Pre-processing

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Overview

The first processing level of the LFI(Planck) Low Frequency Instrument DPCData Processing Center is Level 1. The input data for the Level 1 software includes:

  • raw housekeeping telemetry packets retrieved from different satellite subsystems, namely the LFI(Planck) Low Frequency Instrument instrument, the sorption cooler, the HFI(Planck) High Frequency Instrument instrument, and the Central Data Management Unit (CDMUCommand and Data Management Unit);
  • the LFI(Planck) Low Frequency Instrument raw scientific telemetry;
  • additional auxiliary data provided by the MOC[ESA's] Mission Operation Center [Darmstadt, Germany] and the Flight dynamics, specifically
    • the Attitude History File (AHFAttitude History File),
    • time correlation data (time correlation coefficients and time couples),
    • command history data,
    • sorption cooler out of limit data.

Only a subset of the raw housekeeping telemetry packets is processed daily and converted into TOIs, specifically those relevant to the production of the LFI(Planck) Low Frequency Instrument Daily Quality Report and the estimation of the LFI(Planck) Low Frequency Instrument instrument systematic effects.

Scientific telemetry

Each LFI(Planck) Low Frequency Instrument radiometer provides two analogue outputs, one for each amplifier chain. In a nominal configuration, each output yields a sequence of alternating Vsky, Vload signals at the frequency of the phase switch. By changing the phase switch configuration, the output can be a sequence of either Vsky or Vload signals.

The conversion from analogue to digital form of each radiometer output is performed by a 14-bit analogue-to-digital converter (ADCanalog to digital converter) in the Data Acquisition Electronics unit (DAELFI Data Acquisition Electronics). The DAELFI Data Acquisition Electronics transforms the signal in the range [–2.5V, +2.5V] – first it applies a tunable "offset," ODAELFI Data Acquisition Electronics, then it amplifies the signal with a tunable "gain," GDAELFI Data Acquisition Electronics, in order to make full use of the resolution of the ADCanalog to digital converter, and finally the signal is integrated. To eliminate phase switch rise transients, the integration takes into account a '"blanking time," i.e., a blind time in the integrator where data are not considered. The default value of the blanking time is 7.5μs. The ODAELFI Data Acquisition Electronics, the GDAELFI Data Acquisition Electronics, and the blanking time are parameters set through the LFI(Planck) Low Frequency Instrument on-board software. The equation applied to transform a given input signal Vin into an output Vout is

[math] V_{\rm out} = G_{\rm DAE}(V_{\rm in} + O_{\rm DAE}) + Z_{\rm DAE}, [/math]

with GDAELFI Data Acquisition Electronics = 1, 2, 3, 4, 6, 8, 12, 16, 24, or 48, ODAELFI Data Acquisition Electronics being one of 255 possible offset steps from +0 up to +2.5V, and where ZDAELFI Data Acquisition Electronics is a small offset introduced by the DAELFI Data Acquisition Electronics when applying a selected gain. The values of GDAELFI Data Acquisition Electronics and ODAELFI Data Acquisition Electronics are set by sending, through specific telecommands, the DAELFI Data Acquisition Electronics gain index (DGI) and the DAELFI Data Acquisition Electronics offset index (DOI) associated with the desired values.

The ADCanalog to digital converter quantizes the Vout uniformly in the range –2.5V ≤VADCanalog to digital converter≤+2.5V, so that the quantization step is qADCanalog to digital converter=5.0/214 = 0.30518mV. The quantization formula is

[math] S = \text{round} \left(\frac{V_{\rm out} + 2.5}{q_{\rm ADC}} \right), [/math]

and the output is stored as an unsigned integer of 16 bits.

The digitized scientific data are then processed by the Radiometer Electronics Box Assembly (REBALFI Radiometer Electronics Box Assembly), which runs the LFI(Planck) Low Frequency Instrument on-board software. For each LFI(Planck) Low Frequency Instrument detector, the REBALFI Radiometer Electronics Box Assembly processes the data in the form of time series, which are split into telemetry packets. To satisfy the LFI(Planck) Low Frequency Instrument assigned telemetry budget limit of 53.5 kbps, the REBALFI Radiometer Electronics Box Assembly implements seven acquisition modes (processing types), which reduce the scientific data rate by applying a number of processing steps. The following figure illustrates the main steps of the on-board processing and the corresponding processing types (PTypes).

