# CBM: An IoT Enabled LiDAR Sensor for In-Field Crop Height and Biomass Measurements
## 2 Materials and Methods  
### System Architecture  
There are three main hardware components of IoT that enabled the CBM sensor; solid-state LiDAR sensor (Leddar Tech Vu8, Leddar Tech, Quebec City, QC, Canada), onboard Raspberry Pi 4 computer, and autopilot controller that was used for an embedded GNSS logger (Navio2, Emlid Ltd., Hong Kong).  
For the crop height measurements, a solid-state LiDAR LeddarTech Vu8 module was used as it specifically measures ranges in a given direction within a required FoV. Regular commercial-grade LiDARs scans in every direction (360\({}^{\circ}\)) that produce voluminous data and pose processing challenges. A narrow FoV solid-state LiDAR unit is suitable for scanning only the canopy profile which significantly reduces the data output and processing overload. The selected Leddar Tech Vu8 module sends pulsed light in a horizontal FoV of 48\({}^{\circ}\) with the reflected pulses measured into 8 discrete segments each with 6\({}^{\circ}\) of horizontal FoV coverage; the vertical FoV of the module is 0.3\({}^{\circ}\). The laser emitter operates over a near-infrared wavelength of 905 nm with IEC 60825-1:2014 Class 1 eye safety rating. It works with a 12 V \(\pm\) 0.6 DC input power supply with a power consumption of 2 W. LeddarTech Vu8's carrier board hosts the electrical and communication interface of the module and has two interfacing options, including SPI or USB-CAN-serial (UART/RS-485). The sensor has a programable data refresh rate, measurement accumulation, and sensitivity peak, which help in tuning the sensor to accurately profile different type of targets, including vegetation.  
A Raspberry Pi 4 4 GB Model B single-board computer (SBC) was used as the sensor driver unit. Raspberry Pi is known for its compact size (as small as the size of a credit card) and relatively good processing power- 1.5 GHz quad-core Cortex-A72 (ARM v8) 64-bit System-on-Chip (SoC). It has 4 GB of LPDDR4-3200 Synchronous dynamic random-access memory (SDRAM) storage, 2.4 GHz and 5.0 GHz IEEE 802.11 ac wireless (Bluetooth 5.0, BLE), 2 USB 3.0 ports, 2 USB 2.0 ports, 40 pin general-purpose input/output (GPIO) header, and a micro-SD card slot for loading operating system and data storage. The SBC can be powered with a 5 V DC via USB-C connector or GPIO header. The Raspberry Pi also have low power consumption and plethora of available library packages, which is beneficial in rapid prototyping of new sensor systems. For tagging the LiDAR measurements with positioning logs, a Navio2 GNSS receiver (Emlid Ltd., Hong Kong) was used supporting GPS, GLONASS, Beidou, Galileo, and SBAS satellites constellation systems. Additionally, Navio2 is relatively low-cost, more reliable than other commercial-grade positioning sensing systems.  
A detailed schematic overview of the CBM components is provided in Figure 1. The LeddarTech Vu8 module was interfaced with the USB port on Raspberry Pi 4 over USB-CAN-serial communication. The GNSS receiver on Navio2 was interfaced over the Raspberry Pi 4 with the 40 pin GPIO port. The power supply to the respective units was provided through a voltage regulator circuitry. The regulator provides a steady and filtered DC output power; a 12 V constant output was supplied to the LeddarTech Vu8 module and step-down 5 V output to power the Raspberry Pi and Navio2 modules. The total current consumption was rated around 800 mA in the regular mode of operation.