Criteria for Trajectory and Exterior Orientation

The position and attitude accuracy specifications of various Leica Geosystems airborne sensors with integrated GNSS/IMU system are specified in terms of the Root Mean Square Error (RMSE) of the post-processed trajectory values. RMSE values give a robust indication of the trajectory quality with the assumptions listed below. Average and maximum limits are easily dominated with short term events that would invalidate a complete trajectory if used as pass/fail quality criteria, but do not give a clear indication of the general trajectory quality.  

The recommended values are typical values. If any of the recommended values are exceeded in the processed solution, the processed solution must be reviewed in detail. The reasons for the higher values must be isolated and analyzed first by the user. Leica Geosystems Support can also review the processed solution if necessary. Often reprocessing of the recorded data with changed processing parameters or data from a different base station will improve the solution so that the values are subsequently better than the recommended values.  

The following section outlines the acceptance criteria for a trajectory suitable for misalignment calculation. While the quality of the trajectory for project work can be slightly more relaxed, it should generally adhere as closely as possible to the following values or better:

Minimum requirements for acceptable differential processed tightly coupled solution

The recommendations below are based on a proper and correctly processed differential tightly coupled solution. The processed differential tightly coupled solution must have the following minimum characteristics to qualify as an acceptable solution: 

• Base station data used for processing must: 

  • start before the start of the sensor data airborne recording 

  • end after the end of the sensor data airborne recording 

  • have a minimum data logging frequency of 1 Hz 

  • if a GPS base station is deployed by the customer (manly in remote areas for the duration of Project) the base station should be record data for more then 2h to be able to process a proper position in post processing.

• Base station position, coordinate refence system and datum must be known, correct and used in processing 

• Solar activity and other atmospheric conditions are minimal and acceptable  

• No GNSS outages or poor data 

• Acceptable GNSS constellation factors (acceptable average PDOP values of <2.5, during the recording, optimal is close to 1.0) 

• Maximum aircraft bank angle is less than 25° during the recording 

• Proper and suitable start and end alignment segments were recorded 

• No other external factors that would create erroneous or poor solution 

• Leverarms are measured and applied correctly

If a trajectory exhibits lower quality than described, a misalignment calculation should not be performed without first reducing the QA criteria. The focus should be on further improving the resulting quality of the trajectory.