Controller-Pilot Communications

From pre-flight to landing, all Instrument Flight Rule (IFR) flights are conducted with controller-pilot communications. An IFR flight over a long distance requires many communications with many different controllers.

After the flight plan is filed for a commercial jetliner and the aircraft preflight is completed, the pilot is ready to taxi. A call is made to Clearance Delivery in local control (the airport's control tower) for either verification of the "clearance filed" or to receive a "modified clearance." Pilots are encouraged to file for "preferred" routes, if there are any. Pilots always like to hear "cleared as filed" as this means their flight plan was received without requiring any changes. When pilots receive an amended clearance, they copy and read back to verify. The controllers will warn a flight crew if the new clearance is a long or complicated notation. A clearance delivery controller at Chicago's O'Hare (ORD) airport would warn a pilot of complicated changes with the statement, "Hope you have a sharp pencil handy." The crew receiving the clearance would recognize that they would have to listen carefully and write quickly. After the pilot has clearance, he/she is instructed to contact ground control in local control (the airport's control tower) on the frequency given by the clearance delivery controller. Next, the ground controller clears the pilot to taxi to the takeoff runway. At large airports this can take a considerable amount of time, involving many turns on many taxiways with many stops (for further clearance if the taxi path crosses runways along the way). All clearances have a "cleared to" phrase that gives further directions on how to proceed once the aircraft arrives at that point.Once the pilot is at the takeoff runway in the run-up area, he/she contacts the airport tower. When the tower controller clears the aircraft for takeoff, the controller also instructs the pilot as to the heading and altitude to climb to after takeoff. Clearance for many flights specifies a standard Departure Procedure (DP).

After takeoff and the initial climb out from the departure airport, the local controller hands off the flight to the departure controller located in the Terminal Radar Approach Control (TRACON). The "hand-off" consists of the local controller telling the pilot to contact departure control and giving the radio frequency to which the pilot must switch. This hand-off also takes place electronically as the aircraft's transponder code is received by the controller in the TRACON. The signal appears on the controller's radar screen as a "target" with its data block. The pilot then contacts the departure controller located in the TRACON who then provides necessary altitude or heading changes to position the aircraft for its next flight phase: en route. The departure controller then hands off the flight to a controller in an Air Route Traffic Control Center (ARTCC).

The ARTCC controller then monitors the aircraft along the en route portion of the flight. A coast-to-coast flight will fly through many different ARTCC sections before the flight is handed off to an approach controller. The original flight clearance that was given probably contained a Standard Terminal Arrival Route (STAR) for the arrival phase of the flight. If there are no delays or weather problems, the STAR will be routinely followed.

The Approach Controller gives the pilot descent altitudes and vectors (headings) to a final approach fix. When the aircraft arrives at the final approach fix, it will be cleared to fly a published approach. The flight will next be handed off to the destination airport's tower controller for landing instructions. The tower controller clears the flight to land. Upon landing, the tower controller directs the pilot to an exit taxiway. The pilot also receives the next radio frequency to which he/she must switch the radio in order to contact the ground controller.After exiting the runway, the pilot contacts the ground controller for taxi clearance and gate instructions. The pilot parks the aircraft at the gate, terminating the flight.

NASA Research

Modern aircraft cockpits and air traffic control centers are very complex, high-technology environments in which to work. Understanding and optimizing the ways in which humans and high-technology systems work together are critical to aviation safety and the development of new aviation systems.
The Crew-Vehicle Systems Research Facility at NASA's Ames Research Center was designed for the study of human factors in aviation safety. The facility is used to analyze performance characteristics of flight crews; help develop new designs for future aviation environments; evaluate new and contemporary air traffic control procedures; and develop new training and simulation techniques required by the continued technical evolution of flight systems.

The facility is home to a Boeing 747-400 flight simulator, the Advanced Concepts Flight Simulator, and an air traffic control system simulator. Together, these systems provide full mission flight simulation research capability. Visual systems provide out-the-window cues in both cockpits. The Air Traffic Control System simulator provides a realistic air traffic control environment, including communication with the cockpits allowing the study of air-to-ground communications systems as they impact crew performance. Dedicated experimenter labs for each simulator provide full monitoring and control capability for each simulation system.

