CADAC Primer

Program CADAC, Computer Aided Design of Aerospace Concepts, provides an environment for the development of general purpose, digital computer simulations of time-phased dynamic systems. It manages input and output, generates stochas­tic noise sources, controls state variable integration, and provides postprocessing analysis and display. CADAC has proven its adaptability to many simulation tasks: air-to-ground and air-to-air missiles, ground-to-space and space-to-ground vehi­cles, and airplanes. The CADAC environment is suitable for three-, five-, and six-DoF simulations. It supports deterministic and Monte Carlo runs. Output can be listed or plotted.

CADAC is hosted on an IBM-compatible PC with a minimum of 32 MB of RAM, 25 MB of free disk space, and Microsoft’s Window operating system. The graphics utilities are best displayed on a 1024 x 768-resolution screen with font size set to small.

CADAC consists of CADAC Studio and CADAC Simulations. Both are pro­vided on the CADAC CD. CADAC Studio, written in Visual BASIC, analyzes and plots the output with programs KPLOT and SWEEP and provides utility function for debugging. The vehicle simulations are written in FORTRAN 77 with some common language extensions. The preferred compiler is Compaq’s (formerly Dig­ital) Visual FORTRAN for Microsoft’s Developer Studio. Other compilers (like Silicon Graphics) may require minor modifications of the CADX3.FOR executive routine.

The CADAC Studio comes with four volumes of documentation:

1) Quick Start gets you started with the test case and provides an overview of the input files, executables, and output files.

2) User Documentation addresses all capabilities of the CADAC development environment. It should give answers to most questions that come up during the design of a new trajectory simulation. Tables, examples, and matrix utilities pro­vide useful references. The serious CADAC user should read this document in its entirety.

3) Program Documentation provides details for many subjects: building the input and the header files, the integration routine, generation of stochastic variables, execution of multiruns, sweep runs, Monte Carlo runs; utilities that aid in building, documenting, and analyzing CADAC simulations. It should be used as reference to answer specific questions.

4) Real-time CADAC Documentation addresses the functionality and capabil­ities of the real-time CADAC methodology. It explains how to generate the data files for the real-time version of the modules and the validation procedures.

CADAC is essentially a trajectory program. Over the years features were added to make it more useful. Some of these capabilities are the following:

Vector integration State vectors can be integrated as an entity without break­ing them up into scalar components.

Staging Modes of trajectories, like midcourse or terminal guidance, can be sequenced in sections called stages. They are initiated by IF statements in the INPUT. ASC file.

Multiruns Several trajectories can be combined. For instance, if the sensitivity of a seeker parameter is to be investigated, new parameter values are scheduled in separate group runs and loaded into the global C-array at the appropriate stage with the keywords LOAD and STAGE.

Reinitializing To save execution time, the state of the trajectory can be saved at a certain event, say seeker acquisition, and the following group runs are reini­tialized at this point. The keyword SAVE will write the global C-array to the file CSAVE. ASC.

Sweep runs This feature is used to automate the calculation of launch en­velopes and footprints. The launch position or target location is swept through a polar grid. The SWEEP utility analyzes and displays the results.

Single Monte Carlo runs Stochastic runs are generated by the keyword MONTE, followed by the number of desired runs. The noise sources, like wind gusts, INS errors, and seeker noise, are defined by the keywords GAUSS, UNIF, EXPO, or RAYLE. The output file RANVAR. ASC saves the random values, which can be used to rerun a particular trajectory realization. The MCAP program av­erages the trajectory parameters and calculates means, standard deviations, and correlation coefficients. Histograms and error ellipses are displayed by the KPLOT program.

Sweep Monte Carlo runs Sweep runs can also be executed as a family of Monte Carlo runs. Just introduce the keyword MONTE with its replication number into a sweep run.

Weather Atmospheric conditions, like temperature, density, pressure, and wind, can be specified as tabular functions of altitude using the keyword WEATHER in the INPUT. ASC file.

Real-time execution The CADAC simulation can be converted by the CON – VRT. EXE program into a real-time capable code package, suitable for a man-in – the-loop simulation.

The best way to learn CADAC is by trying. So get the code from the CADAC CD and a FORTRAN compiler, then start running the test cases. To help you over the initial hurdles, I include a primer that gets you going.

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