LaserScan Data

Before writing any code, it is important to understand what scan data is available and how it is structured. The following ROS 2 commands allow you to discover the /scan topic, inspect its message type, and explore the data used for Automatic Emergency Braking (AEB).

Finding the /scan Topic

First, confirm that the lidar scan topic exists.

ros2 topic list
ros2 topic list | grep scan

If /scan appears in the list, the lidar is publishing data and you are ready to proceed.

Checking the Message Type

Next, verify the type of message being published on /scan.

ros2 topic info /scan

You should see output similar to:

  • Type: sensor_msgs/msg/LaserScan

This tells us that scan data is published using the LaserScan message.

Inspecting the LaserScan Message Definition

To see what information is contained in each scan message:

ros2 interface show sensor_msgs/msg/LaserScan

This is one of the most important steps. It shows all available fields, including:

  • ranges[] – distance measurements in meters

  • angle_min, angle_max, angle_increment – used to map array indices to angles

  • range_min, range_max – valid sensor distance limits

  • header.frame_id – the coordinate frame of the scan data

Echoing Live Scan Data

To view raw scan data as it is published:

ros2 topic echo /scan

This produces a large amount of output. More student-friendly options include:

Show only a single scan message:

ros2 topic echo /scan --once

Show a compact portion of the message:

ros2 topic echo /scan --once | head -n 60

Understanding /scan Output (LaserScan)

The /scan topic publishes lidar data using the sensor_msgs/msg/LaserScan message. The most important field for Automatic Emergency Braking (AEB) is ranges[] — an array of distance measurements (in meters) taken at evenly spaced angles across the lidar’s field of view.

What ranges[] Means

  • ranges[] is an array of distance readings in meters.

  • Each element corresponds to one laser ray at a specific angle.

  • The scan spans from angle_min to angle_max in steps of angle_increment.

From the example output:

  • angle_min = -2.356 rad (≈ -135°)

  • angle_max =  2.356 rad (≈ +135°)

  • angle_increment = 0.004363 rad (≈ 0.25° per sample)

This results in approximately a 270° field of view.

How Many Samples Are in One Scan?

The number of samples in ranges[] is approximately:

\[N \approx \frac{\text{angle\_max} - \text{angle\_min}}{\text{angle\_increment}} + 1\]

With the values above, this scan contains roughly 1081 distance measurements.

Mapping an Index to an Angle

Each index in ranges[] maps to an angle using:

\[\text{angle}(i) = \text{angle\_min} + i \cdot \text{angle\_increment}\]

  • ranges[0] corresponds to angle_min (far left)

  • ranges[last] corresponds to angle_max (far right)

“Straight Ahead” Index

The index closest to straight ahead (0 radians) can be estimated by:

\[i_{\text{forward}} \approx \frac{0 - \text{angle\_min}}{\text{angle\_increment}}\]

For this scan, the forward-facing index is approximately 540, near the center of the array.

Interpreting Range Values

  • Values below 1 meter indicate very close obstacles.

  • Values between 1–3 meters represent nearby objects.

  • Readings of inf or values greater than range_max indicate no valid return.

In the example output, a value such as 65.53 exceeds range_max = 30.0 and should be treated as invalid for AEB calculations.

Scan Timing and Health

You can check how often scans are published:

ros2 topic hz /scan

A higher scan rate allows faster reaction times for braking decisions.

You can also inspect bandwidth usage:

ros2 topic bw /scan

This helps explain why ranges[] contains a large amount of data.

Confirming Frame Information

If you are working with angles or transforming scan data, it is important to know the coordinate frame.

ros2 topic echo /scan --once | grep frame_id

This confirms the frame in which the scan data is reported and how it relates to the rest of the robot.

Hardware: Hokuyo UST-10LX LiDAR

The /scan data you’re working with comes from the Hokuyo UST-10LX 2D LiDAR sensor mounted on the RoboRacer vehicle. Understanding the hardware specifications helps you interpret the scan data and design effective safety systems.

Hokuyo UST-10LX LiDAR

Note

Manufacturer Information

Full specifications available at: Hokuyo UST-10LX Product Page

Key Specifications

Measurement Range

  • Maximum Range: 10 meters

  • Minimum Range: 0.06 meters (60 mm)

  • Values beyond these limits in ranges[] should be treated as invalid

Field of View

  • 270° coverage (approximately -135° to +135°)

  • This wide field of view allows detection of obstacles to the sides as well as front

  • Corresponds to the angle_min to angle_max values you see in the scan message

Angular Resolution

  • 0.25° per measurement (0.004363 radians)

  • This is the angle_increment value

  • Results in approximately 1081 distance measurements per scan

Scan Rate

  • 40 Hz scanning frequency

  • Provides scan updates 40 times per second

  • Critical for real-time obstacle detection and emergency braking

Accuracy

  • ±30 mm under standard conditions

  • Important when calculating precise time-to-collision values

  • Range measurements are reliable within this tolerance

Hokuyo UST-10LX Specifications

Physical Characteristics

Compact and Lightweight

  • Dimensions: 62 × 62 × 87 mm

  • Weight: 130 g

  • Minimal impact on vehicle dynamics

Environmental Protection

  • IP65-rated enclosure

  • Dustproof and water-resistant

  • Suitable for indoor robotics environments

Power and Communication

  • 12V DC operation (~8W power consumption)

  • Ethernet interface for high-speed data transfer to ROS 2

  • Ensures low-latency scan data delivery

Connecting Hardware to Software

When you subscribe to /scan, you’re receiving data that reflects these hardware characteristics:

  • The ranges[] array length (~1081 elements) comes from the 270° FoV divided by 0.25° resolution

  • The 40 Hz scan rate determines how quickly your AEB system can react

  • The 10-meter max range defines the maximum detection distance for obstacles

  • The ±30 mm accuracy affects the precision of your TTC calculations

Understanding these specifications helps you make informed decisions about:

  • Threshold values for collision detection

  • Update rates for your safety node

  • Valid range filtering to exclude sensor noise or invalid readings