Formation-Flying CubeSat Constellations for Internal Gravity Wave Tomography
Presenter:
Riley Fitzgerald
Massachusetts Institute of Technology
Atmospheric and Environmental Research, Massachusetts Institute of Technology
Talk
The advent of radio occultation (RO) instruments that use the transmitters of the Global Navigation Satellite Systems (GNSS) aboard CubeSats leads to the possibility of a mission to sound atmospheric internal gravity waves if such satellites are deployed in close-flying constellations. Constellations such as this that spread their satellites into orbits with slightly different inclinations will experience a dispersing effect caused by differences in the rates of regression of nodes which must be countered by propulsion if the constellation is to remain in formation. If maintained, however, such constellations yield clusters of closely-spaced occultation events which enable the reconstruction of internal gravity waves. The results of two studies will be presented, the first completed and the second on-going, along with requests of the research and engineering communities for future development.
In the first study we consider the constellation maintenance problem. We will describe a theoretical approach to the propulsive maneuvering necessary to maintaining a close-flying formation of GNSS RO CubeSats and discuss the results of simulations using comprehensive orbit propagators for four propulsive systems. Cold gas thrusters can permit inclination separations of 1 to 10 degrees while electrospray limits separations to less than 0.2 degrees. Cold gas propulsion, though, expends all of its fuel in maintaining the constellation formation in less than approximately 100 days while electrospray propulsion can achieve lifetimes greater than 1000 days.
In the second study we examine the effectiveness of various constellation arrangements. We have developed two quality metrics for occultation clusters according to their spatial distributions and simulated distributions for multiple constellation arrangements in order to assess the long-term performance over different latitude bands. Results from these simulations indicate relatively even cluster quality across latitudes with the largest number of clusters occurring in the Subtropics. Additionally, the results suggest a slight asymmetry in expected cluster quality between the Northern and Southern Hemispheres; for prograde (retrograde) constellations, the Northern (Southern) Hemisphere is slightly favored.
We recommend that precision orbit determination capabilities be expanded to perform high precision orbit determination during low-acceleration propulsive maneuvers; otherwise, too great a fraction of the mission lifetime will be spent in propulsive maneuvers and too little a fraction of the time available for RO sounding. Secondly, we recommend that electrospray propulsion deliver much greater propulsive thrust. While much more instantaneous power would be necessary for increased thrust, the time-averaged power consumption should be independent of maximum thrust.
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