Satellite Tool Kit^TM (STK)

STK is a powerful COTS analysis tool developed and marketed by Analytical Graphics, Inc. (AGI) of Malvern, PA. The basic STK software package is available free of charge by request, but is somewhat limited in capability. For advanced analysis, AGI has developed a number of specialized add-on modules - available for a fee - that greatly extend the utility of the software. The Professional (PRO) module is a ‘must-have’ for all but the most basic analysis tasks. One of the most powerful and useful is the Visualization Option (VO) that generates a 3D view of the mission scenario – an invaluable advantage for the visually oriented analyst. (For more information concerning these modules, visit the AGI web site at http://www.stk.com.) The NMDB licenses these modules and most of the others offered by AGI and has a number of analysts with extensive experience in using them in concert with other NMDB analysis tools to solve complex real-world problems in spacecraft mission design.

STK is used extensively by the NMDB in support of all phases of the spacecraft mission life cycle from conceptual analysis to decommissioning. Following are three examples of analysis performed by the NMDB primarily with STK.

Optimal Orbit Geometry for TRMM Uncontrolled Re-entry

Using the Connect and MATLAB Interface modules, STK can be used to automate analysis that requires complex or repetitive calculations. An example of this is analysis that was performed in connection with TRMM re-entry planning to determine the orbit ground-track geometry that resulted in the minimum amount of time spent over land for a given number of consecutive orbits. STK has the basic tools to answer this question, but manually performing the analysis in manageable pieces via the GUI and integrating the results into a meaningful whole would have been extremely tedious, time-consuming, and prone to user error. Instead, the analysis was automated by using the Connect and MATLAB Interface modules to drive an STK scenario with a MATLAB script, which also provided for automatic data reduction and presentation of results. In this case, the automation capability eliminated the tedious aspects of the analysis and, except for the required input, the opportunity for user error; however, it was still considerably time-consuming. The script ran just over 15 hours on a 500 MHz Pentium III system to produce the results depicted below.

TRMM Land Mass Coverage Over 17 Days

TRMM Land Mass Coverage vs. Initial Longitude of Ascending Node 3 Orbit Average Over 17 Days

Best Case: 15.5% at LAN of 175.8 Degrees

Worst Case: 42% at LAN of 31.6 Degrees

CGRO Re-entry - Maneuver Constraint Analysis and Impact Zone Targeting Analysis

STK was instrumental in the mission planning and analysis for the CGRO re-entry, the first controlled re-entry of an unmanned NASA spacecraft not designed to survive the re-entry process. Several of the spacecraft subsystems imposed requirements on the re-entry trajectory design. The power subsystem required that the Sun vector be within ±30º of apogee for the final two maneuvers to ensure a power-positive state during the maneuvers. This requirement combined with the impact zone targeting requirements to restrict the timeframe within which the final two maneuvers could be performed to periods when apogee occurred close to orbit noon near the ascending node. Rather than going through the laborious process of sorting through a listing of the pertinent data to search for favorable geometry, an STK scenario was developed to visually confirm when all requirements were met concurrently, as shown in the figure below, and quickly led to the conclusion that 4-day windows of opportunity would present themselves every 54 days.

Sun Coverage for Maneuvers 3 and 4

STK was also utilized to perform impact zone targeting analysis with the High Precision Orbit Propagator option. Burnout vectors from the final maneuver were propagated against a range of ballistic coefficient values to estimate the impact footprint of spacecraft components expected to survive re-entry and ensure that they remained inside the specified target zone. As with the maneuver constraint analysis described above, an STK scenario was developed to visually confirm that the impact footprint conformed to the specified requirements. The figure below depicts the impact footprints for the prime and backup opportunities based on the nominal maneuver plan.

Nominal Impact Footprints for CGRO - June 2000 Re-entry

Aqua Star Tracker Light Baffle Analysis

In late February 2002, a design flaw was discovered in the star tracker light baffles of the Aqua spacecraft. Questions arose as to whether star tracker coverage would still be satisfactory, or if the flaw would substantially impact mission operations by allowing extended periods of concurrent Sun/Moon intrusion in the star tracker fields-of-view. Fortunately, a detailed STK scenario and spacecraft model had already been developed by the Aqua flight operations team for mission analysis and operations. NMDB analysts utilized this pre-existing scenario and used the Connect and MATLAB Interface modules to automate the search for concurrent intrusions. Once the periods of concurrent intrusions were identified, STK’s Visualization Option (VO) was used to view the geometry of the intrusion periods in hope of identifying strategies for ameliorating the impact of the design flaw (see figure below). It was immediately apparent that modifications to the light baffles would be necessary to avoid serious impact to mission operations and objectives, and these were successfully completed prior to launch on May 4, 2002.

Concurrent Sun/Moon Intrusion in Aqua Star Trackers. Note Moon intrusion (white line) simultaneously in both trackers (white cones).