gnss_converter

An Open-source ROS Package for GNSS Preprocessing

This ROS package contains a simple RINEX-to-Rosbag converter written in C++. It decodes observations from a multi-constellation GNSS receiver and augments them with an ephemeris, atmospheric corrections, and the individual satellite clock bias.

The package is used to generate a Rosbag file containing raw GNSS pseudoranges for further research. In addition, it contains an estimated fix of the rover's position and a velocity estimate based on Doppler data, both calculated by the RTKLIB. A Matlab script can ultimately be used to convert the Rosbag file to a .mat file.

Please note that this repository holds just a stripped-down version GraphGNSSLib, which provides a full factor graph back-end. We created this fork for a more straightforward maintainability of the gnss_preprocessor package.

1. Install ROS

Install ROS on a Ubuntu machine. The package is tested under Ubuntu 20.04 and ROS noetic.

2. Build gnss_converter

Clone the repository to the desired folder and build the package:

mkdir -p gnss_converter/src
cd gnss_converter/src
git clone https://github.com/TUC-ProAut/gnss_converter.git
cd ..
catkin_make

Source the built packages:

source ~/gnss_converter/devel/setup.bash

3. Configuration

We employ YAML files for the configuration of the gnss_preprocessor. Many pre-configured examples exist, but new configurations can also be created easily.

3.1 Preconfigured

Example configurations of many different datasets can be found in the config folder.

3.2 Custom

The following parameters have to be adapted to fit to the considered dataset. They are internally used by the RTKLIB and GraphGNSSLib scripts:

• mode: GNSS positioning mode (0: single, 1: DGPS/DGNSS, 2: kinematic)
• frequency: all used frequencies of the GNSS receiver (1: L1, 2: L1+L2, 3: L1+L2+L5)
• soltype: type of the solution (0: forward, 1: backward, 2: combined)
• satellites: list of all GNSS-systems to be used (GPS, GLONASS, Galileo, BeiDou, QZSS, SBAS)
• shared_ephemeris: use one common RINEX file for the broadcast ephemeris (true/false)
• precise_ephemeris: use a SP3 file for the precise satellite orbit solution (true/false)
• ionex_correction: use a IONEX file for ionosphere correction
• custom_atx: use a custom antenna model file
• elevationmask: minimal elevation angle of satellites to be used in degrees

Please see also the documentation of the RTKLIB for further explanations regarding specific parameters.

3.3 Launchfiles

ROS launch files are used to start the gnss_preprocessor. The paths to the config file and all the other GNSS-related files have to be specified there.

Many examples are given inside the launch folder.

4. Getting GNSS related files

All information about downloading the GNSS-related files are summarized in "Getting GNSS relates files".

5. Usage

5.1 Run GNSS preprocessor with your own RINEX file

Run the desired launch file:

roslaunch gnss_preprocessor <datasetname>.launch

While running, you can see the number of available satellites for each system:

[ INFO] [1647857601.020952311]: GPS_cnt [1]   9
[ INFO] [1647857601.020979743]: SBS_cnt [2]   0
[ INFO] [1647857601.020996915]: GLO_cnt [4]   7
[ INFO] [1647857601.021008216]: GAL_cnt [8]   0
[ INFO] [1647857601.021029226]: QZS_cnt [16]  0
[ INFO] [1647857601.021037431]: CMP_cnt [32]  0

The processed data is saved in a rosbag file inside the folder dataset/processed.

5.2 Messages

Different messages are published while the package is running:

• gnss_raw (gnss_msgs::GNSS_Raw_Array): custom message containing the raw GNSS information (pseudoranges and additional information regarding the satellites)
• gnss_fix (sensor_msgs::NavSatFix): receiver position fix calculated by the RTKLIB (reference frame: WGS 84)
• gnss_vel (geometry_msgs::TwistStamped): receiver velocity information calculated by the RTKLIB from doppler data
• gnss_raw_base (gnss_msgs::GNSS_Raw_Array): base GNSS information (only used with relative positioning modes)

5.3 Conversion to a Matlab-file

The file convertRosbags.m can be used to convert the generated Rosbag file to a .mat file for further evaluation. Please note that only a few specific datasets can be used directly. For custom datasets, the filenames have to be entered manually.

6. Acknowledgments

Since this package is a stripped-down and modified version of Weisong Wen's GraphGNSSLib, the credits for initially creating this helpful converter belong to him. The RTKLIB is used internally for GNSS data decoding and atmospheric corrections.

7. License

The source code is released under GPLv3 license.