Important This document is currently under construction.

Overview

The DIRSIG model has been used to model "Space Situational Awareness" (SSA) applications. This handbook attempts to describe the capabilities of the DIRSIG model with respect to this modality.

This application area entails the following scenarios:

  • Looking up from a ground-based or airborne imaging platform through some portion of the atmosphere at an exo-atmospheric object.

  • Looking from an exo-atmospheric imaging platform at another exo-atmospheric object.

This handbook discusses SSA by addressing a select set of topics that differentiate this application area from terrestrial applications.

  • Space-based objects can be attributed using the same material properties.

  • Sat-to-Sat simulations do not involve atmospheric paths.

Technical Readiness

This application area is currently ranked as experimental due to the lack of some key features and lack of extensive use by RIT and the user community.

Model Strengths

  • The user can task a broad-band, multi-spectral, hyper-spectral or any combination of instruments.

  • Model driven (e.g. MODTRAN, MonoRTM, etc.) atmospheric absorption and scattering

    • Atmospheric refraction (bent path) and non-uniform speed of light effects can be incorporated via data-driven temperature, pressure and relative humidity profiles.

  • Usage of detailed surface and volume optical descriptions

    • A suite of mono-directional reflectance distribution (MRDF) and bi-directional reflectance distribution (BRDF) models, including those with support for polarization.

    • Scattering and absorption from participating mediums (clouds, water, fogs, etc.)

  • Complex scene geometries for arbitrary polygon defined terrains and scene objects (natural and man-made)

    • Conversion routines to import geometry from SatAC NSM models, NASA models and Trimble 3D Warehouse (formerly, the Google 3D Warehouse) models are available.

  • Usage of the same platform motion and platform-relative scanning models employed in passive simulations.

    • Support for positioning both the image platform and targets using a variety of static and dynamic descriptors are available (including build-in SGP4 orbit prediction, STK ephemeris and attitude report direct ingestion).

    • The platform motion and platform-relative scanning models support temporally uncorrelated (mean and standard deviation) and temporally correlated (magnitude and phase frequency spectrum) jitter.

    • Platform-relative mounting and scanning allows for both mono-static and bi-static configurations.

Model Weaknesses

  • DIRSIG currently does not have built-in routines to compute the point spread from atmospheric turbulence.

  • A detailed whole earth, "earth shine" model is not supported

    • A whole earth, atmosphere leaving, spectral radiance map could be created outside of DIRSIG and mapped to a Earth sized sphere within a DIRSIG simulation.

    • In DIRSIG5, the EarthGrid plugin provides a partial solution to the need for a whole earth model. However, it currently only supports a RGB representation of the earth. We expect to expand the capabilities of this plugin to support a broader wavelength region in the future.

  • The built-in temperature prediction model (THERM) cannot be used to predict temperatures in space at this time.

  • Support for exo-atmospheric scatterers (space dust) has not been addressed at this time.

  • In DIRSIG 4.5.x and earlier, the location of the Sun (and Moon) was provided by a static pointing vector that was computed for the scene origin. This approach worked fine for small scenes close to the surface of the Earth, however, the relative sun angle for exo-atmospheric objects was inaccurate.

    • In DIRSIG 4.6.x and later, the Sun and Moon locations are stored as ECEF coordinates and the direction vector to the sun is computed as a function of the location.

    • In DIRSIG5, the ephemeris plugins also store the Sun/Moon positions as ECEF coordinates.

  • A streamlined method for incorporating star fields (individual stars) or angularly average background fluxes does not exist at this time.

  • No support for inertial coordinates at this time.

The existence of these limitation is primarily a result of funded research priorities. Our past and current research has not placed a priority on resolving this current set of limitations. As researchers, we are always seeking opportunities to to address known limitations if funding is available.

Differences with DIRSIG5

In DIRSIG4, the ray tracer used double-precision, which allowed the user to create scenes that could span the solar system and still have 1 mm precision. DIRSIG5 adopted a single-precision ray tracer to reduce memory usage and increase speed. However, that prevents building single scenes that include the entire earth and the moon in a single coordinate space. Instead, the approach for building large scenes (including scenes with exo-atmospheric objects) is to make smaller, individual scenes that are positioned with the Earth-Centered, Earth-Fixed (ECEF) coordinate system. The ECEF coordinate system in DIRSIG5 is a double-precision coordinate system that serves as the glue to traverse between various scenes and the sensors.

Important All exo-atmospheric objects must be positioned with the Flex Motion model using a global coordinate system (e.g., geodetic or ECEF).

Target and Platform Positioning

This section will discuss the various ways that the target object and imaging platform can be positioned and oriented as a function of time.

