From the book "Microwaves and Wireless Simplified" by Thomas S. Laverghetta by Artech House
When most people hear the term microwave, they think of microwave ovens. That is natural and perfectly appropriate, since microwave ovens operate at 2.45 GHz, which is in the microwave and wireless frequency band. The microwave oven is a device with a high-power electron tube (magnetron) that generates and radiates electromagnetic energy into food to be prepared. It cooks the food by heating the moisture inside the food, i.e. it cooks from the inside to the outside.
Figure 1 is a diagram of the electromagnetic spectrum illustrating the respective frequency bands for common communications, navigation and radar applications. Some of the more familiar applications are AM and FM broadcast bands for radio, television channels, cellular phones, global positioning systems (GPS), personal communications services (PCS) and direct broadcast satellites (DBS). Each of these applications has a different frequency of operation. They are in an FDMA mode, i.e. operating over a specified band of frequencies all the time. There is no planned time gap and no time sharing of stations or channels.
Some applications, in particular, are summarized below:
A classical radar (see Figure 2) sends out a high-power short duration microwave pulse generated by a transmitter through a beam-focusing antenna. The pulse is controlled by a pulse-forming network and begins a time sequence when it is transmitted. The pulse strikes an object or target that reflects the energy back through the antenna to the radar receiver. The time it takes for the pulse to be transmitted, bounce off an object and be received determines the distance that object is away from the radar antenna. The angle of the antenna beam to the object determines the azimuth and/or elevation.
An important block in Figure 2 is the duplexer, or transmit/receive (Tx/Rx) switch. The duplexer is a circuit that switches the radar’s antenna from the transmitter to the receiver at the proper time so that the transmitted signal does not damage the receiver. It also allows the very low level signal coming back from an object to be routed to the receiver. The duplexer can be a physical switch or a series of transmission lines that perform the switching function.
To further understand radar concepts, it is necessary to define some additional terminology:
- Continuous wave (CW) (see Figure 3a) refers to a signal that is continuously on; i.e. there is no time that the signal is interrupted or is off.
- Pulse transmission (see Figure 3b), is the heart of a radar system and what actually enables the operation. A pulse signal supplies power for only a very short amount of the time (5 to 10 percent or less). This enables high peak powers, usually not possible with CW systems, because of the high average power required to be continuously on. If the power is on all the time, there is also a problem with system components being able to dissipate wasted energy in the form of heat.
- Pulse width (τ) is how long the signal is on (measured in units of time (e.g. seconds, milliseconds, microseconds).
- Pulse repetition rate (PRR), alternatively referred to as pulse repetition frequency (PRF) is the number of times the pulse is repeated each second. Its inverse is the PRT, or pulse repetition time designated as represented by “T” in Figure 3b.
- Duty cycle is the ratio of the pulse width, τ, to the pulse repetition time, T or τ/T. The duty cycle is 5, 6, 7, 10 percent or whatever percentage of time that the signal is present. Looking at it another way, it is the time that the signal is actually doing something, or is on duty, compared to the total amount of time available.
- Peak power is the amount of power present at the top of the pulse. In Figure 3b, the peak power is the amplitude of the pulse over the duration, τ. Peak power is usually high, but it is present for only a short period of time. Many components are characterized with both a peak and CW power specifications for use in either application.
- Average power is defined as the peak power multiplied by the duty cycle. Power is available for the period of time that a pulse is on (peak power). Subsequent pulses contribute to the system average power based on the pulse repetition rate and the pulse width (i.e. the duty cycle).
The applications and functions of radar systems can be categorized as search and warning, tracking and measurement and imaging.
Search and Warning Applications
The search and warning function of radar is the one most people recognize. It is the detection of an object (e.g. an airplane in flight viewed on a radar screen by a ground-based radar or a land mass viewed by a ship-borne or air-borne radar). Such “targets” are usually struck many times by the signal due to many scans (rotations) of the radar antenna. A typical area where you would notice this type of radar system is at an airport. Their antennas are especially noticeable at smaller airports, where they are much more visible and can be seen rotating. At larger airports the antennas, many of them protected by domes, are located in more remote areas. Another area where this type of radar system is visible is at shipping ports and marinas. Freighters, tankers, cruise ships and recreational yachts all have radar systems on them with rotating antennas for navigation and weather monitoring purposes.