Schematic representation of the scientific on-board processing, processing parameters, and processing types for the LFI(Planck) Low Frequency Instrument. The diagram shows the sequence of operations leading to each processing type: coadding; mixing; requantization (Requant); and compression (CMP).
PType 0:
in this mode the REBALFI Radiometer Electronics Box Assembly just packs the raw data of the selected channel without any processing.
PType 1:
consecutive Ssky or Sload samples are coadded and stored as unsigned integers of 32 bits. The number of consecutive samples to be coadded is specified by the Naver parameter.
PType 2:
in this mode, two main processing steps are applied. First, pairs of averaged Ssky and Sload samples, respectively, [math]\overline{S}_{\rm sky}[/math] and [math]\overline{S}_{\rm load}[/math], are mixed by applying two different gain modulation factors, GMF1 and GMF2:
[math] \begin{eqnarray} P_1 & = & \overline{S}_{\rm sky} - \text{GMF1} \cdot \overline{S}_{\rm load}; \\ P_2 & = & \overline{S}_{\rm sky} - \text{GMF2} \cdot \overline{S}_{\rm load}. \end{eqnarray} [/math]
The operations are performed as floating point operations. Then the two values obtained are requantized, converting them into two 16-bit signed integers:
[math] \begin{equation} Q_i = \text{round}\left( q \left( P_i + \text{Offset} \right) \right). \end{equation} [/math]
PType 3:
with respect to PType 2, in this mode only a single gain modulation factor is used, GMF1, obtaining
[math] P = \overline{S}_{\rm sky} - \text{GMF1} \cdot \overline{S}_{\rm load}, [/math]
and analogously to PType 2, the value is requantized obtaining a 16-bit signed integer.
PTypes 4, 5, 6:
with the processing types PType 4, PType 5 and PType 6, the REBALFI Radiometer Electronics Box Assembly performs a lossless adaptive arithmetic compression of the data obtained, respectively, with the processing types PType 0, PType 2, and PType 3. The compressor takes couples of 16-bit numbers and stores them in the output stream up to the complete filling of the data segment for the packet in process.

A set of REBALFI Radiometer Electronics Box Assembly processing parameters — Naver, GMF1, GMF2, q and Offset — is selected for each of the 44 LFI(Planck) Low Frequency Instrument channels. They are also included in a teartiary header of each scientific telemetry packet sent to the ground. The REBALFI Radiometer Electronics Box Assembly can acquire data from a channel in two modes at the same time. This capability is used to verify the effect of a certain processing type on the data quality. So, in nominal conditions, the LFI(Planck) Low Frequency Instrument instrument uses PType 5 for all its 44 detectors and every 15 minutes a single detector, in turn, is also processed with PType 1, in order to periodically check the gain modulation factors and the second quantization parameters. The other processing types are mainly used for diagnostic, testing, or contingency purposes.

Packets generated by the REBALFI Radiometer Electronics Box Assembly follow the ESAEuropean Space Agency Packet Telemetry Standard and Packet Telecommand Standard, the CCSDS Packet Telemetry recommendations, and the ESAEuropean Space Agency Packet Utilization Standard (PUSPacket Utilisation Standard). The packet structure for an LFI(Planck) Low Frequency Instrument scientific telemetry packet is shown in the following figure.

LFI(Planck) Low Frequency Instrument scientific telemetry packet structure. The Packet Header, Data Field Header, and Packet Error Control are specified in the PUSPacket Utilisation Standard standard. The source data field contains a Structure ID (SID), to uniquely determine the format and layout of the field itself. It is followed by a tertiary header containing the detector ID, the phase switch status, and the REBALFI Radiometer Electronics Box Assembly processing parameters used. The subsequent structure depends on the phase switch configuration and REBALFI Radiometer Electronics Box Assembly processing type applied. This example shows a packet with PType 0 data and with the nominal phase switch configuration.

