Real-time identification and positioning technologies

Identify objects of interest and control their location can be different. It all depends on the goals and conditions.
If the goal is to recognize subscribers for the provision of zoned services (for example, weather forecast), then an error of ten kilometers will not play a special role, and if it comes to positioning the chip on the board during automatic assembly, it will be about microns.
If you need to quickly find the right part, the frequency of polling in the system may be minimal - only at the time when this part was required or during inventory. The rest of the time the system can spend in sleep mode. But if it is required to monitor the observance of routes and the speed of the movement of loaders in the workshop, a polling frequency of up to several times per second is required — real-time mode.
A wagon on an intercity route is more logical to track using satellite positioning system, but as soon as it gets to the covered unloading area or to the repair box, communication with the satellites is lost and something else is required.
And there are many such features. Naturally, there are many different types of identification and positioning systems.

This topic will focus on identification and positioning systems. But in order not to drown in a sea of ​​information, we will leave aside the location systems (radio, acoustic, infrared), where the location of the object is determined by the reflected signal. We will not consider robotic assembly systems, where the position of an object is not measured by the system, but is given by it. We will also ignore intelligent video surveillance systems with their object recognition methods.
This topic will focus on positioning systems using individual labels - whether it is the actual label, GPS navigator, Wi-Fi device or cell phone.
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The use of identification and positioning systems (locating) material objects - people, vehicles, moving mechanisms and various objects - is the current direction of optimization of technological and business processes. Such systems are already used in various fields of activity. From monitoring patients, staff, drugs and equipment in clinics to monitoring the location of tools, assembly units and workers on the conveyor. From the search for victims in emergency situations - to monitor the animals with free keeping to identify the diseased.
A variety of areas and uses have generated a variety of technologies.

Positioning System Requirements

Before turning to a comparison of systems, we define the criteria for comparison.

As mentioned above, systems should provide:
a) identification of controlled objects;
b) optimal positioning accuracy;
c) the optimal frequency of the survey.

In addition, important criteria are:
d) radius of action (permissible distance from the tags to the infrastructure elements);
e) noise immunity;
e) resistance to multipath attenuation (effect of reflected signals);
g) small dimensions and weight of tags;
h) low power consumption of tags (in order to save battery power);
i) ease of deployment and operation;
k) electromagnetic compatibility, the need for frequency resolution;
l) the cost of solutions.

Types of positioning systems

For positioning, several technology groups are used.
First of all, these are satellite navigation systems - GPS, GLONASS, Beidou, Galileo and others.
The most numerous group consists of radio frequency technologies, including radio frequency tags - RFID.
Infrared and ultrasonic positioning technologies can be distinguished into a separate group.

Among the radio frequency technologies, we can distinguish the technologies originally intended for the provision of communication services, one way or another adapted for positioning (Wi-Fi, Bluetooth, cellular communication), and those that are physically modulating most suitable for positioning - this is CSS (ISO24730 -5), UWB, NFER and others.

Let's leave the radio frequency technology "for dessert" and start with global navigation.

Global Navigation Systems

We will not dwell on the technological aspects - they are well known. We turn immediately to the characteristics. The best accuracy for today provides GPS. Positioning accuracy is already no worse than six meters. And the new generation of satellites currently being launched will ensure accuracy of at least 60-90 cm.
A common disadvantage of global systems is dependence on conditions of use. It is almost impossible to locate inside buildings, in basements or tunnels, the signal level seriously deteriorates under the cover of foliage of trees and even when it is overcast. GPS reception is affected by interference from ground sources. Since GPS orbits have an inclination of about 55 degrees, accuracy at high latitudes is significantly reduced, because GPS satellites are visible low above the horizon. In this regard, the GLONASS satellites have an advantage - the inclination of their orbits is about 65 degrees (designed for the whole territory of Russia).

