The Equations of Motion project provides an application with astrodynamics related functionality along with a library to aid in the creation of custom tools.
User Guide: https://motoq.github.io/doc/eom.html
Technical Notes: https://motoq.github.io/doc/tech_notes.html
The eomx application is being designed to focus on orbit propagation, orbit determination, relative orbital dynamics, and attitude dynamics. It is a command line program driven by a simple text based modeling language. The library allows rapid development of custom applications.
Current functionality includes orbit propagation and access analysis to earth fixed ground points. General perturbation (GP, "analytic") and special perturbation (SP, numerical integration) orbit propagation techniques are supported. Externally generated SP3 formatted ephemerides (CDDIS) can also be ingested. Ephemerides can be saved in STK compatible .e file formats. Other outputs are written as Matlab (Octave compatible) functions. When run, these .m files will plot the data with appropriate formatting while optionally returning the handle to the figure along with the raw data. For example, the range as a function of time between two orbits can be generated and displayed in a 2D plot. Likewise, relative orbits can be output in an RTC coordinates for a 3D plot.
The IAU 2000A and IAU 2006 precession-nutation theories are supported, including parsing of IERS EOP data. In addition to the GCRF and ITRF reference frames, internal support for TEME (true equator, mean equinox, GMST 1980) exists. This ECI reference frame is included for use with legacy GP propagators (such as the Vinti, traditional secular J2, and SGP based theories).
The library is designed with thread safety in mind. Shared resources (ECF/ECI transformation services, ephemerides, etc.) are available as immutable objects. SP based ephemerides and access analysis are generated taking advantage of parallel loop processing available since C++17.
eom utilizes two mature libraries upon which built-in models rely and external libraries may leverage. The first is the International Astronomical Union Standards of Fundamental Astronomy (IAU SOFA) C library http://www.iausofa.org/. Installation of SOFA-Issue 2015-02-09 or newer is required to build eom. The second library is the Eigen 3 C++ template library for linear algebra http://eigen.tuxfamily.org. In the future, Kitware’s VTK OpenGL library may be used for direct plotting of graphics.
Core concepts behind eom are:
- Provide useful built in astrodynamics models and analysis tools
- Provide convenient access to SOFA functionality
- Supply an interface to external legacy libraries that have fallen out of use simply because the GUI supporting them can no longer be maintained
- Provide a tool that allows for easy comparison of results from different functions and/or libraries
- Provide a framework in which short response analysis can be addressed or algorithms prototyped
C++ was chosen for development because:
- It plays well with legacy C and FORTRAN libraries
- A number of mature and fast C++ math libraries are available
- Code designed to run on hardware can be tested
- Modern C++ is a really great language, despite all the baggage (a C++17 compatible compiler is required)
The Mozilla Public License 2.0 (MPL2) was chosen because it protects the open source nature of this code while still allowing it to be used with proprietary and closed source tools: https://www.mozilla.org/en-US/MPL/2.0/
CMake was chosen over the use of a Makefile to support the potential of directly generating graphics via Kitware’s VTK OpenGL library. In theory, it should aid in building eom under different platforms. So far, eom has only been built under Linux systems. Updates to the CMake configuration will be made as deficiencies are discovered.
In addition to CMake, the Eigen C++ math library must be installed. CMake is currently configured to rely on automatically locating Eigen. If this fails, then CMake must be manually configured to point to the location of the Eigen header files. More information is available at the Eigen site http://eigen.tuxfamily.org.
The final dependency is the IAU SOFA C library. On a typical Linux distribution, this may be the only external dependency that needs to be installed. Download the latest release from http://www.iausofa.org/ and follow the build instructions. Next, search for "SOFA" in the CMakeLists.txt configuration file. Update the path to where the include and library files were installed when building SOFA.
Once CMake, Eigen, and SOFA are installed, check that the line
set(USE_GENPL TRUE)
is commented out in the CMakeLists.txt file. This line enables an external astrodynamics library that is not included with this repository. It is often left uncommented (enabled). The GENPL keyword also serves as an example within both the build file and source code w.r.t. including 3rd party libraries within eom and eomx.
At this point, the standard CMake build process can be followed. Change to the build directory. Type,
cmake ..
make
to build both the eom library and the eomx application. To build only the library,
make eom