Tracking and Measurement Applications
Once a target is detected through a search function (scan), in some cases it is desirable to cease scanning and keep the radar beam focused on the target. This is called tracking. Tracking and measurement radars “lock” or remain on to a target and track it for a certain distance or for a certain time period. While locked to the target, a number of parameters may be measured, such as target size, range, velocity, acceleration and other features that can be used for identification and characterization. Typical military applications of these radar systems are for gun control and missile guidance. Figure 4 shows a typical tracking radar system.
Imaging radar forms an image that is two-or three-dimensional in nature, usually in azimuth and range coordinates (see Figure 5). A typical airborne application is the imaging of terrestrial features. As the radar moves over the ground, multiple radar returns are processed to form the image. This type of system can also be used to analyze mechanical systems for stress fractures and (with very low power transmitters) for some medical applications and can provide a three-dimensional image of a tumor to aid in cancer diagnoses.
Doppler radar was originally conceived for use in areas with mountainous terrain, where it was difficult to detect moving targets. Before the advent of Doppler radar, it was easy for an aircraft to slip into a mountainous region and proceed virtually undetected to a target. Conventional radar would not indicate a moving target, just a target, which could be an actual airplane or one of the mountains.
The Doppler effect, first described by Christian Doppler in 1842, is commonly demonstrated by standing at a train crossing as an incoming train blows its whistle. One notices a change in the pitch of the sound as the train approaches and then passes. From this change in pitch, the train’s velocity can be determined. This is the basis for police “speed traps,” which use very accurate Doppler radar systems (see Figure 6). Such systems are difficult to detect in time for speeding drivers to slow down. Usually by the time a radar’s transmit signal is detected by a radar detector, it is too late; the radar system already has recorded your speed.
Doppler systems detect and track moving targets. A signal is sent from the radar transmitter at a certain frequency. When the signal strikes the target, it reflects the signal back to the receiver; the frequency that comes back to the receiver determines the speed of the target. If the target is moving toward the receiver, the received frequency is higher than the transmitted frequency by an amount proportional to its relative velocity. Similarly, if the target is moving away from the receiver, the received frequency is lower.
In addition to military and police radar applications, systems may be found, for example, in manufacturing, where the position and the speed of a product on an assembly line must be determined for certain operations to be performed at specific times and at specific locations. An application that significantly impacts our modern civilized world is weather radar for predicting the paths of severe weather, such as tornadoes and hurricanes. Doppler weather radar systems can spot a storm and track it. This allows weather bureaus to warn and evacuate people before the storm arrives. Other applications include radars used in automotive collision avoidance systems and backup aids, as well as automatic door openers common in supermarkets and shopping centers.
Telecommunications is a term used to denote many things. The prefix, tele, means far off, distant, remote. Thus, the word telecommunications refers to a process of communicating over a long distance. For our purposes, we mean electronic communications (technically, smoke signals and homing pigeons could also be categorized as modes of telecommunications). Older textbooks on telecommunications describe them as communications over a wire. To some extent, that is still true today, but a great deal of communications are carried over optical fibers and by means of RF and microwave signals (either radiated or conducted through transmission lines. This section deals with telecommunications as being the transmission and reception of information over various distances by means of radiated microwave signals, in particular, applications termed as wireless, including mobile phones and wireless networks.
Mobile Phone (Cellphone)
Mobile telephones originated in the late 1940s but did not find wide use due to high cost and limited frequency allocation. In the 1970s, this last restriction was removed when the 800 to 900 MHz Band was allocated for mobile communications, and, as the technology has advanced, the cost of a mobile communications system (i.e. cellular telephone) has come down considerably, as the user has experienced revolutionary advances in features and performance.
