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Our tutorial paper on backup-based safety filter has been accepted to CDC 2026. One day, I noticed that three safety filters—Backup CBF, MPS, and gatekeeper—looked quite similar. Although they have attracted attention in different research communities, their core ideas are surprisingly alike. In this paper, we revisit the three methods within a unified framework, compare them side by side, and clarify the advantages and limitations of each. The comparison also reveals how gatekeeper can reduce conservatism by searching over the switching time.
Backup-based safety filters are safe, but can be very conservative
How can we achieve this behavior?
Backup-Based Safety Filters: A Comparative Review of Backup CBF, Model Predictive Shielding, and gatekeeper
Authors: Taekyung Kim, Aswin D. Menon, Akshunn Trivedi, Dimitra Panagou
Abstract: This paper revisits three backup-based safety filters—Backup Control Barrier Functions (Backup CBF), Model Predictive Shielding (MPS), and gatekeeper—through a unified comparative framework. Using a common safety-filter abstraction and shared notation, we make explicit both their common backup-policy structure and their key algorithmic differences. We compare the three methods through their filter-inactive sets, i.e., the states where the nominal policy is left unchanged. In particular, we show that MPS is a special case of gatekeeper, and we further relate gatekeeper to the interior of the Backup CBF inactive set within the implicit safe set. This unified view also highlights a key source of conservatism in backup-based safety filters: safety is often evaluated through the feasibility of a backup maneuver, rather than through the nominal policy's continued safe execution. The paper is intended as a compact tutorial and review that clarifies the theoretical connections and differences among these methods.
