan the sky simultaneously and continuously with an angular resolution varying between  ~30 arcmin at the lowest frequencies and  ~5 arcmin at the highest. The array is arranged into two instruments: the detectors of the Low Frequency Instrument (LFI; Bersanelli et al. 2010; Mennella et al. 2011) are pseudo-correlation radiometers, covering three bands centred at 30, 44, and 70 GHz; and the detectors of the High Frequency Instrument (HFI; Lamarre et al. 2010; Planck HFI Core Team 2011a) are bolometers, covering six bands centred at 100, 143, 217, 353, 545 and 857 GHz. The design of Planck allows it to image the whole sky approximately twice per year, with an unprecedented combination of sensitivity, angular resolution, and frequency coverage. The Planck satellite, its payload, and its performance as predicted at the time of launch, are described in 13 articles included in a special issue (Vol. 520) of Astronomy & Astrophysics.

The main objective of Planck is to measure the spatial anisotropies of the temperature of the cosmic microwave background (CMB), with an accuracy set by fundamental astrophysical limits. Its level of performance will enable Planck to extract essentially all the information in the CMB temperature anisotropies. Planck will also measure to high accuracy the polarisation of the CMB anisotropies, which encodes not only a wealth of cosmological information, but also provides a unique probe of the thermal history of the Universe during the time when the first stars and galaxies formed. In addition, the Planck sky surveys will produce a wealth of information on the properties of extragalactic sources and on the dust and gas in our own Galaxy. The scientific objectives of Planck are described in detail in Planck Collaboration (2005).

At the time this paper is being submitted, Planck is close to completing three surveys of the whole sky, and is releasing to the public its first set of scientific data. This data set is the Early Release Compact Source Catalogue (ERCSC), a list of unresolved and compact sources extracted from the first complete all-sky survey carried out by Planck. The ERCSC (Planck Collaboration 2011c) consists of:

• nine lists of sources, extracted independently from each ofPlanck’s nine frequency bands;

• two lists of sources extracted using multi-band criteria targetted at selecting specific types of source, i.e.,

  • “Cold Cores,” cold and dense locations in the insterstellarmedium of the Milky Way, selected mainlybased on their estimated dust temperature;

  • clusters of galaxies, selected using the spectral signature left on the CMB by the Sunyaev-Zeldovich (SZ) effect.

The ERCSC is a high-reliability compilation of sources, released early to give the astronomical community a timely opportunity to follow up these sources using ground- or space-based observatories, most particularly ESA’s Herschel observatory, which has a limited lifetime. The ERCSC is being released by ESA to the public on 11 January 2011 through an online distribution system accessible via http://www.rssd.esa.int/Planck. At the same time, the Planck Collaboration is submitting for publication a package of consisting of:

• this paper (Planck Collaboration 2011a), whichdescribes the history and main performance elements of thePlanck satellite in its first year of life;

• two papers describing the performance of each of Planck’s two instruments (LFI and HFI) within the same period (Mennella et al. 2011; and Planck HFI Core Team 2011a);

• a paper describing the thermal performance of Planck in orbit (Planck Collaboration 2011b);

• two papers describing the data processing, which has been applied to the data acquired by LFI and HFI, to produce the maps used for the ERCSC and the scientific papers in this package (Zacchei et al. 2011; and Planck HFI Core Team 2011b);

• an Explanatory Supplement to the ERCSC (Planck Collaboration 2011v), describing in detail the production and characteristics of the ERCSC;

• a paper summarising the production of the ERCSC, and the main characteristics of the sources that it contains (Planck Collaboration 2011c);

• twelve papers describing in more detail: (a) specific aspects of different source populations contained in the ERCSC (radio sources, infrared galaxies, galaxy clusters, cold cores etc.); and (b) cross-correlation analysis and follow-up observations which form part of the scientific validation and analysis of the ERCSC data. These papers are:

  • 1.

    Planck Collaboration (2011d) describesthe physical properties of the sample of clusters included inthe ERCSC;

  • 2.

    Planck Collaboration (2011e) describes the validation of a subset of the cluster sample by follow-up observations with the XMM-Newton X-ray observatory;

  • 3.

    Planck Collaboration (2011f) analyses the statistical relationship between SZ flux and X-ray luminosity of the ERCSC cluster sample;

  • 4.

    Planck Collaboration (2011g) uses a high signal-to-noise subset of the ERCSC cluster sample to investigate the relationship between X-ray-derived masses and SZ fluxes;

  • 5.

    Planck Collaboration (2011h) studies the relation between SZ flux and optical properties of galaxy clusters by stacking Planck fluxes at the locations of the MaxBCG optical cluster catalogue;

  • 6.

    Planck Collaboration (2011w) studies an exceptionally X-ray luminous and massive galaxy cluster detected by Planck at z ~ 1;

  • 7.

    Planck Collaboration (2011i) analyses the statistical properties of a complete sub-sample of radio sources drawn from the ERCSC;

  • 8.

    Planck Collaboration (2011j) describes the spectral energy distributions and other properties of some extreme radio sources, using Planck ERCSC data and ground-based observations;

  • 9.