The basic cellular concept can best be pictured as a group of automatically switched relay stations. A populated area is divided into many small regions called cells. The cells are linked to a central location, called a mobile telephone switching office (MTSO), which coordinates all incoming calls. Along with coordinating calls between cell sites, the MTSO also generates time and billing information. A diagram of a cellular system is shown in Figure 7. Each cell has a transmitter/receiver combination for a certain section of an area. The main block, the MTSO, is the control area for the system. It is the unit that connects the caller to the party the caller is trying to contact. If the caller is moving (e.g., in a car), the MTSO senses the level of the signals being used and automatically switches the call to the appropriate cell so the transmission is completed with the best clarity possible. The central office provides the same functionality as the central office in a conventional telephone system, that is, it provides a connection between one phone and another.
The cell site is actually a special transmitter/receiver combination. Because it covers only a small geographical area, the unit is relatively low power. That allows other cells to operate on the same frequency, since the power is low enough such that no interference occurs. This feature is important since the many cells in an area would interfere with each other if it not for the low power requirements placed on each cell. Thus, many cells can exist in a geographical area, all operating at the same frequency and nicely coexisting. Figure 8 shows a cellular telephone site.
The 800 to 900 MHz frequency band allocated for cellular telephone service ranges from 825 to 845 and 870 to 890 MHz. For the cellular phone, the lower end (825 to 845 MHz) is used for transmitting, while the upper end (870 to 890 MHz) is used for receiving. At the base units (cell sites), the frequencies are reversed. This approach is logical, because a phone’s transmitter is the cell’s receiver and vice versa. Within the assigned bands, 666 separate channels are assigned for voice and control, 333 in each band. The bandwidth for each channel is 30 kHz. A person making a cellular telephone call enters a local seven-digit number or a long-distance 10-digit number. The caller then presses the send button, which transmits data to a channel. From the cell site, the data is forwarded to the MTSO with the cell site’s identification number. Once the MTSO detects that the cellular phone is on the proper channel, the call is sent to the central office and then to the “callee’s” phone. This may seem like a time-consuming process, but it is accomplished in a very short period of time. When a cellular phone’s signal strength decreases because of the distance it has been traveled, the MTSO searches through the cells to find the one with maximum strength and automatically switches the conversation. This process is called a handoff, which the user never sees or is even aware of.
Most cellular telephones are used in a local area where the phone is originally registered. When it is necessary to operate the phone outside that selected area, the system incorporates a scheme called roaming. Roaming is possible only if the area you are in has cellular service and agreement has been reached between telephone companies and their users. Many areas of the country have roaming capability, thus greatly extending the range of cellular systems. Roaming is a handy feature for those who do a lot of traveling to different parts of the country.
The earliest cell phones used analog utilized analog modulation of voice signals onto a radio frequency carrier. This is known as the first generation of the cellular phone network, or 1G, which established the foundation for where we are today. 1G was provided a fundamental multi-user wireless capability, but it was limited. Because it was analog, it supported only one user per 30 KHz channel and could support only voice communication.
In the early 1990s, phones with second generation cellular (2G) were introduced. Global System for Mobile communications, or GSM uses digital modulation to improve voice quality, but the network offers only limited data service. Time domain multiple access (TDMA) enables multiple users per channel, but requires large frequency gaps between channels (guard bands) to reduce interference. An intermediary phase, 2.5G was introduced in the late 1990s. It uses the General Packet Radio Service (GPRS) standard, which delivers packet-switched data capabilities to existing GSM networks, allowing users to send graphics-rich data as packets. The Enhanced Data rates for GSM Evolution (EDGE) network, a digital mobile phone technology for improved data transmission rates as a backward-compatible extension of GSM, is an example of 2.5G mobile technology.
3G cellular was introduced in 2001. Using code division multiplexing (CDMA) as its foundation, it is an enabler for mobile audio, graphics and video applications. CDMA opens up the entire bandwidth, allowing multiple users to share the same frequency and communicate at the same time. In addition it established a single global network protocol with compatibility over a variety of devices, with potential access to the Internet from any location. Universal Mobile Telecommunications Service (UMTS) is a 3G broadband, packet-based transmission of text, digitized voice, video and multimedia at data rates up to 2 megabits per second (Mbps).
