Patent Number: 062927515
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An operational flow chart of steps for implementing the methods of the present invention is shown in FIG. 1. Initially, an inertial navigation system (INS), which incorporates a conventional IMU, is at rest, 100. A ZVU is performed prior to commencement of motion of the INS to calibrate the IMU, 100. Motion of the INS then commences and the INS operates in an unaided navigation mode, 102. At some time after the commencement of motion a period of interest begins. At the start of the period of interest, the time and position of the INS is recorded, 104. At a later time, the period of interest ends. At this time, the INS receives a stop notification 106 and comes to rest, 108, and the time and position of the INS is recorded, 100. The velocity along two horizontal axes, say north and east, is then recorded, 112. Because the IMU is at rest, any velocity indicated by the IMU is the result of errors accumulated during the time of motion of the IMU. With the information now obtained, an accurate determination of the position of the INS relative to its position at the start of the period of interest can be made 114. The IMU produces measurements of acceleration over time which may be integrated once to produce a velocity history and twice to produce an indication of position. However, the accelerometer output of the IMU contains errors which are random in nature. Since velocity and position are obtained from integration of the IMU ouput, velocity and position errors accumulate over time. In the present invention, the velocity errors are approximated by a function with parameters that can be determined by evaluating the function at two instances of time: at an initial time when the IMU is at rest and the function is set equal to zero, and at the end of the period of interest when the function is set equal to the velocity recorded after the ZVU is performed when the IMU is at rest. Once the function is determined, it can be integrated over the period of interest to determine the error in position that has accumulated during the period of interest. For example, if the accelerometer bias is constant, the velocity error will grow linearly with time, while position error grows as the square of time. FIG. 2 shows velocity error as a function of time, assuming a constant bias in the accelerometer output: EQU V.sub.e (t)=.intg.a.sub.e dt=a.sub.e t+b where V.sub.e (t) is the velocity error and a.sub.e is the constant acceleration error, and b is some initial velocity error. At time, t=0, V.sub.e (t) is zero because a ZVU was performed with the INS at rest. This yields b=0. At a later time, t.sub.1, when a period of interest begins, the time, t.sub.1, and an indicated position, P.sub.1, is recorded. At a still later time, t.sub.2, the period of interest ends, and the time, t.sub.2, and position P.sub.2, is recorded. The INS is stopped and the velocity V.sub.2, is recorded. Since the actual velocity of the INS is zero, V.sub.2, is equal to the velocity error, V.sub.e (t.sub.2), at time, t.sub.2 : EQU V.sub.e (t.sub.2)=a.sub.e t.sub.2 =V.sub.2 From this equation we can determine the constant acceleration error, a.sub.e =V.sub.2 /t.sub.2. The position error can now be determined by integrating the function for velocity error over the period of interest: ##EQU1## where .DELTA.P.sub.e is the position error that accumulated from the beginning of the period of interest to the end of the period of interest. The indicated position, P.sub.2, at time, t.sub.2, is given by: ##EQU2## where P.sub.a (t.sub.2) is the actual position of the INS at the end of the period of interest relative to its position at time t=0. Since P.sub.2, t.sub.2 and V.sub.2 are known, P.sub.a (t.sub.2) can be calculated directly as: ##EQU3## Similarly, the actual position of the INS at the beginning of the period of interest relative to its position at time t=0 can also be calculated: ##EQU4## Thus, the actual change in position during the period of interest is: ##EQU5## Although the velocity error was approximated by a straight line in this example, a person of ordinary skill in the art will recognize that other suitable functions for approximating velocity error may be employed in accordance with the method of the invention as herein disclosed. The calculation of position errors is performed for each orthogonal direction for which motion may take place. For example, if the motion of the IMU is in a plane, position error is calculated for each of the two orthogonal axes that define the plane of motion. The method of the present invention may be implemented by providing a computer processor for performing the computations and logical decisions required to implement the method for correcting position errors described herein. Programming the processor to implement the method is a relatively simple task for a person of ordinary skill in the art. The method of the present invention described herein may be employed in a mine hunter-killer system. Such a system utilizes a vehicle upon which is mounted an INS comprising an IMU and a processor for deriving errors in position from recorded data obtained in accordance with the method herein described. Initially, the vehicle is at rest and a ZVU is performed on the IMU. The vehicle then commences motion. A period of interest begins when the vehicle detects a mine just in front of the vehicle and records its position relative to the vehicle. At that moment, the time and position of the vehicle is recorded. The vehicle then stops with the mine behind the vehicle and in close proximity thereto, so that the mine may be reached by a robotic arm. At that moment, the time and position is again recorded and a ZVU is performed. From this data the precise position of the mine relative to the vehicle can be accurately determined. Using this accurate position data, a robotic arm places a neutralizer atop the mine for later remote detonation. Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made to the embodiments described herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.