    Planck Collaboration (2011k) presents the spectral energy distributions of a sample of extragalactic radio sources, based on the Planck ERCSC and simultaneous multi-frequency data from a range of other observatories;

  • 10.

    Planck Collaboration (2011l) studies the dust properties of nearby galaxies (z < 0.25) present in the ERCSC;

  • 11.

    Planck Collaboration (2011s) presents the statistical properties of Cold Cores as observed by Planck, in terms of spatial distribution, temperature, distance, mass, and morphology;

  • 12.

    Planck Collaboration (2011r) presents the physical properties and discusses the nature of a selection of interesting Cold Cores observed by Planck.

• seven papers describing in more detail selected science results, based on the maps which were used as input for the production of the ERCSC. The results addressed in these papers are characterised by their robustness, a critical element required for publication at a rather early stage in the reduction of the Planck data. These seven are:

  • 1.

    Planck Collaboration (2011m) presentsestimates based on Planck and IRAS data forthe apparent temperature and optical depth of interstellardust in the Small and Large Magellanic Clouds, andinvestigates the nature of the millimetre-wavelength excessemission observed in these galaxies;

  • 2.

    Planck Collaboration (2011n) presents estimates of the angular power spectrum of the cosmic infrared background as observed by Planck in selected regions of the sky;

  • 3.

    Planck Collaboration (2011o) estimates over the whole sky the apparent temperature and optical depth of interstellar dust based on Planck and IRAS data, and investigates the presence of “dark” gas, i.e., gas which is not spatially correlated with known tracers of neutral and molecular gas;

  • 4.

    Planck Collaboration (2011p) constructs the spectral energy distributions of selected regions in the Milky Way, using Planck maps combined with ancillary multi-frequency data, and investigates the presence of anomalous excess emission which can be interpreted as arising from small spinning grains;

  • 5.

    Planck Collaboration (2011q) estimates the radial distribution of molecular, neutral, and ionised gas in the Milky Way, using as spatial templates a wide variety of tracers of the different phases and components of the interstellar medium;

  • 6.

    Planck Collaboration (2011t) presents a joint analysis of Planck, IRAS, and 21-cm observations of selected high-Galactic-latitude fields, and discusses the properties of dust in the diffuse interstellar medium close to the Sun and in the Galactic halo;

  • 7.

    Planck Collaboration (2011u) presents Planck maps of a selection of nearby molecular clouds, and discusses the evolution of the emitting properties of the dust particles embedded in them.

The next release of Planck products will take place in January 2013, and will cover data acquired in the period up to 27 November 2010. It will include:

• cleaned and calibrated data timelines for each detector;

• maps in Stokes I, Q, and U for each frequency band between 30 and 353 GHz, and in Stokes I for the two highest frequency bands (545 and 857 GHz);

• catalogues of compact sources extracted from the frequency maps;

• maps of the main diffuse components separated from the maps, including the CMB;

• scientific results based on the data released.

A third release of products is foreseen after January 2014, to cover the data acquired beyond November 2010 and the end of Planck operations.

This paper is mainly dedicated to describing the history of the mission from launch until 6 June 2010 (the coverage period of the data used to generate the ERCSC). It also discusses some performance aspects of the satellite which are important for the interpretation of its scientific output. It serves therefore as background and reference for the suite of papers described above. In Sects. 2 and 3, we describe the main events and activities which took place before the start of the Planck surveys. In Sect. 4, we describe relevant aspects of the Planck surveys, i.e., the strategy used to scan the sky, its thermal and radiation environment, the pointing performance of the satellite, and the flow of data in the ground segment. Finally, in Sect. 5 we summarise the scientific performance of the payload as estimated from the first year of data and with the current set of available data processing pipelines.

2. Early operations and transfer to orbit

Planck was launched from the Centre Spatial Guyanais in Kourou (French Guyana) on 14 May 2009 at its nominal lift-off time of 13:12 UT, on an Ariane 5 ECA rocket of Arianespace². ESA’s Herschel observatory was launched on the same rocket. At 13:37:55 UT, Herschel was released from the rocket at an altitude of 1200 km; Planck followed suit at 13:40:25 UT. The separation attitudes of both satellites were within 0.°1 of prediction. The Ariane rocket placed Planck with excellent accuracy (semi-major axis within 1.6% of prediction), on a trajectory towards the second Lagrangian point of the Earth-Sun system (“L₂”) which is drawn in Fig. 1. The orbit describes a Lissajous trajectory around L₂ with a  ~6 month period that avoids crossing the Earth penumbra for at least 4 years.

After release from the rocket, three large manoeuvres were carried out to place Planck in its intended final orbit. The first (14.35   ms^-1), intended to correct for errors in the rocket injection, was executed on 15 May at 20:01:05 UT, with a slight over-performance of 0.9% and an error in direction of 1.°3 (a touch-up manoeuvre was carried out on 16 May at 07:17:36 UT). The second and major (mid-course) manoeuvre (153.6   ms^-1) took place between 5 and 7 June, and a touch-up (11.8   ms^-1) was executed on 17 June. The third and final manoeuvre (58.8   ms^-1), to inject Planck into its final orbit, was executed between 2 and 3 July. The total fuel consumption of these manoeuvres, which were carried out using Planck’s coarse (20N) thrusters, was 205 kg. Once in its final orbit, very small manoeuvres are required at approximately monthly intervals (1   ms^-1 per year) to keep Planck from drifting away from its intended path around L₂. The attitude manoeuvres required to follow the scanning strategy require about 2.6   ms^-1 per year. Overall, the excellent performance of launch and orbit manoeuvres will lead to a large amount (~160 kg, or  ~40% of initial tank loading) of fuel remaining on board at end of mission operations.

