Preliminary study of the EChO data sampling and processing

Abstract

The EChO Payload is an integrated spectrometer with six different channels covering the spectral range from the visible up to the thermal infrared. A common Instrument Control Unit (ICU) implements all the instrument control and health monitoring functionalities as well as all the onboard science data processing. To implement an efficient design of the ICU on board software, separate analysis of the unit requirements are needed for the commanding and housekeeping collection as well as for the data acquisition, sampling and compression. In this work we present the results of the analysis carried out to optimize the EChO data acquisition and processing chain.The HgCdTe detectors used for EChO mission allow for non-destructive readout modes, such that the charge may be read without removing it after reading out. These modes can reduce the equivalent readout noise and the gain in signal to noise ratio can be computed using well known relations based on fundamental principles. In particular, we considered a multiaccumulation approach based on non-destructive reading of detector samples taken at equal time intervals. All detectors are periodically reset after a certain number of samples have been acquired and the length of the reset interval, as well as the number of samples and the sampling rate can be adapted to the brightness of the considered source.The estimation of the best set of parameters for the signal to noise ratio optimization and of the best sampling technique has been done by taking into account also the needs of mitigating the expected radiation effects on the acquired data. Cosmic rays can indeed be one of the major sources of data loss for a space observatory, and the studies made for the JWST mission allowed us to evaluate the actual need of the implementation of a dedicated deglitching procedure on board EChO.

The instrument control unit of the EChO space mission: electrical architecture and processing requirements

“…The basic data processing assumption is to sample up-the-ramp pixels in a non-destructive readout manner with a sampling rate up to 8 Hz (Tab. 2) [13]. This rate can be determined on-ground by simulations activities and in flight by dedicated calibration procedures.…”

“…Deglitching pixels arrays from cosmic rays hits is commonly an operation to be performed at pixel-level [13,14].…”

“…Once assessed the post-processing overall system noise level (astrophysical noise plus noise floor) it is possible to define the overall integration time to get a targeted S/N ratio aver a scientific observation, the ramps lengths (within the adopted sampling up-the-ramp or multi-accumulation method) and the number of the samples for a single ramp [13]. The overall data flow (in bit/s) produced by all the scientific modules depends on the sampling rate, the number of involved pixels and their digital representation.…”

The visible and near infrared module of EChO

“…It has been presented for the first time in its complete form by [9,10] and is reported herein below:…”

“…The result has then been compared with the system requirements for the two different detector arrays under study for the EChO mission, see [9]. The aim of the work has been to provide indications on how to optimize the EChO focal plane arrays sampling rate and data processing procedures in order to Fig.…”