Pre-processing

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Overview

In terms of data processing, the HFI(Planck) High Frequency Instrument ground segment handles two types of data, both made available via the MOC[ESA's] Mission Operation Center [Darmstadt, Germany]:

  • telemetry data transmitted from the satellite. These come from the different subsystems of the satellite service module, from the sorption cooler and from the two instruments.
  • auxiliary data. These are data produced by MOC[ESA's] Mission Operation Center [Darmstadt, Germany]. The only 3 products used by the HFI(Planck) High Frequency Instrument DPCData Processing Center are the pointing data, the orbit data, and the time correlation data.

All data are retrieved by the HFI(Planck) High Frequency Instrument level 1 software and stored in the HFI(Planck) High Frequency Instrument database.

Telemetry data

The digitized data from the satellite are assembled on board in packets according to the ESAEuropean Space Agency Packet Telemetry Standard and Packet Telecommand Standard, the CCSDS Packet Telemetry recommendations and the ESAEuropean Space Agency Packet Utilization Standard. The packets are dumped to the ground during the Daily Tranmission Control Period, consolidated by and stored at MOC[ESA's] Mission Operation Center [Darmstadt, Germany]. Telemetry data contain the housekeeping data and the bolometer (ie science) data.

Housekeeping data

For several reasons (systems monitoring, potential impact of the environment, understanding of the bolometer data), the HFI(Planck) High Frequency Instrument level 1 software gathers and stores in its database the satellite subsystems housekeeping parameters:

  • Command and Data Management System
  • Attitude Control & Measurement Subsystem
  • Thermal Control System
  • Sorption Cooler System
  • HFI(Planck) High Frequency Instrument housekeeping parameters

The structure and the frequency of the packets built by these subsystems and the format of the house keeping parameters are described in the Mission Information Bases. The HFI(Planck) High Frequency Instrument L1 software uses these MIBs to extract the house keeping parameters from the packets. Given the status of each subsystem, the parameters are gathered in the HFI(Planck) High Frequency Instrument database in groups. The HFI(Planck) High Frequency Instrument L1 software builds in each group a vector of time (usually named TIMESEC) and a single vector per housekeeping parameter.

Bolometer data

The HFI(Planck) High Frequency Instrument science data is retrieved and reconstructed as described in section 3.1 of Planck-Early-VI[1].

The first stage of HFI(Planck) High Frequency Instrument data processing is performed on-board in order to generate the telemetry. This is described in the data compression section.

On board, the signal from the 72 HFI(Planck) High Frequency Instrument channels is sampled at 180.4Hz by the Read-out Electronic Unit. 254 samples per channel are grouped into a compression slice. The Data Processing Unit then builds a set of several telemetry packets containing the compression slice data and adds to the first packet the start time of the compression slice. When receiving this set of telemetry packets, the L1 software extracts the [math]72 × 254[/math] samples and computes the time of each sample based on the compression slice start time (digitized with [math]2^{-16} s = 15.26[/math] μs quantization steps) and a mean sample time between samples. For the nominal instrument configuration, the sample integration time is measured to be Tsamp = 5.54404 ms

The two plots show the histogram of time differences between two successive samples for the full mission. 6 sample intervals (> 6 ms) correspond to 3 occurrences of packets lost on board when the on board sofwtare of the Command and Data Management Unit has been patched (18, 19 and 20th of August 2009). The right plot is a blow up of the left one. It shows the distribution around the mean value of 5,54 ms and the 15 μs quantization step.

HFI 4 4 1 deltatos.png
HFI 4 4 1 deltatoszoom.png

Transfer functions

In order to ease the reading data in the HFI(Planck) High Frequency Instrument database, so called transfer functions are created. They allow software items to read data on which functions are applied on the fly. A simple example of a transfer function is the conversion of a thermometer data in Analogic to Digital Units (ADU) to Kelvins. Here we show the 2 transfer functions applied on the bolometer samples in the data processing pipelines:


From raw signal to non demodulated signal in Volts (transf1_nodemod)

[math]Signal\_{N\!D}\_V_{bc} = \dfrac{\dfrac{C_{bc}}{N_{sample} - Nblanck_{bc}}- offset_{bc}}{F1_{bc}} \times \dfrac{Gamp_{ETAL}}{Gamp_{bc}} [/math]

where:

