Validation: second AGC removal step. The PDF after AGC removal step 2 versus respectively the PDF of the field-strengths of the raw monitoring data (with AGC) and the PDF of the actual monitoring data (i.e., without AGC). In this step the wide-band artifacts can be removed due to the pre-work of the NMF operation. After the application of the second AGC removal step the PDF contains primarily thermal noise. Due to NMF the thermal noise distribution shows lower variance, however the probability mass herein remained the same (see Table 3).