About 25,000 total electron content (TEC) values obtained in 2 years from Navy Navigation Satellite System (NNSS) differential Doppler were compared with the corresponding TECs obtained from the Global Positioning System (GPS). The method by which the comparison is effected is also described. The resulting statistics are very effective in showing a good agreement between the two techniques. The GPS TEC results are statistically larger than NNSS TEC, namely, 3¿1 TEC units; the lower quartile of the differences is always greater than zero.¿ 1997 American Geophysical Union