STK/Astrogator

The Astrogator module developed by Analytical Graphics, Inc. of Malvern, PA is a mission analysis and operations tool that leverages on the powerful functionality of their Satellite Tool Kit (STK) product. Astrogator with its mission sequence architecture can trace its heritage back to Swingby, developed at NASA/GSFC in the early 90’s.

Astrogator is used to build a mission sequence using a series of propagate and maneuver objects as stopping conditions. The propagate objects can be used to stop at such conditions as orbit apses (perigee, apogee, periselene, etc.), epoch, orbit nodes, true/mean anomaly, altitude/radius, or some other user-defined parameter. Maneuver objects, of either the impulsive or finite type, can be inserted amongst the propagator objects, as needed in a particular mission scenario. Astrogator allows the use of targeter objects to wrap around segments of this sequence to allow for the varying of certain parameters through the use of the maneuvers. The Navigation and Mission Design Branch(NMDB)most recently made use of STK/Astrogator on the Microwave Anisotropy Probe (MAP) mission. NMDB mission designers used STK/Astrogator to design trajectories that took MAP from a highly elliptical orbit, through three phasing loops to a lunar encounter. The lunar encounter was used to propel MAP on its way to small amplitude Lissajous orbit about the Sun-Earth/Moon L2 libration point, 1.5 million km from the Earth opposite the Sun.

In addition to the trajectory design and maneuver planning, STK/Astrogator was used for many analysis tasks. In particular, NMDB analysts used STK/Astrogator to perform Monte-Carlo analyses for MAP. This was made possible through the use of the STK/Connect module. With STK/Connect, the analyst has the capability to interface with STK through a TCP/IP socket and send commands to STK and receive data back from STK. Using Mathworks MATLAB tool, the NMDB analysts wrote a MATLAB script which generated random error sources, applied the errors to the MAP trajectory through the socket, propagated the trajectory using STK/Astrogator, and retrieved the data back into MATLAB for processing. In this fashion, the analysts were able to answer important questions concerning the stability of the MAP trajectory with respect to launch vehicle errors and the effects of maneuver execution errors on the ability to reach the L2 orbit.

In the end, STK/Astrogator was used to design the MAP trajectory, provide launch targets to the launch vehicle provider (Boeing), analyze the stability of the trajectory, and plan and calibrate the maneuvers during mission operations. All of these tasks made it possible for MAP to successfully encounter the Moon which enabled MAP to orbit L2 and collect science to fulfill its mission goal – to develop a high resolution map of the cosmic microwave background radiation, remnants of the Big Bang.

Figure 1: MAP Trajectory to L2