Harmful Interference can be caused in the case of Frequency Sharing, by Out-of-band Emission and by Spurious Emission.
Concering Interference and Harmful Interference the ITU-R Radio Regulations use the following definitions:
Interference: The effect of unwanted energy due to one or a combination of emissions, radiations, or inductions upon reception in a radiocommunication system, or loss of information which could be extracted in the absence of such unwanted energy.
Harmful Interference: Interference which endangers the functioning of a Radionavigation Service or of other safety services or seriously degrades, obstructs, or repeatedly interrupts a radiocommunication service operating in accordance with the ITU-R Radio Regulations.
Harmful Interference to the Radio Astronomy Service:
Interference levels are considered to be harmful to the Radio Astronomy Service when the rms fluctuations of the system noise increase at the receiver output by 10% due to the presence of interference.
ITU-R RA.769-1 gives the levels of harmful interference for the Radio Astronomy Service based on this criterion and the adopted integration time of 2000 seconds. It should be noted that in practice it depends completely on the type of telescope and the operating parameters used whether interference is considered harmful. For example: the conditions are different for single dish observations, radio interferometry and Very Long Baseline Interferometry (VLBI); or whether observations are done of spectral lines or in a broadband continuum mode. Usualy each instruments has a number of different operating parameters. The values given in ITU-R RA.769-1 general “averages” and give an indication of the levels at which an interfering signal degrades the observations significantly.
Percentage of time lost by the Radio Astronomy Service due to harmful interference:
In cases where the strength of an interfering signal varies as a result of time-varying propagation conditions, for example in the case of propagation by tropospheric scatter, the usual practice in interference calculations for radio astronomy is to consider the level for which the propagation loss is exceeded for 90% of time. Thus the harmful threshold would be exceeded for 10% of time, and in removing contaminated data 10% of the observations would be lost. Radio astronomers generally agree that this is the maximum tolerable loss. In general this figure applies to interference which intermittently exceeds the harmful threshold for time periods such that no more than 10% of the data is contaminated. In terms of the time averaging no more than 10% of the initially-averaged data (for example of tens of milliseconds to tens of seconds) should be lost when contaminated data is rejected. Thus, for example, interference from a radar signal, the mean power of which exceeds the harmful threshold, would not be tolerable even if the duty cycle of the transmitter is less than 10% (ITU-R Handbook on Radio Astronomy).