Computation

Introduction

X-pos provides a fully integrated Coordinate Computation service to all users. This service uses files contained in the X-pos repository as reference station data.

Only users with a suitable subscription can benefit from the Coordinate Computation service. Please note that the Computation option is only visible on the SBC Portal if the “Enable Coordinate Computation Service” slider is toggled on the Settings > X-pos > General page.

Coordinate Computation Request

The “Post-Processing > Computation” page enables users to upload RINEX rover files and automatically detect suitable reference stations from the X-pos reference sites. If these have GNSS data for the matching time interval, coordinate computation can be performed to provide users with accurate rover coordinates. RINEX v2.x, 3.02, 3.03 and 3.04 files can be used as rover input files.

The rover files selected by users are parsed to detect the type of antenna and receiver, the type of data to be processed (Static, Kinematic or Stop&Go) and, most importantly, the rover's approximate position. This enables X-pos to locate suitable reference sites for specific rovers in order to perform coordinate computations. The parsed information is also displayed in a summary table on the right-hand side of the “Computation” page. Rover locations and reference station positions are visualised on a map.

Several rover files can be uploaded at a time, whether they refer to the same rover or different ones. It is also possible to use different data types at a time (Static, Kinematic and/or Stop&Go). X-pos will detect the most appropriate reference sites and computation methods for each rover file.

The maximum size of rover data files to be uploaded can be configured in the “Maximum Input file size for X-pos Coordinate Computation” on the Settings > X-pos > General page. The limitations of this field are as follows:

• Default value: 150 MB,

• Minimum value: > 0 MB,

• Maximum value: 2047 MB

Based on the value set by the system administrator, users are limited to that specific value when uploading rover data files on the Coordinate Computation request page.

Users can give a project name to each computation request. This is not mandatory.

Coordinate Computation with Virtual RINEX

When requesting a Coordinate Computation, users have the option to enable the Virtual RINEX feature. When this option is selected, a virtual station will be used as the reference for each uploaded rover file, instead of the surrounding physical stations. Only single baseline results will be delivered for each rover in this case, as only one reference file will be produced for each input file. If multiple files referring to different rovers are uploaded at once, one Virtual RINEX file will be created for each rover, and the same number of baselines will be computed and included in the computation results.

Virtual RINEX requires GNSS network correction data. It is possible to obtain a RINEX file referring to a virtual location for periods of time and positions for which an active cluster has successfully provided and pushed sufficient network correction data to the X-pos data repository. Virtual reference files are created based on the real data available for the X-pos reference stations. When the Virtual RINEX option is enabled before submitting a Coordinate Computation request on the SBC Portal, the real reference stations involved in creating the virtual data are displayed on the map and highlighted in blue. The same display rule applies to real reference stations involved in a Coordinate Computation request where the Virtual RINEX option is not in use.

Enabling the Virtual RINEX option requires an additional step for creating the virtual reference file, so the overall coordinate computation process may take longer. Virtual RINEX files generated as part of the coordinate computation workflow will not be stored in the local X-pos repository.

Only users with a suitable subscription can benefit from the Virtual RINEX service. Please note that the Virtual RINEX option is only available on the SBC Portal if the “Enable Virtual RINEX Service” slider is toggled on the Settings > X-pos > General page.

Target Coordinate System

The Coordinate Computation service allows users to select the coordinate system in which they would like the results to be expressed after post-processing. By default, post-processing results are expressed in WGS84, but the processing kernel of X-pos incorporates a coordinate transformation module so that results can be converted to the target coordinate system selected by the user. The target coordinate system can be chosen for each Computation request on the “Post-Processing > Computation” page, from a group of coordinate systems made available by system administrators in Settings > X-pos > Coordinate Systems.

Selecting a particular target coordinate system before triggering a Computation request affects how the final results are provided on the “Post-Processing > Results” page and in the final report:

• If no target coordinate system other than WGS84 is selected, only WGS84 coordinates will be provided, expressed in both Cartesian and Geodetic coordinates,

• If a target coordinate system other than WGS84 is selected that contains a transformation but no projection, results will be provided in WGS84 (Cartesian and Geodetic) and in Local coordinates (also Cartesian and Geodetic).

• If a target coordinate system other than WGS84 is selected that contains a transformation and a projection, then the results will be provided in WGS84 (Cartesian and Geodetic) and in Local Grid Coordinates (E, N, U), with the latter replacing the local ones.

Validation Checks on Rover Data

Several validations are in place every time a user uploads rover data for coordinate computations. These validity checks cover:

• Unknown antenna in rover file: the computation may be affected by the unknown calibration parameters (see Settings > X-pos > GNSS Antennas for more information), but it is executed anyway.

• Invalid RINEX version, file format, file content: in this case the computation cannot take place.

• Reference site availability: coordinate computations can only take place when at least three reference stations are located in the proximity to the selected rover file(s). If no stations are found within the maximum range of the rover as defined by system administrators (Settings > X-pos > General), the computation will not be executed. If only one or two reference stations are found around the selected rover, the user will also be notified that the processing accuracy may be lower than expected.

Even though there are enough reference stations located for each of the selected rover files, there may be cases when these cannot provide any data for the required time frame. In this case, the coordinate computation is triggered but fails due to a lack of reference data. The user is then notified on the “Post-Processing > Results” page.

Processing Parameters

The Coordinate Computation service uses a set of default parameters for post-processing. The majority of these settings cannot be customised, except for those in the “Coordinate Computation related settings” section of the Settings > X-pos > General page of the SBC Administrator Portal. The settings are summarised as follows:

• Minimum cut-off angle: 10°.

• GNSS constellations: Unless manually changed by administrators, the constellations used for post-processing are determined by the satellite systems tracked in the selected rover RINEX file(s).

• Disabled satellites: The default setting (i.e. unless manually changed by administrators) is 'none'.

• Frequencies: They are determined by the observations contained in the selected RINEX rover file(s) and can be L1 only or L1/L2.

• Tropospheric model (used to correct the measurements errors caused by the troposphere effect): the Vienna Mapping Function is used.

• Ionospheric model (used to correct the measurements errors caused by the ionosphere effect): It is chosen as the best option for the given data, so it can vary.

• Solution type: This defines which solution type should be output by the processing engine. The “Phase fixed” solution type is attempted by default. If the quality of the rover and reference data does not allow a fixed position to be obtained, the solution type will automatically revert to a float solution.

• Possible ambiguities fix: The default value is 80 km. This parameter defines the maximum length of a baseline for which the system should attempt to resolve ambiguities. For baselines exceeding this limit, a float solution will be computed.

• Minimum distance for iono-minimized: The default value is 15 km. This indicates the minimum baseline length for which an iono-minimized solution (e.g. using the ionosphere-free linear combination) will be attempted.