Difference between revisions of "Timelines"

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{{DISPLAYTITLE:2015 timelines}}
{{DISPLAYTITLE: Timelines}}
==General description==
==General description==

Revision as of 13:16, 12 June 2018

General description[edit]

The timelines, or TOIs for "time-ordered information," are vectors of signal or of pointing or of some other quantity, giving the value of that quantity as a function of time during the mission. For LFI, refer to Planck-2015-A02[1], and for HFI, to Planck-2015-A07[2].

The TOIs described here are sampled regularly at the (instrument-dependent) detector sampling frequency and span the full science mission. They thus consist of a large amount of data. This section is concerned with the signal TOIs and their associated pointing TOIs. For selected detectors the DPCs provide a single timeline of cleaned and calibrated signal and three timelines of coordinates, corresponding to the two angular coordinates and one orientation angle.

Obviously these vectors are very long (about 1.38×1010 samples for HFI, from 2.5×106 to 5.5×106 for LFI) and thus need to be split into multiple files for export. Here the data are split by operational day (OD) as follows:

  • HFI has 12 files per OD, one for pointing and one for signal for each channel, covering ODs 91 to 974, all sampled at Fsamp = 180.3737 Hz;
  • LFI has six files per OD, three each for signal and pointing, since the sampling frequency is channel dependent, and covering ODs 91 to 1543, sampled at Fsamp = 32.5079, Fsamp = 46.5455, and Fsamp = 78.7692 Hz.

The signal timelines are encoded as single-precision real values, but the pointing vectors had to be encoded as double-precision reals to maintain the required accuracy. The result is that the total volume of the full dataset is approximately 30 TB. All files also contain the OBT.

The signal timelines have been cleaned of all known instrumental systematic effects, they have been calibrated in flux, and the solar and Earth motion dipole signals have been removed. But they have not been cleaned of the low-frequency, "1/f" noise that needs to be removed via a destriping tool. The methods consists of removing offsets or "baselines" determined by minimizing the differences in the signals at points on the sky where they intersect. For HFI an offset per ring is determined; for LFI the the baseline is computed every 0.246, 0.988, and 1.000 seconds for the 30, 44, and 70 GHz channels, respectively and maintain the same structure of the signal timelines.

In the case of HFI these offsets are determined using the full mission and all the valid detectors per channel, and they are then used for all the maps produced, i.e., for those maps using any fraction of the mission (year, survey) or any subset of the detectors (single detector, detector set), and it has been shown that using offsets determined from a limited part of the mission and/or a subset of the detectors yields maps that are less consistent than otherwise.

In the case of LFI these offsets are determined using the full mission and all the valid detectors per channel; those values have been used for the production of the full mission period maps. Note that baseline used for shorter period maps are determined on those data periods to avoid noise cross-correlation effects and those are not delivered.

The offsets are delivered separately, as described below.

Furthermore, for HFI and LFI, there are three sets of offsets produced: the primary set using the full rings; and the secondary ones using the first and second half rings, only. The differences between the primary and secondary sets are fairly minor, but they are necessary to include to reconstruct the maps as they were built by the HFI-DPC and LFI-DPC. In particular the offsets are useful for users wishing to build a map of a small area of the sky.

The HFI delivers its offsets in a ROI, or "ring-ordered Information" file. That file contains a table of Nrings = 26766 rows by Nbolometers in which each cell contains a 3-element vector containing the full-ring, first half-ring, and second half-ring offsets. The indices of the signal TOI delimiting the rings are given in a second ROI file with the same Nrings rows containing global parameters.

The LFI delivers its offsets in a TOI format, the structure is exactly the same as that used by the science timelines. The user can simply subtract one-by-one the offset timelines from the science timelines and then generate a map using the result. In the case of the half-ring baselines, a vector has been added in the OBT extension; this vector contains "1" or "2," depending which half ring it should be applied to.

