How ranging works, in full.
Four words come up across this site: UWB, anchor, ranging and tag. This page explains each one in plain words, and lets you move a dot through a room to see what ranging measures.
Drag the dot. Each line is a distance.
In each corner of this room sits an anchor. Ranging times a UWB radio signal between the dot and each anchor and turns that time into a distance. Here each dotted line is one of those distances, computed from the drawing; nothing in this demo is measured.
Drag the dot, tap where it should go, or use the sliders under the room. With a keyboard, focus the dot and use the arrow keys; hold Shift to move farther.
Static diagram: the dot rests in one place, and each dotted line shows its distance to an anchor.
Illustrative room, 12 meters on a side.
Illustrative demo. These dots are not real students.
- North-west anchor 9.0 m
- North-east anchor 6.2 m
- South-west anchor 10.8 m
- South-east anchor 8.6 m
The dot is 6.2 meters from the north-east anchor, the nearest.
UWB
UWB stands for ultra-wideband. It is a kind of short-range radio that two devices use to measure the distance between them.
Most radios send a steady signal on one narrow channel. UWB sends very short pulses spread across a wide band of frequencies. A short pulse has a sharp start, so the device that receives it can tell when it arrived. That arrival time is what ranging works from.
Anchors
An anchor is a small radio fixed in place. Because it stays put and its place is known, it is a reference point.
One distance only says how far a tag is from one point. Distances to several anchors, each in a known place, together say where the tag is among them. That is why the room above has an anchor in each corner.
Ranging
Ranging is measuring the distance between two radios by timing how long a signal takes to travel between them. Radio travels at the speed of light, so a travel time converts directly into a distance.
Two radios do not share a clock, so they cannot simply compare times. In two-way ranging, one common method, the radios trade short messages and each notes when a message left and when its reply arrived. The round trip, minus the time the other radio took to reply, is the travel time there and back.
Questions and answers
Are the distances in the demo measured?
No. The room on this page is an illustrative drawing. Its distances are computed from where the dot sits in the drawing; no radio measures anything in it.
What is the difference between an anchor and a tag?
An anchor stays fixed in one place and serves as a known reference point. A tag is the small radio a student wears, and it moves with them. Ranging measures the distance between a tag and an anchor.
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