Constellation

Abstract: A hierarchical modulation scheme is proposed to upgrade an existing digital broadcast system, such as satellite TV, or satellite radio, by adding more data in its transmission. The hierarchical modulation consists of a basic constellation, which is the same as in the original system, and a secondary constellation, which carries the additional data for the upgraded system. The upgraded system with the hierarchical modulation is backward compatible in the sense that receivers that have been deployed in the original system can continue receiving data in the basic constellation. New receivers can be designed to receive data carried in the secondary constellation, as well as those in the basic constellation. Analysis will be performed to show the tradeoff between bit rate of the data in secondary constellation and the penalty to the performance of receiving the basic constellation.

Abstract: There has been resurgent interest in building low Earth orbiting (LEO) constellations of satellites on a new scale. Their aim is Internet for the world with plans for potentially thousands of satellites. Here, we explore how these LEO constellations can be utilized for navigation. Closer to Earth, LEO offers stronger signals, strengthening us against jamming and aiding in indoor and urban environments. Proximity is also its weakness, where satellites have a small Earth footprint requiring many to provide global coverage. We show that the strength of the Broadband LEO constellations is their numbers, where they offer threefold improvement in satellite geometry compared to navigation core-constellations today. This allows for relaxation of the signal-in-space user range error, while still matching the position accuracy of GPS. Coupled with the more benign radiation environment in LEO compared to GPS in medium Earth orbit, this enables a navigation payload designed using commercial-off-the-shelf components.

Abstract: There has been resurgent interest in building large low Earth orbiting (LEO) constellations of satellites on a new and bigger scale. Their aim is delivering Internet to the world by providing truly global and robust broadband coverage. Players such as OneWeb, SpaceX, Samsung, and Boeing all have proposals for such a system. Each plans on launching constellations of 600 to over 4000 satellites, dwarfing the 1400 operational satellites currently in orbit. This sheer number of satellites, along with their global coverage, gives rise to opportunities not only for broadband but also as a platform for providing navigation services. Here we explore how such a LEO constellation could be leveraged for augmentation of GPS for navigation and even as a full standalone backup. LEOs have the advantage of being closer to the Earth compared to GNSS systems in medium Earth orbit (MEO), thus experiencing less path loss and potentially delivering stronger signals. This makes them more resilient to jamming. LEO spacecraft also have much faster motion in the sky, passing overhead in minutes instead of hours in MEO. This gives rise to more multi-path rejection, as reflections are no longer static over short averaging times. The addition of these constellations adds a wealth of geometric diversity. We examine how these LEO constellations can be piggybacked to deliver navigation services in the form of a hosted payload. The full architecture is explored, from user geometry and signal in space ranging errors (SIS URE) to position errors as well as how such a payload could be conceived. This can be done more economically as better satellite geometry allows for degraded SIS URE to achieve comparable performance to GPS today. We explore the use of chip-scale atomic clocks (CSAC) on the satellites for precise timing as well as the GPS ephemeris message to enable backwards compatibility. Constellation-wide orbit determination methods are also discussed. Ultimately, all elements and are brought together to show the possible system performance that is achievable. This is in an effort to leverage infrastructure coming on the horizon to further protect, toughen, and augment the PNT that we are increasingly reliant upon.