Relative orbit determination for unconnected spacecraft within a constellation

Large constellations of spacecraft are set to become increasingly common. Orbit determination (OD) can be a non-trivial task for a large constellation operator, and efficient operations of such constellations is a crucial challenge. This Note aims at addressing one critical issue of large constellations, that is, the relative orbit determination (OD). Relative OD provides basic state information for subsequent spacecraft control and operation. In addition, determining relative orbits can be used to aid certain scientific and operational objectives, such as the gravity recovery in [1] and Earth observation in [2].