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• Description: Key-information digest of the Velodyne VLP-32C ("Ultra Puck") LiDAR User Manual (doc 63-9325) — sensor model, packet rates, data/position packet structure, azimuth/distance encoding, and network access. Reference for parsing raw VLP-32C UDP streams
• My Notion Note ID: K3-1-1
• Created: 2020-06-08
• Updated: 2026-07-18
• License: Free to share: please credit Yu Zhang and link back to yuzhang.io

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Table of Contents

• 1. Overview
• 2. Sensor Operation & Rates
• 3. Data Packet Structure
• 4. Position Packet Structure
• 5. Access & Control
• References

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1. Overview

VLP-32C = 32-channel mechanical spinning LiDAR ("Ultra Puck"), 903 nm, time-of-flight. Reference figures/tables below cite the User Manual (63-9325).

1.1 How LiDAR works (Time of Flight)

• Each IR laser emits a pulse; time-of-shooting + direction registered.
• Pulse travels until it hits an obstacle → part of the energy reflected back → paired IR detector registers time-of-acquisition + received power.
• Distance ← round-trip time; reflectivity ← received power.

1.2 Sensor Origin & Frame of Reference

• Distances reported relative to the sensor in spherical coordinates: radius $r$, elevation $\omega$, azimuth $\alpha$.
• Data origin $(0,0,0)$ is 37.34 mm above the sensor base, on the center axis (Figure 9-1).

1.3 Packet Types

• Data packets — 3D measurements + calibrated reflectivity, a set of azimuths, a 4-byte timestamp, and two factory bytes (identify sensor model + return mode → software auto-adapts to the data format).
• Position packets — echo of the last NMEA message from a GPS time source (if configured) + a byte for the PPS (Pulse Per Second) signal state.

1.4 Calibrated Reflectivity

• Diffuse reflectors: 0–100 (for reflectivities 0%–100%).
• Retro-reflectors: 101–255 (255 = ideal retro-reflector; 101–254 = partially obstructed / imperfect).

1.5 Laser Return Modes

Sensor analyzes multiple returns; selectable output = strongest, last, or both (dual return).

1.6 Precision Azimuth Calculation

• The azimuth $\alpha$ reported per data block = the center line of the firing pattern at the moment the first laser pair fired.
• Lasers are offset from the center line by fixed amounts → to get each point's precise azimuth, apply a horizontal offset per laser (vertical angle + azimuth offset given by the laser's position in the packet, Table 9-2). Lasers are not evenly placed.

2. Sensor Operation & Rates

2.1 Firing Sequence

• Takes 55.296 μs to fire + recharge all 32 lasers (a "firing sequence" / "firing group"). Lasers fire in pairs.
• Time between firings = 2.304 μs. 16 firings, then an idle recharge period of 18.432 μs:

$$(16 \times 2.304\ \mu s) + 18.432\ \mu s = 55.296\ \mu s$$

The manual's prose rounds the idle period to 18.43 μs but uses 18.432 μs in the formula — 18.432 is the exact value.

2.2 Data Packet Rate

• 12 firing cycles per data packet → accumulation delay $12 \times 55.296\ \mu s = 0.663552$ ms/packet.
• $1 / 0.663552\ \text{ms} = 1507$ packets/second.
• $1248\ \text{bytes/packet} \times 1507\ \text{packets/s} = 1{,}880{,}736\ \text{bytes/s}$.

2.3 Position Packet Rate

• Position packets arrive at ≈ 1/14 the data-packet rate:
• $554\ \text{bytes/packet} \times 1507 / 14 = 59{,}634\ \text{bytes/s}$.

2.4 Total Packet Rate

• Single return: $1{,}880{,}736 + 59{,}634 = 1{,}940{,}370\ \text{bytes/s}$.
• Dual return (data rate doubles, position rate unchanged): $(2 \times 1{,}880{,}736) + 59{,}634 = 3{,}821{,}106\ \text{bytes/s}$.

2.5 Laser Measurements per Second (single return)

• $32\ \text{firings/block} \times 12\ \text{blocks/packet} = 384\ \text{firings/packet}$.
• $384 \times 1507 = 578{,}688$ laser measurements/second.