Planck started cooling down radiatively shortly after launch. Heaters were activated to hold the focal plane at 250 K, which was reached around 5 h after launch. The valve opening the exhaust piping of the dilution cooler was activated at 03:30 UT, and the ⁴He-JT cooler compressors were turned on at low stroke at 05:20 UT. After these essential operations were completed, on the second day after launch, the focal plane temperature was allowed to descend to 170 K for out-gassing and decontamination of the telescope and focal plane.

3. Commissioning and initial science operations

The first period of operations focussed on commissioning activities, i.e., functional check-out procedures of all sub-systems and instruments of the Planck spacecraft in preparation for running science operations related to calibration and performance verification of the payload. Planning for commissioning operations was driven by the telescope decontamination period of 2 weeks and the subsequent cryogenic cool-down of the payload and instruments. The overall duration of the cool-down was approximately 2 months, including the decontamination period.

The sequence of commissioning activities covered the following areas:

• on-board commanding and data management;

• attitude measurement and control;

• manoeuvreing ability and orbit control;

• telemetry and telecommand;

• power control;

• thermal control;

• payload basic functionality, including:

  • the LFI;

  • the HFI;

  • the cryogenic chain;

  • the Standard Radiation Environment Monitor (SREM, see Sect. 4.4);

  • the Fibre-Optic Gyro unit (FOG), a piggy-back experiment which is not used as part of the attitude control system.

The commissioning activities were executed very smoothly and all sub-systems were found to be in good health. Figure 2 shows a sketch of the cool-down sequence indicating when the main instrument-related commissioning activities took place. The most significant unexpected issues that had to be addressed during these early operational phases were the following.

• The X-band transponder showed an initialisation anomalyduring switch-on which was fixed by a software patch.

• Large reorientations of the spin axis were imperfectly completed and required optimisation of the on-board parameters of the attitude control system.

• The data rate required to transmit all science data to the ground was larger than planned, due to the unexpectedly high level of Galactic cosmic rays (see Sect. 4.4), which led to a high glitch rate on the data stream of the HFI bolometers (Planck HFI Core Team 2011a); glitches increase the dynamic range and consequently the data rate. The total data rate was controlled by increasing the compression level of a few less critical thermometers.

• The level of thermal fluctuations in the 20-K stage was higher than originally expected. Optimisation of the sorption cooler operation led to an improvement, though they still remained  ~25% higher than expected (Planck Collaboration 2011b).

• The 20-K sorption cooler turned itself off on 10 June 2009, an event which was traced to an incorrectly set safety threshold.

• A small number of sudden pressure changes were observed in the ⁴He-JT cooler during its first weeks of operation, and were most likely due to impurities present in the cooler gas (Planck Collaboration 2011b). The events disappeared after some weeks, as the impurities became trapped in the cooler system.

• The ⁴He-JT cooler suffered an anomalous switch to standby mode on 6 August 2009, following a current spike in the charge regulator unit which controls the current levels between the cooler electronics and the satellite power supply (Planck Collaboration 2011b). The cooler was restarted 20 h after the event, and the thermal stability of the 100-mK stage was recovered about 47 h later. The physical cause of this anomaly was not found, but the problem has not recurred.

• Instabilities were observed in the temperature of the ⁴He-JT stage, which were traced to interactions with lower temperature stages, similar in nature to instabilities observed during ground testing (Planck Collaboration 2011b). They were fixed by exploring and tuning the operating points of the multiple stages of the cryo-system.

• The length of the daily telecommunications period was increased from 180 to 195 min to improve the margin available and ensure completion of all daily activities³.

The commissioning activities were formally completed at the time when the HFI bolometer stage reached its target temperature of 100 mK, on 3 July 2009 at 01:00 UT. At this time all the critical resource budgets (power, fuel, lifetime, etc.) were found to contain very significant margins with respect to the original specification.

Calibration and performance verification (CPV) activities started during the cool-down period and continued until the end of August 2009. Their objectives were to:

• verify that the instruments were optimally tuned and theirperformance characterised and verified;

• perform all tests and characterisation activities which could not be performed during the routine phase;

• characterise the spacecraft and telescope characteristics of relevance for science⁴;

• estimate the lifetime of the cryogenic chain.

CPV activities addressed the following areas:

• tuning and characterisation of the behaviour of the cryogenicchain;

• characterisation of the thermal behaviour of the spacecraft and payload;

• for each of the two instruments: tuning; characterisation and/or verification of performance⁵, calibration (including thermal, RF, noise and stability, optical response); and data compression properties;

• determination of the focal plane footprint on the sky;

• verification of scanning strategy parameters;

• characterisation of systematic effects induced by the spacecraft and the telescope, including:

  • dependence on solar aspect angle;

  • dependence on spin;

  • interference from the RF transmitter;

  • straylight rejection;

  • pointing performance.

