Pre-processing

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Overview

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

  • raw housekeeping telemetry packets retrieved from different satellite subsystems: 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:
    • The Attitude History File (AHFAttitude History File)
    • Time correlation data (time correlation coefficients and time couples)
    • Command History data
    • The Sorption cooler out of limit data

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

Scientific telemetry

Each LFI(Planck) Low Frequency Instrument radiometer provides two analog outputs, one for each amplifier chain. In a nominal configuration, each output yields a sequence of alternating [math]V_{sky}[/math], [math]V_{load}[/math] signals at the frequency of the phase switch. By changing the phase switches configuration, the output can be a sequence of either [math]V_{sky}[/math] or [math]V_{load}[/math] signals.

The conversion from analog to digital form of each radiometer output is performed by a 14 bits Analog-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 [-2.5 V, +2.5 V]: first it applies a tunable offset, [math]O_{DAE}[/math], then it amplifies the signal with a tunable gain, [math]G_{DAE}[/math], 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 raise 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 7.5 [math]\mu s[/math]. Both the [math] O_{DAE}[/math], the [math]G_{DAE}[/math] 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 [math]V_{in}[/math] into an output [math]V_{out}[/math] is:


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


with [math]{G_{DAE} = 1, 2, 3, 4, 6, 8, 12, 16, 24, 48}[/math], [math]O_{DAE}[/math] is one of 255 possible offset steps from +0 up to +2.5 V and where [math]Z_{DAE}[/math] is a small offset introduced by the DAELFI Data Acquisition Electronics when applying a selected gain. The values of [math]G_{DAE}[/math] and [math]O_{DAE}[/math] 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 to the desired values.

The ADCanalog to digital converter quantizes the [math]V_{out}[/math] uniformly in the range [math]-2.5 V \le V_{adc} \le +2.5 V[/math], so that the quantization step is [math]q_{ADC}=5.0/2^{14} = 0.30518[/math] mV. The quantization formula is


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


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

The digitized scientific data is 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 7 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 [math]S_{sky}[/math] or [math]S_{load}[/math] samples are coadded and stored as unsigned integers of 32 bits. The number of consecutive samples to be coadded is specified by the [math]N_{aver}[/math] parameter.
PType 2
in this mode, two main processing steps are applied. First, pairs of averaged [math]S_{sky}[/math] and [math]S_{load}[/math] samples, respectively [math]\overline{S}_{sky}[/math] and [math]\overline{S}_{load}[/math], are mixed by applying two different gain modulation factors, GMF1 and GMF2:
[math] \begin{eqnarray} P_1 & = & \overline{S}_{sky} - \text{GMF1} \cdot \overline{S}_{load} \\ P_2 & = & \overline{S}_{sky} - \text{GMF2} \cdot \overline{S}_{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 \cdot \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}_{sky} - \text{GMF1} \cdot \overline{S}_{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 loss-less 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 — [math]N_{aver}[/math], 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 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 switches status and the REBALFI Radiometer Electronics Box Assembly processing parameters used. The subsequent structure depends on the phase switches configuration and REBALFI Radiometer Electronics Box Assembly processing type applied. This example shows a packet with PType 0 data and with the nominal phase switches configuration.

From packets to raw TOIs

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 most accurately the values of the original (averaged) sky and load samples acquired on-board. Data acquired with PTypes 4, 5 and 6 is first uncompressed. The loss-less 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 (Analog to Digital Unit). Conversion of [math]S_{sky}[/math] and [math]S_{load}[/math] in Volt requires the Data Source Address (DSA), i.e. the radiometer and detector from which the data are generated, the blanking time (indicized by the Blancking Time Index, BTI), the DGI and the DOI. 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 Volt is obtained as:


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


where [math]\tilde{G}_{DAE}(\text{DSA}, \text{BTI}, \text{DGI})[/math], [math]\tilde{O}_{DAE}(\text{DSA}, \text{BTI}, \text{DOI})[/math] and [math]\tilde{Z}_{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}_{DAE}(\text{DSA}, \text{BTI}, \text{DGI}) \approx \dfrac{G_{DAE}}{q_{ADC}} [/math],
  • [math]\tilde{O}_{DAE}(\text{DSA}, \text{BTI}, \text{DOI}) \approx O_{DAE}[/math],
  • [math]\tilde{Z}_{DAE}(\text{DSA}, \text{BTI}, \text{DGI}) \approx \dfrac{Z_{DAE} + 2.5}{q_{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 is just coadded on-board, the division by [math]N_{aver}[/math] 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}_{sky}[/math] and [math]\overline{S}_{load}[/math]:

[math] \begin{eqnarray} \overline{S}_{sky} & = & \dfrac{P_2 \cdot \text{GMF1} - P_1 \cdot \text{GMF2}}{\text{GMF1} - \text{GMF2}} \\ \overline{S}_{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, [math]f_{sampling}[/math], is 8192 Hz. Denoting with [math]i \ge 0[/math] the sample index within the packet, for PType 0 and 4 we have that:


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


where [math]t^{obt}_{0}[/math] is the on-board time of the packet ([math]t^{obt}_{pkt}[/math]) and [math]i = 0[/math] 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 couples of samples have the same time stamp and [math]f_{sampling}[/math] = 4096 Hz.

For averaged data (PTypes 1, 2, 3, 5, 6), the first sample of a scientific packet is the sum (mean) of [math]N_{aver}[/math] samples, and the packet time, [math]t^{obt}_{pkt}[/math], is the time of the first of the [math]N_{aver}[/math] samples. In this case, [math]t^{obt}_{0}[/math] is computed as:


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

and

[math] t^{obt}_{i} = t^{obt}_{0} + i\dfrac{N_{aver}}{f_{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, the out of limit values to be checked for each parameter.

The Level1 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, flags reporting some quality measures (time quality, 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 contains also different type of records: high frequency data records containing the raw attitude data; spin period frequency records, that are derived from the high frequency records by averaging data over a complete spin period; observation frequency records, where data is averaged over a complete observation period. The Level1 software simply reformat all data contained in the AHFAttitude History File, ingesting it in the LFI(Planck) Low Frequency Instrument Level1 data management system.

The MOC[ESA's] Mission Operation Center [Darmstadt, Germany] is also responsible of 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 spacecraft, and the Time Correlation Coefficients, computed by a linear fit of the time couples, are also provided by MOC[ESA's] Mission Operation Center [Darmstadt, Germany] as auxiliary data. The Level1 software uses the time correlation coefficients to check the UTCUniversal Time Coordinate(d) time provided by 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.