Useful Flexible motion model examples of positioning of space-based objects and tracking them:

  • Ground platform tracking an orbiting object example.

  • Orbiting platform tracking an orbiting object example.

Demonstrations

This section outlines the demos that are currently shipped with each DIRSIG release with relation to SSA applications

DIRSIG4 Compatible

Ssa1

This demo follows an exo-atmospheric satellite (Anik-F1) across the sky from a ground-based sensor. The object motion is described with Two-Line element (TLE) using the Flexible motion model. The imaging platform tracking the space object uses a slightly different Flexible motion configuration to drive the orientation of the static ground vs. time. There are separate simulations for DIRSIG4 (see demo_sgp4.sim) and DIRSIG5 (see demo_ecef.jsim) in this demo.

Ssa2

This demo features a space-based platform viewing a Hubble-like space telescope vehicle, that is also in space. A crude model of the Earth is visible in the background and can also be seen reflecting in the underside of the telescope vehicle.

Ssa3

This demo features a imaging platform in a GEO orbit that is tracking a satellite in a LEO orbit. It makes heave

MoonSat1

This demo features an geostationary, exo-atmospheric sphere 280 meters across that is "painted" with a texture map of the Moon. Note that neither the location (35,000 km up vs 384,400 km up) nor it’s size reflect properties of the actual Moon. The simulation captures frames throughout the day as an Earth sized object behind the platform shadows the object.

DIRSIG5 Compatible

Ssa1

This demo follows an exo-atmospheric satellite (Anik-F1) across the sky from a ground-based sensor. The object motion is described with Two-Line element (TLE) using the Flexible motion model. The imaging platform tracking the space object uses a slightly different Flexible motion configuration to drive the orientation of the static ground vs. time. There are separate simulations for DIRSIG4 (see demo_sgp4.sim) and DIRSIG5 (see demo_ecef.jsim) in this demo.

EarthShine1

This demo features an exo-atmospheric reflective sphere above the Earth (modeled via the EarthGrid plugin) and being imaged by an exo-atmospheric sensing platform. The Earth can be seen reflected in the sphere.

Eclipse1

This demo features the Earth (modeled via the EarthGrid plugin) during the April 8th, 2024 total solar eclipse in North America. The moon is modeled as a sphere object and dynamically positioned using the FlexMotion model using ECEF waypoints that were computed using the NASA SPICE library.

MoonSat2

This demo is a clone of the MoonSat1, which looks at an exo-atmospheric "moon-like" object from the ground. However unlike the original DIRSIG4 version, this demo does not position the moon object using the Scene ENU coordinate system because the distances would exceed what can be supported by the single-precision ray-tracer in DIRSIG5. Instead, the moon object in this DIRSIG5 compatible version uses the ECEF coordinate system.

Useful Links

Repositories of 3D Models

This is a list of web-based repositories of 3D geometry models. These repositories contain models in various 3D geometry formats, but most of them can be converted to the OBJ file format (which is easily ingested into DIRSIG) using free or commercial tools.

Trimble 3D warehouse

Must use the free program Sketchup to convert any model from either Collada or Sketchup format to OBJ format. The geometric detail in many of these models is limited, in that the spirit of this repository seems to focus more on texture mapped flat surface models versus highly detailed geometric descriptions of objects.

Archive3D

These models are in an older style 3DS format, which is well interpreted and converted to OBJ by the open source CAD program Blender. Also, the entire catalog of available objects here is also hosted by many other free CAD websites with similar names.

NASA shared 3D models

Quite a variety of 3D satellite objects shared by NASA. There is a large variety of formats represented in this collection including 3Dmax, 3DS, Blender, Collada, and OBJ among others.

AGI 3D models

Currently many low to medium fidelity CAD models of most public satellite objects are available, but contained within their proprietary MDL format. These may be converted to Collada format using a provided AGI utility and then converted over to OBJ from Blender or another tool, but I have not tried it yet.

TF3DM

This place contains freely available objects in a variety of formats, most of which are straightforward to convert to OBJ for DIRSIG use (except I haven’t had luck yet with C4D format). Most of these objects are focused on gaming, but there are still many things here that have been useful over the years.

GrabCAD

This place has many file formats, but most commonly users post in SolidWorks or 3D Max formats requiring that commercial software to export to another format. However, many times the models are useful simply for their provided texture maps and/or they provide the model in up to 10 different file formats at a time!

Blend Swap

This site is 100% focused on users posting freely available Blender files to demonstrate various types of computer rendering that Blender can do. However it is very useful for high geometric detail objects, even if there are only a few relevant to SSA.