USB GNSS RTK Receiver Setup

This setup provides a tutorial on how to use a GNSS RTK Receiver, a Linux Machine, and an RTK Correction Service (or Base Station) to access position data with centimeter-level precision. Many GNSS RTK Receivers will connect to an NTRIP Caster directly utilizing its on-board LoRa,WiFi, Bluetooth, or Cellular Modem. However this tutorial is geared towards using a Linux Machine to connect to an NTRIP Caster to receive the RTK Correction Data and then relay the data to the RTK Receiver via USB. This use case is more relevant to a robotic platform that already has access to the Internet or a Local Network with an NTRIP Caster. Main reason to do it this way is because your Linux Machine may already be using a Cellular Modem or Long Distance Wireless Access Point. This would render the wireless capabilities of the RTK Receiver as redundant or less capable than what the Linux machine is using.

Basic Concepts

GNSS RTK Receiver (Rover)

These devices have recently became available in low-cost forms with comparable performance to higher end products. It is basically a micro-controller with a GNSS antenna and receiver with the capability to run complex calculations that enable it to resolve position data with centimeter precision under the right conditions. The micro-controller may be using a library like RTKLIB to perform the calculations. The most important condition for resolving centimeter-level precision is access to RTK corrections from a nearby(~35km) Base Station.

Base Station

The Base Station is basically a stationary GNSS RTK Receiver that is configured to stream out RTK correction data using a NTRIP Server (COTS products offer this as a single package). A Base Station and Reference Station are synonymous.

Continuously Operating Reference Stations (CORS) network

A CORS Network is a collection of GNSS Base stations combined in a single network to provide wider coverage of valid RTK correction data. Recall that RTK correction data is only valid within a ~35km radius of the Base Station. A CORS network may provide convenient access to RTK correction data from the nearest base station. There also exist advance techniques to combine correction data from multiple base stations to provide correction data from a Virtual Reference Station (VRS) to further reduce location error. See VRS for details

Linux Machine NTRIP Client

RTKLIB provides a suite of CLIs and GUIs that simplify much of the work. In theory could configure your design to perform the RTK calculations on the Linux Machine. However in this tutorial, the Linux Machine is only used to relay the RTK correction data from the NTRIP Caster to the GNSS RTK Receiver. To do this we use the RTKLIB CLI str2str which enables us to connect to a NTRIP caster as a data input source and a GNSS RTK Receiver has a data output target. This employs our Linux machine as a NTRIP Client. Alternatively, instead of using the RTKLIB CLI, we can use the ROS package ntrip_client. See OpenMower's ntrip_client launch file for further details.

Getting Started with RTKLIB str2str

1. Open a terminal and enter the following command to capture the serial port device label e.g. /dev/tty* More info

dmesg --follow

1. Plugin in the USB RTK module and on the Linux terminal you should see this something like the following:

[604400.525618] cdc_acm 1-1.3:1.0: ttyACM0: USB ACM device
                                   ^^^^^^^ make note of this label

1. In the terminal, enter the following.

sudo screen /dev/ttyACM0 9600

sudo cutecom

1. You should be able to observe NMEA strings streamed to the terminal. If not, your device may need to be manually configured to do so. One, alternative method is to configure the GNSS receiver to output data via TCP. Enter the either the following commands to observe the data stream.

nc 192.168.0.20:9002

socat - TCP4:192.168.0.20:9002

1. The str2str rtklib CLI application is use to connect to a NTRIP Caster and recieve

$ sudo ./str2str -in ntrip://username:password@hostname:port/mountpoint -out serial://ttyACM0:115200#52001 -b 1

Access RTK correction service and RTK correction data

• If your using a RTK correction service then we need a convenient way to store the login credentials securely. This is can be done with Environment Variables
• Alternatively you can set up your own base station (this requires an additional Receiver that can stream out RTK corrections).
• Each method will perform similarly. The limitations of each will be based on the following:

Environment Variables

https://www.linode.com/docs/guides/how-to-set-linux-environment-variables/

1. Open new terminal, and append the .bashrc file with the following template and overwriting the characters with ...

sudo vim ~/.bashrc

# InCORS RTK correction service credentials https://incors.in.gov/
# Save the environment variables to to .bashrc i.e `sudo vim ~/.bashrc`
#########################################################
# DO NOT PUBLISH, THIS CONTAINS USERNAMES and PASSWORDS #
#########################################################
export NTRIP_HOSTNAME="..."
# Automatic cells
export NTRIP_PORT_AUTO="9000"
export NTRIP_ENDPOINT_AUTO="RTCM3_MAX"
# single site
export NTRIP_PORT_SINGLE="..."
export NTRIP_ENDPOINT_SINGLE="..."
# secret
export NTRIP_USER="..."
export NTRIP_PASSWORD="..."

