AbstractMars Global Reference Atmospheric Model (Mars-GRAM 2001) is an engineering-level Mars atmosphere model widely used for many Mars mission applications. From 0–80 km, it is based on NASA Ames Mars General Circulation Model (MGCM), while above 80 km it is based on University of Arizona Mars Thermospheric General Circulation Model. Mars-GRAM 2001 and MGCM use surface topography from Mars Global Surveyor Mars Orbiter Laser Altimeter (MOLA). Validation studies are described comparing Mars-GRAM with Mars Global Surveyor Radio Science (RS) and Thermal Emission Spectrometer (TES) data. RS data from 2480 profiles were used, covering latitudes 75°S–72°N, surface to ∼40 km, for seasons ranging from areocentric longitude of Sun (Ls)=70–160° and 265–310°. RS data spanned a range of local times, mostly 0–9 and 18–24 h. For interests in aerocapture and precision landing, comparisons concentrated on atmospheric density. At a fixed height of 20 km, measured RS density varied by about a factor of 2.5 over the range of latitudes and Ls values observed. Evaluated at matching positions and times, average RS/Mars-GRAM density ratios were generally 1±0.05, except at heights above ∼25 km and latitudes above ∼50°N. Average standard deviation (SD) of RS/Mars-GRAM density ratio was 6%. TES data were used covering surface to ∼40 km, over more than a full Mars year (February, 1999 through June, 2001, just before start of a Mars global dust storm). Depending on season, TES data covered latitudes 85°S–85°N. Most TES data were concentrated near local times 2 and 14 h. Observed average TES/Mars-GRAM density ratios were generally 1±0.05, except at high altitudes (15–30 km, depending on season) and high latitudes (>45°N), or at most altitudes in the southern hemisphere at Ls∼90° and 180°. Compared to TES averages for a given latitude and season, TES data had average density SD about the mean of ∼2.5% for all data, or ∼1–4%, depending on time of day and dust optical depth. Average SD of TES/Mars-GRAM density ratio was 8.9% for local time 2 h and 7.1% for local time 14 h. Thus SD of observed TES/Mars-GRAM density ratio, evaluated at matching positions and times, is about three times the SD of TES data about the TES mean value at a given position and season.