The schedule of CPV activities consumed about two weeks longer than initially planned, mainly due to:

• the anomalous switch to standby mode of the⁴He-JT cooler on 6 August (costing6 days until recovery);

• instabilities in the cryo-chain, which required the exploration of a larger parameter phase space to find an optimal setting point;

• additional measurements of the voltage bias space of the LFI radiometers, which were introduced to optimise its noise performance, and led to the requirement of artificially slowing the natural cool-down of the ⁴He-JT stage.

A more detailed description of the relevant parts of these tests can be found in Mennella et al. (2011) and Planck HFI Core Team (2011a). On completion of all the planned activities, it was concluded that:

• the two instruments were fully tuned and ready for routineoperations. No further parameter tuning wasexpected to be needed, except for the sorption cooler, whichrequires a weekly change in operational parameters (PlanckCollaboration 2011b);

• the scientific performance parameters of both instruments was in most respects as had been measured on the ground before launch. The only significant exception was that, due to the high level of Galactic cosmic rays, the bolometers of HFI were detecting a higher number of glitches than expected, causing a modest (~10%) level of systematic effects on their noise properties (see details in Planck HFI Core Team 2011a);

• the telescope survived launch and cool-down in orbit without any major distortions or changes in its alignment;

• the lifetime of the cryogenic chain was adequate to carry the mission to its foreseen end of operations in November 2010, with a margin of order one year;

• the pointing performance was better than expected, and no changes to the planned scanning strategy were required;

• the satellite did not introduce any major systematic effects into the science data. In particular, the telemetry transponder did not result in radio-frequency interference, which implies that the data acquired during visibility periods is useable for science.

The First Light Survey (FLS) was the last major activity planned before the start of routine surveying of the sky. It was conceived as a two-week period during which Planck would be fully tuned up and operated as if it was in its routine phase. This stable period could have resulted in the identification of further tuning activities required to optimise the performance of Planck in the long-duration surveys to come. The FLS was conducted between 13 and 27 August, and in fact led to the conclusion that the Planck payload was operating stably and optimally, and required no further tuning of its instruments. Therefore the period of the FLS was accepted as a valid part of the first Planck survey.

4. Routine operations phase

The routine operations phase of Planck is characterised by continuous and stable scanning of the sky and data acquisition by LFI and HFI. It started with the FLS on 13 August of 2009, at 14:15 UT. In this section we describe the major characteristics of this phase from start until 6 June 2010, i.e., the period over which data were used to generate the ERCSC.

4.1. Mission operations and data flow

A general description of mission operations is provided in Tauber et al. (2010a).

The Planck satellite generates (and stores on-board) data continuously at the following typical rates: 21   kilobits^-1 (kbps) of house-keeping (HK) data from all on-board sources, 44 kbps of LFI science data and 72 kbps of HFI science data. The data are brought to ground in a daily pass of approximately 3 h duration. Besides the data downloads, the passes also acquire real-time HK and a 20 min period of real-time science (used to monitor instrument performance during the pass). Planck utilises the two ESA deep-space ground stations in New Norcia (Australia) and Cebreros (Spain), usually the former. Scheduling of the daily telecommunication period is quite stable, with small perturbations due to the need to coordinate the use of the antenna with other ESA satellites (in particular Herschel).

At the ground station the telemetry is received by redundant chains of front-end/back-end equipment. The data flows to the mission operations control centre (MOC) located at ESOC in Darmstadt (Germany), where it is processed by redundant mission control software (MCS) installations and made available to the science ground segment. To reduce bandwidth requirements between the station and ESOC only one set of science telemetry is usually transferred. Software is run post-pass to check the completeness of the data. This software check is also used to build a catalogue of data completeness, which is used by the science ground segment to control its own data transfer process. Where gaps are detected, attempts to fill them are made as an offline activity (normally next working day), the first step being to attempt to reflow the relevant data from station. Early in the mission these gaps were more frequent, with some hundreds of packets affected per week (impact on data return of order 50 ppm) due principally to a combination of software problems with the data ingestion and distribution in the MCS, and imperfect behaviour of the software gap check. Software updates implemented during the mission have improved the situation such that gaps are much rarer, with a total impact on data return well below 1 ppm.

Redump of data from the spacecraft is attempted when there have been losses in the space link. This has only been necessary on three occasions. In each case the spacecraft redump has successfully recovered all the data.

An operational principle of the mission is to avoid impact on the nominal science of a completely missed ground station pass. Commanding continuity is managed by keeping more than 24 h of commanding-timeline queued on-board. The telemetry resides on board the satellite in a  ~60 h circular buffer in solid-state memory, and can be recovered subsequently using the margin in each pass, or more rapidly by seeking additional station coverage after an event. The lost-pass scenario has in fact occurred only once (on 21 December 2009), when snow on the dish at Cebreros led to the loss of the entire pass. A rapid recovery was made by using spare time available on the New Norcia station. Smaller impacts on the pass occur more often (e.g., the first  ~10 min of a pass may be lost due to a station acquisition problem) and these can normally be recovered simply by restarting a software task or rebooting station equipment. Such delays are normally accommodated within the margin of the pass itself, or during the subsequent pass.