1. After saving edits, close the terminal
2. Open new terminal, Verify environment variables

echo -e '\n'\
'ntrip hostname:        ' $NTRIP_HOSTNAME'\n'\
'ntrip port_auto:       ' $NTRIP_PORT_AUTO'\n'\
'ntrip endpoint_auto:   ' $NTRIP_ENDPOINT_AUTO'\n'\
'ntrip port_sigle:      ' $NTRIP_PORT_SINGLE'\n'\
'ntrip endpoint_single: ' $NTRIP_ENDPOINT_SINGLE'\n'\
'ntrip user:            ' $NTRIP_USER'\n'\
'nitrip password:       ' $NTRIP_PASSWORD

Installing RTKLIB

1. In the terminal install the rtklib

sudo apt install rtklib

1. Attempt to receive RTK corrections.

2. Single Site

sudo ./str2str -in ntrip://$NTRIP_USER:$NTRIP_PASSWORD@$NTRIP_HOSTNAME:$NTRIP_PORT_SINGLE/$NTRIP_ENDPOINT_SINGLE#rtcm3

• Automatic Site

sudo ./str2str -in ntrip://$NTRIP_USER:$NTRIP_PASSWORD@$NTRIP_HOSTNAME:$NTRIP_PORT_AUTO/$NTRIP_ENDPOINT_AUTO#rtcm3

Configure RTK device correction input and position out methods

This project is using the Emlid Reach RTK Module and following documentation. The Emlid is configured with the following settings

[details="Simple system report"]
app version: 28.4-r0
enabled: true
mode: client
correction_input:
  base_corrections:
    io_type: serial
    settings:
      serial:
        baud_rate: 115200
        device: ttyGS0 (USB-to-PC)
        send_position_to_base: true
position_output:
  output1:
    io_type: serial
    nmea_settings:
      serial:
        gga:
          enabled: true
          update_rate: 1
        gsa:
          enabled: false
          update_rate: 1
        gst:
          enabled: false
          update_rate: 1
        gsv:
          enabled: false
          update_rate: 1
        main_talker_id: gn
        rmc:
          enabled: false
          update_rate: 1
        vtg:
          enabled: false
          update_rate: 1
        zda:
          enabled: false
          update_rate: 1
    settings:
      serial:
        baud_rate: 9600
        device: ttyGS0 (USB-to-PC)
        format: NMEA
positioning_settings:
  elevation_mask_angle: 15
  glonass_ar_mode: true
  gnss_settings:
    positioning_systems:
      beidou: false
      galileo: true
      glonass: true
      gps: true
      qzss: true
      sbas: true
    update_rate: 5
  gps_ar_mode: fix-and-hold
  max_horizontal_acceleration: 1
  max_vertical_acceleration: 1
  positioning_mode: kinematic
  snr_mask: 35
[/details]

Creating a persistent label for the RTK GNSS USB device

To use the serial device as non-root user, udev rules must be installed.

1. Open a terminal and enter the following follow commands.

lsusb

Bus 003 Device 063: ID 3032:0014 Emlid Reach
                       ^^^^ ^^^^
                    vendor  product

1. On the Linux terminal you should see this something like the following:

[604400.525618] cdc_acm 1-1.3:1.0: ttyACM0: USB ACM device
                                   ^^^^^^^ make note of this label

1. todo

udevadm info -q all -n /dev/ttyACM0

P: /devices/pci0000:00/0000:00:14.0/usb3/3-1/3-1:1.2/tty/ttyACM0
N: ttyACM0
....
E: DEVPATH=/devices/pci0000:00/0000:00:14.0/usb3/3-1/3-1:1.2/tty/ttyACM0
E: DEVNAME=/dev/ttyACM0
E: MAJOR=166
E: MINOR=0
E: SUBSYSTEM=tty       <----------- make note of this label
E: USEC_INITIALIZED=166592821213
E: ID_BUS=usb
E: ID_VENDOR_ID=3032   <------------
E: ID_MODEL_ID=0014    <------------
E: ID_PCI_CLASS_FROM_DATABASE=Serial bus controller
....

1. todo

cd /etc/udev/rules.d/;
sudo vim 51-emlid.rules

# Copy this udev with "sudo cp 51-emlid.rules /etc/udev/rules.d/"
# When done, do "sudo udevadm control --reload && sudo udevadm trigger"
# Edit it to suit your type of Linux. It's currently set up for modern Ubuntu
ATTRS{idProduct}=="0014", ATTRS{idVendor}=="3032", MODE:="666", GROUP="plugdev", SUBSYSTEM=="tty", SYMLINK+="emlid_rtk"

1. todo

sudo udevadm control --reload && sudo udevadm trigger

Re-plug in USB GNSS receiver and verify NMEA strings streamed to the terminal

sudo screen /dev/emlid_rtk

1. Run the str2str rtklib CLI application

2. To receive Single site corrections via NTRIP and relay to USB GNSS receiver via serial

sudo ./str2str -in ntrip://$NTRIP_USER:$NTRIP_PASSWORD@$NTRIP_HOSTNAME:$NTRIP_PORT_SINGLE/$NTRIP_ENDPOINT_SINGLE#rtcm3 -out serial://emlid_rtk:115200#52001 -b 1

• Alt. to Receive Automatic cell corrections via NTRIP and relay to USB GNSS receiver via serial

sudo ./str2str -in ntrip://$NTRIP_USER:$NTRIP_PASSWORD@$NTRIP_HOSTNAME:$NTRIP_PORT_AUTO/$NTRIP_ENDPOINT_AUTO#rtcm3 -out serial://ttyACM0:115200#52001 -b 1

1. Listen

nc localhost 52001