Abstract. Measurements of Doppler spread, multipath spread, and signal-to-noise ratio have been made on four high-latitude high-frequency (HF) communications paths. The measurement system and analysis techniques are outlined, and an analysis of the data pertinent to the design of robust HF data modems is presented. A summary of the spreads that are exceeded for 5% of time is presented for each path. Doppler spreads range from 2 to 55 Hz, while multipath spreads range from 1 to 11 ms. Physical interpretations of the data are made, and the data are related to the measured performance characteristics of an HF data modem to estimate the modem availability on the paths considered. When there is mode support, availabilities range from 64% to 100% for a signal-to-noise ratio of 0 dB, although the data indicate that the availabilities can generally be increased by optimizing frequency selection.

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Abstract. The high-latitude HF channel has been measured and characterized in terms ofDoppler spread• delay spread, and signal-to-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 characterizations. Thereby the percentage of time that the modem works satisfactorily (availability), during the measured channel conditions, is determined. Two 75 bits/s modems (compliant with STANAG 4415 and STANAG 4285), a 2400 bits/s modem (compliant with STANAG 4285), Morse code, and voice have been selected for analysis. The two 75 bits/s modems show 60-75% higher availability than the 2400 bits/s modem on the measured paths. The 2400 bits/s modem is degraded in performance during a geomagnetic disturbance whereas the 75 bits/s modems maintain approximately the same performance level. The paper also addresses the necessary frequency pool (at a specific time) required for an HF circuit to achieve the best possible communications availability. For the 75 bits/s modems, independent of path length, a one-frequency pool is all that is required during summer and winter when the ionospheric conditions are quiet. A four frequency pool is necessary during disturbed ionospheric conditions. For the 2400 bits/s waveform, three to four frequencies are needed on the 200 km west-east path, and four to six frequencies are required on the north-south 2000 km path. This applies to all seasons and geomagnetic conditions analyzed.

This paper describes observations of the reliability of the ionospheric HF communication channel within and close to the northern auroral zone. Automatic equipment has been developed to measure reliability and virtual reflection height as a function of frequency in an hourly cycle. Measurements have been made over two propagation paths within Norway. The data show diurnal and seasonal variations as well as dependence of the measured parameters on geomagnetic conditions. The observations have been compared with predictions obtained from the Ionospheric Communication Enhanced Profile Analysis and Circuit Prediction Program (ICEPAC) prediction code. This code includes a model of the high‐latitude and polar ionospheres and their dependence upon geomagnetic disturbances. The results of the comparison show that ICEPAC represents an improvement over previous codes but that transmission losses are not properly included.

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