All the data downloaded from the satellite, and processed products such as filtered attitude information, are made available each day for retrieval from the MOC by the LFI and HFI Data Processing Centres (DPCs). Typically, the data arrive at the LFI (resp. HFI) DPC 2 (resp. 4) hours after the start of the daily acquisition window. Automated processing of the incoming telemetry is carried out each day by the LFI (resp. HFI) DPCs and yields a daily data quality report which is made available to the rest of the ground segment typically 22 (resp. 14) hours later. More sophisticated processing of the data in each of the two DPCs is described in Zacchei et al. (2011) and Planck HFI Core Team (2011b).

4.2. Scanning strategy

The strategy used to scan the sky is described in Tauber et al. (2010a). The spin axis follows a cycloidal path on the sky as shown in Fig. 3, by step-wise displacements of 2 arcmin approximately every 50 min. The dwell time (i.e., the duration of stable data acquisition at each pointing) has varied sinusoidally by a factor of  ~2 (see Fig. 3). Planck’s scanning strategy results in significantly inhomogeneous depth of integration time across the sky; the areas near the ecliptic poles are observed with greater depth than all others. This is illustrated in Figs. 4 and 5. Table 1 shows more quantitatively the coverage of the sky at three representative frequencies.

The major pre-planned deviation from the nominal spin axis path took place in the period 1 to 19 March 2010. During this time, the average daily progression speed of the spin axis (normally 1   deg   day^-1) was temporarily increased, to gain a margin with respect to the attitude constraints imposed by the Sun and the Earth at the time that the Crab Nebula, Planck’s main polarisation calibrator, was being observed. This increased margin would have allowed Planck to re-observe the Crab if a significant problem had been encountered, but none occurred. A corresponding deceleration was included to rejoin the normal scanning path after the Crab had been observed by all detectors. The whole operation (clearly visible in Fig. 3) also resulted in a deviation of the solar aspect angle.

Orbit maintenance manoeuvres were carried out at approximately monthly intervals⁶. Although the manoeuvres only required a few minutes, preparations, post-manoeuvre mass-property calibration, and re-entry into scientific slewing mode increased the overhead to several hours. The manoeuvres were carried out without disturbing the path of the spin axis from its nominal scanning law. The dwell times of pointings before and after the execution of the manoeuvre were reduced to allow all pre-planned pointings to be carried out.

The main unplanned deviations from the basic scanning strategy included the following.

• An operator error in the upload of the on-board commandtimeline led to an interruption of the normal sequence ofmanoeuvres and therefore to Planck pointing tothe same location on the sky for a period of 29 hbetween 20 and21 November 2009(“the day Planck stood still”).Observations of the nominal scanning pattern resumed on22 November, and on23 November a recovery operation was applied tosurvey the previously missed area. During the recovery period theduration of pointing was decreased to allow the nominal law to becaught up with. As a side effect,the RF transmitter was left on for longer than24 h, which had a significant thermal impact on thewarm part of the satellite(see Fig. 6).

  Fig.6 The main long-timescale thermal modulation is a seasonal effect driven by the solar power absorbed by the satellite. The evolution of the solar heat input is traced by the top figure which shows the total current produced by the solar panels; the long-term variation is largely a reflection of the distance from the Sun, with a very small modulation due to variations in the satellite’s aspect angle to the Sun. The top and second panels show the temperature variation at two representative locations in the room-temperature service module (SVM), i.e., on one of the (HFI) helium tanks and on one of the LFI back-end modules (BEM). The bottom panel shows the temperature evolution of VG3, the coldest of three stacked conical structures or V-grooves which radiatively isolate the warm SVM from the cold payload module. The seasonal effect is not dominant in the evolution of VG3, demonstrating the high thermal isolation of the payload from the SVM. Most variations on VG3 are due to weekly power input adjustments of the sorption cooler (see also Fig. 7), which is heat-sunk to VG3. The main operational disturbances during the routine phase which had a thermal impact can be seen in the middle two panels (see text for more detail): a) the “catbed” event between 110 and 126 days after launch; b) the “day Planck stood still” 191 days after launch; c) the change in temperature and its daily variation starting 257 days after launch, due to the RF transmitter being turned permanently on; and d) two star-tracker reconfiguration events, 242 and 288 days after launch.

• Very minor deviations from the scanning law include occasional (on the average about once every two months) under-performance of the 1-N thrusters used for regular manoeuvres, which implied the corresponding pointings were not at the intended locations. These deviations, visible in Fig. 12, had typical amplitudes of 30′′, and have no significant impact on the coverage map.

• During the coverage period, the operational star tracker switched autonomously to the redundant unit on two occasions (11 January 2010 and 26 February 2010); the nominal star tracker was restored a short period later (3.37 and 12.75 h, respectively) by manual power-cycling. Although the science data taken during this period have normal quality, they have not been used because the redundant star tracker’s performance is not fully characterised.

