Spectrum quietness metrics for radio astronomy

We review metrics to assess the radio quietness of sites used for radio astronomy. Concise metrics are needed to compare candidate sites for new telescopes, to monitor the quality of existing sites, and to design telescopes to work well at a given site. Key points of assessment are the receiver dynamic range required for the strongest interferers and the expected fraction of spectrum available for sensitive astronomical measurements. We propose three metrics: (1) total radio frequency interference power, (2) interference-to-noise power ratio and (3) time-frequency occupancy. Box plots of these metrics summarise large quantities of information, highlight expected ranges of interfering signal properties, and aid comparisons of sites and other factors of interest. We provide examples for Square Kilometre Array phase one deployment in Australia based on measurements made for the selection of this site. The Square Kilometre Array will be the largest radio telescope in the world.