When a spectrum manager is making a frequency assignment it may be necessary to estimate the minimum transmitter power level that is consistent with the achievement of specified link performance objectives. It is seldom feasible to use direct measure ments for this purpose. Similarly, the impact on the reception of a wanted signal of interference from known sources may have to be estimated. For point-to-area services such as broadcasting or land mobile, the power level required to set up a specified signal strength anywhere within a specified service area may have to be estimated. Accurate estimates enable the best use to be made of the spectrum. Accurate estimates require knowledge of the local topography and of various char acteristics of the radio stations involved, including the gain, polarisation characteris tics and effective height of the antennas, the modulation parameters of the emissions involved, the bandwidth of the receivers and the power of interfering transmitters. In the absence of such knowledge it will be necessary to make assumptions about them, and standards recommended by ITU-R are often used for this purpose. But above all it is necessary to be able to estimate the transmission loss arising from propa gation between the transmitting antennas and the receiving antennas. It may also be necessary to estimate radio noise levels.