Accuarcy improvement of low cost INS/GPS for land applications

Abstract

High performance Inertial Navigation Systems (INSs) provide continuously very accurate attitude, velocity and position information. However, it is usually very expensive and because the navigation solutions are obtained by integration, they drift at low frequencies. To obtain very accurate outputs at all frequencies, the INS should be updated periodically using external measurements. On the other hand, the GPS measurements, under ideal conditions, are consistent in accuracy throughout a survey mission. However, such conditions do not often exist. Independent GPS navigation requires at least four satellites with good geometry. The major drawback of GPS is, therefore, the accuracy degradation due to poor satellite geometry, cycle slips, satellite outages, and dynamic lag during maneuvers. This is especially prevalent in urban centres and when encountering highway overpasses/tunnels. Because of their complementary characteristics, INS is often integrated with GPS. The integration of GPS and INS provides a system that has superior performance in comparison with either a GPS or an INS stand-alone system. For instance, GPS derived positions have approximately white noise characteristics over the whole frequency range. The GPS-derived positions and velocities are therefore excellent external measurements for updating the INS, thus improving its long-term accuracy. Similarly, the INS can provide precise position and velocity data for GPS signal acquisition and reacquisition after outages. Recently, focus has been made on developing integrated GPS and low-cost inertial measuring units (IMUs) for commercial applications, for example car navigation. However, due to the low quality of the IMU, huge amount of navigation errors can be generated in very short time intervals. Therefore, low-cost IMUs have shown to be usable only for few seconds without additional aiding. Furthermore due to the large drift rate of the gyros used in low-cost IMUs, self-alignment cannot be applied for some systems. This paper introduces some of the techniques that overcome the limitations of lowcost IMUs, i.e. the in-filed calibration, the velocity matching alignment, and the use of non-holonomic constraints. Tests were conducted using the NovAtel Black Diamond System (BDSTM). A new field calibration method was developed and tested successfully. The new calibration method does not require the IMU to be aligned to the local level frame. Furthermore, the bias estimation of the calibration method is not affected by the reference gravity error. Almost half of the positioning error could be removed with the accelerometer calibration information. The mechanization and navigation Kalman filter were implemented based on the navigation frame to test the velocity matching alignment and non-holonomic constraints. The velocity matching alignment technique was tested for the IMUs to which stationary alignment technique cannot be applied. All attitude components converged within three minutes with RMS 0.03. Non-holonomic constraints dramatically reduced the horizontal positioning error, within 40 m for 20 minutes operation. Therefore, low cost INSs can be used as a stand-alone positioning system during the GPS outages of over 10 minutes.