A method of measuring noise power spectral density in the UHF SATCOM downlink band

It is difficult to make accurate noise power spectral density measurements on digital modulation waveforms in the UHF SATCOM downlink band because of the wide variety of terrestrial noise sources present. Many of these noise sources have unknown distributions, and their presence is a strong function of both geographical location and time. Incorporating the effect of such noise in a link budget is a challenge, but ignoring it can be disastrous. Our approach involves using a cooperative channel modulated with bit error rate tester data, together with a calibrated UHF receiver. From this setup, we develop a time record of effective signal-to-noise ratio (E_b/N_o) at the receiver input. Since downlink carrier power (and therefore E_b) is relatively easy to measure, this leads to a time record of effective N_o which can be analyzed statistically. The resulting distribution may be treated as a representation of system noise temperature or otherwise employed to assess the impact of terrestrial noise on the particular waveform in use. The results of a noise measurement made in this way are presented.