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Surrogate Basic Safety Messages for V2X Testing and Evaluation: An Inside Look at SAE J2945/8-2024
⚠️ Important Note: SAE J2945/8-2024 is an Information Report intended solely for testing and evaluation purposes. The functional requirements described here must not be applied in production V2X deployments, as they are designed to support controlled experiments in mixed-equipage environments.
1. The Need for Surrogate BSM in Mixed-Equipage Environments
Vehicle-to-Everything (V2X) communication holds great promise for improving road safety, but the benefits are limited when only a fraction of vehicles are equipped. In initial deployments, it is critical to evaluate how V2X-equipped vehicles can interact with non-equipped ones. The SAE J2945/8-2024 standard addresses this need by defining a Surrogate Basic Safety Message (SBSM) that an equipped host vehicle (eHV) or roadside equipment (RSE) can broadcast on behalf of an unequipped remote vehicle (uRV). This allows testing and evaluation of V2X applications in a mixed-equipage environment without requiring all vehicles to be retrofitted.
The rationale behind SBSM is to compensate for low penetration rates and enable coexistence with legacy vehicles. By using on-board sensors (radar, camera, LiDAR) combined with V2X data, an eHV can detect an uRV and generate a basic safety message that mirrors the uRV’s state, enabling other equipped vehicles to “see” the non-equipped vehicle in their V2X awareness picture. The standard clearly states that this is a testing tool only, helping industry partners validate the effectiveness of V2X in realistic, less-than-ideal integration scenarios.
2. Key Functional Requirements for Surrogate BSM
J2945/8 establishes a set of functional requirements to ensure that SBSM transmissions are safe, accurate, and respectful of network capacity and privacy. The standard covers detection and characterization of unequipped vehicles, minimum transmission criteria, data accuracy, and security. Below are some of the central requirements.
Detection and Characterization of Unequipped Vehicles
The equipped vehicle must use its perception sensors (radar, camera, LiDAR) and cross-reference with V2X messages to reliably detect an uRV. The system must determine at least the uRV’s position, speed, heading, and vehicle dimensions with sufficient accuracy to produce a meaningful SBSM. A key requirement is that SBSM generation should only be attempted when the detection confidence is high; transmitting with low confidence could introduce errors rather than improve safety.
Minimum Transmission Criteria (MINTX)
| Criterion | Requirement |
|---|---|
| Detection Confidence | High confidence in uRV presence, position, and dynamics. |
| Duplicate Prevention | Do not transmit if another SBSM from a different eHV is already representing the same uRV within a configurable range and time window. |
| Transmission Rate | Limited to avoid network congestion; may be tied to the uRV’s dynamics (e.g., update every 100 ms when moving). |
| Positioning Accuracy | Typically sub-meter (e.g., 1.0 m or better) for uRV absolute position to enable effective safety applications. |
| PSID | Use the Test Provider Service Identifier (e.g., 0x7F) to distinguish surrogate messages from actual V2X messages. |
Data Element Accuracy
The standard proposes accuracy requirements for key SBSM data elements. For example, heading should be within 5 degrees, speed within 1 m/s, and vehicle length and width within 0.5 m. These values ensure that the surrogate message is ‘good enough’ for safety applications while recognizing the limitations of surrogate detection.
Security and Privacy
SBSM should be signed using the Security Credential Management System (SCMS) using a Test PSID to separate surrogate messages from authentic V2X messages. Privacy must be respected; the surrogate message must not reveal information that could track the unequipped vehicle without consent (e.g., avoid using permanent identifiers). The standard also recommends mechanisms to prevent misuse, such as transmission constraints and the use of short-term certificates.
🔍 Engineering Design Insight: J2945/8 deliberately bases the SBSM structure on the existing Basic Safety Message (BSM) defined in SAE J2735. This reuses the same data frames and ASN.1 definitions, minimizing changes to the V2X protocol stack. For testing, this means existing BSM parsers can understand SBSM with only small modifications to account for the Test PSID.
3. Design Insights and Frequently Asked Questions
Implementing a surrogate BSM system for testing requires careful attention to coordination, accuracy, and adherence to the standard’s constraints. Below we address some common questions and design considerations.
Frequently Asked Questions
Q1: What is the main purpose of SAE J2945/8?
The standard provides functional requirements for a V2X-equipped host vehicle (eHV) or Roadside Equipment (RSE) to send a Surrogate Basic Safety Message (SBSM) on behalf of an unequipped remote vehicle (uRV). Its sole purpose is to enable testing and evaluation of V2X applications in mixed-equipage environments.
Q2: How does an SBSM differ from a regular BSM?
A regular BSM comes from an equipped vehicle’s own systems and uses a standard Provider Service Identifier (PSID) (e.g., 0x20 for V2V safety). An SBSM is generated by a different entity (eHV or RSE) using a Test PSID (e.g., 0x7F) to explicitly mark it as a surrogate. The data content is similar, but the source and context are different.
Q3: When should an equipped vehicle start sending an SBSM?
The system should only generate SBSM when the uRV is detected with high confidence and the system is operating in a test/evaluation mode. The standard defines “Minimum Transmission Criteria” (MINTX) that include confidence thresholds, and it requires checking that no other eHV is already providing an SBSM for the same uRV in the area (to reduce duplicates).
Q4: What prevents multiple vehicles from sending duplicate SBSMs for the same vehicle?
J2945/8 includes a dialog-based coordination mechanism in its reference architecture. An eHV that detects an uRV should first listen for existing SBSMs covering that uRV (identified by position and time). If one is present, the eHV should refrain from transmitting its own SBSM for that uRV. This can be implemented via a distributed “claiming” procedure or by using a centralized coordination service (e.g., at an RSE).
🛠️ Design Consideration: One of the common mistakes when testing with SBSM is over-reliance on low-quality sensor data. Ensure that your detection pipeline (e.g., fusion of radar and camera) provides the accuracy levels expected by the standard—especially for position and dynamics. Otherwise, the surrogate message may mislead other equipped vehicles and degrade safety instead of improving it.
For teams involved in V2X testing and evaluation, SAE J2945/8-2024 offers a practical and well-structured approach to emulate the presence of unequipped vehicles. By following the functional requirements on detection, transmission, and security, testers can gather valuable insights while maintaining safety and network integrity.
