This paper describes the conditions for observation by absorption spectroscopy of the homosphere between 20 and 100 km from a space platform such as Spacelab. Using the nominal orbital elements, and taking the launch date and time as parameters of the problem, formulas are established that give the data necessary for determining the flight plan. The geographical coordinates of the tangential points of the solar illumination boundaries are also calculated. Thus it is shown that the latitude coverage of the observations is closely related to the launch conditions. Finally, expressions for the pointing angles referred to an instantaneous terrestrial trihedron are given. These angular elements are needed to determine the real pointing angles referred to a reference system linked to Spacelab. 1 ref.