The main output of the KIO RTLS is precise, real-time coordinates. These are based on trilateration calculations, using the distance measurements between the tag and the anchors (often called beacons or satellites). Similar to GPS, it is possible to calculate the exact location of the tag if you have at least four reference points or anchors.
Both the ToF and TDoA methods rely on measuring the time it takes for the radio waves to travel a distance in the air. As radio waves travel at the speed of light, it is easy to calculate the distances we need from these time measurements. In reality, however, it’s a real challenge to overcome the uncertainties of radio wave propagation and measure these really tiny time slots precisely every single time.
The difference between ToF and TDoA lies in what distances we actually measure and how we use them to calculate the position of the tag.
When using the time-of-flight method, the tag measures how long it takes for the radio signal to travel from the tag to the anchor and back to the tag. We can then calculate the distance from the tag to each anchor. Once we have the distances to at least four anchors, we can use trilateration to calculate the X, Y and Z coordinates of the tag.
When using the time-difference-of-arrival method, the tag sends out data packets, sometimes called “blinks”. All the nearby anchors pick up this blink message but do not respond back. As the anchors are located at different distances from the tag, the message does not reach every anchor at the exact same moment in time. These time differences between the anchors are the basis for the multilateration calculation to determine the X, Y and Z coordinates of the tag. The key point here is that all the anchors must work in sync; otherwise, the results will be meaningless.
The characteristics of both methodologies translate into real-life advantages and disadvantages for an RTLS application.
TDoA requires additional hardware
As previously stated, one of the essential prerequisites of the TDoA method is keeping the anchors in sync. I cannot emphasize this enough, that as we measure the flight times of radio wave propagation, the synchronization must be really precise. This can be a real challenge, both because the anchors are usually far from each other and our synchronization signals can’t move any faster than the speed of light.
The solution for this precise synchronization problem lies in special hardware, which makes the whole RTLS setup more expensive. There are special clock generation units needed, as well as approximately 20-40% more expensive anchors, which should be able to extract and use the synchronization signals. Overall, the TDoA method has a direct negative impact on the RTLS’ payback time.
TDoA allows you to track more tags, more often
The ToF method is based on 2-way communication between the tag and the anchor, while in TDoA the communication only goes 1 way: the tag sends out a signal, but doesn’t need to wait for a reply. This means there is more airtime for locating additional tags, or positioning the existing tags more often. Several applications (for example, in sports) require a high density of tags. Here, TDoA has a clear advantage over ToF.
The battery-powered tag lasts longer with TDoA
As mentioned earlier, ToF is based on 2-way communication between the anchor and the tag. This means that the tag needs to emit and receive signals from the anchors several times. This is a power-hungry way of positioning compared to just emitting a signal in TDoA ranging. As a result, ToF measurement requires the user to charge the battery more frequently. In large-scale applications, charging hundreds of batteries every other week can be a real inconvenience.
TDoA is more difficult to set up and maintain
To minimise the risk of measurement errors due to clock synchronization, the anchors are usually connected in a wired system, which makes the overall installation clumsier. With a TDoA based RTLS, each installation needs specific calibration after the initial set-up. This process includes fixing reference tags to known locations and running a special calibration software. It is also important to know that any anchor or even cable replacement will trigger the need for a new calibration.
We have chosen to use ToF instead of TDoA because it is, by far, a more robust technique.
TDoA has its own advantages—it saves energy and allows you to track more tags—however, there is too much that can go wrong with time synchronization. In real-life applications, where systems need to provide guaranteed accuracy, robustness is often the most desirable quality.