HFI indexing[edit]

Each FITS file contains keywords giving the first and last index of the section of the full TOI contained in the file. These indices are those of the internal DPC representation; they begin at 0, corresponding to the switch-on of the instrument, and run to some very large number (about 25 billion for HFI) representing the full mission. Only the science part of the mission is exported, which is about 60% of the total for HFI. Left out are the early mission phase (cool-down and validation phases) and the warm phase of HFI, during which LFI continued to collect data. The HFI keywords also indicate which rings are included in each OD, but note that normally an OD does not begin at the same time (with the same index) as a ring.

Note that for historical reasons, the OD definitions of the two DPCs differ; for LFI they occur at the transition between pointing periods, whereas for HFI they do not. However, this has no significance for the user, since this splitting is somewhat arbitrary anyway, and what counts is the full vector once it is rebuilt in the user's own work space.

HFI timelines at the DPC are indexed from 0 to 25×109, which correspond to instrument switch-on and switch-off, respectively. Of these the indices 1.4×109 to 151.5×109 correspond to the science mission and are exported and delivered. Each file contains a keyword giving the first and last index of the data in that file, and EndIndex(OD)+1 = BeginIndex (OD+1). The ROI "Global" file gives the Begin/End Indices of each Ring, or Pointing Period, and can be used to destripe the signal TOIs with the offsets provided in the "Destriping-Offsets" ROI file.

Production process[edit]

HFI processing[edit]

The input TOIs are in ADUs, representing the voltage signal at the end of the electronics (see Detection chain for details). The processing applied to remove instrumental effects and to calibrate them is described in detail in the HFI TOI processing section, and in Planck-2015-A07[2] we give a very brief summary here for convenience. This pipeline performs the following operations.

ADC correction
This corrects for the uneven size of the ADC bins (see ADC correction).
This is performed around a variable level, which is determined from the valid input data (using a validity flag from a previous version of the processing), and the data are converted to engineering units (V) using known conversion coefficients.
The redundancy within a ring is used to determine where glitches occur. Once identified, the glitches are fitted with templates. A glitch flag is produced that identifies the strongest part of the glitches, and a timeline of glitch tails. produced from the template fits, is subtracted from the demodulated timeline. Finally, the flagged regions are replaced with data from an average over the pointing period in order to not leave holes in the timeline that would perturb the usage of Fourier methods on them
Dark template removal
The two dark bolometers are demodulated and despiked as above; the resulting timelines are then smoothed and used as an indicator of the overall temperature variations of the bolometer plate. Where the variations are consistent with each other, they are combined and removed from the bolometer signal timelines using appropriate coupling coefficients. The few percent of the data where they are not consistent are flagged as not valid on the timelines.
Conversion to absorbed power
The timeline is converted to watts of absorbed power using the bolometer function. This includes a nonlinearity correction and removal of the 4-K cooler lines (i.e., the electromagnetic interference of the 4-K cooler with the bolometer readout wires, which induces some sharp lines in the signal power spectra at frequencies of the 4-K cooler's fundamental and its multiples, folded by the signal modulations). Fourier coefficients of the relevant lines are determined on a per-ring basis, and then removed from the data. The quality of the removal depends on the bolometer.
Deconvolution by the time transfer function
This is done to correct for the non-instantaneous time response of the bolometers. The function itself is modeled using up to eight time constants, which are adjusted primarily on planet data and also from comparisons of the northward and southward scans of the Galactic Plane. It is then removed using Fourier techniques.
Jump correction
This corrects some rare jumps in the signal baseline (there are on average around 0.3 jumps per bolometer per pointing period). The jumps are detected and characterized on smoothed TOIs, and corrected by adding a constant to part of the signal timeline. The origin of the jumps is not known.

The results of this set of processing steps are a timeline of signal (in absorbed watts) and a "valid data" flag timeline for each of the 50 valid bolometers that are processed, out of the 52 HFI bolometers; these timelines contain the full-sky signal, i.e., including the solar and orbital dipoles, the Zodiacal light, and contributions from the far sidelobes. The dipoles are necessary for the flux calibration and are removed at the mapmaking stage. The remaining two bolometers (143-8 and 545-3) show semi-random jumps in the signal level, typically jumping over 2 to 5 different "pseudo-baseline" levels, a behaviour known as "random telegraphic signal" (RTS), so that these are commonly called the RTS bolometers. Finally, ring-level statistics of different types (mean, median, rms, kurtosis, etc.) are determined on a per-ring basis for all timelines, and a selection based on these statistics is used to discard anomalous rings, which are recorded in a ring-level flag for each valid bolometer timeline (see the Discarded rings section).