Positioning in cellular networks

Positioning in cellular networks appeared one of the first (long before global positioning). This is due to the widespread cellular communication and the relative simplicity of the Cell Of Origin method — at the location of the cell to which the subscriber has connected. The accuracy of this positioning is determined by the radius of the cell. For picos, this is 100-150 meters, for most base stations - a kilometer or more.
For more accurate determination of coordinates using data from several base stations. There are several such methods.
Angle of arrival - direction to the subscriber. The method is based on the fact that the base station has from three to six antenna arrays, each of which serves its own sector (at its own frequency). The location is determined at the intersection of the sectors of several stations. The more sectors in a cell, the narrower each sector and the smaller the area of ​​intersection of sectors. So, higher accuracy. Usually the accuracy is 100-200 meters.
Time of arrival - time of arrival. With this method, the time of arrival of a signal from a subscriber to at least three base stations is measured. To achieve accuracy, synchronization of base stations is required using atomic clocks or satellite signals. The accuracy of the method is about one hundred meters.
The hybrid method comes down to equipping a mobile phone with a GPS receiver.
In addition to these, there are a number of proprietary technologies:
Mobile Positioning System (Ericsson) - accuracy 100 m;
RadioCameraTM - accuracy 50 m;
SnapTrackTM (Wireless Assistant GPS) - accuracy up to 15 m;
CursorTM (CPS) - accuracy 50 m;
Finder (CellPoint) - accuracy 75 m.
The price of the solution is the higher, the more accurate the positioning.
Identification of the object in cellular networks is possible, but usually this task is not set.

WiFi positioning

If we consider that the number of devices equipped with WiFi in 2011 reached 1.2 billion, including 513 million smartphones and 230 million computers, the rapid spread of Wi-Fi positioning systems is quite natural.
The easiest way of positioning in WiFi networks, like cellular, is through the base station to which the subscriber is connected. The method is used to provide various services, depending on the type of device connected and its location. The range of WiFi access points is usually 30-200 meters. This determines the positioning accuracy.
To improve the accuracy of positioning, the signal power is measured, its propagation time from the subscriber to the access point, and direction to the signal source.
But even in such systems, positioning accuracy is relatively low. In ideal conditions, it is 3-5 meters, in real - 10-15 meters.
As in the case of cellular networks, identification of an object is possible in Wi-Fi networks, but usually such a task is not set.

“Local” positioning systems

Local positioning systems include optical (usually infrared) and ultrasound systems. Their radius of action is small - 3-10 meters.
Their advantage is that since light and sound practically do not pass through walls and doors, they guarantee “room level accuracy” - the fact that a controlled object is located in a particular room. This is important, for example, in medicine.
Infrared positioning
A mobile tag in an infrared positioning system emits infrared pulses that are received by system receivers with fixed coordinates. The location of the tag is calculated by Time-of-flight (ToF) - the time of signal propagation from the source to the receiver. The disadvantage of the method is sensitivity to interference from sunlight. The use of IR laser increases the range, accuracy, but unfortunately also the cost. The accuracy of the positioning of this method is 10-30 centimeters.
Ultrasound positioning
Ultrasonic positioning systems use frequencies from 40-130 kHz. To determine the coordinates, tags are usually measured with ToF up to four receivers.
The main disadvantage is the sensitivity to signal losses in the presence (appearance) of even “light” obstacles, to false echoes and to interference from ultrasound sources, for example, from ultrasonic flaw detectors, ultrasonic cleaning devices in manufacturing, ultrasound in the hospital. To eliminate these shortcomings, it is necessary to carefully plan the system.
The advantage of ultrasonic systems is the highest positioning accuracy, reaching three centimeters.
“Local” positioning systems are used quite rarely, and their use is declining with the development of radio frequency technologies.

Positioning systems using passive radio frequency identifiers (RFID)

The main purpose of systems with passive RFID tags is identification. They are used in systems that traditionally used bar codes or magnetic cards — in systems for identifying goods and goods, identifying people, in access control systems (ACS), etc.
The system includes RFID tags with unique codes and readers and works as follows. The reader continuously generates radio emission of a given frequency. The tag CHIP, falling within the range of the reader, uses this radiation as a power source and transmits an identification code to the reader. The range of the reader is about one meter.
The cost of systems with passive RFID tags is higher than the cost of systems with bar codes or magnetic cards, but the use of passive RFID significantly relieves operators.