2.6 Rotation Speed

• Motor: 300–1200 RPM inclusive, in increments of 60 RPM (300, 360, 420, …, 1140, 1200).

3. Data Packet Structure

3.1 Laser Channel

• A single 903 nm laser emitter + detector pair, fixed at a particular elevation angle, with its own Laser ID.
• Elevation doesn't change → not stored in the packet; inferred from the data point's position in the block.
• In VLP-32C, no laser channel is vertically inline with the azimuth in a firing group; each channel is offset by one of four different azimuth offsets.

3.2 Data Point

• 3 bytes = 2 bytes distance + 1 byte calibrated reflectivity.
• Distance = unsigned int with 4 mm granularity → a raw value 25,154 represents $25{,}154 \times 4 = 100{,}616$ mm = 100.616 m.
• Elevation angle $\omega$ inferred from position within the data block.

3.3 Azimuth

• 2-byte unsigned int, in hundredths of a degree → raw 27,742 = 277.42°.
• Valid range 0–35,999. One azimuth reported per data block.

3.4 Data Block

• Holds one firing sequence of 32 lasers; 12 data blocks per data packet; one azimuth per block.
• 100 bytes = 2-byte flag 0xFFEE + 2-byte azimuth + 32 data points ($2 + 2 + 32 \times 3 = 100$).
• For time-offset calculations, number the blocks 0–11.

3.5 Time Stamp

• 4-byte, 32-bit unsigned int marking the first data point of the first firing sequence of the first block.
• Value = microseconds since the top of the hour, range 0 – 3,599,999,999 (μs in one hour).

3.6 Factory Bytes

• Two bytes indicating how azimuths + data points are organized in the packet (values/meanings in Table 9-1). Together with the model byte they let parsing software auto-adapt.
• (VLP-32C firmware at time of manual = 4.3.0.0.)

3.7 Overall Data Packet

• 1248 bytes, sent via UDP on port 2368: 42-byte protocol header + 12 data blocks + 4-byte timestamp + 2 factory bytes.
• Two formats: Single Return (strongest or last) and Dual Return.
• In dual return the sensor sends a pair of blocks per azimuth firing: odd blocks (1, 3, …, 11) = strongest or second-strongest; even blocks (0, 2, …, 10) = last return. If strongest = last, the second-strongest is provided; if only one return, even|odd pairs are identical. Data rate doubles.

4. Position Packet Structure

• 554-byte UDP packet on port 8308: an echo of the NMEA sentence from an external time source + PPS status + a timestamp of when the position packet was assembled.
• Field layout (Table 9-3, offsets from start of packet). Several fields the older draft marked "unused" are actually sensor telemetry:

Bytes	Offset (hex)	Description
42	0x0000	UDP header
187	0x002A	Reserved
11	0x00E5	Board temperatures + ADC calibration status/timing
4	0x00F0	Timestamp (μs since top of hour)
1	0x00F4	Pulse Per Second (PPS) status
3	0x00F5	Thermal status, last-shutdown temp, power-up temp
128	0x00F8	NMEA sentence (GPRMC/GPGGA), fixed 128-byte field, null-padded
178	0x0178	Reserved

Total = $42 + 187 + 11 + 4 + 1 + 3 + 128 + 178 = 554$ bytes.

5. Access & Control

• Default IP: factory-set to 192.168.1.201. To talk to the sensor directly, put the host on the same subnet.
• Web interface: connect Ethernet → power the Interface Box (two green LEDs) → sensor starts scanning/transmitting ~30 s after power-up → browse to http://192.168.1.201. The web UI exposes most control settings.
• Visualization: VeloView (open-source, Velodyne-tailored) for live view + recording; ROS / PCL can substitute.
• Scripted control: HTTP curl against the sensor's control endpoints (see manual).

References

• Velodyne LiDAR. VLP-32C User Manual (63-9325). — the source manual; §6 (data structures), §8 (firing timing), §9 (packet formats), Appendix C (mechanical). Firmware 4.3.0.0.