Measurements of the pre-deployment voltage spectral density (in dB) of TNR (crosses) and HFR (dots) to evaluate the receivers system noise for the three dipole sensors V1-V2 (blue), V2-V3 (red) and V3-V1 (orange). Thick black dashed lines correspond to the lower envelope of the background spectral density representing the reference receiver pre-deployment background. Thin black dashed lines correspond to the background noise measured during the on-ground tests in Meudon and including the effects of the preamplifiers. The red dotted line indicates the HFR system noise obtained after the fitting procedure described in Sect. A.2.