  • [math]bc[/math] refers to a bolometer usually labelled by its electronic belt and channel. Eg. bc=00 refers to the a part of the first 100 GHz Polarized Sentivite Bolometer.
  • [math]C_{bc}[/math] is the bolometer sample in Analogic Digital Units (ADU)
  • [math]N_{sample}[/math] is the number of samples in half a modulation period. This parameter is common to all bolometer channels and - although kept fixed during the whole mission - is read from the housekeeping parameter. [math]N_{sample}=40[/math]
  • [math]Nblanck_{bc}[/math] is the number of samples suppressed at the beginning oft each half modulation period. Although kept fixed during the whole mission, it is read from the housekeeping parameter. For bc=00, [math]Nblanck_{00}=0[/math]
  • [math]F1_{bc}[/math] is a calibration factor. For bc=00, [math]F1_{00} \simeq 1.8\times10^7[/math].
  • [math]offset_{bc}[/math] is close to 32768
  • [math]Gamp_{ETAL}[/math] is the gain amplifier mesured during the calibration phase. For bc=00, [math]Gamp_{ETAL}=1[/math]
  • [math]Gamp_{bc}[/math] is the current gain amplifier. Although kept fixed during the whole mission, it is read from the housekeeping parameter. For bc=00, [math]Gamp_{00}=1[/math]


From raw signal to demodulated signal in Volts (transf1)


The same formula as above is used but with the demodulation and a 3-point filter computed as:

[math]Signal\_V_{bc} [n] = - \tfrac{1}{4} (-1)^{parity_{n-1}} Signal\_{N\!D}\_V_{bc} [n-1] + \tfrac{1}{2} (-1)^{parity_{n}} Signal\_{N\!D}\_V_{bc} [n] - \tfrac{1}{4} (-1)^{parity_{n+1}} Signal\_{N\!D}\_V_{bc} [n+1][/math]

where:

  • [math]n-1[/math], and [math]n+1[/math] refer to the samples before and after the given [math]n[/math] sample to demodulate
  • [math]parity[/math] is computed by the HFI(Planck) High Frequency Instrument L1 software

Statistics on the telemetry data

The table gives some statistics about the data handled at the pre-processing level:

Nominal mission(1) Full mission(1) From launch to the end of full mission(1)
Duration [math]473[/math] days [math]884[/math] days [math]974[/math] days
Number of HFI(Planck) High Frequency Instrument packets generated onboard (HSKHouse-Keeping data/science)(2) [math]19~668~436[/math] / [math]376~294~615[/math] [math]37~762~493[/math] / [math]704~852~262[/math] [math]41~689~4090[/math] / [math]765~043~713[/math]
Number of HFI(Planck) High Frequency Instrument packets lost (3) (HSKHouse-Keeping data/science) [math]2[/math] / [math]20[/math] [math]2[/math] / [math]20[/math] [math]2[/math] / [math]20[/math]
Ratio of HFI(Planck) High Frequency Instrument lost packets vs generated on board (HSKHouse-Keeping data/science) [math]1\times10^{-7}[/math] / [math]5\times10^{-8}[/math] [math]5\times10^{-8}[/math] / [math]3\times10^{-8}[/math] [math]5\times10^{-8}[/math] / [math]3\times10^{-8}[/math]
Number of different housekeeping parameters stored in the database (HFI(Planck) High Frequency Instrument/SCSSorption Cooler Subsystem (Planck)/sat) [math]3174[/math] / [math]708[/math] / [math]12390[/math]
Number of science samples stored in the database [math]530~632~594~653[/math] [math]991~929~524~565[/math] [math]1~090~125~748~960[/math]
Number of missing science samples [math]2~537~499[/math] [math]6~634~491[/math](4) [math]7~521~758[/math]
Ratio of missing science samples vs samples stored [math]5\times10^{-6}[/math] [math]7\times10^{-6}[/math] [math]7\times10^{-6}[/math]
  • (1) : mission periods are defined in this page.
  • (2) : science packets refer to the number of telemetry packets containing science data (ie: bolometer data and fine thermometer data) when the instrument is in observation mode (Application Program IDentifier = 1412). HSKHouse-Keeping data packets refer to the number of HFI(Planck) High Frequency Instrument non essential housekeeping telemetry packets (Application Program IDentifier = 1410).
  • (3) : all lost packets have been lost on board ; no HFI(Planck) High Frequency Instrument packet has been lost at ground segment level.
  • (4) : this amount of lost science samples is distributed as
    • 0.3% are due to Single Event Unit
    • 18.5% have been lost during the 3 CDMUCommand and Data Management Unit patch days and the consecutive clock resynchronisation
    • 29.1% have been lost because of compression errors
    • 52.1% have been lost due to the EndOfSlew buffer overflow being triggered by solar flare events

Pointing data

The pointing data are built by the MOC[ESA's] Mission Operation Center [Darmstadt, Germany] Flight Dynamics team. The pointing data are made available to the DPCData Processing Center via AHFAttitude History File files. See AHF description document and AHF files repository. All data contained in the AHFAttitude History File files are ingested in the HFI(Planck) High Frequency Instrument database. The present section describes the steps to produce the HFI(Planck) High Frequency Instrument pointing solution.

