We present the first joint comparison of global measurements of F region characteristics with three models used widely in the specification of the ionospheric-thermospheric system. The models, the International Reference Ionosphere (IRI), the field line interhemispheric plasma (FLIP) model, and the Thermospheric-Ionospheric General Circulation Model (TIGCM), represent a unique set of capabilities with major differences in approaches to the prevailing physics and different levels of computational complexity. The database was developed by a global network of 53 ionosonde stations operating around-the-clock for the period March 22 through April 4, 1992 in collaboration with the ATLAS 1 mission. The emphasis is on the F region characteristics of peak heights (hmF2) and densities (NmF2), their climatological (i.e., average) behavior during the ATLAS 1 period, and associated cause-effect relationships. We explore latitudinal and local time variations with attention to the influences of meridional winds and plasmaspheric fluxes in the maintenance of different domains in the ionospheric-thermospheric system. We find that all three models tend to underestimate the values of hmF2 and NmF2 with the largest discrepancies in NmF2 resulting in the FLIP and TIGCM representations at night. These discrepancies can grow to levels as large as 110% near 0400 LT, a ''rediscovery'' of the old but unsettled issue of maintenance of the nighttime ionosphere. This nighttime discrepancy is traceable in first order to model underestimates of prevailing meridional winds. The contributions of plasmaspheric fluxes are also considered, with the conclusion that they are of secondary importance, but substantially more work is necessary to uniquely quantify their role. In contrast to their nighttime characteristics, the FLIP and TIGCM generally have excellent agreement (i.e., 6¿6%) with daytime observations of NmF2, and the IRI tends to underestimate the observed values of NmF2 by a nominally LT insensitive level of 28¿6%. Other campaign results are reviewed in this issue, with a focus on regional responses to the prevailing conditions and their characterization in terms of latitudinal distributions of F region heights and densities. ¿ American Geophysical Union 1996