A solid state tilt meter for current meter attitude determination

Earth coordinate determination of current requires measurement of heading and tilt of the sensor to rotate the instrument frame measurement of velocity into Earth frame coordinates. Gimbaled magnetometer coils allow the magnetic heading to be determined but solid state three-axis magnetometers require tilt in addition to the Earth's magnetic vector to resolve heading. A measure of tilt is also desirable if the current meter cannot be assured of a vertical orientation. Two-axis MEMS accelerometers by Analog Devices, Inc. are capable of providing this tilt measurement but require temperature correction to remove a significant error at zero tilt. The determination and application of this temperature coefficient to correct the error has enabled a versatile attitude sensor to be incorporated in the modular acoustic velocity sensor (MAVS) for conversion of flow measurements by the sensor into Earth-coordinate currents. Residual tilt error is less than 0.3 degrees from 5degC to 28degC. Absolute accuracy of the sensor is better than 1deg between -50deg and +50deg and is better than 3deg between -70deg and +70deg. Because the accelerometer works equally well inverted, the current meter can be mounted upside down to place the velocity sensor above instead of below the housing without opening the case. A second socket for the accelerometer at right angles to the first permits a horizontal mounting of the current meter with only minor internal rearrangement of a part