From packets to raw TOI

On a daily basis, the LFI(Planck) Low Frequency Instrument Level 1 software pipeline retrieves the housekeeping and scientific telemetry packets dumped from the satellite on-board memory through the MOC[ESA's] Mission Operation Center [Darmstadt, Germany] Data Disposition System (DDSMOC's Data Distribution System). The Level 1 software has to recover, as accurately as possible, the values of the original (averaged) sky and load samples acquired on-board. Data acquired with PTypes 4, 5, and 6 are first uncompressed. The lossless compression applied on-board is simply inverted and the number of samples obtained is checked with the value stored in the tertiary header.

The digitized data, processed by the REBALFI Radiometer Electronics Box Assembly, are not in physical units but in ADU (Analogue-to-Digital Units). The conversion of Ssky and Sload in volts requires the Data Source Address (DSA), i.e., the radiometer and detector from which the data are generated, the blanking time (indexed by the Blancking Time Index, BTI), the DGI (DAELFI Data Acquisition Electronics Gain Index) and the DOI (DAELFI Data Acquisition Electronics Offset Idex). The DSA and BTI values are recovered from the packet tertiary header, while the DGI and DOI values are recovered from the LFI(Planck) Low Frequency Instrument HKHouse Keeping telemetry. Hence, the value in volts is obtained as

[math] V_i = \dfrac{S_i \cdot q_{\rm ADC} - Z_{\rm DAE} - 2.5}{G_{\rm DAE}} - O_{\rm DAE} \approx \dfrac{S_i - \tilde{Z}_{\rm DAE}(\text{DSA}, \text{BTI}, \text{DGI})}{\tilde{G}_{\rm DAE}(\text{DSA}, \text{BTI}, \text{DGI})} - \tilde{O}_{\rm DAE}(\text{DSA}, \text{BTI}, \text{DOI}) [/math]


where [math]\tilde{G}_{\rm DAE}(\text{DSA}, \text{BTI}, \text{DGI})[/math], [math]\tilde{O}_{\rm DAE}(\text{DSA}, \text{BTI}, \text{DOI})[/math], and [math]\tilde{Z}_{\rm DAE}(\text{DSA}, \text{BTI}, \text{DGI})[/math] are look-up tables estimated during the LFI(Planck) Low Frequency Instrument ground calibration campaign with:

  • [math]\tilde{G}_{\rm DAE}(\text{DSA}, \text{BTI}, \text{DGI}) \approx \dfrac{G_{\rm DAE}}{q_{\rm ADC}} [/math];
  • [math]\tilde{O}_{\rm DAE}(\text{DSA}, \text{BTI}, \text{DOI}) \approx O_{\rm DAE}[/math];
  • [math]\tilde{Z}_{\rm DAE}(\text{DSA}, \text{BTI}, \text{DGI}) \approx \dfrac{Z_{\rm DAE} + 2.5}{q_{\rm ADC}}[/math].

This conversion is the only processing required by PTypes 0 and 3 and it is the last step in the processing of all the other processing types. Since PType 1 data are just coadded on-board, the division by Naver is performed by the Level 1 software. PType 2 and 5 data have to be dequantized by

[math] P_i = \dfrac{Q_i}{q} - \text{Offset} [/math]

and then demixed to obtain [math]\overline{S}_{\rm sky}[/math] and [math]\overline{S}_{\rm load}[/math]:

[math] \begin{eqnarray} \overline{S}_{\rm sky} & = & \dfrac{P_2 \cdot \text{GMF1} - P_1 \cdot \text{GMF2}}{\text{GMF1} - \text{GMF2}}; \\ \overline{S}_{\rm load} & = & \dfrac{P_2 - P_1}{\text{GMF1} - \text{GMF2}}. \end{eqnarray} [/math]

On-board time reconstruction

The On-Board Time (OBTOn-Board Time) reconstruction for scientific data has to take into account the phase switch status and the processing type applied on-board. If the phase switch is off, it means that the packet contains consecutive values of either sky or load samples, and the sampling frequency, fsampling, is 8192 Hz. Denoting with i ≥ 0 the sample index within the packet, for PType 0 and 4 we have that

[math] t^{\rm obt}_{i} = t^{\rm obt}_{0} + i\dfrac{1}{f_{\rm sampling}}, [/math]

where t0obt is the on-board time of the packet (tpktobt) and i = 0 denotes the first sample in the packet. If the switching status is on, either consecutive pairs of (sky, load) samples or (load, sky) samples are stored in the packets. Hence, consecutive pairs of samples have the same time stamp and fsampling = 4096 Hz.