Introduced in 2009, 4G provides transmission rates 10x 3G, or higher. It prioritizes traffic according to the application and dynamically adjusts resources to optimize needs. While GSM uses TDMA and 3G uses CDMA, 4G uses orthogonal frequency division multiple access (OFDMA). Orthogonal frequency division multiplexing splits a serial data stream into several parallel streams transmitted through separate closely-spaced narrowband channels. This has the effect of reducing interference and crosstalk without sacrificing throughput. Long term evolution (LTE) is an instantiation of 4G that employs IP addresses, based upon the TCP/IP networking scheme; each device on the wireless network has a unique address. The LTE standard was created by the Third Generation Partnership Project (3GPP) and has been adopted by most worldwide wireless carriers.
5G LTE promises a revolution in wireless communications technology to enable the large scale implementation of the Internet of Things (IoT). It extends the existing approved RF spectrum into the mmw region to provide wider bandwith for more subscribers, higher speeds and lower latency in order to open up new applications for smart homes, cities and infrastructures, virtual reality and autonomous transportation (see Figure 9). Firm dates have not been established for the development of 5G, but a number of companies and organizations have their own timelines. Initial 5G standards were approved by the 3GPP in December 2017.
Local Area Networks
Understanding the wireless local-area network (WLAN) requires first knowing something about its predecessor, the local area network (LAN). Before wireless, there were wired LANs. This is a group of computers and associated devices that share a common communications line, usually a coaxial cable. These networks span a relatively short area such as a single building or group of buildings. LANs can be connected to other LANs over a greater distance by way of telephone lines. This increases the coverage area greatly and offers much more flexibility and the ability to communicate with locations beyond the single building or group of buildings.
WLAN is the next generation of the LAN. This is a flexible data communications system implemented as an extension to, or an alternative for, the wired LAN. WLAN is not a replacement for the typical LAN, but rather, an enhancement. Many have adopted WLAN for improved mobility. Installation speed is another advantage as well as installation flexibility, i.e. the wireless network can go where wires cannot go. Long-term cost benefits are much better with wireless networks, although the initial costs may be higher. Some who benefit from WLAN are medical personnel in hospitals using hand-held units to obtain information instantly, students and professors on college campuses accessing information for a variety of needs, training personnel at corporations exchanging information during seminars and short courses, and warehouse workers working with central databases to increase productivity.
Most WLANs employ spread spectrum techniques for security. The two methods used are frequency-hopping spread spectrum (FHSS) and direct-sequence spread spectrum (DSSS). Frequency hopping employs a carrier that changes frequency in a pattern that is determined by a random signal generator. The pattern is known only to the transmitter and the intended receiver. The direct sequence scheme generates a redundant bit pattern for each bit (called a chip) to be transmitted. The longer the chip, the greater the probability that the original data can be recovered.
The Federal Communications Commission (FCC) has approved specific frequency bands for WLANs. These are 902 to 928 MHz, 2.4 to 2.483, 5.15 to 5.35 and 5.725 to 5.875 GHz. WLANs are governed by certain standards. The Institute of Electrical and Electronic Engineering (IEEE) has a specific standard, which is 802.11. There are different levels of this standard such as 802.11 (data up to 2 Mbps in the 2.4 GHz band), 802.11a (data up to 54 Mbps in the 5 GHz band), 802.11b (data up to 11 Mbps in the 2.4 GHz band) and 802.11g (data up to 54 Mbps in the 2.4 GHz band). 802.11n improves on 802.11g in the amount of bandwidth supported by utilizing multiple wireless signals and antennas, i.e. multiple input-multiple output (MIMO) technology. The newest generation, 802.11ac, utilizes dual-band wireless technology, supporting simultaneous connections on both the 2.4 and 5 GHz Wi-Fi bands. 802.11ac offers backward compatibility to 802.11b/g/n and bandwidth rated up to 1300 Mbps on the 5 GHz band plus up to 450 Mbps on 2.4 GHz.
