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GNSS Sensors#

Configuration details for supported GNSS receivers.

Supported Receivers#

Model Type Accuracy RTK Support Interface Config
u-blox ZED-F9R Multi-band GNSS ~2cm (RTK) Yes USB gnss_receiver:=ublox
Garmin GPS 18x Standard GPS ~3-5m No USB (FTDI) gnss_receiver:=garmin
Septentrio Multi-band GNSS ~2cm (RTK) Yes USB/Serial gnss_receiver:=septentrio

Recommendation: Use u-blox ZED-F9R for outdoor precision positioning with RTK.

u-blox ZED-F9R (RTK)#

Hardware#

Board: SimpleRTK2B Fusion (u-blox ZED-F9R module) Antenna: Must have clear sky view (mount on roof/highest point) Connection: USB to Jetson

USB Device Setup#

Create udev rule for consistent device naming:

File: /etc/udev/rules.d/99-ublox-gps.rules 

SUBSYSTEM=="tty", ATTRS{idVendor}=="1546", ATTRS{idProduct}=="01a9", SYMLINK+="ublox-gps", MODE="0666"

Apply rules: 

sudo udevadm control --reload-rules
sudo udevadm trigger

# Verify
ls -l /dev/ublox-gps  # Should link to /dev/ttyACMx

Driver Package#

Package: ublox_gps (ROS 2 Humble) Installation: sudo apt install ros-humble-ublox-gps

RTK with NTRIP#

RTK (Real-Time Kinematic) achieves ~2cm accuracy using correction data from a base station.

NTRIP streams corrections over internet: 

Internet → NTRIP Client → RTCM Corrections → u-blox Driver → Receiver

Default NTRIP Service: e-GNSS Taiwan VRS - Server: 210.241.63.193:81 - Mountpoint: Taiwan - Coverage: Taiwan region

Configuration: ntrip.launch.xml 

<node pkg="ntrip_client" exec="ntrip_client_node">
  <param name="host" value="210.241.63.193"/>
  <param name="port" value="81"/>
  <param name="mountpoint" value="Taiwan"/>
  <param name="username" value="[username]"/>
  <param name="password" value="[password]"/>
  <remap from="/ntrip/rtcm" to="/sensing/gnss/ntrip/rtcm"/>
</node>

Enable RTK#

# Launch with NTRIP corrections
make launch ARGS="gnss_receiver:=ublox use_ntrip:=true"

Monitor RTK Status#

Check RTCM corrections: 

ros2 topic hz /sensing/gnss/ntrip/rtcm  # ~1 Hz

Check fix quality: 

ros2 topic echo /sensing/gnss/ublox/nav_sat_fix
# Look for status.status: 2 (RTK fix) or 1 (GNSS fix)

Fix status values: - -1: No fix - 0: GPS fix - 1: DGPS fix - 2: RTK fix (best, ~2cm accuracy)

Troubleshooting#

No GNSS fix: - Ensure clear sky view (outdoor, no obstructions) - Wait 1-5 minutes for satellite acquisition - Check antenna connection

No RTK fix: - Verify RTCM topic: ros2 topic hz /sensing/gnss/ntrip/rtcm - Check internet connection (NTRIP requires network) - Verify NTRIP credentials in ntrip.launch.xml - Wait 1-10 minutes for RTK convergence

Poor accuracy: - Improve antenna placement (higher, less obstruction) - Reduce multipath (avoid reflective surfaces) - Test in open area (NTU COSS courtyard recommended)

Garmin GPS 18x#

Hardware#

Receiver: Garmin GPS 18x (standard GPS, ~3-5m accuracy) Update Rate: 5 Hz Interface: USB (FTDI serial converter) Output: NMEA 0183 sentences

USB Detection#

# Check device detected
sudo dmesg | grep tty
# Should show: FTDI USB Serial Device converter attached to ttyUSB0

Driver: gpsd#

Installation: 

sudo apt install gpsd gpsd-clients

Start daemon: 

sudo /usr/sbin/gpsd -n -G -b /dev/ttyUSB0

Verify signal: 

# Terminal GPS viewer
cgps

# Graphical GPS viewer
xgps

# Wait for "3D Fix" (takes 1-5 minutes on cold start)

ROS 2 Integration#

Configuration: gnss.launch.xml 

<include file="$(find-pkg-share gpsd_client)/launch/gpsd_client-launch.py">
  <arg name="config_file" value="$(find-pkg-share gpsd_client)/config/gpsd_client.yaml"/>
</include>

gpsd client config: 

/**:
  ros__parameters:
    host: "localhost"
    port: 2947
    frame_id: "gnss_link"

Test#

# Start gpsd
sudo /usr/sbin/gpsd -n -G -b /dev/ttyUSB0

# Launch ROS 2 client
ros2 launch gpsd_client gpsd_client-launch.py

# Verify
ros2 topic echo /fix
ros2 topic hz /fix  # ~5 Hz

Troubleshooting#

No GPS signal: - Move to open area (clear sky view) - Wait 5-10 minutes for cold start - Check antenna cable connection

Device not found: - Check /dev/ttyUSB0 exists - Try different USB port - Verify USB cable provides power

Indoor Operation (No GNSS)#

For indoor environments without satellite signal:

# Disable GNSS, use NDT localization only
make launch ARGS="use_gnss:=false"

Use RViz "2D Pose Estimate" tool to manually set initial position.

Quick Reference#

# Launch commands
make launch ARGS="gnss_receiver:=ublox use_ntrip:=true"   # u-blox RTK
make launch ARGS="gnss_receiver:=garmin"                  # Garmin
make launch ARGS="use_gnss:=false"                        # Indoor (no GNSS)

# u-blox verification
ls -l /dev/ublox-gps
ros2 topic hz /sensing/gnss/ublox/nav_sat_fix
ros2 topic hz /sensing/gnss/ntrip/rtcm  # RTK corrections

# Garmin verification
cgps                                     # Check GPS signal
ros2 topic hz /fix

# Check fix quality
ros2 topic echo /sensing/gnss/ublox/nav_sat_fix --once
# status.status: -1=no fix, 0=GPS, 1=DGPS, 2=RTK