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Indoor locations such as large shopping malls, hospitals, and airports often make it difficult for people to quickly find their destination due to the complex spaces. The emergence of indoor positioning technology has effectively addressed this issue, providing precise guidance for people's movement within indoor spaces. So, what are the methods for indoor positioning?
Positioning Methods Based on Wireless Signals
Wi-Fi Positioning
Wi-Fi positioning technology is currently a widely used indoor positioning method. It primarily achieves positioning through two methods: triangulation, which uses the signal strength between a mobile device and three or more wireless network access points, combined with a differential algorithm, to calculate the device's location. The other method is "fingerprinting," which records the signal strength of a large number of identified locations and creates a database. When a new device connects, its location is determined by comparing its signal strength with the database.
The advantages of Wi-Fi positioning are low cost and ease of scalability. The widespread availability of Wi-Fi routers and mobile terminals allows positioning systems to share the network with other clients. However, Wi-Fi signals are easily affected by the surrounding environment, and positioning accuracy is generally around 2 meters. It is suitable for locating and navigating people or vehicles in settings such as medical institutions, theme parks, factories, and shopping malls.
Bluetooth Positioning
Bluetooth positioning technology utilizes Bluetooth Low Energy (BLE) beacons for positioning. After Bluetooth beacons are deployed in an area, they continuously broadcast signals and data packets to the surrounding area. When a mobile device enters the beacon's signal range, it measures the received signal strength and, combined with its built-in positioning algorithm, estimates the distance between the device and the beacon. When three or more beacons are near a device, trilateration can be used to calculate the device's location.
Bluetooth positioning offers the advantages of small size, short range, low power consumption, and ease of integration into mobile devices such as mobile phones. However, in complex spatial environments, Bluetooth systems are less stable and susceptible to interference from signal noise and obstructions. Furthermore, Bluetooth components and equipment are relatively expensive. Bluetooth positioning is primarily used for small-scale positioning, such as in single-story halls and stores.
Ultra-Wideband (UWB) Positioning
UWB positioning is an emerging wireless positioning technology that transmits data by sending and receiving extremely narrow pulses in the nanosecond or microsecond range, with bandwidths in the order of gigahertz. Ultra-wideband positioning utilizes pre-deployed anchor nodes and bridge nodes with known locations to communicate with newly added tags, determining their location through methods like Time Difference of Arrival (TDoA).
Ultra-wideband positioning technology offers advantages such as strong penetration, low power consumption, excellent multipath mitigation, high security, and low system complexity. It can provide accurate positioning within 0.1 to 0.5 meters. It is suitable for industrial applications requiring high-precision positioning, such as battlefield soldier location detection and robot motion tracking.
Radio Frequency Identification (RFID) Positioning
RFID positioning technology utilizes radio frequency (RF) for contactless, two-way communication to exchange data, enabling mobile device identification and location. A fixed set of readers reads the characteristic information (such as ID and received signal strength) of target RFID tags and uses methods such as proximity, multilateration, and received signal strength to determine the tag's location.
RFID positioning technology has a short range, typically only tens of meters, but it can provide centimeter-level positioning accuracy within milliseconds, has a wide transmission range, and is relatively low-cost. However, RFID is difficult to integrate into mobile devices and has poor interference immunity, limiting its applicability. RFID positioning technology has been widely used for locating goods and merchandise flows in warehouses, factories, and shopping malls, as well as in emergency rescue, asset management, and personnel tracking.
Sensor-Based Positioning Methods
Inertial Navigation Positioning
Inertial navigation positioning is a purely client-side technology that primarily uses motion data collected by terminal inertial sensors (such as accelerometers and gyroscopes) to measure an object's speed, direction, acceleration, and other information. Based on dead reckoning, it performs various calculations to determine the object's position.
The advantage of inertial navigation positioning is that it does not rely on external signals and can operate in any environment. However, as travel time increases, inertial navigation positioning errors accumulate. Therefore, inertial navigation is often combined with Wi-Fi fingerprinting. Regularly requesting indoor location information via Wi-Fi is used to correct for inertial navigation errors. This technology is currently widely used in applications such as sweeping robots.
Ultrasonic Positioning
Ultrasonic positioning primarily uses reflection ranging. The system consists of a main rangefinder and several receivers. The main rangefinder can be placed on the target to be measured and transmit a signal of the same frequency to a receiver mounted indoors. The receiver receives the signal and then reflects it back to the main rangefinder. The distance is calculated based on the time difference between the echo and the transmitted wave, thereby determining the location.
Ultrasonic positioning offers high overall accuracy, reaching the centimeter level. Its structure is relatively simple, its penetration is reasonable, and ultrasonic waves themselves have strong anti-interference capabilities. However, ultrasonic waves experience significant attenuation in air, making them unsuitable for large-scale applications. Furthermore, reflection ranging is significantly affected by multipath and non-line-of-sight propagation, and requires substantial investment in underlying hardware, resulting in high costs. Ultrasonic positioning technology is primarily used for locating objects in unmanned workshops and digital pens.
Other Positioning Methods
Infrared Positioning
There are two main methods for infrared positioning: one uses infrared markers as moving points, emitting modulated infrared rays that are received by optical sensors installed indoors for positioning; the other uses multiple pairs of transmitters and receivers to create an infrared network covering the measurement space, directly locating moving targets. Infrared positioning technology offers relatively high accuracy, but because infrared light can only travel within line of sight, its penetration is extremely poor and it is easily affected by environmental factors such as lighting and smoke. It also fails to function properly when obscured by obstructions, and requires the installation of sensors in every room and hallway, resulting in a high overall cost. Infrared positioning technology is particularly suitable for accurately recording the trajectory of simple objects in the laboratory and for locating the position of autonomous robots indoors.
Geomagnetic Positioning
Geomagnetic positioning technology achieves positioning by measuring the strength and location of the Earth's magnetic field. Modern buildings often use structures such as reinforced concrete, which perturb the Earth's magnetic field, resulting in varying magnetic characteristics at different locations. Similar to Wi-Fi fingerprinting technology, geomagnetic positioning requires manual collection of indoor geomagnetic distribution to create a baseline map of magnetic field characteristics. Then, algorithms such as particle filtering are used to estimate a person's location.
Geomagnetic positioning technology requires no hardware installation and is low-cost. Magnetic sensors are already standard equipment in smartphones and tablets, making it widely applicable. However, magnetic signals are susceptible to interference from constantly changing electrical and magnetic sources in the environment, resulting in unstable positioning results and reduced accuracy.
LED Visible Light Positioning
LED visible light positioning technology codes each LED light and modulates its ID onto the light, which then continuously emits its ID. Users use their phone's front-facing camera to identify these codes and use the acquired identification information to determine the corresponding location in a map database, completing positioning.
LED visible light positioning technology offers high density, low cost, a rich spectrum, high confidentiality, and high speed. Currently, this technology has been applied in aircraft, automobiles, and smart home applications, and represents a promising indoor positioning technology.
There are various indoor positioning methods, each with its own unique advantages and disadvantages and applicable scenarios.
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