#3788 Ranging with WiFi and Bluetooth

#3788 Can a wifi or bluetooth signal be used for ranging

Yes, both Wi-Fi and Bluetooth signals can be used for ranging, which is the process of estimating the distance between two devices.

The accuracy and methods, however, differ between the two technologies.

Wi-Fi Ranging

Wi-Fi ranging uses the IEEE 802.11mc standard, which includes a feature called Fine Timing Measurement (FTM), often referred to as Wi-Fi Round Trip Time (RTT).

How it Works: RTT measures the time-of-flight—the time it takes for a Wi-Fi signal to make a round trip between two devices (a client and an access point). Since radio waves travel at the speed of light, the distance can be calculated from this time measurement.
Accuracy: Modern Wi-Fi RTT can achieve meter-level to sub-meter accuracy (down to a few decimeters in ideal conditions), significantly better than older methods like Received Signal Strength Indicator (RSSI).
Applications: It's primarily used for indoor positioning systems (IPS), asset tracking, and enabling services like "hyperlocal" geofencing.

Bluetooth Ranging

Bluetooth has evolved with several methods for ranging and localization, with the most advanced being Bluetooth Channel Sounding.

1. Bluetooth Channel Sounding (High-Accuracy Distance Measurement)

This is the latest development in Bluetooth ranging, promising high precision.

How it Works: It uses a technique called Phase-Based Ranging (PBR), which measures the phase shift of the radio signal across multiple frequencies (channels). This phase shift is a precise indicator of the distance traveled. Some implementations also incorporate Round Trip Time (RTT) measurements for security and reliability.
Accuracy: This new feature aims for centimeter-level accuracy, making it suitable for secure access (like digital car keys) and high-precision asset tracking.

2. Received Signal Strength Indicator (RSSI)

This is a simpler, less accurate, and widely available method.

How it Works: It estimates distance based on the strength of the received signal. As the signal travels, it attenuates (gets weaker), which can be correlated with distance.
Accuracy: This method is highly susceptible to environmental factors (walls, obstacles, human bodies) and only provides a rough distance estimate, typically accurate to several meters. It's primarily used for proximity detection (e.g., "is the device close by?") rather than precise ranging.

3. Direction Finding (Angle of Arrival/Departure)

While not direct ranging, this feature allows for high-accuracy positioning when combined with ranging or triangulation.

How it Works: Bluetooth Direction Finding uses antenna arrays to determine the Angle of Arrival (AoA) or Angle of Departure (AoD) of the signal, which indicates the direction of the transmitting device.

In summary, both technologies are actively used for ranging, with newer standards in both Wi-Fi (RTT) and Bluetooth (Channel Sounding) offering much higher accuracy than the older, ubiquitous RSSI method.

Describe how a transceiver that rotates at 1 RPM could be used for ranging and plotting

A transceiver that rotates at 1 RPM (Revolution Per Minute) would be used for ranging and plotting by combining Time-of-Flight (ToF) distance measurement with Direction Finding (DF), a principle fundamental to Radar (Radio Detection And Ranging).

A rotation rate of 1 RPM means the transceiver completes a full $360^\circ$ scan every 60 seconds.

Principles of Ranging and Plotting

1. Ranging (Distance Measurement)

The distance to a target is determined by measuring the Time-of-Flight (ToF) of a radio pulse.¹

Pulse Transmission: The directional antenna/transceiver emits a short burst of radio energy (a pulse).
Echo Reception: The pulse travels at the speed of light ( $c$ ), hits an object, and is reflected back as an echo to the transceiver.
Distance Calculation: The system measures the precise time elapsed between transmission and reception (2Δt).3 The range (distance, R) is calculated using the formula:
$R = \frac{c \cdot \Delta t}{2}$
The time is divided by two because the pulse travels the distance R twice (out and back).

2. Plotting (Angular Measurement)

The angular position (or bearing) of the target is determined by the rotational position of the antenna at the moment the echo is received.

Directional Beam: The transceiver uses a directional antenna that focuses its energy into a narrow beam. This beam acts like a searchlight.
Angular Position: The system is synchronized with the mechanical rotation of the antenna. Since the antenna rotates at a known, constant speed (1 RPM, or $6^\circ$ per second), a precision encoder measures the azimuth angle ( $0^\circ$ to $360^\circ$ ) of the antenna head.
Target Identification: When an echo is detected, the system records the current azimuth angle. This angle is the bearing of the target relative to a fixed reference (e.g., True North).

How 1 RPM Rotation is Used for Plotting

The 1 RPM rotation rate determines the Scan Time and Update Rate for the entire $360^\circ$ area:

Parameter	Value	Calculation
Rotation Rate	1 RPM	1 revolution per 60 seconds
Scan Time	60 seconds	$60\ \text{seconds}/\text{revolution}$
Angular Speed	$6^\circ/\text{second}$	$360^\circ / 60\ \text{seconds}$

As the transceiver slowly rotates, it continuously transmits pulses, listens for echoes, and measures:

Range ( $R$ ): From the time-of-flight.
Bearing ( $\theta$ ): From the antenna's instantaneous angular position.

These polar coordinates ( $R, \theta$ ) are then used to plot the position of the target on a display, creating an image similar to a Plan Position Indicator (PPI) display used in marine or air traffic control radar. The slow 1 RPM rate would provide a relatively long "dwell time" (time the beam spends on a target), which helps with long-range detection, but results in a slow update rate (a full image is updated only once per minute).

You can see a video showing the practical application of this ranging and plotting concept in a maritime environment in How to Use a Navigational Plotter.

Would WiFi or Bluetooth be better for indoor ranging?

It depends entirely on your priority: accuracy or infrastructure cost/availability.

For indoor ranging, the modern, precise methods for both Wi-Fi and Bluetooth offer comparable performance, but they have different trade-offs.

Quick Comparison of Modern Methods

Feature	Wi-Fi (802.11mc/az)	Bluetooth (BLE 5.1+ Direction Finding / Channel Sounding)
Accuracy (Best)	$0.4 - 1$ meter (using FTM/RTT)	Sub-meter / Centimeter-level (using AoA/AoD or Channel Sounding)
Method	Time-of-Flight (ToF) / Round Trip Time (RTT)	Angle of Arrival/Departure (AoA/AoD) or Phase-Based Ranging (PBR)
Infrastructure	High Availability (Uses existing Wi-Fi Access Points)	Requires Deployment (Needs dedicated Bluetooth beacons/anchors)
Cost	Lower initial cost (if Wi-Fi is already in place)	Higher deployment cost (new hardware needed)
Power Use	Higher	Very Low (BLE is battery-friendly)

Detailed Breakdown

1. Wi-Fi: Better for Leveraging Existing Infrastructure¹

Wi-Fi's strength lies in its ubiquity.

Technology: It uses Fine Timing Measurement (FTM), also known as Wi-Fi RTT (Round Trip Time), defined in the IEEE 802.11mc/az standards.² This measures the actual time-of-flight of the radio signal.³
Accuracy: It offers very good accuracy, typically $1-2$ meters with compatible routers and devices, and is expected to improve to sub-meter accuracy with newer standards like 802.11az (Wi-Fi Ranging).
Advantage: If you already have a dense network of modern, compatible Wi-Fi Access Points, Wi-Fi is the most cost-effective option because it requires no new hardware deployment.⁴

2. Bluetooth: Better for Higher Accuracy and Low Power⁵

Bluetooth Low Energy (BLE) excels in precision and low energy consumption.⁶

Technology (Legacy): Traditional Bluetooth ranging using Received Signal Strength Indicator (RSSI) is highly inaccurate, often providing only $5-15$ meter precision or just a rough "proximity" estimate.
Technology (Modern): New specifications, like Bluetooth 5.1 (Direction Finding) and the upcoming Channel Sounding technology, enable much higher accuracy.⁷
- Direction Finding (AoA/AoD): Achieves sub-meter accuracy by measuring the angle of the signal's arrival or departure.⁸
- Channel Sounding: The latest development, which uses phase-based ranging, promises centimeter-level accuracy, making it ideal for highly precise applications like secure access (digital car keys).⁹
Advantage: Bluetooth is the superior choice for pinpoint accuracy (especially with Channel Sounding) and for battery-powered devices (due to low power consumption).¹⁰ The drawback is that it requires deploying a new network of dedicated Bluetooth beacons or anchors.¹¹

What would be the optimal rotation rate?

The concept of an "optimal" rotation rate for a rotating transceiver is a trade-off between two primary, competing factors: Update Rate and Detection Performance (Range/Accuracy).

The optimal rate is therefore determined by the intended application and the required maximum range.

Key Factors and the Trade-Off

1. Update Rate (Higher RPM is Better)

Definition: The frequency at which a target's position is re-measured.
Need: A higher rotation rate (e.g., 12 RPM or higher) is necessary for tracking fast-moving targets (like aircraft or high-speed vessels) or targets in a dense, close-range environment (like a harbor) where timely updates are critical for collision avoidance.¹
- Example: An antenna rotating at 12 RPM completes a scan in 5 seconds, providing a target update every 5 seconds. Marine radar may use 20–40 RPM for short ranges.

2. Detection Performance (Lower RPM is Better)

Definition: The ability to detect weak signals from distant targets and accurately measure their parameters.
Need: A lower rotation rate (e.g., 6 RPM or less) is necessary for increasing the maximum detection range and improving signal processing.
- Dwell Time: Slower rotation increases the dwell time (the amount of time the directional beam is focused on a target).² A longer dwell time allows the receiver to integrate the energy from more transmitted pulses ("hits per scan"), which significantly improves the Signal-to-Noise Ratio (SNR), making distant or weak targets easier to detect.
- Example: A surveillance radar searching for long-range air traffic might operate in the 5–12 RPM range.³

Conclusion: Optimal Rates for Typical Applications

Since the user's initial prompt mentioned 1 RPM, which is extremely slow, here is how the optimal rate usually breaks down in real-world radar-like systems:

Application	Optimal Rotation Rate (RPM)	Primary Focus
Long-Range Air Surveillance	5–8 RPM	Maximizing detection range (longer dwell time) and reliable tracking.
Terminal/Airport Surveillance	12–15 RPM	Balancing range and providing frequent updates for approaching aircraft.
Short-Range Marine/Harbor	20–40 RPM	Maximizing update rate to manage high traffic density and high relative speeds for collision avoidance.
1 RPM (Very Slow)	< 2 RPM	Suitable only for extremely long-range systems or applications where the targets move very slowly (or not at all) and the need for frequent updates is minimal. In most active ranging systems, 1 RPM is considered sub-optimal due to the excessively long 60-second update interval.

Applying Bluetooth 5 and optimal RPM, provide a MAKER project plan for developing this system

Building a rotating ranging and plotting system using Bluetooth 5.1's Angle of Arrival (AoA) is an excellent MAKER project. Instead of a traditional radar's Time-of-Flight, you'll use Direction Finding combined with rotation to create a continuous 360° map.

The key is to leverage the directional capability of AoA on a static array, and then use the mechanical rotation to sweep the azimuth.

I. Optimal Rotation Rate: The Trade-Off

The optimal rotation rate is a compromise between angular resolution and update frequency.

Parameter	Recommended Value	Rationale
Rotation Rate ( $\text{n}$ )	$\mathbf{12\ \text{RPM}}$ (1 rev per 5 seconds)	This is a standard rate for high-update surveillance (e.g., airport radar). It offers a good balance between a high update rate and signal dwell time.
Angular Speed ( $\mathbf{\omega}$ )	$72^\circ/\text{second}$	$12\ \text{RPM} \times \frac{360^\circ}{60\ \text{seconds}} = 72^\circ/\text{s}$
Angular Resolution	$1^\circ$ (Goal)	The system needs to accurately read the motor's position and the AoA calculation must be precise enough to distinguish targets $1^\circ$ apart.

