When using the positioning and orientation system to locate the sensor to the ground, the sources of error mainly include the following aspects.
Sensor position
The placement of the sensor is a very important task that affects the performance of the entire system. For the placement position of the sensor, the following two conditions must be met: 1 the correction error has the least influence on the offset correction between the sensors; 2 there can be no slight displacement between the sensors. To this end, the image of the first condition can be improved by reducing the distance between the sensors, but the latter is relatively more difficult to overcome.
Time synchronization
Because the positioning and orientation system is an integrated application of GPS and inertial measurement systems, the GPS can continuously receive external data, so that the inertial measurement device can be modified during the motion, so that the error accumulated over time can be controlled. At the same time, GPS will encounter the problem of cycle slip and signal loss in the dynamic environment, which can be solved by the high-precision inertial measurement information in a short time, and can also help the GPS receiver to improve the anti-interference ability, making it The ability to track and capture satellite signals is enhanced. However, it is very difficult to synchronize the time between GPS and inertial measurement system under normal conditions. The first problem is the synchronous use of GPS data and inertial measurement data. The time synchronization requirement between the two systems. It will increase with the improvement of precision requirements. If this problem cannot be handled properly, it will become a serious error source because it directly affects the running track of the carrier and thus affects the determination of the external orientation elements.
Initial correction
The initial calibration process is generally completed before the measurement. It is the process of converting the inertial system to the ground level system through the rotation matrix, usually including two stages of rough correction and precise correction. Rough correction approximates the attitude parameters by the raw output data of the sensor and a single consideration of the Earth's rotation and gravity field hypothesis model. Since low-precision inertial systems cannot be calibrated in a static environment, more optimal alignment accuracy can be achieved by aircraft motion. If the motion of the aircraft can bring enough horizontal acceleration, the uncertainty of the misalignment error will be quickly observed by the velocity error, and the Kalman filter can be used to estimate the size according to the speed update of DGPS.
System check
Since the direct sensor orientation does not utilize the ground control point, but the ground point coordinates are extrapolated by the projection center, the correction system is an indispensable task. The accuracy of the obtained ground point coordinates depends mainly on the accuracy of the system calibration. System calibration mainly includes single sensor calibration and calibration between sensors.
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