For averaged data (PTypes 1, 2, 3, 5, and 6), the first sample of a scientific packet is the sum (mean) of Naver samples, and the packet time, tpktobt, is the time of the first of the Naver samples. In this case, t0obt is computed as

[math] t^{\rm obt}_{0} = t^{\rm obt}_{\rm pkt} + \dfrac{N_{\rm aver}-1}{2} \dfrac{1}{f_{\rm sampling}} [/math]

and

[math] t^{\rm obt}_{i} = t^{\rm obt}_{0} + i\dfrac{N_{\rm aver}}{f_{\rm sampling}}, \text{for} \ \ i \ge 1. [/math]

Housekeeping telemetry handling

The structure of telecommand and housekeeping telemetry packets of the entire satellite is defined by the Mission Information Base (MIB), a database formed by a set of ASCII tables formatted according to the ESAEuropean Space Agency Mission Control System interface control documents. The MIB information includes the type and structure of the telemetry packets, the location, type, and format of the monitoring parameters within the packets, the calibration curves to convert each parameter raw value into an engineering value, and the out-of-limit values to be checked for each parameter.

The Level 1 software uses the MIB information to group the housekeeping packets according to their type (PUSPacket Utilisation Standard service type, sub-sytem, periodicity). A subset of the housekeeping packets that are relevant for the daily instrument quality verification and the scientific data analysis are further processed; samples of each parameter are extracted, grouped into timelines and saved as TOIs. Each TOI contains, for each parameter sample, the on-board time, the UTCUniversal Time Coordinate(d) time, the parameter raw and engineering value, and flags reporting some quality measures (time quality, or out-of-limit checking result).

Auxiliary data handling

The MOC[ESA's] Mission Operation Center [Darmstadt, Germany] Flight Dynamics team daily provides the Attitude History information as an ASCII file (AHFAttitude History File), automatically delivered through the Planck File Transfer System. For stable pointing periods, the AHFAttitude History File provides quaternions describing, at given on-board times, the orientation of the Planck body reference frame with respect to the Ecliptic inertial reference system, and additional information, such as wobble angles, spin phase angle, and rate. An AHFAttitude History File file also contains different types of records: high frequency data records containing the raw attitude data; spin period frequency records, which are derived from the high frequency records by averaging data over a complete spin period; and observation frequency records, where data are averaged over a complete observation period. The Level 1 software simply reformats all data contained in the AHFAttitude History File, ingesting it into the LFI(Planck) Low Frequency Instrument Level 1 data management system.

The MOC[ESA's] Mission Operation Center [Darmstadt, Germany] is also responsible for computing the correlation between the On-Board Time (OBTOn-Board Time) and the ground Coordinated Universal Time (UTCUniversal Time Coordinate(d)) and providing the UTCUniversal Time Coordinate(d) time of each telemetry packet. Time couples (OBTOn-Board Time, UTCUniversal Time Coordinate(d)), generated by processing the Standard Time Source packets received from the spacecraft, and the Time Correlation Coefficients, computed by a linear fit of the time couples, are also provided by the MOC[ESA's] Mission Operation Center [Darmstadt, Germany] as auxiliary data. The Level 1 software uses the time correlation coefficients to check the UTCUniversal Time Coordinate(d) time provided by the MOC[ESA's] Mission Operation Center [Darmstadt, Germany]. Moreover, the time couples are used to recompute the UTCUniversal Time Coordinate(d) time of each packet with a variation of the MOC[ESA's] Mission Operation Center [Darmstadt, Germany] fitting procedure, in order to reduce the time correlation variance.