Throughout this processing, some planet signals are masked in the timeline in order to avoid ringing effects in the processing. This concerns only Jupiter at 100 and 143 GHz, Jupiter and Saturn at 217 GHz, and Jupiter, Saturn, and Mars at 353, 545, and 857 GHz (thanks to various improvements in the despiking and in the transfer functions it is no longer necessary to flag these three planets in the CMB channels, as was done in the 2013 release). Since they move on the sky, the portion of the sky masked during one survey is observed during another one, and no hole is left in the final map. In parallel, the planet data are processed in a similar way, but with different parameters for the despiking step. These results are processed separately to determine the beam shapes and the focal plane geometry.

Next, the TOIs are calibrated in astrophysical units using the results of the calibration pipeline (see the Map-making and calibration section) and the best estimates of the zero-point offsets (a constant level for each bolometer). These values are given in the RIMO. Also, the solar and Earth-motion dipole signals are computed and removed.

These TOIs are accompanied by several flags that are described below. The most important one is the "Total" flag, which identifies all the samples that were discarded by the DPC mapmaking, as described in HFI TOI processing. This flag includes the portion beyond 72 min for the rings that are longer; this is not used for science analysis because of the slight drift of the satellite pointing direction (spin axis) during these long acquisition periods.

At this point the TOIs still contain the low frequency (1/f) noise, which should be removed before projection onto a map. That cleaning step is called "destriping" or "baseline removal". The HFI-DPC does its destriping at ring-level, meaning that a constant is added to the signal of each ring in order to minimize the difference at the ring crossings, where the signal should be the same for all detectors (with the necessary precautions as described in Map-making and calibration section); should the user want to use the DPC's offsets, they are provided separately (see ROI files below). Maps produced from these TOIs, and after subtraction of the DPC's destriping offsets, are not identical to the maps that are delivered. This is discussed in Section A.2 of Planck-2015-A08[3].

As indicated above, the brightest planets are masked in the TOI in order to avoid ringing problems. For users specifically wanting to study these planets, we provide separate timelines covering just the planet transits. These timelines also include transits of Uranus and Neptune, which are not masked in the regular TOIs. They are produced with less agressive deglitching options in order to work on the rapidly changing baselines. These are the data used the reconstruction of the focal plane geometry and also for the determination of the scanning beams.

The table below lists the 16 transits, giving for each the begin/end ring number and the begin/end OD in which that ring is found. Note that transit 15 is split into two parts because some tests were done during the transit. The last row is a region processed in the same manner but without a planet transit, which is included for comparison.

Planet transits
Number Beg Ring End Ring Beg OD End OD Comment
1 2700 3211 160 176 Jupiter, Mars, Neptune
2 3892 4099 203 212 Uranus
3 4575 4775 233 240 Saturn
4 7900 8150 330 337 Mars
5 8979 9188 367 376 Neptune
6 9550 9750 392 402 Saturn
7 9940 10300 411 425 Jupiter, Uranus
8 14018 14227 537 544 Neptune
9 14820 15190 567 583 Jupiter, Uranus
10 15925 16275 612 624 Saturn
11 20016 20225 734 743 Neptune
12 20900 21650 775 804 Saturn, Uranus
13 21780 22150 808 818 Jupiter
14 24992 25202 916 921 Neptune
15a 25830 25849 937 938 Mars, Uranus
15b 25864 26225 947 957 Mars, Uranus
16 26650 27005 968 974 Jupiter
17 12000 12150 479 483 background

The pointing (see common Detector Pointing section for details) is determined from the AHF produced by the MOC, which gives the direction and orientation of the LOS of a fiducial position in the focal plane at frequencies of 8Hz during stable pointing and 4Hz during manoeuvres. This is interpolated to the timestamps of data samples, corrected for the wobble and other time-dependent offsets determined from the observed positions of a large number of sources around the sky, and finally converted to the LOS of each detector using the quaternions in the IMO. These pointing data are valid for both the regular and the planet TOIs.

