Analysis of the radiometer-reference load system on board the Planck/LFI instrument

The Low Frequency Instrument aboard the Planck satellite will employ pseudo-correlation radiometers, operating over three broad bands centered at 30, 44, 70GHz. This choice is oriented to maximize the instrumental stability reducing non-white noise effects produced inside the radiometers. The radiometer scheme allows the simultaneous comparison of two input signals, one coming from the telescope and the other coming from a reference blackbody (the "reference load" RL) at a stable temperature near 4 K. In order to minimize non-white noise, typically exhibiting a I If spectrum, the Reference Load temperature must be as close as possible to the sky temperature (about 2.7 K). The accuracy of the measurement is tightly related to the ability of keeping under control and minimizing all systematic effects potentially. perturbing the RL signal, particularly those not easily removable in software during data analysis. A model of the,radiometer and RL system interface, based on the finite elements method, has been developed, together with a detailed thermal modeling, to characterize the physical system and to optimize its performance. The function of the radiometric chain, the modeling and the changes made in the baseline configuration to improve the system performances are described. The impact of systematic effects on the optimized configuration are also quantitatively investigated. (C) 2003 Elsevier B.V. All rights reserved.