Most real-time location systems make a quiet compromise. They pick either accuracy or scalability. Either robustness or low cost. Either flexibility or synchronisation simplicity. Eliko’s Active-Passive Two-Way Ranging (AP-TWR) protocol was designed specifically to avoid that compromise.
This article explains what AP-TWR is, how it works, why it outperforms both standard TWR and TDoA, and why it is uniquely suited to the most demanding real-world environments — from forklifts navigating factory floors to performers tracked across a live stage.
First: what problem does a ranging protocol actually solve?
Ultra wideband (UWB) location tracking works by measuring how long it takes a radio pulse to travel between a tag (the device being tracked) and a set of fixed anchors. Time-of-flight becomes distance; distances from multiple anchors become a position.
The ranging protocol is the ruleset governing how that timing information is exchanged. Get it right, and you have an accurate, scalable, resilient system. Get it wrong, and you have congestion, clock drift errors, single points of failure, or a system that works in a demo room but not on a real factory floor.
There are two dominant approaches in the UWB RTLS market: Two-Way Ranging (TWR) and Time Difference of Arrival (TDoA). Both have well-documented strengths. Both have serious limitations. AP-TWR was built to take the best of each while eliminating the core weaknesses of both.
Standard TWR: accurate but constrained by airtime
Standard two-way ranging works through a three-message handshake: the tag sends a poll to an anchor, the anchor replies, and the tag sends a final message. The anchor measures the round-trip time, subtracts its own processing delay, and derives a precise range.
The result is a highly accurate distance measurement that requires no clock synchronisation between anchors — each exchange is self-contained. This makes TWR deployments robust and simple to configure and troubleshoot.
The problem is airtime. In standard TWR, every tag must complete this three-message handshake with every anchor it ranges to. With six anchors, that is eighteen messages per position update — per tag. As tag counts grow, the radio channel fills rapidly. The result is packet collisions, degraded update rates, and a hard ceiling on scalability. Adding more anchors to improve coverage or accuracy makes the congestion problem worse, not better.
In standard TWR, each additional anchor costs airtime. In AP-TWR, additional passive anchors cost almost nothing.
TDoA: scalable but fragile
TDoA takes the opposite approach. Tags broadcast short pulses — called blinks — and multiple anchors record when each pulse arrives. Position is computed from the time differences between anchor receptions. Tags never receive a response, so airtime per tag is minimal: one message per update cycle regardless of anchor count.
That scalability advantage is real. But TDoA has a fundamental architectural dependency: all anchors must share a common, nanosecond-precision time reference. If any anchor’s clock drifts — even slightly — position accuracy degrades across the entire zone it serves.
Maintaining this synchronisation in practice requires dedicated hardware (more expensive anchors with synchronisation circuitry), wired connections or strict line-of-sight between anchors for wireless sync, and recalibration after any anchor replacement or infrastructure change. As a result, TDoA anchor planning has more restrictions because every tracking cell must have a master anchor — creating a single point of failure that, when it fails, produces not a degraded result but no result at all.
TDoA also cannot support two-way communication. Because tags never receive a reply, low-latency safety features — proximity alarms, geofence triggers sent back to a tag in real time — are not possible. In safety-critical applications, this is a disqualifying limitation.
How AP-TWR works: the best of both
Eliko’s AP-TWR protocol introduces a third type of node into the network: the active-passive anchor. This anchor simultaneously participates in a standard TWR exchange with the tag and listens to the exchanges of other active-passive anchors — computing passive range estimates without any additional messages.
The key insight is that when anchor A and a tag exchange ranging messages, anchors B, C, D, and E can all overhear those messages and extract their own time-of-flight information — without transmitting anything. This passive ranging gives the system multiple range measurements per update cycle without consuming additional airtime.
The formal framework for this approach was developed and validated by Eliko’s engineers. In the 2020 IEEE ICSPCS paper, our engineers in collaboration with TalTech proposed the generalized AP-TWR method and demonstrated that a configuration of 5 active-passive anchors and a single passive anchor achieved a 7.4% improvement in positioning RMSE and a 12.5% improvement in airtime efficiency compared to a standard 6-anchor SS-TWR setup.
In the subsequent 2022 IEEE Sensors Journal paper, our team expanded this framework to six distinct AP-TWR variants — pairing active-passive anchor roles with single-sided (SS), symmetrical double-sided (SDS), and alternative double-sided (AltDS) TWR methods. Simulation results showed that AP2-TWR surpasses the performance of the best purely active TWR method using only 6 active-passive anchors.
A 2022 performance evaluation paper by the same team further showed that AP-TWR weighting methods (IDW, DWE) outperformed standard SS-TWR ranging RMSE by up to 14.3% in line-of-sight conditions and 19.08% in non-line-of-sight conditions — exactly the scenarios that cause other systems to degrade most severely.
What this means in practice
The protocol improvements above are not abstract. They translate directly into system capabilities that matter for real deployments:
1. No synchronisation infrastructure
Because AP-TWR is built on two-way ranging, each ranging exchange is self-contained. There are no master anchors, no dedicated clock generation hardware, no wired synchronisation lines required. Replacing an anchor does not require system recalibration. Adding coverage to a new area is a matter of placing more anchors and configuring them — not redesigning a synchronisation topology.
2. Asymmetric anchor layouts are supported
TDoA systems require anchors to surround the tracking zone from multiple directions to maintain geometric quality. AP-TWR — because it uses bidirectional ranging rather than arrival-time differences — can operate reliably with anchors on one side only. This is designed for environments like tunnels, aisles, or deployments where only one wall is available for installation. You can read more about this in our support article about positioning modes.
3. Up to 8× more data per position update
Because passive anchors extract range from existing transmissions, each active TWR exchange generates a matrix of range measurements rather than a single value. Independent analyses of Eliko deployments in industrial environments have found this produces up to 8× more data points per positioning cycle — directly improving trajectory smoothness, reducing the impact of individual packet loss, and enabling more reliable position estimates in challenging RF conditions.
4. Two-way communication and low-latency alarms
AP-TWR preserves the two-way channel that TDoA discards. This means the system can push alarms to tags in real time — with latency as low as 20 ms according to Eliko’s product documentation. Tags can receive proximity warnings, trigger external devices, or respond to geofence events as they move through the environment. This is a prerequisite for any safety-critical application.
Why AP-TWR is built for dynamic industrial and entertainment environments
Most RTLS protocols were designed with static or semi-static use cases in mind: tracking assets in a warehouse, locating equipment in a hospital. The assumption is that the environment is predictable, the tag population is stable, and the load on the network is roughly uniform.
The hardest real-world deployments share none of those properties. A forklift accelerating through a busy aisle, a performer moving across a stage, a player in an immersive game running unpredictably through a space — these environments impose exactly the conditions that expose the weaknesses of standard TWR and TDoA.
Industrial: forklift and loader tracking
Forklift and heavy vehicle tracking is one of the most demanding RTLS applications. Vehicles move fast, change direction abruptly, and operate in RF-hostile environments — metal racking, concrete floors, other radio systems. The consequences of position error are not just inaccurate data; they are safety incidents.
AP-TWR handles this environment in several ways that other protocols cannot:
- Dynamic geofences — AP-TWR’s two-way channel enables real-time proximity zones around moving vehicles. If a forklift enters within a dangerous radius of a pedestrian, the tag receives an alarm within milliseconds. TDoA cannot do this.
- Asymmetric layouts — in a warehouse aisle, anchors may only be mountable on one wall and because of the shape of the aisle, the geometry for creating a standard network is bad. AP-TWR’s corridor mode maintains reliable tracking in exactly this configuration. See the Krah Pipes use case for more
- Resilience under load — as vehicles cluster in a loading zone, standard TWR would suffer congestion. Passive anchors reuse the same transmissions, keeping airtime efficient regardless of local traffic density.
- No recalibration after anchor changes — in industrial environments, anchors get knocked, replaced, or moved when layouts change. AP-TWR requires no global recalibration after individual anchor changes.
Eliko’s deployment at Bosch Thermotechnology and the video case study of Krah Pipes demonstrates these properties in a real manufacturing environment.
Entertainment: immersive games, live shows, and interactive exhibitions
Entertainment and experience design applications demand something different from industrial ones: not just accuracy, but real-time responsiveness to unpredictable human movement, tight latency for triggering effects, and deployment flexibility in venues that change configuration between shows.
AP-TWR suits this environment specifically because:
- The two-way channel enables instantaneous triggers — when a performer steps into a defined zone, a microphone activates, a lighting cue fires, a sound effect plays.
- High update rates up to 75 Hz or simultaneously 320 Hz for a small 50×50 m space are achievable without network congestion, supporting smooth trajectory tracking for interactive systems.
- The passive anchor architecture allows venues to deploy broader coverage without proportionally increasing the active radio traffic — important in spaces where RF crowding from audience devices is already present.
- Asymmetric and flexible layouts accommodate the architectural constraints of heritage buildings, temporary installations, and venues where anchor placement is restricted.
At Cosmic CAMP, a dark room immersive experience, Eliko’s AP-TWR-based system tracked participants through an interactive environment with sufficient precision to trigger personalised experience elements based on real-time position — a use case that requires not just accuracy, but also the stability and latency performance that only a robust ranging protocol can provide.
The engineering behind the system
AP-TWR is a research outcome. The protocol was developed and validated at Tallinn University of Technology (TalTech) by Eliko’s own engineering team, and the results have been independently peer-reviewed and published in IEEE journals and conferences.
The system has since been validated in real industrial environments, including a dataset captured at a thermoplastic pipe manufacturer Krah Pipes using the Leica DISTO S910 laser distance meter as ground truth — the gold standard for indoor spatial measurement.
Eliko’s system is built entirely in-house, from hardware to firmware to positioning software. Machine learning algorithms and advanced filtering ensure that the coordinate output is stable and precise even as raw ranging data varies. The result — reliable 30cm ±20cm tracking in complex environments — is documented across multiple independent industrial deployments.
AP-TWR vs the market: a protocol built for the real world
The comprehensive review paper published in Applied Sciences (2024) surveyed UWB-based indoor positioning systems across the literature and identified AP-TWR as a distinct and significant advance in the field, noting its dual active-passive anchor role architecture and the experimental validation conducted using the Eliko UWB RTLS platform.
Independent evaluations by technology partners and system integrators have reinforced this assessment. eInfochips, a global engineering services company, ran comparative tests of available RTLS solutions and concluded: they are confident in recommending the TWR-based Eliko RTLS as a robust UWB solution. Flowcate, a leading industrial RTLS middleware provider, certified Eliko’s system as DeepHub Connected — confirming it integrates into Omlox-compliant industrial infrastructure.
The case for AP-TWR is straightforward: it is the only UWB ranging protocol that simultaneously achieves high accuracy, high scalability, two-way communication for safety applications, no synchronisation dependency, and flexible anchor layout support. No other protocol on the market combines all five.
AP-TWR is the only UWB protocol that achieves high accuracy, scalability, safety alarms, no synchronisation dependency, and asymmetric layout support — simultaneously.
Get in touch if your use case requires robust accuracy across a complex space with real-time tracking.
