Survey scanning and performance

The baseline observation strategy

The Planck observation strategy, or scanning strategy, makes use of the characteristics of the Planck spacecraft and orbit in order to achieve the survey goals in terms of sky coverage and scanning directions.

The Planck focal plane scans the sky in the way explained in the figure below.

The 353 GHz row scans the sky in small circles with a 85 deg. bore-sight angle from the spin axis. 545 GHz, 857 GHz, 143 GHz bolometers as well as LFI 25-26 radiometers have a slightly smaller bore-sight angle, while the other part of the Planck focal plane have larger ones (up to ~ 89 deg.). The Planck spacecraft spins at a rate of 1 rpm.

Planck, being at L2, moves along the Ecliptic at ~ one degree per day, and needs to keep the Solar aspect angle below ~ 9 degrees at all times. In practice, during the surveys, the limit considered is 8 deg. Another celestial constraint is that the angle between the Planck spin axis and the anti-Earth direction cannot exceed 15 deg. (relaxed to 17 deg. from Survey 5 onward). These constraints have a very direct influence on the chosen scanning strategy.

The path followed by the Planck spin axis is defined with respect to the Ecliptic plane. It corresponds to a motion in longitude which maintains an anti-Sun pointing (about one degree per day), to which is added a cycloidal motion (precession) of the spin axis around the anti-Sun position (fiducial point). The cycloidal path is defined by the following functions:

• λ = θ sin((-1)n ω(t-t0) + φ) (eq. 1)

• β = -θ cos((-1)n ω(t-t0) + φ) (eq. 2)

where λ is the angular distance from the fiducial point in Ecliptic longitude, β the angular distance from the fiducial point in Ecliptic latitude, θ the spin axis precession amplitude, ω the pulsation of the precession, φ its phase, n the parameter which controls the motion direction of the precession, t is the time, and t0 is the first time during the Planck survey at which the fiducial point crosses the 0 Ecliptic longitude line.

The reasons to decide to use such a precession of the spin axis are the following :

• An excursion of the spin axis from the anti-Sun direction is required to fully observe the whole sky (no excursion would leave a large area unobserved around the Ecliptic poles)

• The precession motion allows to keep the Sun aspect angle constant during the whole survey, therefore minimizing the thermal constraint variations on the spacecraft

The following precession parameters have been chosen for the BSS:

• amplitude θ = 7.5 deg. (this value is the lowest possible which allows to cover the whole sky with all detectors)

• n = 1 (anti-clockwise motion as seen from the Sun)

• pulsation ω = 2π / (half a year). Faster precession was considered, and has interesting advantages, but is not possible given the constraints if one wants to keep a 7.5 deg. amplitude (which is necessary to cover the whole sky).

• The phase of the precession was decided according to this set of criteria (in order of importance): a) respecting the operational constraints, b) allowing the largest possible angle between two scans on the Crab, c) avoiding null dipole amplitude for the whole mission, d) optimizing the position of the planets in the beginning of the survey and with respect to the feasibility of their recovery, e) placing the two “deep fields” where Galactic foregrounds are minimum, f) allowing a reasonable survey margin

The phase is 340 deg. for Surveys 1 to 4, and 250 deg. for Surveys 5 to 8. The change of phase between Survey 4 and Survey 5 occurred because it was found essential especially on the HFI side that scanning direction crossings are increased in the Planck survey. By choosing a phase 90 deg. away from the original one, one optimizes the scanning strategy in this respect.

The details of the justifications for these parameters can be found in PL-WG9-TN-001.

The scanning strategy parameters are input to the PSO Survey Planning and Performance Tool, which is the software which generates the series of pointing records to be sent out to MOC for implementation.

Spin axis manoeuvres and exact pointings

The motion of the spin axis is not continuous. Every change in the spin axis position is initiated by a manoeuvre which requires less than 5 minutes time to complete.

The duration between spin-axis manoeuvres is hereafter referred to as “dwelling times"; and the angular distance between manoeuvres along the spin axis path is hereafter referred to as “spacing”.

