Speaker
Sun Hur-Diaz
(Emergent Space Technologies, Inc.)
Description
The orbit determination (OD) requirement for a lunar CubeSat mission is examined. The motivation for this study is the NASA CubeQuest Challenge, for which the winning 6U CubeSats in the competition are offered a launch on the Exploration Mission (EM) 1 mission as secondary payloads. All such CubeSats will be disposed into a high-energy trajectory that will fly by the moon. Unless high-impulse chemical propulsion system is allowed on the CubeSats, most designs will involve some form of a low-thrust propulsion system to achieve lunar orbit. In order to determine the OD strategy for such a mission, the OD accuracy requirement needs to be understood. One driver for the OD accuracy is its contribution to the delta-V budget and hence the spacecraft’s ability to achieve the target lunar orbit. Typically, this type of analysis is done using Monte Carlo simulations, but the large number of cases required to achieve a statistically significant result is often prohibitive.
In this paper, we examine the use of unscented transforms1 to determine the impact of OD accuracy on the delta-V budget. We take a candidate low-thrust propulsion trajectory from EM-1 disposal to lunar orbit and a candidate ground station architecture using one-way Doppler measurements to determine the OD accuracy requirement. This method is not unlike the linear covariance analysis2, however, its use of sigma points extends its usefulness beyond the linear region, especially for the highly nonlinear problem of the low-thrust transfer to the moon.
Two open-source tools from NASA Goddard Space Flight Center are utilized to perform this analysis: the General Mission Analysis Tool (GMAT)3 is used for the low-thrust maneuver planning and the Orbit Determination Toolbox (ODTBX)4 is used for the orbit determination. GMAT and ODTBX are interfaced with a function that generates the sigma points of the unscented transformation from the orbit estimate covariance matrix obtained from the orbit determination. Another separate function combines the sigma points of the delta-V penalty. The efficacy of the unscented transformation method is demonstrated by comparing the results of this technique with the results from a small-scale Monte Carlo simulation.
References:
1. Julier, S.J. and J.K. Uhlmann, “Unscented Filtering and Nonlinear Estimation,” Proc. IEEE, vol. 92, pp. 401 – 402, Mar. 2004.
2. Geller, D., "Linear Covariance Techniques for Orbital Rendezvous Analysis and Autonomous Onboard Mission Planning," Journal of Guidance, Control, and Dynamics, Vol. 29, No. 6, Nov-Dec 2006, p. 1404-1414.
3. https://gmat.gsfc.nasa.gov/
4. http://opensource.gsfc.nasa.gov/projects/ODTBX/
| Applicant type | First author |
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Primary author
Sun Hur-Diaz
(Emergent Space Technologies, Inc.)
Co-author
Ravishankar Mathur
(Emergent Space Technologies, Inc.)
