Figure 3: (a) SNR in ground-based observation of the SED of an AME cloud. The calculation includes the contributions of typical atmospheric and system noise and a typical spectrum of a warm ionized medium region. The dips in the SNR are due to atmospheric water (around 22 GHz) and oxygen (near 60 GHz). High SNR can be achieved with a long integration time and a large (50 m class) telescope. (b) Error in the estimate of the spectral index of the AME SED above, assuming that the same receiver is used at two frequencies 10% apart. This measurement is important if one wants to discriminate between AME and other contaminants (synchrotron or free-free or thermal dust). (c) Merit function computed as the ratio between the SNR and the error in the determination of the spectral index, normalized to the maximum. This indicates which is the optimal frequency to measure both the brightness of the AME and its spectral index.