Pre-processing LFI

Overview

The first processing level of the LFI DPC 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 instrument, the Sorption cooler, the HFI instrument and the Central Data Management Unit (CDMU).
• the LFI raw scientific telemetry
• Additional auxiliary data provided by the MOC and the Flight dynamics:
  • The Attitude History File (AHF)
  • Time correlation data (time correlation coefficients and time couples)
  • 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 DQR production and the estimation of the LFI instrument systematic effects.

The LFI scientific telemetry

Each LFI 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 (ADC) in the Data Acquisition Electronics unit (DAE). The DAE 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 ADC, 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 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 DAE 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 DAE Gain Index (DGI) and the DAE Offset Index (DOI) associated to the desired values.

The ADC 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 (REBA) which runs the LFI on-board software. For each LFI detector, the REBA processes the data in the form of time series which are split into telemetry packets. To satisfy the LFI assigned telemetry budget limit of 53.5 Kbps, the REBA 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. 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 REBA 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 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 REBA 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 REBA processing parameters — [math]N_{aver}[/math], GMF1, GMF2, q and Offset — is selected for each of the 44 LFI channels.They are also included in a teartiary header of each scientific telemetry packet sent to ground. The REBA 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 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 REBA follow the ESA Packet Telemetry Standard and Packet Telecommand Standard, the CCSDS Packet Telemetry recommendations and the ESA Packet Utilization Standard (PUS). The packet structure for an LFI scientific telemetry packet is shown in the following figure.

LFI scientific telemetry packet structure. The Packet Header, Data Field Header and Packet Error Control are specified in the PUS 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 REBA processing parameters used. The subsequent structure depends on the phase switches configuration and REBA 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 Level 1 software pipeline retrieves the housekeeping and scientific telemetry packets dumped from the satellite on-board memory through the MOC Data Disposition System (DDS). 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 REBA, are not in physical units but in ADU (Analog to Digital Unit). Conversion of [math]\overline{S}_{sky}[/math] and [math]\overline{S}_{load}[/math] in Volt requires the data source address, 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. These values are recovered from the packet tertiary header. Hence, the value in Volt is obtained as: