Concept renderUWB, or ultra-wideband, is a short-range radio that sends very short pulses across a wide band, typically 500 MHz or more. Devices time those pulses to measure distance and, with two or more antennas, direction. In a tracker, UWB handles the close-range search, while Bluetooth LE handles discovery, setup and data.
Ultra-wideband shows up on tracker spec sheets as the feature that points a phone at a lost item. For a device maker it is a second radio with its own chip, antenna, rules and phone support list. This guide covers how UWB measures distance, the standards and US rules, and when Bluetooth LE alone is enough.
This is an engineering summary, not legal advice. The IEEE and FiRa specifications are paywalled or member-only, and we quote no value we did not read on a public page.
What is UWB, and how does ultra-wideband measure distance?
UWB is a radio that spreads short pulses across a very wide band. The US rule defines a UWB transmitter by bandwidth. In 47 CFR 15.503, it has a fractional bandwidth of 0.20 or more, or a UWB bandwidth of 500 MHz or more regardless of the fractional bandwidth (eCFR, as of November 2026).
The wide band matters because it makes the pulses short. FiRa, the industry group behind UWB interoperability, says ranging uses time-of-flight measurements between devices, calculated from the round-trip time of challenge and response packets. It says the 500 MHz channel and short pulses of about 2 ns help achieve centimeter accuracy (FiRa, as of November 2026).
Signal strength methods guess distance from how weak a signal is, and walls and bodies change it. The Bluetooth SIG says distance measurements based on RSSI and path loss are not accurate enough for some applications (Bluetooth SIG, as of November 2026).
How do two-way ranging, TDoA and angle of arrival differ?
Phone-facing trackers mostly use two-way ranging. Time difference of arrival needs infrastructure you install.
| Method | How it works | What it needs | Typical use |
|---|---|---|---|
| Two-way ranging | Two devices trade timed packets. The round trip, minus reply delays, gives the time of flight. | A UWB radio on each side, plus a link such as Bluetooth LE to set up the session. | Phone to tag finding, access control. |
| Time difference of arrival | Anchors at known positions exchange messages with the device. Arrival time differences place it. | Several installed anchors with known coordinates. | Indoor positioning on a site you control. |
| Angle of arrival | The receiver compares the signal across two or more antennas to find its direction. | More than one receive chain and a calibrated antenna layout. | Pointing a phone or reader toward a tag. |
Apple documents a downlink version of TDoA. Installed anchors send messages, an iPhone listens, and the app works out its location from the arrival time differences across several anchors. Apple describes it as able to give sub-meter location in a well-defined area, and says the anchors comply with IEEE 802.15.4z. The app needs location permission (Apple developer documentation, as of November 2026).
Which standards define UWB technology: IEEE 802.15.4z and FiRa?
The radio layer comes from IEEE 802.15.4. The 802.15.4z-2020 amendment, published August 25, 2020, added coding options to increase the integrity and accuracy of ranging, and extended the MAC to control time-of-flight ranging. The IEEE site now lists 802.15.4z as superseded. IEEE 802.15.4-2024 is the active base standard, and its abstract says it provides modes for precision ranging (IEEE 802.15.4z and IEEE 802.15.4-2024, as of November 2026). A further amendment, P802.15.4ab, is listed as a draft.
FiRa builds interoperability and certification on top. It says 802.15.4z adds PHY-level protection against known attacks on legacy UWB radio. It supports 500 MHz channels from Channel 5 to Channel 14, and Channel 9 is the only one it calls mandatory across FiRa Certified devices (FiRa, as of November 2026). Android asks IoT devices that range with phones to be FiRa MAC 1.3 compliant (Android developers, as of November 2026).
Android compares two ways to key a session. A static scrambled timestamp sequence (STS) leaves the session open to replay and relay attacks. A provisioned one, with a session key provisioned for each session, mitigates them, and Android supports it on all UWB-enabled devices running Android 14 or higher (Android developers, as of November 2026). For a tag that unlocks something, plan for provisioned STS.
How do phones use UWB, from Precision Finding to third-party accessories?
Apple lists the devices with its UWB chips by generation. The second-generation chip is in iPhone 15 and later, excluding iPhone 16e and 17e, and in iPhone Air and some Apple Watch models. The first-generation chip is in iPhone 11 through 14 models. Apple also says UWB is not available in some countries and regions, and lists them (Apple Support, published March 27, 2026, as of November 2026).
Precision Finding on AirTag
Apple's tech specs for the first-generation AirTag list Bluetooth for proximity finding, the U1 chip for Ultra Wideband and Precision Finding, and a CR2032 coin cell (Apple Support, as of November 2026). On January 26, 2026, Apple said the second-generation AirTag uses its second-generation UWB chip and that Precision Finding guides users from up to 50 percent farther away than the previous generation. Apple adds that Precision Finding is not available where UWB is restricted (Apple Newsroom, as of November 2026).