Radar communication system

Radar

Radar is actually an acronymthat stands for RAdio Detection And Ranging. It was developed in the early 1940s. Radar uses the echo principle.Radar equipment emits a high energy radio signal from an antenna. The signal travels out from the source untilit is reflected back by contact with an object. The radar antenna relays this signal to a scope where the imageis displayed. Using the time it takes for the emitted signal to reach the object and reflect back to its source,the distance to the object can be computed. The radar signal is moving at the speed of light and can make sucha trip in microseconds.

In aviation, a ground radarantenna sends radio signal pulses into the sky. These signals are reflected back by aircraft flying in the airspace.The radar scope displays the direction and distance from which the signals are reflected back. This coupled witheach aircraft's transponder signal identifies the aircraft on the radarscope. Also, all airliners are equippedwith radar equipment in the aircraft's nose. Short bursts of radio signals are emitted from the nose cone of theaircraft. These signals reflect off clouds ahead of the aircraft. The on-board computer calculates the distanceand displays the object (the cloud) on the on-board radar screen.

The Flight PlanCommercial airline companies employ flight planners who perform all the necessary data gathering and analyses necessaryto complete a flight plan. These flight plans are then given to the pilots during a flight briefing before thepilot begins the aircraft preflight check. These flight plans contain information similar to what is required fora small aircraft pilot's flight plan. Small aircraft pilots and charter pilots perform their own flight planning and submit their flight plans to the Flight Service Station (FSS) that services their departure airport. The FSS enters the flight plan information into their system. Among the many services offered by the FSS, it is responsible for processing flight plans. After a pilot files a flight plan with an FSS facility, a record of the flight plan is made that includes the aircraft description and tail numbers, departure and destination airports, route of flight, estimated time of departure (ETD), estimated time of arrival (ETA) and number of people on board. About an hour before takeoff or once airborne, the pilot "opens" the VFR flight plan. This ensures that the FSS will keep track of the airplane's ETA. Along the route the pilot radios the FSS with occasional position reports. This helps the FSS to track the route. If the pilot gets disoriented along the way, an FSS specialist could locate the aircraft with a VHF direction finder or use radar. Within thirty minutes of completing a flight, the pilot needs to close the VFR flight plan. If the pilot changes the final destination or will be at least 15 minutes later than estimated, the pilot needs to inform the FSS facility accordingly. If the pilot does not close the flight plan or indicate changes to the FSS, the FSS will initiate search and rescue procedures believing the aircraft has been "lost".

Flight Tracking Strip and Data BlockUpon acceptance of a flight plan for a commercial jetliner flight, a "flight tracking strip" is generated in the departure control tower. This strip contains essentially the same information from the flight plan, but in an abbreviated format. This strip communicates to air traffic controllers along the route information about the flight that assists controllers in directing the pilot. This strip is physically handed off from controller to controller within the same air traffic management facility (such as the local control tower). It is also electronically handed off from one air traffic management facility to another as the flight moves from one airspace sector to another.Each air traffic management facility has a slightly different look for their flight tracking slips.

Every commercial flight is equipped with a transponder. This electronic device is connected to the on-board computer. It transmits coded radio signals to the controller's radar receiver. These signals contain information about the flight: aircraft's identification letters or flight number and its altitude. Upon departure, pilots receive a 4-digit transponder code and set their transponder to that code. The terminology is "Squawk" 1200. The standard transponder code for VFR flights is 1200. When the code is set, the radar "blip" for that flight shows as an enhanced signal on the controller's radar screen. The aircraft is shown in motion on the screen and is followed by a box with the flight's information in it: the data block. This way controllers can visually track each flight as it flies through their designated airspace.

