Abstract :
Past observations, indicating that electron density profiles derived from topside ionograms have a tendency to be slightly too low, are reviewed and discussed. This problem is then examined in the light of more recent and better controlled experiments in which vertical electron density profiles were obtained essentially simultaneously by topside and ground-based soundings. These new measurements confirmed past observations and indicated that the error had a tendency to increase with satellite altitude. From a study of the ground echoes, which are often seen on topside ionograms, it is shown that the above discrepancies can be partially attributed to systematic errors (0 to +30 km) in the topside ionogram height markers. Horizontal electron density gradients can also contribute to this discrepancy. The errors found in the N(h) profile (although systematic) are usually too small to detract significantly from the general usefulness of topside ionograms. Also, the portion of the error due to incorrect height markers can often he calculated from a ground trace analysis and the ionogram data can be corrected accordingly.
