The present invention relates generally to navigation and velocity estimation in crafts and vehicles. More particularly the invention relates to a spatial velocity meter, a navigation arrangement, a method for spatial velocity estimation and a navigation method. The invention also relates to computer programs and respective computer readable media.
In the last decades, navigation systems have been developed which facilitate the determination of a position dramatically compared to the earlier known methods. These navigation systems determine positions on basis of accurate radio signals received from a plurality of satellites, and are commonly referred to as global navigation satellite systems (GNSS:s).
An alternative or complementary way to navigate and determine a craft's movement can be provided by an inertial measurement system (INS). The INS registers relative movements of the craft. Based on these movements, in turn, conclusions may be drawn as to the craft's speed and course. In case certain initial conditions are known, such as data defining a starting position and an initial speed, it is also possible to produce updated position information for the craft. The INS normally contains an inertial measurement unit (IMU), which in turn includes accelerometers and rate gyros. The accelerometers measure specific forces which are translated to a known coordinate frame with aid of angular measurements performed by the rate gyros. Newton's laws of motion are applied to generate velocity and attitude information.
The U.S. Pat. No. 6,449,559 describes a fully coupled positioning solution, wherein INS data is integrated with GPS data by means of a Kalman filter. Thereby, a positioning accuracy may be achieved, which is better than what would be possible to attain with a GPS receiver only.
The U.S. Pat. No. 6,170,344 discloses a purely IMU-based system for monitoring distortions in a pipeline. A so-called pipeline pig is here used to register parameters relating to accelerations and angular velocities. Based on these data, and a set of initial conditions in the form of an initial attitude matrix and an initial velocity vector, a Kalman filter is used to provide i.a. a velocity vector.
Hence, the prior art includes various solutions for positioning and determination of a craft's velocity based on IMU data. However, there is yet no example of a solution through which an accurate velocity vector may be determined without the input of artificially generated external information. Namely, either reception of GPS signals (or equivalent GNSS signals) is required, or an initial velocity vector must be supplied. This renders the known solutions comparatively vulnerable and sensitive to disturbances and spoofing.