LFI processing[edit]

The input TOIs are in ADUs, representing the voltage signal at the output of the electronics (see Radiometer Chain Assembly (RCA) for details). The processing applied to remove instrumental effects and to calibrate them is described in detail in the LFI TOI processing section, and in Planck-2015-A03[1]; here we give a very brief summary for convenience. That pipeline performs the following operations.

ADC correction
Due to ADC nonlinearity under certain condition, this instrumental effect is removed by applying well know templates directly to the diode signal.
Electronics spikes
This is caused by the interaction between the electronics clock and the scientific data lines. The signal is detected in all the LFI radiometer time-domain outputs as a 1-s square wave with a rising edge near 0.5s and a falling edge near 0.75s, synchronous with the onboard time signal. In the frequency domain it appears as a spike signal at multiples of 1Hz. The 44-GHz channels are the only LFI outputs significantly affected by this effect. Consequently the spike signal is removed from the data only in these channels.
Each LFI diode switches at 4096Hz between the sky and the 4-K reference load. The data acquired in this way are dominated by 1/f noise that is highly correlated between the two streams; differencing those streams results in a strong reduction of 1/f noise. The procedure applied is described in LFI TOI processing, taking into account that the gain modulation factor R was computed on timestreams with the Galaxy and point sources masked to avoid strong sky signals.
Diode combination
Two detector diodes provide the output for each LFI receiver channel. To minimize the impact of imperfect isolation of the data stream of each diode, we perform a weighted average of the time-ordered data from the two diodes of each receiver. The procedure applied is detailed in LFI TOI processing; and the weights used are kept fixed for the entire mission.
Scientific calibration
This step calibrates the timelines to physical units KCMB, fitting the total CMB dipole convolved with the 4π beam representation, without taking into account the signature due to Galactic stray light.
Gain regularization
The calibration constants computed using the model of the dipole signal suffer from large uncertainties when the Planck spacecraft is badly aligned with the dipole axis. To reduce the noise, we apply an adaptive smoothing algorithm, which is also designed to preserve the discontinuities caused by abrupt changes in the working configuration of the radiometers (e.g., sudden temperature changes in the focal plane).
Removal of solar and orbital dipole signal
The combined solar and orbital dipole is convolved with the 4π beam representation of each radiometer and then removed from its timeline.
Removal of Galactic stray light
The light incident on the focal plane without reflecting on the primary mirror (stray light) is a major source of systematic effects, especially when the Galactic plane intersects the direction of the main spillover. This effect is corrected by removing the estimated straylight signal from the timelines. This signal is computed as the convolution of Galaxy simulation with the beam sidelobes (see details in LFI TOI processing).

After these processing steps the timelines are used for the production of the maps.

The pointing (see common Detector Pointing section for details) is determined from the AHF produced by the MOC, which gives the direction and orientation of the LOS of a fiducial position in the focal plane at frequencies of 8Hz during stable pointing and 4Hz during manoeuvres. This is interpolated to the timestamps of data samples, corrected for the wobble and other time-dependent offsets determined from the observed positions of a large number of sources around the sky, as well as temperature sensors, and finally converted to the LOS of each detector.

File Names[edit]

The file names are of the form



  • fff denotes the frequency,
  • SCI denote signal TOIs,
  • PTG denote pointing TOIs,
  • TAI denotes the OBT-MJD correlation TOI,
  • OFF denotes the baseline TOI,
  • R2.nn is the version, and
  • ODxxxx indicates the OD.

Regarding the HFI TOIs for the 100 to 217GHz channels, the R2.00, made public in Jan 2015, contained only the unpolarized bolometer timelines, while R2.02, made public in July 2015, contains all bolometers.

The HFI planet timelines are named


and are provided for all 50 valid bolometers, i.e., SWBs and PSBs.

FITS file structure[edit]

All FITS files begin with a primary extension containing no data and a minimal header, which is followed by one or more BINTABLE extensions containing data, and with a description of the data in the header keywords. In what follows we will usually ignore the primary extension, and count only the extensions containing data.