II. Project Plan: Rotating Bluetooth 5.1 AoA System

Phase 1: Core Hardware and Assembly ( $\sim 40\%$ )

Component	Function	Detail
1. BLE AoA Locator (Receiver)	Determines the Angle of Arrival ( $\phi$ )	Use a commercially available Bluetooth 5.1 AoA Locator/Receiver Module (e.g., from Nordic, Silicon Labs, or u-blox) with a built-in multi-antenna array. This simplifies the complex IQ sampling and angle calculation.
2. BLE Beacon (Transmitter)	Transmits the position signal	Standard Bluetooth 5.1 Beacon/Tag capable of transmitting the Constant Tone Extension (CTE). This is the target being tracked.
3. Rotation Platform	Provides mechanical scanning	Stepper Motor (NEMA 17 recommended for precision) or a powerful Servo Motor for the rotation platform.
4. Angle Encoder	Measures the platform's absolute angle ( $\theta$ )	High-resolution Absolute Rotary Encoder (or a simple Index Sensor if using a stepper motor for relative position tracking) to know the exact $360^\circ$ orientation.
5. Controller/Hub (Raspberry Pi)	Manages control and data	Raspberry Pi 5 or similar single-board computer to control the motor, read the angle encoder, and communicate with the AoA locator via Ethernet or USB.

Assembly Steps:

Mount the AoA Locator (Antenna Array) securely atop the rotating shaft of the stepper motor.
Align the angle encoder's zero position with a known reference point ( $0^\circ$ or True North).
Wire the motor controller and angle encoder to the Raspberry Pi's GPIO pins.

Phase 2: Firmware and Ranging Logic ( $\sim 40\%$ )

Step	Action	Description
1. Motor Control	Implement $\mathbf{12\ \text{RPM}}$ sweep logic.	Write a Python/C++ script on the Raspberry Pi to command the stepper motor driver to rotate at a constant $72^\circ/\text{second}$ rate.
2. Azimuth Synchronization	Combine rotation angle ( $\theta$ ) and AoA ( $\phi$ ).	The AoA locator provides the angle ( $\phi$ ) of the beacon relative to its own orientation. The absolute bearing of the target is: $\text{Bearing} = \theta + \phi$ .
3. Ranging (Distance Estimation)	Use RSSI for rough range (initial method).	Since AoA only provides angle, use the Received Signal Strength Indicator (RSSI) from the beacon to perform a rudimentary distance calculation. This will be highly inaccurate but provides the required polar coordinate ( $R$ ). (For better ranging, you'd need Bluetooth Channel Sounding, which requires more specialized hardware.)
4. Data Stream	Create a consistent data stream.	Continuously capture data packets: $(\text{Timestamp}, \theta, \phi, \text{RSSI}) \rightarrow (\text{Timestamp}, \text{Bearing}, \text{Distance})$ .

Phase 3: Plotting and Visualization ( $\sim 20\%$ )

Step	Action	Description
1. Data Server	Run a local web server (Flask/Node.js).	Create a simple local server to receive the data stream from the Raspberry Pi.
2. Real-Time Plotter	Develop a Plan Position Indicator (PPI).	Use a visualization library (e.g., p5.js or D3.js for web, or Matplotlib for desktop) to plot the targets in real-time on a circular azimuth map.
3. Plotting Logic	Visualize targets as polar coordinates.	Convert the (Bearing, Distance) data into Cartesian coordinates ( $\mathbf{x}, \mathbf{y}$ ) and plot them as persistent "blips" on the screen, fading old blips as the antenna sweeps past.

From <https://gemini.google.com/app/> Google Gemini 2.5 Pro

Search This Blog

Learning from ChatGPT