Another standard that is widely used is Bluetooth. This was named after Harald Bluetooth who was king of Denmark in the late 10th century. He united Denmark and part of Norway into a single kingdom and then introduced Christianity into Denmark. The choice of this name for a wireless standard indicates how important companies from the Baltic region are to the communications industry. Bluetooth operates in the 2.4 GHz band, which has been set aside by international agreement for use in industrial, scientific and medical (ISM) areas. In the U.S. and Europe, the frequency range is 2.400 to 2.483.5 GHz with 79 1 MHz channels (The range of 2.402 to 2.480 GHz is actually used). In Japan, the frequency range of 2.472 to 2.479 GHz with 23 1 MHz channels used. It provides an approach which enables various devices to communicate with each other within approximately a 10 m range. Its primary purpose is to unify connections within small work areas so that the electronic devices in close proximity can communicate effectively with one another.
Bluetooth provides a universal short-range wireless network that is available worldwide for unlicensed low-power use. If, for example, two Bluetooth devices are within 10 m of one another, they can share up to 720 Kbps of capacity. Bluetooth can transmit data, audio, graphics and video. Applications include headsets, cordless telephones, home stereos and digital MP3 players. Some of the things that are possible or currently operational are making calls from wireless headsets to cell phones, the elimination of cables between computers and peripheral equipment, the connection of MP3 equipment to other machines for music download, monitoring systems for home appliances and the control of many home appliances. A piconet is a small network with eight devices communicating within this network. A Bluetooth radio can have 10 piconets working together within the same range of coverage.
Wireless LAN Technology
Current wireless technology falls into three categories: infrared (IR), at optical wavelengths extending from 770 nm and upward, spread spectrum systems and narrowband microwave systems. IR applications are rather limited in range since they involve infrared light that cannot penetrate opaque walls. IR communication is therefore practical only within a single room. Spread spectrum systems operate solely within the ISM bands. Within these bands, there is no FCC license required. Narrowband microwave WLAN does not use spread spectrum techniques and when used in bands other than ISM bands, it requires an FCC license. Applications for narrowband microwave in addition to WLAN include land mobile radio and wireless backhaul.
Infrared technology parameters and characteristics are shown in Table 1. The parameters are shown for both diffused infrared (which uses a source, such as a light emitting diode (LED), which spreads the light transmission over an area) and direct beam infrared (which uses a more focused beam of light). Spread spectrum is broken down further into frequency hopping and direct sequence (see Table 2). Narrowband microwave is shown in Table 3.
|Data Rate (Mbps)||1 to 4||1 to 10|
|Range (m)||15 to 60||25|
|Wavelength (nm)||800 to 900||800 to 900|
|Parameter||Frequency Hopping||Direct Sequence|
|Data Rate (Mbps)||1 to 3||2 to 20|
|Range (m)||30 to 100||30 to 250|
|Frequency||—||902 to 928 MHz|
|—||2.4 to 2.435 GHz|
|—||5.725 to 5.85 GHz|
|Radiated Power||< 1 W||< 1 W|
|Data Rata (Mbps)||10 to 20|
|Range (m)||10 to 40|
|Frequency||902 to 928 MHz|
|5.2 to 5.775 GHz|
|18.825 to 19.205 GHz|
|Radiated Power||25 mw|
RFID is a generic term for any combination of circuitry that uses RF or microwave energy to provide a means of identification. It could be used to identify people or objects for a number of applications. A typical system uses a microchip that stores information (a serial number, for example, or other information about a person or product) attached to an antenna for transmission. This combination is called an RFID tag. The tag contains a unique serial number but can have other information (account number, license number and so forth). RFID tags can be active, passive or semipassive.