  • During the stable pointing period (ie during the dwell), data sampling rate is given at 8 Hz while it is at 4 Hz during the satellite slews. Those data are thus interpolated to the bolometer sampling rate using a spherical linear interpolation as described in section 3.4 of of Planck-Early-VI[1].
  • The pointing solution is then amended from the wobble effect as delivered by MOC[ESA's] Mission Operation Center [Darmstadt, Germany] Flight Dynamics in the AHFAttitude History File files.
  • A final correction is then applied based on the study of main planets and point sources seen by the bolometers.


Note: an HFI(Planck) High Frequency Instrument ring corresponds to each stable pointing period, when the spin axis is pointing towards an essentially fixed direction in the sky and the detectors repeatedly scan the same circle on the sky. More precisely the HFI(Planck) High Frequency Instrument ring start time is defined as the time of the first thurster firing. The end time of the HFI(Planck) High Frequency Instrument ring is the start time of the following ring.


The plot below shows the evolution of the ring duration along the whole mission. It mainly reflects the scanning strategy and few long rings due to operational constraints or tests.

ring duration

Orbit data

Satellite orbit velocity is built by the MOC[ESA's] Mission Operation Center [Darmstadt, Germany] Flight Dynamics team and made available to the HFI(Planck) High Frequency Instrument DPCData Processing Center via orbit files. See Orbit description document and orbit files repository. As those orbit files contain both effective and predictive data, they are regularly ingested updated in the HFI(Planck) High Frequency Instrument database.
Note: these same data are also ingested in parallel into the JPL Horizons system under ESAEuropean Space Agency’s responsibility.

The satellite orbit data preprocessing is the following.

  • The sampling of the MOC[ESA's] Mission Operation Center [Darmstadt, Germany] provided orbit velocity data is appromximately 1 every 5 mn. These data are interpolated to the time of the middle of the HFI(Planck) High Frequency Instrument rings.
  • The reference frame of the orbit data is translated from the MOC[ESA's] Mission Operation Center [Darmstadt, Germany] given Earth Mean Equator and Equinox J2000 (EME2000) reference frame to the ecliptic reference frame in cartesian coordinates.
  • The earth velocity provided by the NASA JPL Horizons system is interpolated to the time of the middle of the HFI(Planck) High Frequency Instrument rings. It is then added to the satellite velocity data.

The use of the satellite orbit velocity is three-fold:

  1. computation of the CMBCosmic Microwave background orbital dipole (see calibration section),
  2. computation of the positions of solar system objects (see data masking section),
  3. computation of the abberation correction.

Time correlation data

The MOC[ESA's] Mission Operation Center [Darmstadt, Germany] is responsible for providing information about the relationship between the satellite On Board Time (OBTOn-Board Time) and Coordinated Universal Time (UTCUniversal Time Coordinate(d)). This information comes via several measurements of OBTOn-Board Time and UTCUniversal Time Coordinate(d) pairs each day, measured during the satellite ranging. See Time Correlation document.

The following plot shows the (OBTOn-Board Time, UTCUniversal Time Coordinate(d)) data couples provided by MOC[ESA's] Mission Operation Center [Darmstadt, Germany] since Planck launch. The x axis is in number of days since the 17th of May 2009 while the y axis is the difference of OBTOn-Board Time and UTCUniversal Time Coordinate(d) in seconds. The very slow drift is approximately 0.05 seconds per day. The small wiggles are due to the satellite global temperature trends due to its orbit and distance to the sun.

Note: The isolated point on the 1st of June 2010 (just above 400 in abscisse) is not significant and due to a misconfiguration of the ground station parameters.

OBTOn-Board Time-UTCUniversal Time Coordinate(d) correlation data

An order 3 polynomial is then fitted to the data. As the whole HFI(Planck) High Frequency Instrument data management and processing uses OBTOn-Board Time, the OBTOn-Board Time-UTCUniversal Time Coordinate(d) information is only used when importing orbit data from Horizons.

References

  1. 1.0 1.1 Planck early results. VI. The High Frequency Instrument data processing, Planck HFI Core Team, A&A, 536, A6, (2011).