TDOA vs. Ranging in UWB
This article is a very high level guide to help LocoPoint users understand some of the key technologies employed in it’s positioning system.
In Ultra-Wideband (UWB) positioning systems, two common methods for determining the location of an object are Time Difference of Arrival (TDOA) and Ranging. Both methods are based on measuring the time it takes for a signal to travel between UWB devices, but they differ in how this timing is used to calculate distance or position.
1. Ranging (Time of Flight – ToF)
Ranging measures the absolute time it takes for a UWB signal to travel from one device (often a tag) to another (often an anchor or reference point). This method is also referred to as Time of Flight (ToF).
- Direct distance measurement: Ranging measures the time the signal takes to travel between two devices, which is then converted into distance using the known speed of light.
- Two-way communication: To achieve this, ranging typically requires two-way communication. The first device sends a signal, and the second device responds. The round-trip time is then used to compute the distance.
- Latency: Because two-way communication is required, ranging can sometimes introduce latency.
- Fewer devices needed: A single distance measurement can give a rough estimate of location, but for precise 3D location, multiple anchor points are typically used.
Applications:
- Used in systems where direct measurement of distance is needed, for example, asset tracking in warehouses.
- Also common in UWB-based proximity sensors.
2. Time Difference of Arrival (TDOA)
TDOA measures the difference in the time it takes for a signal to reach multiple receiving devices (often anchors) at different locations. Instead of calculating the absolute time of travel, TDOA relies on the relative timing difference.
- Relative distance measurement: TDOA does not require measuring the absolute distance between devices. Instead, it uses the difference in arrival times at multiple anchor points.
- One-way communication: The tag or object only needs to send a signal, and the difference in signal arrival times at multiple anchors is used to calculate the object’s location. The object itself doesn’t need to know the time.
- Scalability: TDOA systems can scale well in environments with many anchors because the tag or object doesn’t need to engage in two-way communication with every anchor.
- More precise for positioning: TDOA is typically more accurate than ranging in large, complex environments where multiple anchors can give precise positioning based on timing differences.
Applications:
- Commonly used in real-time location systems (RTLS) for tracking objects in large areas (factories, hospitals, etc.).
- More suitable for systems requiring continuous tracking of multiple objects with low latency.
Comparison
Ranging (ToF): Measures the absolute time for a signal to travel between two devices, requiring two-way communication.
TDOA: Measures the time difference of signal arrival at multiple receiving points (anchors), requiring only one-way communication.
Ranging is often simpler but requires more interactions between devices, while TDOA is highly scalable and efficient in environments with multiple anchors, especially for real-time tracking.
Here is a diagram that visually explains the two UWB positioning techniques:
- On the left side, Ranging (Time of Flight – ToF) shows how a signal is sent from a tag to an anchor. The distance is calculated by measuring the time the signal takes to travel between them.
- On the right side, Time Difference of Arrival (TDOA) illustrates how a tag sends a signal to multiple anchors. The difference in signal arrival times at each anchor is used to calculate the tag’s location.
If you’d like to know more or find out how you can benefit from LocoPoint UWB, please contact us. System Loco has a number of integration partners in US who can help advise you on your real-time indoor positioning journey.