Digital Systems

As demand for mobile telephone service has increased, service providers found that basic engineering assumptions borrowed from wireline (landline) networks did not hold true in mobile systems. While the average landline phone call lasts at least 10 minutes, mobile calls usually run 90 seconds. Engineers who expected to assign 50 or more mobile phones to the same radio channel found that by doing so they increased the probability that a user would not get dial tone—this is known as call-blocking probability. As a consequence, the early systems quickly became saturated, and the quality of service decreased rapidly. The critical problem was capacity. The general characteristics of time division multiple access (TDMA), Global System for Mobile Communications (GSM), personal communications service (PCS) 1900, and code division multiple access (CDMA) promise to significantly increase the efficiency of cellular telephone systems to allow a greater number of simultaneous conversations

The advantages of digital cellular technologies over analog cellular networks include increased capacity and security. Technology options such as TDMA and CDMA offer more channels in the same analog cellular bandwidth and encrypted voice and data. Because of the enormous amount of money that service providers have invested in AMPS hardware and software, providers look for a migration from AMPS to digital analog mobile phone service (DAMPS) by overlaying their existing networks with TDMA architectures.

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Time Division Multiple Access (TDMA)

North American digital cellular (NADC) is called DAMPS and TDMA. Because AMPS preceded digital cellular systems, DAMPS uses the same setup protocols as analog AMPS. TDMA has the following characteristics:
IS–54 standard specifies traffic on digital voice channels
initial implementation triples the calling capacity of AMPS systems
capacity improvements of 6 to 15 times that of AMPS are possible
many blocks of spectrum in 800 MHz and 1900 MHz are used
all transmissions are digital

TDMA/FDMA application 7. 3 callers per radio carrier (6 callers on half rate later), providing 3 times the AMPS capacity

TDMA is one of several technologies used in wireless communications. TDMA provides each call with time slots so that several calls can occupy one bandwidth. Each caller is assigned a specific time slot. In some cellular systems, digital packets of information are sent during each time slot and reassembled by the receiving equipment into the original voice components. TDMA uses the same frequency band and channel allocations as AMPS. Like NAMPS, TDMA provides three to six time channels in the same bandwidth as a single AMPS channel. Unlike NAMPS, digital systems have the means to compress the spectrum used to transmit voice information by compressing idle time and redundancy of normal speech. TDMA is the digital standard and has 30-kHz bandwidth. Using digital voice encoders, TDMA is able to use up to six channels in the same bandwidth where AMPS uses one channel.

Extended Time Division Multiple Access (E–TDMA)

The E–TDMA standard claims a capacity of fifteen times that of analog cellular systems. This capacity is achieved by compressing quiet time during conversations. E–TDMA divides the finite number of cellular frequencies into more time slots than TDMA. This allows the system to support more simultaneous cellular calls.

Fixed Wireless Access (FWA)

FWA is a radio-based local exchange service in which telephone service is provided by common carriers. It is primarily a rural application—that is, it reduces the cost of conventional wireline. FWA extends telephone service to rural areas by replacing a wireline local loop with radio communications. Other labels for wireless access include fixed loop, fixed radio access, wireless telephony, radio loop, fixed wireless, radio access, and Ionica. FWA systems employ TDMA or CDMA access technologies.

Personal Communications Service (PCS)

The future of telecommunications includes PCS. PCS at 1900 MHz (PCS 1900) is the North American implementation of digital cellular system (DCS) 1800 (GSM). Trial networks were operational in the United States by 1993, and in 1994 the Federal Communications Commission (FCC) began spectrum auctions. As of 1995, the FCC auctioned commercial licenses. In the PCS frequency spectrum, the operator's authorized frequency block contains a definite number of channels. The frequency plan assigns specific channels to specific cells, following a reuse pattern that restarts with each nth cell. The uplink and downlink bands are paired mirror images. As with AMPS, a channel number implies one uplink and one downlink frequency (e.g., Channel 512 = 1850.2-MHz uplink paired with 1930.2-MHz downlink).

Code Division Multiple Access (CDMA)

CDMA is a digital air interface standard, claiming 8 to 15 times the capacity of analog. It employs a commercial adaptation of military, spread-spectrum, single-sideband technology. Based on spread spectrum theory, it is essentially the same as wireline service—the primary difference is that access to the local exchange carrier (LEC) is provided via wireless phone. Because users are isolated by code, they can share the same carrier frequency, eliminating the frequency reuse problem encountered in AMPS and DAMPS. Every CDMA cell site can use the same 1.25-MHz band, so with respect to clusters, n = 1. This greatly simplifies frequency planning in a fully CDMA environment.

