The Range Safety Algorithm software encapsulates the various constructs and algorithms required to accomplish Time Space Position Information (TSPI) data management from multiple tracking sources, autonomous mission mode detection and management, and flight-termination mission rule evaluation. The software evaluates various user-configurable rule sets that govern the qualification of TSPI data sources, provides a pre-launch autonomous hold-launch function, performs the flight-monitoring-and-termination functions, and performs end-of-mission safing.

Rule types are tailored to the Range Safety problem domain and are based on existing range practices with human-in-the-loop flight-termination systems. This module provides a cleanly deployable software library for autonomously executing a number of real-time range safety decisional functions. Its key strength is its ability to emulate the substantial variety of human-in-the-loop flight safety mission rules using a comparatively small set of flight-termination rule types.

These four rules are:

  1. Generic Parameter Threshold Limit – This rule is used to fire a terminate condition in response to one or more threshold conditions or Boolean truth conditions. This rule may carry with it an arbitrary number of interpolative look-up tables. It can implement flight azimuth constraints, erratic flight and attitude rate limits, and vehicle performance limits.
  2. Coordinate Boundary Rule – This rule is used to determine whether or not a specified point is contained within a simple closed boundary represented by a table of coordinates. The state of a variable is evaluated using a ray crossing point-in-polygon algorithm. During each update cycle, the closest distance between the specified coordinate point and coordinate boundary is computed. These can be applied to present position, instantaneous impact point coordinates, flight corridor inclusion limits, protected area exclusion enforcement limits, and upper-stage commit limits.
  3. Two-Point Gate Rule – This rule is used to determine whether or not a specified point has crossed a gate formed by a line between two points, and whether or not a specified point is ahead of or behind the gate at any point in time. These gates may be statistically located, or dynamically assigned via interpolative lookup tables. The state of a variable is computed by looking for intersections between the gate and a line formed from the previous and current locations of the target point. Another variable is computed by comparing the relative location of the specified coordinate set to the gate.
  4. Trajectory Adaptable Green-Time Rule – This rule is used to establish the permissible time of flight with no valid data, and to flag a destruct condition when this time has been exceeded. Algorithms are used to compute the worst-case instantaneous impact point velocity as a function of the current state, and of vehicle acceleration.

The software is configured by the user via an XML configuration file. The algorithms are implemented within a C++ container class that can be embedded in and serviced by host application. Presently, the software possesses approximately 5,500 lines of code distributed over 28 source files.

This work was done by Raymond J. Lanzi of Goddard Space Flight Center and James C. Simpson of Kennedy Space Center. For more information, download the Technical Support Package (free white paper) at under the Software category. GSC-15594-1

NASA Tech Briefs Magazine

This article first appeared in the July, 2010 issue of NASA Tech Briefs Magazine.

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