Although the present invention is described with specific reference to the Global Positioning System (GPS) as an example of a satellite-based positioning and navigation system, it should be understood that this is only a specific example that does not limit the scope of the invention and that other positioning or navigational systems may be used.
The global positioning system (GPS) is currently used by many applications to determine the location of a receiver. GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the GPS receiver to compute position, speed, and heading. Four GPS satellite signals are used to compute positions in three dimensions and a time offset in a receiver clock.
The GPS receiver determines location by using triangulation; triangulation requires a location of at least four satellites and the distances from the receiver to each of those four satellites. The GPS receiver determines the locations of the satellites and the distances by analyzing the specially coded satellite signals that are high frequency, low-power radio signals.
In a poor signal-reception environment, such as, for example, “urban canyons”, one or more satellite signals may be blocked or distorted, to the point where there may hot be enough measurements with which to formulate a navigation solution. Dead reckoning (DR) has been used to supplement GPS in poor signal-reception environments. In dead reckoning, if the vehicle or platform starts a trip from a known location, the distance and direction from the known location can be used to determine the current location of the vehicle. In the terrestrial environment, such as for automobiles, ships, boats, and aircraft, dead reckoning uses such simple “inertial navigation” sensors such as, for example, an odometer sensor and a vibrational gyroscope. Typical inertial navigation sensors suffer from error accumulation. For example, a gyro bias error can cause a gyroscope to output a non-zero value even if the angular velocity is zero. Gyro bias is observable when the vehicle is not moving or when it is moving in a straight line.
Both GPS and DR suffer from limitations, for example, the GPS signal may not be available in obstructed areas such as urban canyons or tunnels while the DR system can drift over time and accumulate errors. However, the integration of GPS and DR yields a positioning system that is superior to either GPS or DR alone. The two systems are integrated in such a way that the GPS subsystem inputs control the drift and error accumulation of the DR subsystem, and the DR subsystem becomes the main positioning system during GPS outages. The result is an integrated system that is superior to either alone.
The current state-of-the-art vehicle navigation systems have integrated GPS and DR with a digital road map, frequently with user friendly and ergonomic enhancements. This integration of GPS with digital mapping, supplemented by DR, is especially useful in urban canyons and tunnels. These navigation systems may provide “turn-by-turn” instructions to a driver. The “turn-by-turn” travel times are determined by approximating real-time position information augmented by re-routing if the driver misses a turn.
In these integrated navigation systems, the GPS is typically used to periodically correct gross positioning errors in the overall system position solution. The position solution is generally derived from an integration of a map-matching system and a DR system in which the DR system is calibrated by matching the actual DR path to map street patterns.
Although this technology has proven to be useful, it would be desirable to present additional improvements. There are several drawbacks to conventional navigation systems. Conventional pattern-matching algorithm used to calibrate the DR sensors in navigation systems tend to be sophisticated, heuristic, and very complex, requiring a large amount of processing resources.
In addition, significant GPS sub-system position inaccuracies may be present in the urban canyon environment, requiring yet another set of non-trivial, complex heuristics. The navigation system thus is required to frequently examine the relative quality of different navigation data sources, and to ultimately adjudicate which data source provides the output navigation state (for example, position, speed, and heading).
Furthermore, the complexity of a map-matching problem in a conventional navigation system generally requires the use of severe filtering that introduces a perceivable lag between the actual position and the solution of the system. Consequently, the system may not “recognize” that it has turned a corner until many seconds after the fact. This makes turn-by-turn navigation difficult to manage.
Thus, in conventional navigation systems, the GPS function is used to correct gross navigation errors. However, in the urban canyon environment, stand-alone GPS may not be available to provide this correcting function. In addition, DR sensors used in conventional navigation systems have errors that grow over time. It is then to be expected that in the urban canyon environment, the conventional navigation systems, which rely on the combination of DR and map-matching, have errors in position and heading that accumulate to the point where turn-by-turn navigation becomes unreliable.
What is therefore needed is a system and an associated method for a augmenting a satellite-based navigation system that comprises a location determination system integrating map-matching with a position indicating and direction indicating navigation system, such as stand-alone GPS or combined GPS and DR, such that the output navigation parameters are consistent with turn-by-turn navigation in all the navigation environments in which a vehicle, a platform, or a SPS receiver might operate. The need for such a solution has heretofore remained unsatisfied.