CDMA is an interference-limited system. Unlike AMPS/TDMA, CDMA has a soft capacity limit; however, each user is a noise source on the shared channel and the noise contributed by users accumulates. This creates a practical limit to how many users a system will sustain. Mobiles that transmit excessive power increase interference to other mobiles. For CDMA, precise power control of mobiles is critical in maximizing the system's capacity and increasing battery life of the mobiles. The goal is to keep each mobile at the absolute minimum power level that is necessary to ensure acceptable service quality. Ideally, the power received at the base station from each mobile should be the same (minimum signal to interference).

4. North American Analog Cellular Systems

Originally devised in the late 1970s to early 1980s, analog systems have been revised somewhat since that time and operate in the 800-MHz range. A group of government, telco, and equipment manufacturers worked together as a committee to develop a set of rules (protocols) that govern how cellular subscriber units (mobiles) communicate with the cellular system. System development takes into consideration many different, and often opposing, requirements for the system, and often a compromise between conflicting requirements results. Cellular development involves the following basic topics:

frequency and channel assignments
type of radio modulation
maximum power levels
modulation parameters
messaging protocols
call-processing sequences

The Advanced Mobile Phone Service (AMPS)

AMPS was released in 1983 using the 800-MHz to 900-MHz frequency band and the 30-kHz bandwidth for each channel as a fully automated mobile telephone service. It was the first standardized cellular service in the world and is currently the most widely used standard for cellular communications. Designed for use in cities, AMPS later expanded to rural areas. It maximized the cellular concept of frequency reuse by reducing radio power output. The AMPS telephones (or handsets) have the familiar telephone-style user interface and are compatible with any AMPS base station. This makes mobility between service providers (roaming) simpler for subscribers. Limitations associated with AMPS include the following:
low calling capacity
limited spectrum
no room for spectrum growth
poor data communications
minimal privacy
inadequate fraud protection

AMPS is used throughout the world and is particularly popular in the United States, South America, China, and Australia. AMPS uses frequency modulation (FM) for radio transmission. In the United States, transmissions from mobile to cell site use separate frequencies from the base station to the mobile subscriber.

Narrowband Analog Mobile Phone Service (NAMPS)

Since analog cellular was developed, systems have been implemented extensively throughout the world as first-generation cellular technology. In the second generation of analog cellular systems, NAMPS was designed to solve the problem of low calling capacity. NAMPS is now operational in 35 U.S. and overseas markets, and NAMPS was introduced as an interim solution to capacity problems. NAMPS is a U.S. cellular radio system that combines existing voice processing with digital signaling, tripling the capacity of today's AMPS systems. The NAMPS concept uses frequency division to get 3 channels in the AMPS 30-kHz single channel bandwidth. NAMPS provides 3 users in an AMPS channel by dividing the 30-kHz AMPS bandwidth into 3 10-kHz channels. This increases the possibility of interference because channel bandwidth is reduced.

5. Cellular System Components

The cellular system offers mobile and portable telephone stations the same service provided fixed stations over conventional wired loops. It has the capacity to serve tens of thousands of subscribers in a major metropolitan area. The cellular communications system consists of the following four major components that work together to provide mobile service to subscribers.
public switched telephone network (PSTN)
mobile telephone switching office (MTSO)
cell site with antenna system
mobile subscriber unit (MSU)

PSTN

The PSTN is made up of local networks, the exchange area networks, and the long-haul network that interconnect telephones and other communication devices on a worldwide basis.
Mobile Telephone Switching Office (MTSO)
The MTSO is the central office for mobile switching. It houses the mobile switching center (MSC), field monitoring, and relay stations for switching calls from cell sites to wireline central offices (PSTN). In analog cellular networks, the MSC controls the system operation. The MSC controls calls, tracks billing information, and locates cellular subscribers.

The Cell Site

The term cell site is used to refer to the physical location of radio equipment that provides coverage within a cell. A list of hardware located at a cell site includes power sources, interface equipment, radio frequency transmitters and receivers, and antenna systems.
Mobile Subscriber Units (MSUs)
The mobile subscriber unit consists of a control unit and a transceiver that transmits and receives radio transmissions to and from a cell site. The following three types of MSUs are available:
the mobile telephone (typical transmit power is 4.0 watts)
the portable (typical transmit power is 0.6 watts)
the transportable (typical transmit power is 1.6 watts)
The mobile telephone is installed in the trunk of a car, and the handset is installed in a convenient location to the driver. Portable and transportable telephones are hand-held and can be used anywhere. The use of portable and transportable telephones is limited to the charge life of the internal battery.

