Two-Way Ranging (TWR)

What is Two-Way Ranging (TWR)?

While in TDoA multiple fixed anchors work together to determine the location of a mobile object, TWR primarily uses two-way communication between two devices, such as location tracking tags deployed to people, objects, or vehicles, for real-time monitoring of the distance between them. This allows you to power two-way communication-based applications like autonomous collision awareness systems, proximity detection, and more without any additional infrastructure.

With TWR, when a device is in close proximity to another, the two devices will start ranging with each other to determine their distance. The time it takes for signals to travel between them is then multiplied by the speed of light and used to continuously determine their relative positions, often to enable location-aware communication and services.

In order to measure the distance, three messages need to be exchanged:

• A ranging device like an Inpixon RTLS tag initializes TWR by sending a ‘Poll’ message to a nearby tag or other RTLS node in its range.
• The nearby device receives and then records the time of reception for the “Poll” and replies with a “Response” message.
• The tag, upon receiving the response message, then records the time of response reception.

Based on the time of reception of this final message, the tags can then determine the time-of-flight (ToF) and distance between them.

The determined location from one device to another is then harnessed depending on the specific application. TWR can also be used by fixed anchors and UWB or Chirp devices, however the TWR process can only use one ranging partner to locate the device at a time. If multiple tagged objects need to range with the specific device (multiple mining vehicles with a miner) they need to do that one by one in a so called “round robin” manner. Inpixon’s UWB and Chirp modules have a comparably short ranging cycle so that the ranging latency can be kept fairly short - low 2-5 ms latency and high update rates up to 100Hz and further.

Inpixon’s Patented SDS-TWR Technology

Inpixon owns the patented Symmetrical Double-Side Two-Way Ranging (SDS-TWR) technique.

• Symmetrical — Measurements from Station A to Station B are a mirror-image of the measurements from Station B to Station A (ABA to BAB).
• Double-sided — Two stations are used for ranging measurement
• Two-way — Two packets, a data packet and an ack packet, are used.

It is a methodology that uses two delays that naturally occur in signal transmission to determine the range between two stations. These signals are Signal Propagation Delay between two wireless devices and Processing Delay of acknowledgements within a wireless device. Because of its simple methodology, SDS-TWR can be easily adapted to many location-aware purposes.