# EarthShine1

Keywords: space situational awareness

## Summary

This demo shows how to setup an exo-atmospheric sensor that is looking at a second exo-atmospheric object (a satellite). In this scene, the earth serves as a backdrop for the satellite. Specifically, the earth is reflected in the bottom of the satellite. This makes the earth a secondary source of radiance for that object.

## Details

Some of the key elements of this simulation inolve the positioning of exo-atmospheric objects. In this case, both the imaging platform and our satellite will be exo-atmospheric. The Earth-Centered Earth-Fixed (ECEF) coordinate system is employed, which is a more natural choice than Scene ENU coordinate system for this scenario. Therefore, this simulation uses GLIST files rather than a ODB files because the GLIST file allows you to specify the location and orientation using the ECEF coordinate system.

The two ECEF locations are key to this simulation. They were not selected to mimic any real-world scenario.

Imaging Satellite Location

  ECEF -689390.0, 6340312.0, 2741500.0

Target Satellite Location

  ECEF -689350.0, 6340300.0, 2741500.0

## Important Files

Some of the important aspects of this simulation scenario include the geometry of the scene and the approach used to map materials onto the earth object.

### Geometry

This scene employs a few tricks to make the geometry for the scene. All of the geometry is in the `geometry` folder.

crinkle_tube.obj

  This OBJ file is a simple cylinder that has been slightly deformed to resemble the "crinkle" of the multi-layer insulation (MLI) usually applied on the exterior of spacecraft.

simple_sat.odb

  A "simple satellite" is the target object being observed by our imaging platform in the simulation. The object is created using primitive constructive geometry. It is composed of two instances of the `crinkle_tube.obj` file (a cylinder). One instance has the scales set such that it is slightly longer but has a smaller diameter than the other.

simple_sat.glist

  This is the file that positions our simple satellite in the scene. It has a single instance that places the `simple_sat.odb` file at the ECEF coordinate defined earlier.

earth.glist

  A simple earth object sits in the background (behind the simple satellite) in this simulation. The geometry for this object utilizes the GLIST file’s sphere geometry primitive. The radius of the sphere is set to the earth’s equatorial radius. The static instance for the sphere performs two tasks: (1) it scales the sphere in the Z dimension to make the sphere an oblate spheroid and (2) is places the sphere center at the ECEF origin. The Z axis rotation can be used to rotate the Earth so that different parts of the Earth appear behind the satellite.

### Materials

The Earth sphere (defined in the `earth.glist` file) has the surface materials defined by MODIS derived class map (reprocessed to create 4 material classes). This material map setup is defined in the `.scene` file.

### Atmosphere

This scenario utilizes the Uniform atmopshere model, which permits positioning of the Sun directly above the satellite.

### Platform Positioning

The platform positioning (motion) for this scenario uses the Flexible Motion model. The setup locates the image platform at one ECEF location and the orientation is defined to look at the ECEF location where the target satellite is located.

    <motion type="flexible">
      <locationengine type="fixed">
        <location type="ecef">
          <x>-689390.0</x><y>6340312.0</y><z>2741500.0</z>
        </location>
      </locationengine>
      <orientationengine type="lookat">
        <locationengine type="fixed">
          <location type="ecef">
            <x>-689350.0</x><y>6340300.0</y><z>2741500.0</z>
          </location>
        </locationengine>
      </orientationengine>
    </motion>

## Setup

The target location, camera location, camera focal length and pixel pitch are purely notional, and not meant to represent any currently existing system.

To run the simulation, perform the following steps:

1. Run the DIRSIG `demo.sim` file

2. Load the resulting `demo.img` file in the image viewer.

## Results

The resulting RGB image using the Selected Two Percent Scaling in the DIRSIG built-in image viewer is shown below. The target satellite appears in the middle of the image. The illumination on the underside of the satellite is light reflected by the Earth below.

Figure 1. The resulting RGB image with the two percent scaling