3. Cellular System Architecture

Increases in demand and the poor quality of existing service led mobile service providers to research ways to improve the quality of service and to support more users in their systems. Because the amount of frequency spectrum available for mobile cellular use was limited, efficient use of the required frequencies was needed for mobile cellular coverage. In modern cellular telephony, rural and urban regions are divided into areas according to specific provisioning guidelines. Deployment parameters, such as amount of cell-splitting and cell sizes, are determined by engineers experienced in cellular system architecture.
Provisioning for each region is planned according to an engineering plan that includes cells, clusters, frequency reuse, and handovers.

Cells

A cell is the basic geographic unit of a cellular system. The term cellular comes from the honeycomb shape of the areas into which a coverage region is divided. Cells are base stations transmitting over small geographic areas that are represented as hexagons. Each cell size varies depending on the landscape. Because of constraints imposed by natural terrain and man-made structures, the true shape of cells is not a perfect hexagon.

Clusters

A cluster is a group of cells. No channels are reused within a cluster. Figure 4 illustrates a seven-cell cluster.

Frequency Reuse

Because only a small number of radio channel frequencies were available for mobile systems, engineers had to find a way to reuse radio channels to carry more than one conversation at a time. The solution the industry adopted was called frequency planning or frequency reuse. Frequency reuse was implemented by restructuring the mobile telephone system architecture into the cellular concept.

The concept of frequency reuse is based on assigning to each cell a group of radio channels used within a small geographic area. Cells are assigned a group of channels that is completely different from neighboring cells. The coverage area of cells is called the footprint. This footprint is limited by a boundary so that the same group of channels can be used in different cells that are far enough away from each other so that their frequencies do not interfere

Cells with the same number have the same set of frequencies. Here, because the number of available frequencies is 7, the frequency reuse factor is 1/7. That is, each cell is using 1/7 of available cellular channels.

Cell Splitting

Unfortunately, economic considerations made the concept of creating full systems with many small areas impractical. To overcome this difficulty, system operators developed the idea of cell splitting. As a service area becomes full of users, this approach is used to split a single area into smaller ones. In this way, urban centers can be split into as many areas as necessary to provide acceptable service levels in heavy-traffic regions, while larger, less expensive cells can be used to cover remote rural regions

Handoff

The final obstacle in the development of the cellular network involved the problem created when a mobile subscriber traveled from one cell to another during a call. As adjacent areas do not use the same radio channels, a call must either be dropped or transferred from one radio channel to another when a user crosses the line between adjacent cells. Because dropping the call is unacceptable, the process of handoff was created. Handoff occurs when the mobile telephone network automatically transfers a call from radio channel to radio channel as a mobile crosses adjacent cells

During a call, two parties are on one voice channel. When the mobile unit moves out of the coverage area of a given cell site, the reception becomes weak. At this point, the cell site in use requests a handoff. The system switches the call to a stronger-frequency channel in a new site without interrupting the call or alerting the user. The call continues as long as the user is talking, and the user does not notice the handoff at all.

Cellular Communications

Definition

A cellular mobile communications system uses a large number of low-power wireless transmitters to create cells—the basic geographic service area of a wireless communications system. Variable power levels allow cells to be sized according to the subscriber density and demand within a particular region. As mobile users travel from cell to cell, their conversations are handed off between cells to maintain seamless service. Channels (frequencies) used in one cell can be reused in another cell some distance away. Cells can be added to accommodate growth, creating new cells in unserved areas or overlaying cells in existing areas.

