Operational history

(Note: Adapted from Planck Collaboration 2011, A&A 536, A1)

Provide a description of the product Planck Operational State History.

The major operational phases and milestones are:

Launch and transfer to orbit: (provide date, and summary overview)

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 13:40:25UT. The separation attitudes of both satellites were within 0.1 deg of prediction. The Ariane rocket placed Planck with excellent accuracy (semimajor axis within 1.6% of prediction), on a trajectory towards the second Lagrangian point of the Earth-Sun system (L2). 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 overperformance of 0.9% and an error in direction of 1.3 deg (a touch-up manoeuvre was carried out on 16 May at 07:17:36 UT). The second and major (mid-course) manoeuvre (153.6ms−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 (58.8ms−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.

Commissioning: (provide date, and summary overview)

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;
• 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. 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 anomaly during 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, 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 activities.

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: (provide date, and summary overview)

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 their performance 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 science;
• estimate the lifetime of the cryogenic chain.

CPV activities addressed the following areas:

• tuning and characterisation of the behaviour of the cryogenic chain;
• characterisation of the thermal behaviour of the spacecraft

and payload;

• for each of the two instruments: tuning; characterisation and/or verification of performance, 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 the 4He-JT cooler on 6 August (costing 6 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 routine operations. No further parameter tuning was expected to be needed, except for the sorption cooler, which requires a weekly change in operational parameters (Planck Collaboration 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.

Nominal Mission: (provide date, and summary overview)

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 First Light Survey (FLS) on 13 August of 2009, at 14:15 UT.

The 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.