An Inertial Measurement Unit Application for a GPR Tracking and Positioning
Abilities of an inertial measurement unit (IMU) for ground penetrating radar (GPR) tracking and positioning are presented at this paper. The IMU operation principle is based on calculating of the device's orientation in space via measurement of linear accelerations and angular velocities. Features of the IMU, GPS and wheel odometer are compared. Principal scheme of the IMU consisted of low-g three-axis MEMS accelerometer: three acoustic gyroscopes and CPU with USB interface are described. It is shown that the IMU providing six coordinates (Cartesian and angular) allows to improve GPR trace positioning in space and to approach GPR profile presentation to real surface configuration. The IMU application simplifies the 3D mapping of the GPR surveying because of reducing of preparation time of the GPR system for measurements execution and fulfilling the field sounding in arbitrary mode. Since each radar waveform is matched with corresponding coordinate set the radar profiles reflect real surface and sounding direction. The IMU provides high precision GPR positioning for various test areas including tunnels, basements, caves, rough surfaces, etc. Experimental results of the IMU prototype examination are presented also.
Quality of the GPR surveying is depended on conformity of sounding traces to real position on the surface. It is desired to know topographic coordinates of the sounding points. And knowledge of the GPR antenna system location concerning to sounding surface provides more data for the GPR profile reconstruction.
The GPR data positioning influences on the GPR profile quality representation. Examples of the profile representation with different topography data representation are shown on Fig. 1.
The simplest tracking unit is odometer (measurement wheel), which provides one coordinates -linear movement of the GPR across survey surface. In this case the GPR profile displays set of traces with the same zero level. This simplification is acceptable for roads sounding and similar applications where surface tilts are insignificant. Otherwise distortion of a target area response is observed (Fig. lb). At present the method is widely used owing to use simplicity.
The Global Positioning System (GPS) application proposes more flexibility for the GPR surveying. It provides three Cartesian coordinates that allow the GPR traces arrangement in according with relief peculiarities. However some drawbacks limit the GPS applicability. There are insufficient precisions of commercial GPS. high prices of differential GPS. inabilities of operation in closed or radio opaque spaces (basements, tunnels, mines, caves, forests, etc.) And again the GPS doesn't provide information about the GPR antenna system position on the surface (yaw, pitch and roll angles). Therefore the GPS applicability is still limited by plain open spaces. The target shape and position is disfigured when uneven surface is examined (Fig. 2c).
An inertial measurement unit (IMU) presents complete information about the antenna system location in the space . There are three Cartesian coordinates and three angular coordinates (turns) that present true antenna system location and orientation concerning the surface. The IMU is able to provide new quality level of the GPR data positioning. It provides maximum agreement between GPR profile and sounding medium (Fig. 1c).