Ytelsen av standard HF-modemer på norske breddegrader
The high latitude HF channel has been measured and characterised in terms of Doppler spread, delay spread and signalto-noise ratio. The performance of data modems has been determined over a comprehensive range of simulated channel conditions. A comparison is made between the channel measurements and the modem characterisations, and modem availabilities during the measured channel conditions are determined. Two 75 bps modems (compliant with STANAG 4415 and ST ANAG 4285), a 2400 bps modem (compliant with ST ANAG 2400 bps), Morse and Voice have been selected for analysis. The two 75 bps modems show 60-75% higher availability than the 2400 bps modem on the measured paths. The 2400 bps modem is degraded in performance during a geomagnetic disturbance whereas the 75 bps modems maintains approximately the same performance leve!. The paper also addresses the number of frequencies required for a HF circuit to achieve maximum communications availability. For the 75 bps modems, one frequency is required except for geomagnetically disturbed periods where four frequencies are needed . For the 2400 bps modem, 3-4 frequencies are needed on a 200 km path and 4-6 frequencies on a 2000 km path.