Spaceborne atmospheric chemistry sensors provide unique access to the distribution and variation of the concentration of many trace species on the global scale. However, since the measurements and the retrieval algorithms are sensitive to a variety of instrumental as well as atmospheric sources of error, they need to be validated carefully by correlative measurements. The quality control and validation of satellite measurements on the global scale, as well as in the long term, is one of the goals of the Network for the Detection of Stratospheric Change (NDSC). Started in 1991, at the present time the NDSC includes five primary and two dozen complementary stations distributed from the Arctic to the Antarctic, comprising a variety of instruments such as UV-visible spectrometers, Fourier transform infrared spectrometers, lidars, and millimeter-wave radiometers. After an overview of the main sources of uncertainty which could perturb the measurements from space, and of the ground-based data provided by the NDSC for their validation, this paper will focus, as an example, on the measurement of total ozone by Earth Probe Total Ozone Mapping Spectrometers (TOMS), ADEOS TOMS and ERS-2 Global Ozone Monitoring Experiment (GOME) and their validations. The data recorded between summer 1996 and April 1997 by 16 Système d'Analyse par observations zénithales (SAOZ)/UV-visible spectrometers distributed over a range of latitudes from the Arctic to the Antarctic, and by Dobson and Brewer spectrophotometers operating at selected sites of the NDSC alpine and Antarctic stations, are used to investigate the solar zenith angle (SZA) dependence, the dispersion, the time-dependent drift, and the possible differences of sensitivity of the space-based sensors. Although the comparison demonstrates an excellent agreement to within ±2%-4% between all space- and ground-based instruments at northern middle latitudes, it also reveals significant systematic features, such as a SZA dependence with TOMS beyond 80°, a seasonal SZA dependence with GOME beyond 70°, a systematic bias of a few percent between satellite and SAOZ observations of low ozone columns in the southern Tropics, a difference in sensitivity to ozone between the GOME and ground-based sensors at high latitudes, and an interhemispheric difference of TOMS with the ground-based observations.