The Velodyne VLP-16, or "Puck", is a 3D LiDAR laser scanning system ideal for use in UAV aerial mapping applications. In order to create an adjusted, properly georeferenced pointcloud suitable for feature extraction and analysis, an inertial navigation system (INS) is required. When synchronised with a LiDAR system, the GNSS+IMU data fusion provides accurate, robust trajectory information that combines with the mobile LiDAR data.
The xNAV550 INS is an ideal companion to the Puck since it is also small and lightweight, but incorporates survey-grade dual GNSS receivers and high-grade MEMS inertial sensors for maximum position accuracy and precise heading and orientation measurements.
In order to integrate the xNAV550 and Puck, the following equipment is needed:
- xNAV550 system
- xNAV user cable
- 2 x GNSS antennas
- VLP-16 system
- Velodyne interface box
- Power supply
- Laptop running NAVsuite and Veloview
- Ethernet cable
Velodyne Interface box
The Puck comes attached with an interface box that provides protection against over voltage and reverse voltage, as well as providing a standard Ethernet and power connector for easy connection.
In order to synchronise the laser data to the GPS time from the xNAV, a one-pulse-per-second (PPS) and $GPRMC NMEA message must be output from the xNAV to the Puck. In order to wire a connection to the interface box, the top face can be removed to allow access to the connectors as seen below.
Connecting the xNAV
The xNAV user cable splits into a number of different connectors. The relevant connectors for connecting to the Interface Box are the J4 RS-232 serial connector and the J5 Digital I/O connector.
Pin 3 of the J4 connector should be wired to the GPS RECEIVE screw terminal of the interface box. This pin transmits the serial data over RS-232, which will be configured to send the required NMEA message later.
Pin 5 of the J4 connector should be wired to the GROUND screw terminal of the interface box. This pin provides a reference level for the RS-232 signal and the PPS pulse from the xNAV.
Pin 1 of the J5 connector should be wired to the GPS PULSE screw terminal of the interface box. This pin transmits the PPS output for synchronisation to the scanner.
Configuring the xNAV
This guide won’t cover the full process of configuring the xNAV, just the steps relevant to working with the VLP-16. Full details for configuration can be found in the xNAV user manual.
The xNAV is configured using the NAVconfig software supplied. The configuration can be created offline without being connected to the xNAV, but in order to commit the configuration to the system you will need to connect to it with the Ethernet connector on the user cable. All the settings relevant for this integration are found on the Options page, seen below.
Serial 1 output
The VLP-16 requires a once-per-second GPRMC NMEA message to timestamp the laser firing. The NMEA messages are configured on the serial output.
Select the Serial 1 output option and click the "..." button to open the settings window.
On the General tab, select NMEA from the Packet dropdown box and 9600 from the Baud rate dropdown box.
When NMEA is selected, the NMEA tab will appear. Click the tab to configure the NMEA message settings.
On the NMEA tab, the GPRMC message should be set to 1 Hz.
By default, the GPGGA and GPHDT messages may also already be set to 1 Hz. If this is the case, these should be disabled as the VLP-16 will not accept any other NMEA messages and they may be misinterpreted.
The checkboxes for the triggers should be left blank.
Click OK to accept the changes.
A one-pulse-per-second (1PPS) output is used to synchronise the VLP-16 data to GPS time.
On the NAVconfig Options page, scroll down to the 1PPS setting. Click the setting and select rising edge from the dropdown box.
Capturing LiDAR data
The VLP-16 doesn't require any configuration or setup in order to start producing laser data once powered on. However, in order to log the data, the UDP packets must be captured using the Ethernet connector on the interface box. Some examples to do this are to use Velodyne's open source VeloView software, which can be downloaded from http://www.paraview.org/Wiki/VeloView, or the application Wireshark which can be downloaded from https://www.wireshark.org/.
To record PCAP data, it may be necessary to change the network settings of the Ethernet adapter connected to the sensor. The IP address should be set to the following:
- IP address: 192.168.1.xx where xx can be any number except 0, 255, or 201.
- Gateway: 255.255.255.0
Any firewall restrictions should also be disabled.
With the sensor streaming data to VeloView, click Tools > Record to start logging a PCAP file.
A handy way to check the hardware interface is working correctly is to look for NMEA messages logged in the PCAP file. This is possible using Wireshark to analyse the PCAP file. They are 512 bytes (or 554 with UDP headers).
Creating a georeferenced pointcloud
To create a georeferenced 3D pointcloud, the LiDAR data needs to be combined with the INS trajectory data. There are a number of third-party tools that can do this, or OxTS have developed a custom tool OxTS Georeferencer to combine our trajectory with the Velodyne LiDAR data. OxTS Georeferencer is currently in beta, click here for more information.