Pre-processing Checks

After conversion to GPB, pre-processing checks are performed in order to help ensure the file is ready for post-processing. Functions performed by the pre-processing checks include:

  • Ensuring positions are present in the GPB file. If no position records have been requested during data logging, pre-processing computes a code only single point solution. This computed position is then written to the GPB file. This position is used to display the unprocessed trajectory and determine whether ionospheric processing will be automatically engaged. The latter option depends on the scanned baseline distance prior to processing.

  • Ensuring an accurate clock shift has been decoded to the GPB file. This is needed in order to correctly process and export results relative to GPS time. If this value is incorrect by a large amount it can result in gross processing errors.

  • Automatic rejection of impossibly large or small pseudorange observations, which can occur due to unusual receiver signal tracking issues. Also automatically rejected are any duplicate PRN numbers (which will cause a processing failure), duplicate epochs and other unusual raw data problems.

  • Computing missing Doppler measurements. A common issue with some RINEX data is that Doppler measurements are provided as 0 for the entire file. As the Doppler is used for cycle slip detection, this would result in large processing errors if it is uncorrected. Pre-processing checks ensure any missing Doppler measurements are recomputed from the C/A code.

  • Static/Kinematic detection. The pre-processing checks attempt to set the static/kinematic flag appropriately. For surveys with significant position changes from epoch to epoch (such as would be the case for a kinematic survey) data is converted as kinematic. Conversely, if no significant movement is detected from epoch to epoch, portions of the data may be converted as static. The ability of the pre-processing check to reliably detect static data largely depends on the noise level of the unprocessed (or computed) position data.

  • Processing environment detection. The unprocessed position records are scanned in order to determine if the dynamics are characteristic of aerial, ground vehicle or marine surveys. The detected environment is written to the header of the GPB file, which an appropriate processing profile (GNSS Airborne, GNSS Ground Vehicle, or GNSS Marine) to automatically load the first time the processing dialog is accessed for a project.