Third-party accessories on iPhone and Android
Apple's public route for other makers is the Nearby Interaction accessory protocol. Apple tells accessory makers to implement it with a Nearby Interaction-enabled UWB chipset and to confirm feature support with the chipset vendor (Apple Nearby Interaction, as of November 2026). An app sets up each session. It establishes a data link with the accessory and exchanges configuration data. In the foreground it ranges freely. In the background it keeps ranging only if the accessory is Bluetooth LE paired and connected, or, from iOS 18.4, if the app starts a Live Activity (Apple developer documentation, as of November 2026).
On Android, UWB is available only through the Jetpack library, on Android 12 or higher. The app discovers the tag and exchanges ranging parameters over an out-of-band link such as Bluetooth LE. Google's device list, dated January 2025, names Pixel Pro, Fold and Tablet models, a Motorola model and several Samsung lines (Android developers, as of November 2026).
Chip vendors state their phone support on their own pages. NXP says its SR040 is designed for coin cell trackers and tags, is FiRa certified, and lists interoperability with the Apple U1 chip with firmware v3.14.0 (NXP SR040, as of November 2026). Confirm the claim for your exact part and firmware.
Finding networks such as Apple Find My and Google Find Hub are a separate layer with their own specifications. See Google Find Hub accessory requirements. Plan for unwanted-tracking alerts too, covered in DULT unwanted tracker rules.
What do FCC Part 15 Subpart F rules require of a UWB tracker?
In the US, UWB transmitters operate under 47 CFR Part 15 Subpart F. The key sections, from the eCFR as of November 2026:
| Section | Covers | What it says |
|---|---|---|
| 47 CFR 15.517 | Indoor UWB systems | Transmitters used solely indoors. UWB bandwidth between 3100 and 10,600 MHz. No outdoor-mounted antennas. The device must state that it may only be operated indoors. |
| 47 CFR 15.519 | Hand-held UWB systems | Small devices that are primarily hand held while operated and use no fixed infrastructure, indoors or outdoors. Transmit only when sending to an associated receiver, and stop within 10 seconds without its acknowledgment. Antennas only on the device. |
| 47 CFR 15.521 | All UWB devices | UWB devices may not be used to operate toys. Operation onboard an aircraft, a ship or a satellite is prohibited. |
For both indoor and hand-held devices, the average EIRP limit from 3100 to 10,600 MHz is −41.3 dBm in a 1 MHz resolution bandwidth. Both sections add −85.3 dBm limits in 1164 to 1240 MHz and 1559 to 1610 MHz, measured in a resolution bandwidth of no less than 1 kHz, and a peak limit of 0 dBm EIRP in a 50 MHz bandwidth around the strongest emission. Between 1610 and 3100 MHz and above 10,600 MHz, the hand-held limits in 15.519 are 10 dB lower than the indoor ones (eCFR, as of November 2026).
Three points shape the product. Under the 10-second acknowledgment rule in 15.519, a hand-held tag cannot beacon UWB on its own. It ranges only with a receiver that answers. Which section a tag falls under depends on how it is used, so ask your test lab early. And 15.521 prohibits operation onboard an aircraft, so a tracker that may travel in luggage needs a plan for that rule. Settle it with your lab and counsel before launch.
UWB vs Bluetooth: when is BLE enough for a tracker?
Bluetooth now has its own ranging feature. Channel Sounding appears in the Bluetooth Core Specification v6.0. It supports phase-based ranging and round-trip timing, and the SIG says phase-based ranging can measure up to about 150 meters before the distance becomes ambiguous, with accuracy that depends on the environment (Bluetooth SIG, as of November 2026).
| Question | BLE signal strength | BLE Channel Sounding | UWB |
|---|---|---|---|
| How distance is found | Signal strength and a path-loss estimate | Phase-based ranging plus round-trip timing | Time of flight of very short pulses |
| Accuracy, as published | Not accurate enough for some applications (Bluetooth SIG) | Depends on the environment and on implementation (Bluetooth SIG) | Centimeter level (FiRa); 10 cm (Android) |
| Direction | No direction from RSSI alone | Not described in the SIG overview; ask your chip vendor | With angle of arrival. Apple gives direction on iOS, distance only on watchOS. |
| Phone support | Broad | Check each target phone | Listed models only |
| Extra hardware on a tag | None beyond the BLE radio | A BLE chip that supports Channel Sounding | A UWB chip and antenna, plus BLE for discovery and setup |
Sources for the table: Bluetooth SIG Channel Sounding overview, FiRa, Android developers and Apple developer documentation, all as of November 2026.
