Difference between revisions of "Pre-processing LFI"
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Revision as of 09:37, 18 October 2012
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 , signals at the frequency of the phase switch. By changing the phase switches configuration, the output can be a sequence of either or 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, , then it amplifies the signal with a tunable gain, , 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 . Both the , the and the blanking time are parameters set through the LFI on-board software. The equation applied to transform a given input signal into an output is:
with , is one of 255 possible offset steps from +0 up to +2.5 V and where is a small offset introduced by the DAE when applying a selected gain. The values of and 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 uniformly in the range , so that the quantization step is mV. The quantization formula is
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).
- PType 0
- in this mode the REBA just packs the raw data of the selected channel without any processing.
- PType 1
- consecutive or samples samples are coadded and stored as unsigned integers of 32 bits. The number of consecutive samples to be coadded is specified by the parameter.
- PType 2
- in this mode, two main processing steps are applied. First, pairs of averaged and samples, respectively and , are mixed by applying two different gain modulation factors, GMF1 and GMF2:
- The operations are performed as floating point operations. Then the two values obtained are requantized, converting them into two 16-bit signed integers:
- PType 3
- with respect to PType 2, in this mode only a single gain modulation factor is used, GMF1, obtaining:
and analogously to PType 2, the value is requantized obtaining 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 string up to the complete filling of the data segment for the packet in process.
(Planck) Low Frequency Instrument
Data Processing Center
(Planck) High Frequency Instrument
Command and Data Management Unit
[ESA's] Mission Operation Center [Darmstadt, Germany]
Attitude History File
Daily Quality Report
analog to digital converter
LFI Data Acquisition Electronics
LFI Radiometer Electronics Box Assembly