Effect of skywave interference on coverage of radiobeacon DGPS stations

Coastal radiobeacon stations, which have long served marine direction-finding receivers, are increasingly being employed to transmit differential satellite navigation system (DSNS) correction messages to marine users. The range of these 300 kHz data transmissions extends beyond the nominal ranges of the radiobeacons into regions where skywave effects become significant, especially at night. The paper demonstrates by statistical analysis and experimental measurements that skywave-propagated components interact with the wanted groundwaves, causing deep signal fading that can lead to loss of data. Also, in Europe, skywave signals from other radiobeacons on the same, or adjacent, channels cause severe interference. A coverage and performance prediction model that takes these effects into account, together with atmospheric noise and attenuation due to ground losses, is used to illustrate the reduction of usable range caused by skywave propagation. The paper argues that it is essential to allow for skywave factors when predicting the operating areas of beacons and when allocating frequencies within the radiobeacon DSNS service