Your phone has known where it is for years thanks to satellites, mobile masts and Wi-Fi networks. But none of those systems can answer a far humbler question that, for many jobs, is far more useful: how many centimetres separate you from that thing right in front of you. That is the job of another radio almost nobody talks about, UWB (ultra-wideband), and its trick is to transmit almost nothing: fire very short pulses and time how long they take to come back.
A two-nanosecond pulse and an enormous spectrum
Wi-Fi and Bluetooth send waves that stay switched on for as long as the message lasts. UWB does the opposite: it emits a radio pulse lasting about two nanoseconds — two billionths of a second — and then goes quiet. It repeats that millions of times per second, but always in extremely short bursts. The consequence lies in the Fourier transform, the mathematical tool that translates between what happens in time and what a signal occupies in frequency: the shorter the pulse, the wider the range of frequencies it fills.
A two-nanosecond pulse spans roughly 500 MHz of spectrum, and that is the key to this technology being both legal and useful. Regulators — the US FCC from 2002, Europe afterwards — reserved between 3.1 and 10.6 GHz for it with one condition: it must occupy at least 500 MHz. In exchange, the maximum permitted power is minus 41.3 dBm per MHz, an absurdly weak emission, on the order of the background noise produced by any electronic device. That is why a UWB tag can coexist with GPS, Wi-Fi and satellite links without disturbing them in the slightest.
Two channels, half a gigahertz each
In the IEEE 802.15.4z standard, the one used by the chips inside phones, two channels are especially common: channel 5, centred on 6,489.6 MHz, and channel 9, on 7,987.2 MHz. Each occupies 499.2 MHz of bandwidth and transmits at a symbol rate — what the standard calls a chip — of 499.2 million per second, that is, one signal element every 2.003 nanoseconds. Data rates run from 110 kbit/s to 6.8 Mbit/s, laughable next to Wi-Fi: nobody here wants to move files, only to exchange a few measurement frames carrying identifiers and timestamps. The goal is distance, not bandwidth.
The stopwatch, not the map
UWB does not triangulate and does not read satellite signals: it measures time. The basic method is called two-way ranging. Device A fires a pulse; B receives it and answers after a delay it knows in advance; A measures the total elapsed time, subtracts that delay and halves the remainder. Since radio travels at the speed of light, every nanosecond of clock error equals 30 centimetres of distance error. The entire precision of the system is born there.
How do you time something that finely? A simple threshold detector is not enough. The receiver knows the exact sequence of pulses it expects and correlates it with what arrives: it slides the incoming signal over the known pattern looking for a peak of coincidence, with early and late tracking gates that refine the position of the maximum. With that, a consumer chip drops below 10 centimetres under normal conditions.
Then there is the clock problem. Two devices never tick in step: an ordinary quartz oscillator drifts by about 20 parts per million, and if the reply delay lasts a millisecond, that drift is already enough to be several metres off. The fix is double-sided two-way ranging: both devices measure, so their clock mismatch cancels to first order and the residual error stays in the centimetre range.
Why an echo cannot fool it
In a living room with walls, furniture and people, every pulse arrives several times: the direct signal plus a collection of reflections. In narrowband radios those echoes add and subtract from one another, causing fades that wreck any calculation based on received power — the famous RSSI that Bluetooth and Wi-Fi rely on. With UWB, by contrast, the echo arrives late in time: every extra nanosecond of flight is 30 additional centimetres of path, so a bounce off a wall a metre and a half away shows up separated from the direct pulse, distinguishable. The receiver rebuilds the channel response, samples it at intervals of roughly one nanosecond and keeps the first edge, the one belonging to the direct path. UWB does not ignore multipath: it resolves it along the time axis.
Not even a repeater can pretend you are closer
This is why the car industry has thrown itself behind this radio. If an attacker wants to open your car without the key, they can amplify and repeat the signal with faster equipment to fake the key being next to the door. With Bluetooth that attack works; with UWB it does not. The 802.15.4z standard added the STS (Scrambled Timestamp Sequence): the preamble pulses are scrambled with an AES-encrypted sequence known only to the two devices. A relay cannot reply any earlier without knowing that sequence, so distance measurement becomes a cryptographic proof of proximity instead of a mere estimate. It is the scheme the Car Connectivity Consortium adopted for its digital key: Bluetooth wakes the system up, UWB measures, and NFC is the fallback if the phone has run out of battery.
Three antennas and an angle
Knowing the distance is enough to point the user at where an object is, but drawing a three-dimensional arrow also needs direction. The UWB chips in phones carry several antennas spaced half a wavelength apart — about two centimetres at 6.5 GHz — and compare the phase at which the same pulse reaches each of them. That phase difference yields the angle of arrival, and distance plus angle gives a position in three dimensions. That is where the screen comes from that shows you the lost key fob under a cushion with an accuracy Bluetooth could never reach.
And a radar on the side
Because the pulses are so short in time, the same chip can detect tiny movements: a chest rising and falling with each breath slightly changes the path of the echoes, and the receiver sees it. That capability is being used to detect a child left behind in the back seat — something protocols such as Euro NCAP’s already reward — and to tell whether a room is occupied without any cameras. The consortia pushing the technology, with FiRa in the lead alongside the UWB Alliance, are also working on Aliro, a standard for opening home and office doors by bringing the phone close, without touching the handle.
It is not a technology without limits: at 6.5 GHz, walls, water and the human body absorb the signal, and useful ranges stay within tens of metres. But in the gap between “it is close, according to Bluetooth” and “it is here”, UWB has found a spot no other radio had occupied: measuring time with a stopwatch so fine that distance stops being an estimate and becomes a reading.
