Geostationary satellites with radio beacons suitable for ionospheric investigations are becoming scarce. In this situation it is important to plan for other possibilities. Differential Doppler data from the polar-orbiting Navy Navigation Satellite System (NNSS) have been used successfully from a network of receiving stations in Europe. It would be worthwhile to use differential Doppler on a larger basis. Modern receiving equipment for geodetic purposes based on NNSS signals should be considered as an important source of data for ionospheric research. We try to demonstrate the importance and usefulness of this source, stressing specifically investigations of the major geomagnetic storm which had its sudden commencement on April 11, 1981. During this event we could gain data from four additional “geodetic” stations, which improved considerably the resolution in longitude. Unusual longitudinal differences in storm response of the ionospheric electron content were detected. This effect would have remained unnoticed without the additional receiving stations.
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