TOI files[edit]

The signal FITS files contain N+1 , "BINTABLE", data extensions, where N is the number of detectors in that frequency channel. The first extension contains the OBT and its flags, called the "global" flags since they apply to all detectors, which is followed by one extension for each detector, containing the signal and its "local" flag. The flags columns are written as "byte" in which each of the 8 bits (maximum) encodes one flag timeline. The meaning of each bit is given in the header comments and is subject to change. For the global flag they include the following list.

  • For HFI:
    • unstable pointing, where 1 = pointing is not stable (e.g., during repointing manoeuvres);
    • dark correlation, where 1 = darks are uncorrelated and data are flagged;
    • first/Second half ring, which samples are in which half (only covers the stable pointing part of the ring);
    • HCM, whether in HCM mode (unstable pointing);
    • and more (see extension header for details).
  • For LFI:
    • bit 0, unstable pointing, where 1 = pointing is not stable;
    • bit 1, time correlation quality, where 1 = outside specification;
    • bit 2, special observation, where 1 = special observation, such as a deep scan.

For the local flag they include the following list.

  • For HFI:
    • total flag, being a combination of the various flags that is the one finally used in the mapmaking (all samples with "total flag" different from zero should not be used);
    • data not valid, e.g., glitched samples;
    • despike common (for PSBs only), corresponding to a glitch on the current or the other of a PSB pair;
    • strong signal, i.e., on the Galactic plane;
    • strong source, i.e., on a point source;
    • and more (see extension header for details).
  • For LFI:
    • bit 0, data not valid, where 1 = science sample not valid;
    • bit 2: planet crossing, where 1 = science sample containing planet;
    • bit 3: moving objects, where 1 = minor Solar System object (not yet used);
    • bit 4: gap, where 1 = this sample was artificially included due to a gap in the data.

For HFI the header extension gives more details on the flags and the meaning of "1" and "0".

TOI file data structure
1. EXTNAME = "OBT" : Data columns
Column name Data type Units Description Comment
OBT Double 2-16 sec Onboard time
FLAG Byte None Various bit-level flags
Keyword Data Type Value Description Comment
OD Int OD covered (as in filename)
BEGIDX Int First sample index of given OD Only HFI
ENDIDX Int Last sample index of given OD Only HFI
BEGRING Int First ring in given OD Only HFI
ENDRING Int Last ring in given OD Only HFI
TIMEZERO String 1958-01-01z00:00 Origin of OBT Only HFI
TIMEZERO Float 106744000000000. Origin of OBT Only LFI
n. EXTNAME = DETNAME : Data columns
Column name Data type Units Description Comment
SIGNAL Real*4 Kcmb or MJy.sr-1 Value of signal Comment
FLAG Byte None Various bit-level flags
Keyword Data type Value Description Comment
UNIT String Units of signal Only HFI, LFI always Kcmb
DESTRIPE 1/0 Whether timeline is destriped Only HFI
OD Int OD covered (as in filename)
BEGIDX Int First sample index of given OD Only HFI
ENDIDX Int Last sample index of given OD Only HFI
BEGRING Int First ring in given OD Only HFI
ENDRING Int Last ring in given OD Only HFI

The HFI Planet files have the same structure, but the local flags contain a single "Data not valid" flag. Also, the first and last ODs of each transit are usually not complete, since they contain only the rings that are included in the transit observations.

The pointing files have a similar structure, except that DETNAME extensions contain three columns of Real*8 variables with the φ, θ, and ψ Galactic spherical coordinates of each sample in radians, as shown in the table below. There is no local flag for the coordinates.

Pointing TOI file DETNAME extension structure
n. EXTNAME = DETNAME : Data columns
Column name Data type Units Description
PHI Real*8 Radian Longitude
THETA Real*8 Radian Colatitude
PSI Real*8 Radian Roll angle

HFI ROI files[edit]

The files provided by HFI are

  • HFI_ROI_GlobalParams_RelNum_full.fits,
  • HFI_ROI_DestripeOffsets_RelNum_full.fits,

which are described below.