Active RFID Positioning Systems

Active RF tags are used when it is necessary to track objects over relatively large distances (for example, on the territory of a sorting yard). The operating frequencies of active RFID are 455 MHz, 2.4 GHz or 5.8 GHz, and the range is up to one hundred meters. Powered active tags from the built-in battery.
There are two types of active tags: radio beacons and transponders. Transponders are enabled, receiving a signal from the reader. They are used in the AS of fare collection, at checkpoints, entry portals and other similar systems.
Radio beacons are used in real-time positioning systems. The beacon sends packets with a unique identification code on command or with a specified frequency. Packets are received by at least three receivers located around the perimeter of the controlled area. The distance from the beacon to the receivers with fixed coordinates is determined by the angle of direction to the Angle of arrival beacon (AoA), by the time of arrival of the Time of arrival signal (ToA) or by the propagation time of the signal from the beacon to the Time-of-flight receiver (ToF).
The infrastructure of the system is built on the basis of a wired network and in the latter two cases requires synchronization.
The term "active RFID" covers an extensive class of diverse products. Most radio frequency positioning systems use active RFID to identify and position objects. Therefore, the characteristics of active RFID tags, including positioning accuracy and cost, vary greatly, depending on the manufacturer.

Positioning on technology "near field"

Near-field electromagnetic range measurement (NFER) technology uses tag transmitters and one or more receivers. In NFER systems, the receiver for determining the distance measures the phase difference between the electrical and magnetic components of the electromagnetic field emitted by the tag. Since this difference varies from 90 ° near the radiating antenna to zero at the half-wave distance, it is the length of the half-wave that determines the range of the system. At a frequency of 1 MHz, the wavelength is 300 m, and the range is –150 m, at a frequency of 10 MHz, 30 and 15 m, respectively.
Accuracy of positioning in real conditions is about a meter at a distance of up to 30 meters.
The relatively low frequency of radio waves facilitates their passage in complex production environments. Radio waves go around obstacles are not reflected. Therefore, NFER technology has advantages in a complex configuration of rooms with a large number of obstacles.
The disadvantage of the NFER system is due to the low efficiency of the antenna. For effective operation, the antenna must be commensurate with the wavelength. In fact, it is hundreds of times smaller, which requires an increase in transmitter power, and accordingly, the size and weight of the tags.

Ultra Wideband (UWB) positioning

UWB (ultra-wideband) technology uses short pulses with a maximum bandwidth at the lowest center frequency. For most manufacturers, the center frequency is several gigahertz, and the relative bandwidth is 25-100%. The technology is used in communications, radar, distance measurement and positioning.
This is ensured by the transmission of short pulses, broadband in nature. An ideal impulse (a wave of finite amplitude and infinitely small duration), as shown by Fourier analysis, provides an infinite bandwidth. The UWB signal is not like a modulated sinusoidal wave, but resembles a series of pulses.

Manufacturers offer different options for UWB technology. Different forms of pulses. In some cases, relatively powerful single pulses are used, in others hundreds of millions of low-power pulses per second are used. It is used as coherent (sequential) signal processing, and non-coherent. All this leads to a significant difference in the characteristics of UWB systems from different manufacturers.

The advantages of the technology: reliable operation, high accuracy, resistance to multipath attenuation.
Limitations: the difficulty of creating a transmitter of substantial power (typical power - 50 µW, the most powerful - 10 mW).
In addition, there are restrictions on the part of the frequency regulation bodies (systems, as a rule, have to be used in rooms where their low-power signal is practically not detected against the background of noise).

The infrastructure of the system is based on a wired network and requires synchronization.

Positioning system using CSS and SDS-TWR

Details about the positioning using CSS and SDS-TWR I wrote in the topics (1) and (2) .
Such a system provides three-meter positioning accuracy and a range of 50 meters, has high noise immunity and multi-path attenuation resistance, is characterized by low label power consumption, does not require synchronization.
But the deployment of infrastructure is complicated by the need to build a wired data network to each base station.

The same, but using ZigBee network and MEMS accelerometers

I also already wrote about such a positioning system in the topic (3) .
Read more here and here .
I note only that these improvements have simplified the deployment of infrastructure and allowed to increase the accuracy of up to one meter.

Technology comparison

Comparative characteristics of the described technologies are given in the table: