Knowing the flux (signal strength) of these sources are particular frequencies, and knowing the distribution of radio noise from our own galaxy, is crucial for our measurements.
Two recent works that assist with this are 'A model of diffuse Galactic Radio Emission from 10 MHz to 100 GHz' and 'A broadband flux scale for low frequency radio telescopes'.
For the first, de Oliveira-Costa et al. have looked at the diffuse emission from not just discrete sources, but also our own Galaxy. This allows a complete sky model to be produced, which aids our calibration.
For the second, Scaife & Heald pick six well studied low-frequency radio sources and collate years of studies that have included them. From this, flux curves can be produced that provide a useful spectral comparison.
The references for these papers are:
- http://arxiv.org/abs/0802.1525v3 -- de Oliveira-Costa et al. 2008
- http://arxiv.org/abs/1203.0977 -- Scaife & Heald, 2012
De Oliveira-Costa also has a great webpage which displays this data. The link is: https://www.cfa.harvard.edu/~adeolive/gsm/