Validating NRLMSIS Using Atmospheric Densities Derived from Spacecraft Drag: Starshine Example

Model validation is an important component of NRL’s program to develop and test improved semi-empirical models of upper atmosphere neutral densities for operational use. The new NRLMSISE-00 model has been formulated from a database that now includes total mass densities from satellite accelerometers and orbit determinations, temperatures from incoherent scatter radar, and O2 densities from solar ultraviolet occultation. Few direct measurements of thermospheric neutral density exist to validate NRLMSIS in recent times. However, information about total mass densities can be extracted from records of spacecraft orbits archived as Two-Line-Element (TLE) sets that were not included in the model formulation. The TLEs provide details of spacecraft position, velocity, mean motion and its derivative. The mean motion derivative provides an excellent estimate of energy dissipation rate, from which atmospheric total mass density can be deduced, providing the spacecraft ballistic coefficient is known. The Starshine spacecraft are especially suitable for this task since they are mirrored spheres for which ballistic coefficients are essentially independent of orientation with respect to the direction of motion. Starshine I, launched in June 1999, completed about 4,000 Earth orbits at altitudes between 200 and 400 km, during 258 days near the maximum of solar cycle 23. Subsequent Starshine spacecraft were launched in 2001, also into low Earth orbits. We have derived total neutral atmospheric mass densities along the Starshine 1 trajectory from TLEs and compare these with the corresponding NRLMSIS determinations. In addition to the TLEs, Special Perturbations solutions were performed for November-December 1999. These determinations were used to validate the accuracy of the TLE data and to provide information on shorter time scales than the typical TLE fit span of three days. Use of a one-day fit span was optimal, enabling study of the thermosphere’s time response. In addition, a mean value of the B term was computed from the Special Perturbations data and used to generate an experimental area-to-mass ratio to compare with the theoretical value. The experimental value was about 5% lower than the theoretical value, which we attribute to the fact that the effective drag coefficient is less than the expected 2.2. The densities exhibit altitude-dependent monthly time-scale modulations associated with solar rotational modulation of extreme ultraviolet (EUV) radiation. We use these density fluctuations to quantify uncertainties associated with the model’s use of the 10.7 cm solar EUV proxy. Starshine 1 Two-Line Elements Starshine 1, a small spherical satellite of mass 39 kg and diameter of 48 cm, is shown schematically in Figure 1, its outer surface covered with 878 polished mirrors. The Starshine 1 Two Line Element (TLE) sets provide parameters derived from General Perturbation (GP) theory for epochs that are spaced at daily intervals or less. The TLEs we used were obtained from http://www.celestrak.com, which posts USSPACECOM TLEs for selected objects (T. Kelso, private communication, 2001). Figure 1. The Starshine 1 spacecraft. In addition to the six Brouwer elements of inclination, right ascension of the ascending node, argument of perigee, eccentricity, mean motion and mean anomaly at epoch, a seventh term in the TLEs relates to drag, and is called B*. As Figure 2 shows, Starshine 1’s 51.6o inclination orbit was approximately circular, with an eccentricity of about 0.001. The small nonzero eccentricity induced altitude variations of ±7 km at the TLE epoch relative to the orbital mean altitude. These variations tracked the drift in the right ascension of the ascending node as the orbit evolved. AIAA/AAS Astrodynamics Specialist Conference and Exhibit 5-8 August 2002, Monterey, California AIAA 2002-4736 This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. 2 American Institute of Aeronautics and Astronautics Atmospheric Densities Derived from TLEs Information about the total mass density, ρ, of the upper atmosphere in the vicinity of Starshine 1 is extracted from the TLEs according to