GPS Landscape: What is Missing?
The Global Navigation Satellite Systems (GNSS) are dominated in the US by the Global Positioning System (GPS). The GPS system provides users with Position, Navigation & Timing (PNT). GPS satellites transmit reference signals to both civilian and military GPS receivers that are used to estimate the distance between satellite and receiver. Triangulation then provides a reasonably accurate 3-dimensional (3D) position estimate of geographic coordinates for the receiver.
It is surprising to many that GPS does not directly measure headings or vehicle attitudes. This is because the difference between two antenna locations clearly indicates a direction, but there really is no indication whether a vehicle is backing up rather than going forward. A moment's thought clarifies that platform location and platform orientation are two different things, since one is where you are and the other is what direction you are facing, or whether you are laying down or standing up or even rolling over. We count this as 3 additional degrees of freedom.
Six Degrees of Freedom (6DOF) Solution: Extend the 3D location capabilities of GPS receivers to use the directional gain to sense the antenna orientation relative to the satellites. Use this 6DOF orientation sensing as an attitude reference for calibrating the Inertial Navigation System (INS) and identifying sources of Radio Frequency (RF) interference and obstruction of the signals. We call this concept GPS6D.