While the Planck detectors are scanning the sky, they also naturally observe celestial calibrators. The main objects used for this purpose are:

• the Crab Nebula, used to calibrate polarisation properties of thedetectors, was observed in September 2009,March 2010 and September 2010;

• the brighter planets, used to map individual detector beams:

  • Jupiter, observed in October 2009 andJuly 2010;

  • Saturn, observed in January 2010 and June 2010;

  • Mars, observed in October 2009 and April 2010.

The use of these observations for beam and time response calibration is described in Zacchei et al. (2011) and Planck HFI Core Team (2011b).

The scanning strategy for the second year of Routine Operations (i.e., Surveys 3 and 4) is exactly the same as for the first year, except that all pointings are shifted by 1 arcmin along the cross-scanning direction, in order to provide finer sky sampling for the highest frequency detectors when combining two years of observations.

4.3. Thermal environment

The satellite design and its location at L₂ provide an extremely stable thermal environment (see Figs. 6 and 7). The main temperature variation on long timescales is driven by the total radiative power absorbed by the solar panels, which varies depending on distance from the Sun and the solar aspect angle (i.e. the angle between the solar direction and the spin axis). On shorter timescales, temperature variations are driven by active thermal regulation cycles. Both seasonal and shorter-timescale variations are observed across the satellite’s service module (SVM), but are heavily damped and almost unobservable within the payload module (PLM).

Specific operations and deviations from the scanning strategy have a thermal influence on the satellite and payload. Some significant effects are clearly visible in Fig. 6 and listed below.

• The thruster heaters were unintentionally turned off between31 August and16 September 2009(the so-called “catbed” event).

• As planned, the RF transmitter was initially turned on and off every day in synchrony with the daily visibility window, in order to reduce potential interference by the transmitter on the scientific data. The induced daily temperature variation had a measurable effect throughout the satellite. An important effect was on the temperature of the ⁴He-JT cooler compressors, which caused variations of the levels of the interference lines that they induce on the bolometer data (Planck HFI Core Team 2011a). Therefore the RF transmitter was left permanently on starting from 25 January 2010 (257 days after launch), which made a noticeable improvement on the daily temperature variations (Fig. 7).

• A significant thermal effect arises from the (approximately) weekly adjustments to the operation of the Sorption Cooler (Fig. 7).

The thermal environment of the payload module is – by design – extremely well decoupled from that of the service module (Fig. 6). As a consequence, in spite of the significant thermal perturbations originating in the SVM, the thermal variability affecting the detectors is essentially completely due to the operation of the cryogenic cooling chain (described in detail in Planck Collaboration 2011b), which ensures their cold environment.

4.4. Radiation environment

The Standard Radiation Environment Monitor on board Planck (SREM, Buehler et al. 1996) is a particle detector which is being flown on several ESA satellites. The SREM consists of several detectors sensitive to different energy ranges, which can also be used in coincidence mode. In particular, the SREM measures count rates of high energy protons (from  ~10 MeV to  ~300 MeV) and electrons (~300 keV to  ~6 MeV).

Particle fluxes measured by the SREM on board Planck are shown in Fig. 8. The radiation environment of Planck is characterised by the current epoch near the minimum in the solar cycle. As a consequence, the particle flux is dominated by Galactic cosmic rays, rather than by the solar wind. The time evolution of the SREM measurements is well correlated with that of identical units flying simultaneously on other satellites (e.g., Herschel, Rosetta) and with indicators of Galactic cosmic rays, and is anti-correlated with solar flare events and with the solar cycle (Fig. 8). More importantly for Planck, the SREM measurements are very well correlated with the heat deposition on the coldest stages of the HFI, and with glitch rates measured by the detectors of HFI. A more detailed interpretation of these data is provided in Planck HFI Core Team (2011a).

4.5. Pointing performance

Redundant star trackers on board Planck (co-aligned within 0°̣2 with the instrument field-of-view) provide absolute attitude measurements at a frequency of 8 Hz. These measurements are used by the attitude control computer on board Planck to execute the commanded reorientations of the satellite. The star tracker data are further processed on the ground on a daily basis, to provide:

• filtered attitude information at a frequency of 4 Hz during the stable observation periods. The filtering algorithm basically suppresses high-frequency components of the measurement noise (i.e., at frequencies well above the nutation frequency);

• reconstructed attitude parameters averaged over each spin period (60 s) and each stable observation period (or dwell, typically of 50 min length).

The daily filtered attitude information is used by the data processing centres to estimate the location of each detector beam with respect to the satellite reference frame, based mainly on observations of planets (as described in Planck HFI Core Team 2011a). The attitude data during the periods that the satellite is slewing are not filtered on the ground and are therefore much noisier.

Planck rotates about its principal axis of inertia at 1 rpm, with a precision of  ±0.1% (see Fig. 9). The observed variation of the spin rate is very systematic due to the following operational features.

