Trans-ionospheric radio propagation effects resulting in ranging errors are examined for a potential orbital network of communications and navigational satellites at Mars. Using recent results from the radio science experiment on board the Mars Global Surveyor (MGS) spacecraft and a photochemical model of Mars' ionosphere, we study the total electron content (TEC) at Mars to investigate how its latitude, local time, and solar cycle patterns would contribute to errors in positioning on the planet. In addition, we examine the relationship between TEC and peak density (Nmax) and find that their ratio, called the equivalent slab thickness, shows that integral preserving distortions of the Ne(h) profile can be rather substantial, implying that neutral atmosphere dynamics can have strong effects upon Mars' photochemical ionosphere. We use MGS observations to validate modeling results and determine the extreme cases for TEC at Mars (i.e., when the planet is at perihelion during solar maximum years and at aphelion during solar minimum years). If a proposed Mars Communication and Navigation (MC&N) System used UHF/L-band (1--2 GHz) transmission frequencies similar to those used for the terrestrial Global Positioning System (GPS) satellites, upper limits to the magnitude and variability of the martian ionosphere (TEC < ~few ¿ 1016 el m-2, with σ ~ 10%) would not be of concern unless extremely precise positional information were required (<1 m). The impact of the ionosphere would be greater along slanted ray paths, and especially if lower frequency UHF beacon frequencies (e.g., 400 MHz) were selected for use. Indeed, such effects could be used as a diagnostic for the global structure of Mars' ionosphere, much in the same way as GPS measurements are used in terrestrial ionospheric physics.