The dwelling times and spacings are defined as follows:

• The spacings are fixed to 2’ which roughly corresponds to Nyquist criterion for sampling the highest-frequency HFI detectors.

• The dwell times vary from 2360 s to 3904 s.

The exact series of coordinates and dwell times are set by equations 1 and 2 when one has fixed the spacings.

Data gap recoveries

Gaps in the data flow can occur because of various reasons:

• Anomalies may occur at spacecraft or instrument level, but also due to failures in the ground-to-spacecraft link or the ground segment itself. The adverse effects of such anomalies may be the loss of scientific data corresponding to one or several planned pointings, which have either not been acquired and/or stored in memory.

• Failure to acquire proper scientific data may occur to one or both instruments

In case a gap occurs, scientific criteria are applied to decide whether to recover the data through the Small-Gap Recovery procedure.

The Small-Gap Recovery (SGR) procedure can be applied by MOC or PSO:

• MOC-triggered SGR: Within its nominal scheduling procedure for an OD, the MOC searches the PPL for PREF corresponding to the OD being processed and any pointings which are subject of a Small Gap Recovery. As the PREF are sorted in chronological order according to their nominal start time, those corresponding to a small gap will naturally be the earliest. After checking attitude constraints, all observable pointings are listed and their dwell times reduced (initially) to the Minimum Dwell Time (MDT) as given in the PPL.

Having included all valid PREF the required slew times are calculated. With the required slew times and minimum dwell times for all pointings, the OD schedule may be under-populated; in this case the dwell times of all scheduled pointings will be increased equally to make use of all available time in the OD. If the resulting schedule is over-populated, even with MDT for all pointings, the recovery cannot be constrained to a single OD. The sequence of pointings for the ODs involved will be optimized in PREF scheduling and dwell times in order to effect the recovery and revert to the nominal timeline as quickly as possible.

• PSO-triggered SGR:

PSO declares a small gap when pointings that have been executed result in data of insufficient quality or inadequate frequency coverage. This assessment is based on data quality and instrument health information provided by the DPCs. The algorithm that is used by the PSO to check the scientific validity of a pointing is implemented within the SPPT software tool. The SPPT ingests on a weekly basis a Weekly Health Report (WHR) for the HFI and LFI containing information on the current working status of each detector and their expected condition over the next four weeks. On a daily basis, a Daily Quality Report (DQR) provided by the DPCs for both instruments, includes a set of parameters that describe the quality of the detection achieved by each individual detector for every pointing executed.

The scheme for declaring a Small-Gap Recovery from PSO is the following:

- If a pointing fails to meet the chosen requirement, it becomes an SGR candidate

- If there are at least 12 consecutive pointings which are declared SGR candidates, and the problem has been identified and solved, then an SGR is triggered As an additional constraint from the 1-N thruster problems, recoveries will only be carried out if the recovery slewing out is guaranteed to be in OCM. MOC will carry out this analysis and decide whether they perform the SGR accordingly.

- The start and end times of the gap and the list of the pointing numbers (or just the numbers of the first and last pointings to be recovered) are then sent to MOC by E-mail, with the notification to start a Small-Gap Recovery

- The MOC implements the Small-Gap Recovery, provided the pointings can still be scheduled with respect to their latest start time, using the same scheme as described in the MOC-triggered SGR section

Special observations

The special observations are:

• special scanning on Solar System objects for calibration purposes

This concerns the following objects (planets): Mars, Jupiter, Saturn, Uranus and Neptune. The Baseline Scanning Strategy will be maintained, but a special case will be set for Small-Gap Recovery algorithm application (see Planck/PSO/2005-016).

• special scanning on other (non-moving) sources (Galactic plane, etc) for calibration purposes

This concerns mostly the Crab Nebula for polarization calibration purposes.

• special pointings during the calibration phase, not relative to celestial sources

(*) The 1-N thrusters problems make it more difficult to recover gaps, since the 20-N thrusters must now be used to initiate a recovery. During roughly half of the year (in two 3-month periods), the recovery is not feasible because the margin with respect to the celestial constraints is too small. Therefore, we try to optimize the cycloid phase in order to have the most important planets in a recoverable area.

