Many modern vehicles (e.g., automobiles or aerial vehicles) or mobile computer devices, such as smartphones, tablet computers or other general or special purpose machines, include one or more components for determining information regarding positions, orientations, velocities or accelerations of such vehicles or devices, and for providing navigational advice or instructions based on such information. For example, some vehicles and computer devices include Global Positioning System (or “GPS”) transceivers for determining positions using data received from one or more orbiting satellites, or cellular telephone equipment configured to estimate (e.g., triangulate) a position using signals received from one or more cellular telephone network towers or other network sources.
A GPS-enabled vehicle, device or other system or component may determine its position by interpreting signals that are received from multiple GPS satellites. A distance between a GPS receiver and a GPS satellite may be determined by calculating a “time of flight” between the GPS satellite and the GPS receiver for each such signal, which is assumed to travel at approximately the speed of light. Where three or more such signals are interpreted, the GPS receiver may be determined to be located at a specific point on the planet to within a certain degree of accuracy or tolerance, commonly on the order of two to ten meters.
Occasionally, position information determined using GPS satellites and GPS-enabled equipment may be inaccurate, irrelevant or unavailable. For example, like any computer device, most GPS-enabled equipment requires an initialization period during which GPS position information obtained or determined by such equipment is unreliable. Furthermore, where an environment includes many natural or artificial obstructions, such as tree limbs, office towers, mountains, walls or ceilings, the receipt of GPS signals by a GPS-enabled device may be delayed or otherwise interpreted as having arrived in an untimely manner. Moreover, the use of GPS satellites and GPS-enabled equipment necessarily relies on the viability of the GPS system as a whole. Like any computer-based system or communications system, the GPS system is at risk, at least theoretically, of hacking, spoofing or jamming, and may be taken offline or rendered unavailable for one or more legitimate or illegitimate purposes from time to time.