BLE alone is often enough. To show that a bag is nearby or in which room, a sound alert plus signal strength can do the job. A pet tracker that reports location over LTE-M and GNSS gains little from UWB while the owner's phone is out of UWB range. UWB earns its cost when the product promises on-screen direction and distance at close range, or unlocks something based on presence. For the BLE side of the design, see what Bluetooth Low Energy is.
What does adding UWB change in the hardware design?
UWB adds a chip, an antenna, firmware and a test plan. Phone-facing products keep their BLE radio. Android exchanges ranging parameters over an out-of-band link such as Bluetooth LE, and on iPhone, background ranging needs the accessory to be BLE paired and connected unless the app runs a Live Activity.
Power
NXP positions the SR040 for coin cell trackers and tags (as of November 2026), and the first-generation AirTag runs both radios on a CR2032. Plan firmware so UWB stays off until BLE shows the phone is near and the user starts a search. The vendor pages we read give no current figures, so measure on your own board before you write a battery claim. For a test method, see how to test and state battery life honestly.
Antennas and direction
Angle of arrival needs more than one receive chain. NXP lists dual receive chains for AoA and 3D AoA capability on its SR150, a part it positions for larger infrastructure (NXP SR150, as of November 2026). On a small tag, direction usually comes from the phone side. The tag still needs a UWB antenna kept clear of the battery and metal and placed alongside the BLE and any GNSS or LTE antennas. Reserve that area before layout starts. Keep-out, matching and detuning in the housing are covered in PCB antenna design for small devices.
Questions for your hardware team
Bring these questions to the first architecture review, before anyone picks a UWB chip.
- Does the product answer which room, how far, or which direction? Write the accuracy and conditions you need.
- Which phones must work with the tag, and is each one on the Apple or Android UWB list?
- Does the UWB chip support the Apple accessory protocol, FiRa MAC 1.3, or both? Get it in writing.
- Which Subpart F section does the test lab expect, and how does firmware meet the 10-second acknowledgment rule?
- How long is UWB on per find, and what does that cost the battery on the real board?
- Where does the UWB antenna sit relative to the battery and the other antennas?
- Is the session key provisioned per session, or static?
For how a tracker project runs with us, see LTE and GNSS tracker development. If you are weighing UWB for a tracker, tell us about your device.
Questions about UWB
How accurate is UWB?
The published figures are in centimeters. FiRa says the 500 MHz channel and pulses of about 2 ns help achieve centimeter accuracy, and Android describes ranging to an accuracy of 10 cm (Android developers, as of November 2026). Your product gets its own figure, so test on your board, in your enclosure, at real distances.
Does a UWB tag need FCC approval?
The UWB radio falls under Part 15 Subpart F, and the tag also carries a BLE radio with its own rules (see FCC Part 15B vs 15C for a BLE wearable). Section 15.519 covers hand-held devices and 15.517 covers indoor-only devices (eCFR, as of November 2026). Ask your test lab which section fits before the design is frozen.
How can Duobay help with a UWB or BLE tracker?
Duobay develops custom trackers for US companies with a Shenzhen engineering team: electronics, firmware, enclosure, an optional app, samples and small batches. FCC and module rules are planned from Gate 0, and source files are assigned to you at every paid gate. Initial calls are free, and we do not publish prices. Tell us about your tracker.
Sources. All pages accessed November 6, 2026. Specifications and phone support lists change, so confirm current values before you decide.
- eCFR, 47 CFR Part 15 Subpart F, Ultra-Wideband Operation, sections 15.503, 15.517, 15.519 and 15.521 (link)
- IEEE 802.15.4z-2020, Enhanced Ultra Wideband PHYs and Associated Ranging Techniques, status superseded (link)
- IEEE 802.15.4-2024, Standard for Low-Rate Wireless Networks, published December 12, 2024 (link)
- FiRa Consortium, How UWB works (link)
- Apple, Nearby Interaction framework documentation (link)
- Apple, Nearby Interaction with UWB, accessory and chipset pages (link)
- Apple, Downlink time difference of arrival ranging (link)
- Apple Support, Learn about Ultra Wideband availability, published March 27, 2026 (link)
- Apple Support, AirTag tech specs (link)
- Apple Newsroom, Apple introduces new AirTag, January 26, 2026 (link)
- Android developers, Ultra-wideband communication (link)
- NXP, Trimension SR040 product page (link)
- NXP, Trimension SR150 product page (link)
- Bluetooth SIG, Bluetooth Channel Sounding technical overview (link)
Planning a tracker? Tell us what it needs to do. A Duobay engineer replies within 1 business day. Send a project brief →