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The 1999 version of the NASA/Marshall Space Flight Center Global Reference Atmospheric Model (GRAM-99) and version 3.8 of the Mars Global Reference Atmospheric Model (Mars- GRAM) are the latest in two series of computer programs for calculating selected physical properties of the atmospheres of Earth and Mars, respectively. GRAM-99, like prior versions of GRAM, implements an amalgamation of empirical models that represent geographical, seasonal, and monthly variations of the state of the terrestrial atmosphere (thermodynamic variables and horizontal and vertical wind components) at all altitudes from the ground up to those of spacecraft orbits. Mars-GRAM provides engineering estimates of density, temperature, pressure, and wind components in the Martian atmosphere as functions of latitude, longitude, altitude, and time.

The 1995 version of GRAM (GRAM- 95) and the underlying model were described in “NASA/MSFC Global Reference Atmospheric Model — 1995 Version” (MFS-31105) NASA Tech Briefs, Vol. 21, No. 3 (March 1997), page 68. For altitudes from 0 to 27 km, GRAM-99 utilizes either of two sets of data: the binary- formatted Global Upper Air Climatic Atlas (GUACA) [which was also used in GRAM-95] or the newer ASCIIformatted Global Gridded Upper Air Statistics (GGUAS). For altitudes from 20 to 120 km, GRAM-99 uses, as did GRAM-95, a specially developed set of data based on Middle Atmosphere Program (MAP) data. Above 90 km, GRAM-99 uses the 1999 version of the Marshall Engineering Thermosphere (MET) model [also known as the Jacchia model]. Fairing techniques assure smooth transitions among the models and sets of data in the overlap height ranges of 20 to 27 km and 90 to 120 km.

Like GRAM-95, GRAM-99 estimates concentrations of water vapor and of O3, N2O, CO, CH4, CO2, N2, O2, O, A, He, and H. For altitudes above 90 km, the Jacchia model can also provide concentrations of N2, O2, O, A, He, and H. Atmospheric-constituent profiles of the Air Force Geophysics Laboratory are also used extensively for altitudes up to 120 km.

GRAM-95 incorporated a then new variable-scale perturbation model that provides both large-scale (wave) and small-scale (stochastic) deviations from mean values for thermodynamic variables and horizontal and vertical wind components. Such perturbations are characterized by time scales of less than a month and are associated with turbulence, storms, atmospheric tides, and other phenomena. GRAM-99 incorporates improvements in the small-scale perturbation model for representing intermittent phenomena.

A major new feature in GRAM-99 is an option to substitute Range Reference Atmosphere (RRA) data for conventional GRAM climatological data when a trajectory passes sufficiently near any RRA site. This feature makes it possible, for example, to simulate the atmosphere, starting from takeoff at one RRA site (e.g., Edwards Air Force Base), where RRA data are used, then make a smooth transition to an atmosphere characterized by GRAM climatology en route, then make another smooth transition to an atmosphere characterized by RRA data at a landing site (e.g., White Sands, New Mexico).

A complete user’s guide for running GRAM-99 is available. The user’s guide includes sample input and output. Also included is an example that shows how to incorporate GRAM-99 as a subroutine in another program (e.g., a trajectory code).

Mars-GRAM implements a mathematical model based on surface and atmospheric measurements taken during the Mariner and Viking (orbiter and lander) missions. At altitudes above about 120 km, the Mars-GRAM model is based on the Stewart (1987) Mars thermospheric model. Mars-GRAM can be used as a stand-alone program or can be incorporated into an orbit-propagator or trajectory code.

Version 3.8 of Mars-GRAM incorporates some new features that were suggested by users engaged in the design and operation of spacecraft on missions of robotic exploration of Mars. Mars- GRAM 3.8 uses new values of planetary reference ellipsoid radii, gravitation terms, and the rate of rotation, (consistent values now used by NASA’s Jet Propulsion Laboratory), and includes centrifugal effects on gravity. The model now uses the NASA Ames Global Circulation Model low-resolution topography. Curvature corrections are applied to winds, and limits based on speed of sound are applied. The altitude and molecular weight of the ionization peak of the F1 layer of the Martian ionosphere and the density scale height (including the effects of the change of molecular weight with altitude) are computed. A check is performed to dis- allow temperatures below the sublimation temperature of CO2.

The user’s guide for Mars-GRAM 3.8 summarizes the changes made for this version and includes an appendix that summarizes changes made for versions 3.5 through 3.7. It includes instructions for running the program, plus sample input and output files.

These programs were written by Carl G. Justus of Computer Sciences Corp. for Marshall Space Flight Center. For further information, contact Carl G. Justus at Jere.Justus @msfc.nasa.gov. MFS-31461/62

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