• The thruster used for a manoeuvre is selected depending onwhether the spin rate preceding the manoeuvre is below or aboveits nominal value. Each thruster has a slightly different“minimum thrust level”, which determines the spin rate actuallyachieved. Therefore, the spin rate after each manoeuvre willtoggle between two different spin rate states which bound thenominal value. If one of these states is very close tothe nominal value, drift during the dwell period(see next item) could cause it to change from oneside to the other of the nominal value, and thus to toggle on the nextmanoeuvre to a “third” spin rate state.

  Fig.9 The top panel shows the measured spin rate over the coverage period, which is always within  ± 0.1% of 1 rpm. The spin rate typically adopts one of three values as can be seen in the middle panel, which is a zoom into a period of several days. In the bottom panel we show the typical drift of the spin rate within a dwell, due to small residual torques applied to the satellite.

  Fig.10 Top: evolution of the measured offset between the principal axis of inertia and the geometric axis of the satellite (one point is plotted for each dwell period of approximately 50 min). The time variations observed are not real changes in the offset angle, but are instead due to thermoelastic deformations in the panel that supports the star trackers. Bottom: zooming in on short periods of time before (left) and after (right) the RF transponder was turned permanently on, reveals periodic variations (before, dominated by the daily RF On-Off switching; after, dominated by thermal control cycling of nearby units with typically one hour periodicity).

• Within a dwell period, the spin rate drifts slightly (typically 10^-6   deg   s^-1 per minute) due to residual torques on the satellite, caused by solar radiation pressure and exhaust of helium from the dilution cooler system.

The principal axis of inertia, about which Planck rotates, is offset from the geometric axis by  ~28.6′ (see Fig. 10). The time evolution of the measured offset angle shows a long-timescale variation which is clearly linked to the seasonal power input variations on the solar array; this effect is not a real variation of the offset angle but is instead due to a thermoelastic deformation of the SVM panel that holds the star trackers⁷. Other thermoelastic deformations that give rise to similar effects are related to specific operations which have a thermal impact (see Fig. 10). The dominant (false) offset angle variation before 25 January 2010 is due to the daily thermal impact of the RF transponder being switched on and off, and after that date it is related to thermal control cycles in electronic units located near the star trackers; the peak-to-peak amplitude of the effect is of order 0.15′ before and 0.08′ after 25 January. These effects can be correlated to the temperature of the units responsible. They easily mask the real variation of the offset angle, which is due to gradual depletion of the fuel and helium tanks, and is of order 2.5′′ per month⁸. This real variation of the wobble causes a corresponding change over time of the radius of the circle which each detector traces on the sky.

As described in Tauber et al. (2010a), Planck’s spin axis is displaced by 2′ approximately every 50 min. A typical sequence of manoeuvres and dwells is illustrated in Fig. 11. Each manoeuvre is carried out as a sequence of three thrusts spaced over three minutes (1st impulse – two minutes wait – 2nd impulse – one minute wait – 3rd impulse), designed to cancel nutation as much as possible. Each manoeuvre lasts an average of about 220 s, as defined by on-board software mode transitions⁹, which are used on the ground to trigger the end and start times of attitude filtering.

The pointing achieved after each reorientation is of course not exactly the commanded one; the difference, which varies between 2 and 8′′ during the coverage period, is shown in Fig. 12. This variation is systematically related to the duration of the dwell preceding the manoeuvre, since the preceding pointing drifts due to solar radiation pressure, and the angular amplitude of the following manoeuvre changes correspondingly. The thrust sequence required for each manoeuvre is computed on-board based on the known mass properties of the satellite and known thruster response functions; the error made on each manoeuvre is mainly driven by the (imperfect) on-board knowledge of these properties, and therefore depends systematically on the amplitude of the manoeuvre. On very few occasions (visible in Fig. 12), the thruster sequence performance did not execute as planned, and resulted in a much larger manoeuvre error.

Although the thruster sequence is designed to damp nutation, it does not do so perfectly. The peak amplitude of the residual nutation is typically 3′′ and does not vary significantly in time (see Fig. 13). The period of the nutation is 5.425  ±  0.010 min, determined by the inertial properties of the satellite. Neither the amplitude nor the period of the nutation are observed to drift during a dwell period.

The last major characteristic of pointing within dwells is the drift due to solar radiation pressure. The total amplitude of the drift within each dwell varies between  ~5 and  ~12′′, depending on the duration of each dwell (see Fig. 14). The rate of drift varies between  ~4 and  ~10′′ h^-1, weakly correlated with the Solar Aspect Angle (which varies by very small amounts throughout the mission).

The pointing characteristics described above are summarised in Table 2.

