Assymetric TWR vs Synced OWR

During a development of new ranging system we had another discussion about assymetric TWR versus synced OWR systems.

Let’s briefly go over both of them and then let’s see what the main differences are. Assymetric TWR system uses a system of three messages, poll, respond and final message. This method does not need clocks to be synchonized since time drift is would be zeroed by our equeasion that has two possitive and two negative time drifts. In other hand synced OWR system would have synced clocks and will need just one message to determine location of the object. Now let’s get into details.

OWR-TWR-RTLS Assymetryc TWR system is based on TOA(time of arrival) technology, which means that we calculate the time that signal travels between the points and multiply by speed of light. This way we will get pretty accurate number, which again, most likely will be limited by hardware accuracy. Downside of this system is that we need 3 messages to determine a location of the object. And information stays on the other side of equation, meaning that if anchor sends our sygnal first, then all the location info will end up on tag(Anchor-tag-anchor-tag).

Synced OWR system uses TDOA(time difference of arrival) technology which requered more complex calculation system. It also requires that all anchors clock will be synced, otherwise even tiny time difference would ruin accuracy. Anchor clock have to be synced eighter using wires or wirelessly. Both ways require a lot of prep work. Final accuracy might suffer based on anchors allocation. In cases when anchors are located closely to each other, the further tag goes away from them – lower accuracy is measured. For perfect calculations anchors have to be allocated around the tag. Final information stays on anchor, so you have to send it over to tag if needed.

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