The near-circular orbit shown in the figure is a typical MGS mapping
orbit. MGS finally achieved such an orbit on February 19, 1999. The
altitude of the mapping orbit ranges from 230 to 274 miles, and the
period of the orbit is just under two hours. The latitude of the entry
and exit occultations varies over the course of the mission. Details about
the latitude coverage of the radio occultation measurements may be found
The locations of each of the retrieved profiles may be seen on the martian weather maps elsewhere on this site. Weather records and atmospheric profiles may be accessed by clicking the occultation points on these maps.
The signal phase is modified by propagation through the martian atmosphere because the velocity of the radio signal in the atmosphere is less than it is in the free space between Earth and Mars, and because the path length through the atmosphere increases up until the moment of occultation entry and decreases following the moment of occultation exit. The physical process where an electromagnetic wave is slowed when propagating through a dense medium is called refraction. The changing phase of the MGS signal at the times of the occultations produces a Doppler shift in the signal frequency. At occultation entry it appears as if the spacecraft is moving away from Earth, and the frequency of the radio signal decreases. At occultation exit it appears as if MGS is moving toward Earth, and the frequency of the radio transmission increases.
Once it is determined how the phase of the signal was modified by propagation through the martian atmosphere, a computer program calculates a profile of the refractivity of the atmosphere. The refractivity profile indicates the velocity of the radio signal at many altitudes in the martian atmosphere relative to the velocity of an electromagnetic wave in free space (the so-called speed of light). A density profile of the atmosphere of Mars is computed directly from the refractivity profile based on knowledge of the propagation of X-band radio signals (3.6 centimeter wavelength). The density profile describes how the thickness of the atmosphere varies with height above the surface.
Finally, the program uses formulas from basic physics (the ideal gas law and assumption of hydrostatic equilibrium) and knowledge of the composition of the martian atmosphere to compute profiles of atmospheric temperature and pressure from the profile of martian atmospheric density. It is necessary to assume that the temperature is known at some altitude high in the atmosphere. That is not actually the case, however, so there are errors in the temperature and pressure profiles which decrease rapidly as the altitude above the surface decreases from the height at which the boundary condition was specified.
The temperature and pressure profiles contain the same information that could be gained by releasing a weather balloon from the surface of Mars and measuring the temperature and pressure at regular intervals as the balloon drifted higher and higher into the atmosphere. Because it is not possible to release weather balloons on Mars every few hours for a couple of years, it is fortunate that radio scientists like those on the MGS Radio Science Team have developed remote means to make meteorological observations of other planets!