5. Payload performance

The performance of the payload (i.e., two instruments and telescope) is described in detail in (Mennella et al. 2011, LFI) and (Planck HFI Core Team 2011a, HFI), and summarised in Table 3. We note that:

• the angular resolution measured on planets is withina few per cent of thatpredicted on the ground (Tauberet al. 2010b);

• the instantaneous sensitivity of the Planck LFI (Mennella et al. 2011) and HFI (Planck HFI Core Team 2011a) channels is estimated to be approximately 10% larger than that measured on the ground and extrapolated to launch conditions (Tauber et al. 2010a). For LFI the excess is understood to be due to inaccuracy in the calibration constants as measured in ground tests (Mennella et al. 2011); for HFI it is expected to be due to systematic effects remaining in the data at the current level of processing (Planck HFI Core Team 2011a);

• the photometric calibration uncertainty quoted is conservatively based on the current knowledge of systematic effects and data processing pipelines (Zacchei et al. 2011; Planck HFI Core Team 2011b). There is no reason to believe that the mission goals (1% in CMB channels and 3% at the highest frequencies) will not be reached for all Planck channels in due time;

• the point source sensitivities quoted correspond to the fluxes of the faintest sources included in the ERCSC (Planck Collaboration 2011c). Since the ERCSC is a high-reliability catalogue, based on very robust extraction from only the first all-sky survey, these levels will certainly improve substantially in the legacy catalogues which will be delivered in January 2013.

Table 3 confirms the findings of the early CPV activities, namely that the basic performance parameters of the scientific payload of Planck are very close to what was expected based on the measurements made on the ground before launch.

The ultimate performance of Planck also depends on its operational lifetime. Planck is a cryogenic mission (Planck Collaboration 2011b), whose nominal lifetime in routine operations (i.e., excluding transfer to orbit, commissioning and performance verification phases) was initially set to 15 months, allowing it to complete two full surveys of the sky within that period. Its actual lifetime is limited by the active coolers required to operate the Planck detectors, as listed hereafter.

• A ³He-⁴He dilution refrigerator, which cools the HFI bolometers to 0.1 K. The ³He and ⁴He gas are stored in tanks and vented to space after the dilution process. In-flight measurements of tank depletion predict that the ³He gas will run out at the end of January 2012 (Planck Collaboration 2011b).

• A hydrogen sorption refrigerator, which cools the LFI radiometers to 20 K and provides a first pre-cooling stage for the HFI bolometer system. Its lifetime is limited by gradual degradation of the sorbent material. Two units fly on Planck: the first has provided cooling until August 2010; the second came into operation thereafter and is currently predicted to allow operation until December 2011 (Planck Collaboration 2011b). A further increase of lifetime may be obtained by applying on board the process of “regeneration” to the material.

Overall, the cooling system lifetime is at least one year above the nominal mission span, and no other spacecraft or payload factors impose additional limitations. Therefore, barring unexpected failures, Planck will continue surveying the sky until at least the end of 2011.

6. Conclusions

This paper summarises the performance of the Planck satellite during its first year of survey operations, in the areas most relevant for scientific analysis of the Planck data. Detailed descriptions of all aspects of the payload are provided in accompanying papers in this issue. It can be concluded that the major elements of the satellite’s performance exceed their original technical requirements, and the scientific performance approaches the mission goals.

After an astoundingly smooth first year of survey operations, Planck continues to observe the sky and gather high quality scientific data, and is expected to do so as long as the cryogenic chain can keep the 100 mK stage near its nominal temperature, i.e., to the end of 2011 or possibly early 2012. Following the end of mission operations, the next major milestone in the project will be the release of the first set of timeline and map data products, currently foreseen in January 2013.

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Acknowledgments

Planck is too large a project to allow full acknowledgement of all contributions by individuals, institutions, industries, and funding agencies. The main entities involved in the mission operations are as follows. The European Space Agency operates the satellite via its Mission Operations Centre located at ESOC (Darmstadt, Germany) and coordinates scientific operations via the Planck Science Office located at ESAC (Madrid, Spain). Two Consortia, comprising around 100 scientific institutes within Europe, the USA, and Canada, and funded by agencies from the participating countries, developed the scientific instruments LFI and HFI, and continue to operate them via Instrument Operations Teams located in Trieste (Italy) and Orsay (France). The Consortia are also responsible for scientific processing of the acquired data. The Consortia are led by the Principal Investigators: J.-L. Puget in France for HFI (funded principally by CNES and CNRS/INSU-IN2P3) and N. Mandolesi in Italy for LFI (funded principally via ASI). NASA’s US Planck Project, based at JPL and involving scientists at many US institutions, contributes significantly to the efforts of these two Consortia. A third Consortium, led by H. U. Norgaard-Nielsen and supported by the Danish Natural Research Council, contributed to the reflector programme. The author list for this paper has been selected by the Planck Science Team from the Planck Collaboration, and is composed of individuals from all of the above entities who have made multi-year contributions to the development of the mission. It does not pretend to be inclusive of all contributions. A description of the Planck Collaboration and a list of its members, indicating which technical or scientific activities they have been involved in, can be found at (http://www.rssd.esa.int/index.php?project=PLANCK\& page=Planck_ Collaboration). The Planck Collaboration acknowledges the support of: ESA; CNES and CNRS/INSU-IN2P3-INP (France); ASI, CNR, and INAF (Italy); NASA and DoE (USA); STFC and UKSA (UK); CSIC, MICINN and JA (Spain); Tekes, AoF and CSC (Finland); DLR and MPG (Germany); CSA (Canada); DTU Space (Denmark); SER/SSO (Switzerland); RCN (Norway); SFI (Ireland); FCT/MCTES (Portugal); and DEISA (EU).

References

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