The RFID active tag contains a battery that is used to power the microchip and transmit a signal to a reader. These tags can be read from 300 ft or more and are usually more expensive than other tag types. The military uses active tags to track supplies. The RFID passive tag has no battery. When RF waves from a reader are received by the chip’s antenna, the energy is converted to an electrical signal that can power the chip and enable it to send back information. These tags are very low-priced and lend themselves very well to many commercial applications. The semipassive tag is similar to the active tag except that the battery is used to power the microchip but is not used to communicate with the reader. Many semipassive tags are in a dormant stage until they are activated by a signal. This conserves battery life. Some documents refer to these tags as battery-assisted tags.
Most of the RFID tags hold digital information in a microchip, but there are other chipless tags that use material to reflect back a portion of the signal beamed toward them. These tags use plastic or conductive polymers instead of silicon-based chips. Still others use materials that completely reflect a portion of the wave beamed at them. A computer then generates an image of the reflected energy and uses it like a fingerprint to identify the item incorporating the tag.
RFID tags can be found in various sizes and shapes. Some are readily identified, such as the plastic tags attached to merchandise in stores. Other types are used for animal tracking. These are implanted under the skin to locate family pets or to keep track of endangered species in the wild. They are very small and probably would not be easily recognized. RFID tags have been incorporated in credit cards, as well.
When people hear of RFID, they relate it to bar codes because it is a form of tracking and identification, as well. The bar code has been the traditional method used for both of these functions. The RFID tag, however, is fast becoming the preferred method of identification and tracking because of its read/write capability. This expands identification and tracking to include interactive applications. Also, the RFID tag can be read from a distance away from the product or person, as previously mentioned. It also has the ability to transmit through substances such as snow, fog, ice, paint and other items.
RFID systems can be grouped into categories: electronic article surveillance (EAS) systems, portable data capture systems, networked systems and positioning systems. The EAS system is the most recognizable as the one in retail stores. It uses tags attached to garments with readers by the entry/exit doors. The portable data capture system uses portable readers that enable it to be used at many various locations. The networked system has fixed position readers connected to a centralized information system. The transponders are on people or moveable items, such are trucks and freight cars. The positioning system is for automated location identification of any tagged item.
RFID operates at various frequencies. Common frequencies are 13.56 MHz and frequencies in the UHF frequency band from 0.3 to 3.0 GHz (300 to 3000 MHz). The RFID band used in North America is 862 to 928 MHz (this takes into account the very popular frequency band of 902 to 928 MHz). The European frequencies are 862 to 870 MHz. There are also microwave tags that operate at 5.8 GHz. These can be read up to 30 ft away, utilize more power, and are more expensive.
Two other types of tags are the read-only tag and the read-write tag. The read-only tag contains data that can be changed only if the chip is reprogrammed. The read-write tag can store new information. This is applicable for reusable containers that must frequently have new information associated with them. This capability is, of course, more costly. The cost of RFID systems is a challenge to widespread adoption.
Another challenge to widespread adoption is privacy and security. The ability to track people, products, vehicles, and currency (such as credit cards) is a concern to many. For example, there are readers now that can read data transmitted by many different RFID tags which means that if a person enters an establishment carrying several tags, one reader can read all of these tags; not only the ones for that business. This might allow that business to put together a profile of that individual, such as their buying habits, currency they carry and their product preferences.
Navigation is defined as “the science of locating a position and plotting a course.” The GPS shows you your exact location anywhere on Earth (global), any time and in any weather. The system consists of 24 satellites in orbit 20,000 to 26,000 nautical miles above the earth. There is a range of distance because the orbit is not completely circular. It is, rather, more elliptical in nature. The satellites are arranged so that there are four satellites placed in each of six orbital planes. Each satellite takes 12 hours to complete one orbit around the Earth. To receive accurate navigation information, data from a minimum of four of these satellites is required. In many instances there are up to six or ten that can supply information to a receiver. There are worldwide ground stations that monitor the satellites continuously. With a suitable GPS receiver, the signals transmitted from the satellites can be detected and location displayed. This system is a one-way, receive only, system; i.e. the only transmission that takes place is that from the satellite(s).
