Real-time identification and positioning technologies
There are many ways to identify objects of interest and control their location. It all depends on the goals and conditions.
If the goal is to recognize subscribers for the provision of regionalized services (for example, weather forecasting), then an error of tens of kilometers will not play a special role, and if we are talking about positioning the chip on the board during automatic assembly, we will talk about microns.
If you need to quickly find the required spare part, the frequency of the survey in the system can be minimal - only at the moment when this spare part was required or during the inventory. The rest of the time the system can sleep. But if you need to monitor compliance with routes and high-speed mode of movement of loaders in the workshop, you will need a polling frequency of up to several times per second - real-time mode.
It is most logical to track a truck on an intercity route using a satellite-based positioning system, but as soon as it enters an indoor loading dock or repair box, communication with the satellites is lost and something else is required.
And there are many such application 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 system (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 set by it. We will also ignore intelligent video surveillance systems with their methods of object recognition.
This topic will focus on positioning systems using individual tags - be it the tag itself, GPS navigator, Wi-Fi device or cell phone.
The use of identification and positioning systems (positioning) of material objects - people, vehicles, mobile mechanisms and various objects - is an actual 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 searching for victims in emergency situations - to observing animals in free keeping to identify sick people.
A variety of areas and directions of use have generated a variety of technologies.
Requirements for Positioning Systems
Before proceeding to the comparison of systems, we determine 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 labels to infrastructure elements);
e) noise immunity;
e) resistance to multipath attenuation (the influence of reflected signals);
g) small dimensions and weight of labels;
h) low power consumption of tags (in order to save battery power);
i) ease of deployment and operation;
j) electromagnetic compatibility, the need for frequency resolution;
k) the cost of decisions.
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 largest group consists of radio-frequency technologies, including radio-frequency tags - RFID.
In a separate group, infrared and ultrasonic positioning technologies can be distinguished.
Among the radio-frequency technologies, we can distinguish technologies that were originally intended for the provision of communication services, one way or another adapted for positioning (Wi-Fi, Bluetooth, cellular communication), and those that are most suitable for positioning by the physical properties of modulation are 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 technological issues - they are well known. We proceed immediately to the characteristics. The best accuracy for today is provided by GPS. Positioning accuracy is now no worse than six meters. A new generation of satellites, currently launched, will provide an accuracy of at least 60-90 cm.
A common drawback of global systems is their dependence on conditions of use. It is practically impossible to determine the location inside buildings, in basements or tunnels, the signal level seriously deteriorates under the cover of tree foliage and even with high cloud cover. GPS signal reception is affected by interference from terrestrial 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, GLONASS satellites have an advantage - their orbits have an inclination of about 65 degrees (calculated for the entire territory of Russia).
Positioning in cellular networks was one of the first (long before global positioning). This is explained by the widespread use of cellular communications and the relative simplicity of the Cell Of Origin method - by the location of the cell to which the subscriber is connected. The accuracy of this positioning is determined by the radius of the cell. For "picocells" it is 100-150 meters, for most base stations - a kilometer or more.
For a 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 sectors of several stations. The more sectors in a cell, the narrower each sector and the smaller the intersection area of sectors. And that means higher accuracy. Usually the accuracy is 100-200 meters.
Time of arrival - time of arrival. With this method, the signal arrival time from the subscriber to at least three base stations is measured. To achieve accuracy, the synchronization of base stations using an atomic clock or satellite signals is required. The accuracy of the method is about one hundred meters.
The hybrid method boils down to equipping a mobile phone with a GPS receiver.
In addition to the above, there are a number of proprietary technologies:
Mobile Positioning System (Ericsson) - accuracy of 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 solution price is higher, the more accurate the positioning.
Identification of the object in cellular networks is possible, but usually this task is not posed.
Given that the number of WiFi-equipped devices 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 simplest 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 connected device and its location. The range of WiFi access points is usually 30-200 meters. This determines the accuracy of positioning.
To improve positioning accuracy, measure the power of the radio signal, its propagation time from the subscriber to the access point, 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 conditions - 10-15 meters.
As in the case of cellular networks, in Wi-Fi networks, object identification is possible, but usually this task is not set.
"Local" positioning systems
Local positioning systems include optical (usually infrared) and ultrasound systems. Their range 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 in a particular room. This is important, for example, in medicine.
The mobile tag in the infrared positioning system emits infrared pulses that are received by the system receivers having fixed coordinates. The location of the label is calculated by Time-of-flight (ToF) - the propagation time of the signal from the source to the receiver. The disadvantage of this method is its sensitivity to interference from sunlight. The use of an IR laser increases range, accuracy, but unfortunately the cost. The accuracy of positioning by this method is 10-30 centimeters.
Ultrasonic Positioning Ultrasonic positioning
systems use frequencies from 40-130 kHz. To determine the coordinates of a tag, ToF is typically measured for up to four receivers.
The main disadvantage is the sensitivity to signal loss in the presence (appearance) of even "light" obstacles, to false echo signals and to interference from ultrasound sources, for example, from ultrasonic flaw detectors, ultrasonic cleaning devices at the factory, and ultrasound in the hospital. To eliminate these shortcomings, you need to carefully plan the system.
The advantage of ultrasound systems is the highest positioning accuracy, reaching three centimeters.
“Local” positioning systems are rarely used, and their use is reduced with the development of radio frequency technologies.
Passive Radio Frequency Identification (RFID) Positioning Systems
The main purpose of systems with passive RFID tags is identification. They are used in systems that have traditionally used barcodes or magnetic cards - in systems for recognizing goods and goods, for recognizing people, in access control and management 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 chip chip, falling into the reader's coverage area, uses this radiation as a power source and transmits an identification code to the reader. The range of the reader is about a meter.
The cost of systems with passive RFID tags is higher than the cost of systems with barcodes or magnetic cards, but the use of passive RFIDs significantly unloads operators.
Active RFID Positioning Systems
Active RFID tags are used when you need to track objects at relatively large distances (for example, on the territory of the sorting site). The active RFID frequencies are 455 MHz, 2.4 GHz or 5.8 GHz, and the range is up to one hundred meters. Active tags are powered by the built-in battery.
There are two types of active tags: beacons and transponders. The transponders turn on, receiving a reader signal. They are used in AC fare payment, at checkpoint, entry portals and other similar systems.
Beacons are used in real-time positioning systems. The beacon sends packets with a unique identification code at the command or at specified intervals. 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 beacon Angle of arrival (AoA), by the time of arrival of the Time of arrival (ToA) signal, or by the time of propagation of the signal from the beacon to the Time-of-flight (ToF) receiver.
The system infrastructure is built on the basis of a wired network and in the last two cases requires synchronization.
The term “active RFID” encompasses an extensive class of diverse products. Most RF positioning systems use active RFIDs to identify and position objects. Therefore, the characteristics of active RFID tags, including positioning accuracy and cost, vary greatly, depending on the particular manufacturer.
Positioning in the "near field" technology
Near-field electromagnetic ranging (NFER) technology uses tag transmitters and one or more receivers. In NFER systems, a distance receiver measures the phase difference between the electrical and magnetic components of an electromagnetic field emitted by a label. Since this difference varies from 90 ° near the radiating antenna to zero at a half-wave distance, it is the half-wave length that determines the radius of the system. At a frequency of 1 MHz, the wavelength is 300 m, and the radius of action is 150 m, at a frequency of 10 MHz, 30 and 15 m, respectively.
Actual positioning accuracy 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 room configuration with many obstacles.
The disadvantage of the NFER system is associated with low antenna efficiency. For effective operation, the antenna must be commensurate with the wavelength. In fact, it is hundreds of times smaller, which requires an increase in the transmitter power, and accordingly the dimensions and weight of the tags.
Ultra Wideband (UWB) Positioning
UWB (ultra wideband) technology uses short pulses with maximum bandwidth at a minimum center frequency. Most manufacturers have a center frequency of several gigahertz, and a relative bandwidth of 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 short duration), as Fourier analysis shows, provides an infinite passband. The UWB signal is not like modulated sine waves, but resembles a series of pulses.
Manufacturers offer different options for UWB technology. The shapes of the pulses are different. In some cases, relatively powerful single pulses are used, in others hundreds of millions of low-power pulses per second. Both coherent (sequential) signal processing and non-coherent are applied. All this leads to a significant difference in the characteristics of UWB systems from different manufacturers.
Advantages of the technology: reliable operation, high accuracy, resistance to multipath attenuation.
Limitations: the complexity of creating a transmitter of significant power (typical power is 50 μW, the most powerful is 10 mW).
In addition, there are restrictions on the part of frequency regulation bodies (systems, as a rule, have to be used in rooms where their low-power signal is practically not detected against noise).
The infrastructure of the system is based on a wired network and requires synchronization.
CSS and SDS-TWR Positioning System
I wrote in detail about positioning using CSS and SDS-TWR in topics (1) and (2) .
Such a system provides positioning accuracy of three meters and a range of 50 meters, has increased noise immunity and resistance to multipath attenuation, is characterized by low power consumption of tags, does not require synchronization.
But the deployment of infrastructure is complicated by the need to build a wired data network to each base station.
Same thing with ZigBee network and MEMS accelerometers
I also wrote about such a positioning system in the topic (3) .
More details can be read here and here .
I only note that these improvements have simplified the deployment of infrastructure and allowed to increase accuracy to one meter.
Comparative characteristics of the described technologies are given in the table: