Control Networks on the Galilean Satellites: Solutions for Size and Shape

A control network is a series of identifiable points on a surface and a table of their coordniates (latitude, longitude, radius). To create a planetary control network, points (usually craters) are identified on pixtures and their image coordinates are measured in pixels. Each control point must be measured on two or more images. The navigation team supplies discrete spacecraft positions and approximate camera-pointing angles. An analytical triangulation program is used to compute the coordinates of the control points and to improve the camera-pointing angles. A control network supports the compilation of maps of a particular reregion or an entire body. The maps may abe planimetric or topographic. A reference surface is used to approximate the shape of a body or to measure elevations. The reference surface is usually a sphere or spheroid, so the map can be displayd in many popular projections. Ideally, the reference surfaces for the Galilean satellites should beellipsoids, because they are in synchronous orbits and experience strong tidal forces. However, most popular projections such as Mercator, Lambert, and stereographic lose their elegant and convenient properties when the reference surface is an ellipsoid. Experiments were made to solve for the three axes as indipendent variables in the analytical triangulation. The results are of little use, as the control points are not uniformly distributed and the image resolutions vary greatly. However, the control networks can be used to study planetary shapes when combined with gravity data to constrain models of internal structure