GPS was first conceived around 1974, with the first satellite being launched in 1978, and was originally designed to be used only for military applications. Initial applications were strictly military and that was where the technology was perfected. Later it was made available for commercial and civilian applications. Dual-use systems can now be accessed by both types of users. This is another example of technology developed for military applications that is later adopted for commercial use. Another example, mentioned earlier, is spread spectrum, now used in secure commercial telecommunications.
GPS consists of three segments (see Figure 10). These segments are: the control segment, the space segment and the user segment.
The control segment consists of a worldwide network of tracking stations. There are five tracking stations throughout the world, with the master control station being located in Colorado Springs, Colo (The other stations are in Hawaii, Kwajalein, Diego Garcia and Ascension Islands). As its name implies, the master control station is where the main control functions are carried out. The primary function is the tracking of the satellites to determine and predict satellite locations, the behavior of their atomic clocks and atmospheric data. Figure 10 shows that the control segment transmits an upload and receives a download. This is done with two carriers of different frequencies. These are designated as L1 (1575.42 MHz) and L2 (1227.60 MHz). With these signals, GPS corrects a major error, ionospheric delay. This error is due to the fact that in the upper part of the atmosphere there are ultraviolet rays and X-rays coming to Earth from the Sun that interact with gas molecules and atoms that are above the Earth. The area in which they interact is the ionosphere (50 km to1,000 km above the Earth). This interaction causes a delay in the signal which is measured and corrected using L1 and L2.
The GPS space segment consists of the 24-satellite group that was discussed previously. Each satellite transmits a signal that consists of: two sine waves (the carriers discussed above), two digital codes and a navigational message. The codes and navigation message are added to the carriers as binary modulations. The combination of the carriers and codes are used to determine the distance from a user’s receiver to the satellites. The navigation message contains the coordinates (location) of the satellites as a function of time. The signals that are transmitted from the satellites are controlled by very accurate on-board atomic clocks.
The user segment consists of the receivers of the signals from the satellites orbiting the Earth. These receivers may be hand-held devices, units that are placed in a car or truck, airborne units or units that may be mounted in a variety of fixed locations. A receiver intercepts a signal from the satellite, which travels at the speed of light. The signal takes a certain finite time to reach the receiver from the satellite. The difference between the time that the signal is sent and the time that it arrives at the receiver is multiplied by the speed of light, enabling the receiver to calculate its distance to the satellite. In order to determine its precise longitude, latitude and altitude, the receiver measures the propagation time from at least four separate satellites. If there are more satellite signals available, accuracy is improved. If the exact distance from a satellite in space is known, it determines location somewhere on the surface of an imaginary sphere with a radius equal to the distance to the satellite. Knowing the exact distance from two satellites determines location somewhere on a line where the two spheres intersect. With a third satellite, there are only two possible points, and one of them is usually impossible (see Figure 11). Accuracy increases with the measurement of more satellite positions. An important aspect to note is that GPS is based on a system of coordinates called the World Geodetic System 1984 (WGS 84). This system is similar to the latitude and longitude lines on a typical world map. The WGS 84 system, with several more recent updates, provides a built-in frame of reference for all geo-location activities.
An area of extensive growth in the 21st century is the use of GPS in everyday lives. It is used, for example, by police, fire and emergency medical units to locate the closest unit to an emergency and enable dispatchers to expedite units to a precise location. Such areas as fleet vehicles, public transportation, delivery trucks and courier services use GPS receivers to monitor vehicle locations. The use of GPS in automobiles and smartphones is ubiquitous. It is central to the operation of ride-share services such as Uber and Lyft, and is a key component of autonomous vehicle development.
Many mapping and surveying companies also use GPS. Such areas as wildlife management and threatened species areas have GPS receivers and small transmitters attached to certain species of wildlife to help determine population distribution patterns and also some sources of diseases in these animals. Archaeologists and explorers also are using the GPS to carry out their research in areas that may not be very accurately mapped.
There are many applications of RF and microwave technology, military and commercial, across the entire spectrum. Figure 12 is a comprehensive summary of current commercial applications, frequency allocations and the governing wireless communications standards. Specialized texts and technical publications present a great deal more information on these topics; this tutorial is designed to be a general overview.