Data collection for the first ground-based ionospheric tomography campaign in North America was conducted over a 48-hour period in mid-November 1991. The data consist of records of ionospheric total electron content (TEC) from a number of passes of the U.S. Navy Navigation Satellite System spacecraft over a chain of ground-based receiving stations. Data collection and reduction techniques are discussed; these include the determination of absolute TEC from the different phase advances induced by the ionosphere in each component of the dual-frequency spacecraft signal. The use of tomographic methods to reconstruct ionospheric electron densities over a two-dimensional (2-D) region of the Earth's ionosphere at a number of different times is demonstrated. Specifically, two distinct tomographic methods, the algebraic reconstruction technique and a maximum entropy method, are used to mathematically invert the records of TEC. The resulting 2-D contour ''maps'' of ionospheric electron density are then compared to similar maps produced by the Millstone Hill incoherent backscatter radar facility located at Westford, Massachusetts. Both qualitative and quantitative measures of agreement among the different reconstructions and the radar maps are presented. The behavior of the ionosphere over the course of the experiment is discussed. ¿ American Geophysical Union 1995