To test rigorously the efficiency of different compressors the best solution is to generate a realistically simulated signal for different mission hypotheses and apply to them the given compressors. To be realistical the simulation of the signal generation should contain both astrophysical and instrumental effects. It would be helpful that the final simulation would be able to given a hint about the influence of the various signal components and their variance. Of course it is useless to reproduce in full detail the LFI to obtain a signal simulation accurate enough to test compressors. A simplified model of the LFI, its front-end electronics and its operations will be enough.
At the base of the simplified model is the concept of acquisition pipeline. This pipeline is composed by all the
modules which process the astrophysical signal: from its
collection to the production of the final data streams which are
compressed and then sent to Earth. In the real LFI, the
equivalent of the acquisition pipeline may be obtained following
the flow of the astrophysical information,
from the telescope through the front-end electronics
and the main Signal Processing Unit (SPU) to the memory of the
Data Processing Unit (DPU) which is in charge to downlink it to
the computer of the spacecraft and then to Earth. The acquisition
pipeline is represented in Fig. 1.
The simulated microwave signal from the sky is collected and compared with the temperature of a reference load which, in our simulations, is supposed to have exactly the CMB temperature T0=2.725 K (Mather et al. 1999). The difference expressed in is sampled along a scan circle producing a data stream of 60 scan circles with 8640 samples (pointings).
Signal detection is simulated by
(Bersanelli et al. 1996; Maris et al. 1998; Maris et al. 1999)
The analog to digital conversion (ADC) is described by the formula:
The simplified model of the Low Frequency Instrument is composed of four acquisition pipelines, one for each frequency, each one being representative of the set of devices which form the full detection channel for the given frequency. The overall data-rate after loss-less compression for LFI should be obtained summing the contribution expected from each detector. Since in the real device each radiometer for a given frequency channel, will be characterized by different values of and , the distribution of these parameters has to be taken in account computing the overall compression efficiency. In particular a greater attention should be devoted to the distribution of the parameter since the compression efficiency is particularly sensitive to it. However, since the distribution of operating conditions and instrumental parameters are not yet fully defined, we assumed that all the detectors belonging to a given frequency channel are identical and located at the telescope focus.
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