The survey requirements and survey goals

The survey requirements are numbers which are required to be achieved at the end of the Planck nominal mission. They correspond to the values obtained from 12 months of observation, with the sensitivity which is measured in-flight during CPV phase. The survey requirements are the following:

• mean integration time over the sky, in seconds / square degree

• percentile of sky not observed

• percentile of sky below half mean integration time

• percentile of sky above four times mean integration time

• percentile of sky above nine times mean integration time (i.e. “deep fields”)

• percentile of sky in which coverage suffers from sharp gradients (> 500 in grad(sec/deg2) )

• percentile of sky in which scanning directions are at least 5 over 16

See Planck/PSO/2006-009 section 4.2.2 for more details on these quantities.

Meaning of these requirements: - mean: the integration time averaged over the sky at the end of the mission should be at least the one mentioned in the table, for each row / frequency - the precentile of sky not observed should be zero - (no strong requirement from the three central columns)

Survey goals are numbers which are wished to be achieved at the end of the Planck nominal mission. Survey goals correspond to the values obtained after a complete nominal mission (15 months of survey with nominal noise – as predicted from ground, and no gaps). The survey goals are the same quantities as the survey requirements. The tables below present these quantities the same way as for the survey requirements.

Start and end of surveys

The completion of a given “survey” is declared when the logical AND of all frequency-coverage maps is greater than 95 %, OR the sky has been surveyed over a consecutive period of 7.5 months. Note that this definition is that of the Project Scientist and Planck Science Office. Data Processing Centres do not use the same concept of surveys to create "survey" maps.

In practice, since the gaps during the surveys have always been very small, it results in ~ six months per survey.

• Survey 1: starts August 13, 2009, ends Feb. 13, 2010

• Survey 2: starts Feb. 14, 2010, ends Aug. 13, 2010

• Survey 3: starts Aug. 14, 2010, ends Feb. 13, 2011

• Survey 4: starts Feb. 14, 2011, ends July 29th, 2011

This survey was shortened in ordered to start earlier with the new scanning strategy (see above).

• Survey 5: starts July 29th, 2011, ends January 30th, 2012

• Survey 6 (LFI only): starts January 30th, 2012, ends July 31st, 2012

• Survey 7 (LFI only): starts July 31st, 2012, ends January 31st, 2013

• Survey 8 (LFI only): starts February 1st, 2013, ends August 1st, 2013

Planets and calibrators during the surveys

• Crab: Sept. 16, 2009 – Sept. 22, 2009

• Mars: Oct. 17, 2009 – Oct. 29, 2009

• Jupiter: Oct. 25, 2009 – Nov. 1, 2009

• Neptune: Nov. 1, 2009 – Nov. 8, 2009

• Uranus: Dec. 6, 2009 – Dec. 16, 2009

• Saturn: Jan. 2, 2010 – Jan. 8, 2010

• Crab: March 6, 2010 – March 12, 2010

• Mars: April 9, 2010 – April 18, 2010

• Neptune: May 15, 2010 – May 23, 2010

• Saturn: June 13, 2010 – June 22, 2010

• Uranus: June 27, 2010 – July 5, 2010

• Jupiter: July 1, 2010 – July 9, 2010

• Crab: Sept. 15, 2010 – Sept. 21, 2010

• Neptune: Nov. 3, 2010 – Nov. 11, 2010

• Jupiter: Dec. 9, 2010 – Dec. 18, 2010

• Uranus: Dec. 12, 2010 – Dec. 21, 2010

• Saturn: Jan. 15, 2011 – Jan. 22, 2011

• Crab: March 7, 2011 – March 12, 2011

• Neptune: May 18, 2011 – May 26, 2011

• Saturn: June 30, 2011 – July 9, 2011

• Uranus: July 3, 2011 – July 11, 2011

• Jupiter: Aug. 9, 2011 – Aug. 15, 2011