Global parameters
This ROI file contains a single "BINTABLE" extension with three columns that give, for each ring, the index number of the first sample, the ESA pointing ID of the ring, and the ring start time. All three are encoded as Int*8, and the time is given in units of 2-16 sec from "TIMEZERO", 1958-01-01z00:00, as in the TOI files. Rings are numbered from 240 to 27005, as given in the "BEGRING" and "ENDRING" keywords.
Destriping offsets
This ROI file contains a single "BINTABLE" extension with 3 columns that give, for each ring, the offsets to subtract from each bolometer signal timeline or vector. There are three offsets available for each bolometer (full-ring, first half-ring, and second half-ring), for building the full-ring or the two half-ring maps, respectively. The offsets must be subtracted from Index(N) to Index(N+1)-1 of the corresponding signal timeline, where N is the ring number, and the indices are given in the "global" parameters file. The offsets are given in the same units as the signal vectors, that is Kcmb for the 100- to 353-GHz channels, and MJy.sr-1 for the 545- and 857-GHz channels. All values are encoded as Real*4. The rings for which the offsets are set to 0.000 are "badrings" which are rejected in the mapmaking process. These rings are also flagged by the "TotalFlag", as are the portions of rings longer than about 72.5 min, where the drift in the satellite's spin axis becomes important.

TAI TOI files[edit]

The TAI TOI files contain one extension with two columns, the first is the OBT value (exactly the same as reported in the SCI TOI), the second is the corresponding modified Julian day. Note that leap seconds were not added.

Time TAI TOI file data structure
1. EXTNAME = "OBT-MJD" : Data columns
Column name Data type Units Description Comment
OBT Int*8 2-16 sec Onboard time
MJD Real*8 day Modified Julian day

LFI OFF TOI files[edit]

The OFF (baseline) TOI files adopt the same file structure as the Science TOI files. Note that in case of OFF timelines related to the half-ring, an additional column is included in the OBT extension to define for each sample if it belongs to half-ring 1 or half-ring 2.

LFI HouseKeeping files[edit]

House keeping timelines are:

  • LFI_TOI_DAE-FAST-HK_R2.nn_ODxxxx.fits;
  • LFI_TOI_DAE-SlowConfiguration-HK_R2.nn_ODxxxx.fits;
  • LFI_TOI_DAE-SlowCurrent-HK_R2.nn_ODxxxx.fits;
  • LFI_TOI_DAE-SlowPhaseSwitch-HK_R2.nn_ODxxxx.fits;
  • LFI_TOI_DAE-SlowVoltage-HK_R2.nn_ODxxxx.fits;
  • LFI_TOI_REBA-HK_R2.nn_ODxxxx.fits;
  • SCS_TOI_EssentialASW-HK_R2.nn_ODxxxx.fits;
  • SCS_TOI_NonEssentialASW-HK_R2.nn_ODxxxx.fits.

Each file contains two extensions, the first is the OBT (values are sampled every 1 or 10 seconds), while the second contains a variable number of columns equivalent to twice the number of Housekeeping timelines stored. Each Housekeeping datum is accompanied by its flag (normally "0" means that the value was considered invalid or out of limits). The Housekeeping names are the ones defined in the LFI Instrument Operation Manual.


(Planck) Low Frequency Instrument

(Planck) High Frequency Instrument

Operation Day definition is geometric visibility driven as it runs from the start of a DTCP (satellite Acquisition Of Signal) to the start of the next DTCP. Given the different ground stations and spacecraft will takes which station for how long, the OD duration varies but it is basically once a day.

On-Board Time

Data Processing Center

Ring-Ordered Information (DMC group/object)

Flexible Image Transfer Specification

analog to digital converter

random telegraphic signal

Cosmic Microwave background

reduced IMO

Attitude History File

[ESA's] Mission Operation Center [Darmstadt, Germany]

Line Of Sight

Angular momentum Control Mode

European Space Agency

LFI Data Acquisition Electronics

House Keeping

LFI Radiometer Electronics Box Assembly

Sorption Cooler Subsystem (Planck)