Overview

This tutorial discusses the basics of radio telephony systems, including both analog and digital systems. Upon completion of this tutorial, you should be able to describe the basic components of a cellular system and identify digital wireless technologies

1. Mobile Communications Principles

Each mobile uses a separate, temporary radio channel to talk to the cell site. The cell site talks to many mobiles at once, using one channel per mobile. Channels use a pair of frequencies for communication—one frequency (the forward link) for transmitting from the cell site and one frequency (the reverse link) for the cell site to receive calls from the users. Radio energy dissipates over distance, so mobiles must stay near the base station to maintain communications. The basic structure of mobile networks includes telephone systems and radio services. Where mobile radio service operates in a closed network and has no access to the telephone system, mobile telephone service allows interconnection to the telephone network

Early Mobile Telephone System Architecture

Traditional mobile service was structured in a fashion similar to television broadcasting: One very powerful transmitter located at the highest spot in an area would broadcast in a radius of up to 50 kilometers. The cellular concept structured the mobile telephone network in a different way. Instead of using one powerful transmitter, many low-power transmitters were placed throughout a coverage area. For example, by dividing a metropolitan region into one hundred different areas (cells) with low-power transmitters using 12 conversations (channels) each, the system capacity theoretically could be increased from 12 conversations—or voice channels using one powerful transmitter—to 1,200 conversations (channels) using one hundred low-power transmitters.

2. Mobile Telephone System Using the Cellular Concept

Interference problems caused by mobile units using the same channel in adjacent areas proved that all channels could not be reused in every cell. Areas had to be skipped before the same channel could be reused. Even though this affected the efficiency of the original concept, frequency reuse was still a viable solution to the problems of mobile telephony systems.
Engineers discovered that the interference effects were not due to the distance between areas, but to the ratio of the distance between areas to the transmitter power (radius) of the areas. By reducing the radius of an area by 50 percent, service providers could increase the number of potential customers in an area fourfold. Systems based on areas with a one-kilometer radius would have one hundred times more channels than systems with areas 10 kilometers in radius. Speculation led to the conclusion that by reducing the radius of areas to a few hundred meters, millions of calls could be served.

The cellular concept employs variable low-power levels, which allow cells to be sized according to the subscriber density and demand of a given area. As the population grows, cells can be added to accommodate that growth. Frequencies used in one cell cluster can be reused in other cells. Conversations can be handed off from cell to cell to maintain constant phone service as the user moves between cells.

The cellular radio equipment (base station) can communicate with mobiles as long as they are within range. Radio energy dissipates over distance, so the mobiles must be within the operating range of the base station. Like the early mobile radio system, the base station communicates with mobiles via a channel. The channel is made of two frequencies, one for transmitting to the base station and one to receive information from the base station.

COMPONENTS FOR COMMUNICATIONS SATELLITES

Basic Communications Satellite Components

Every communications satellite in its simplest form (whether low earth or geosynchronous) involves the transmission of information from an originating ground station to the satellite (the uplink), followed by a retransmission of the information from the satellite back to the ground (the downlink). The downlink may either be to a select number of ground stations or it may be broadcast to everyone in a large area. Hence the satellite must have a receiver and a receive antenna, a transmitter and a transmit antenna, some method for connecting the uplink to the downlink for retransmission, and prime electrical power to run all of the electronics. The exact nature of these components will differ, depending on the orbit and the system architecture, but every communications satellite must have these basic components.

Transmitters

The amount of power which a satellite transmitter needs to send out depends a great deal on whether it is in low earth orbit or in geosynchronous orbit. This is a result of the fact that the geosynchronous satellite is at an altitude of 22,300 miles, while the low earth satellite is only a few hundred miles. The geosynchronous satellite is nearly 100 times as far away as the low earth satellite. We can show fairly easily that this means the higher satellite would need almost 10,000 times as much power as the low-orbiting one, if everything else were the same. (Fortunately, of course, we change some other things so that we don't need 10,000 times as much power.)

For either geosynchronous or low earth satellites, the power put out by the satellite transmitter is really puny compared to that of a terrestrial radio station. Your favorite rock station probably boasts of having many kilowatts of power. By contrast, a 200 watt transmitter would be very strong for a satellite.

Antennas

One of the biggest differences between a low earth satellite and a geosynchronous satellite is in their antennas. As mentioned earlier, the geosynchronous satellite would require nearly 10,000 times more transmitter power, if all other components were the same. One of the most straightforward ways to make up the difference, however, is through antenna design. Virtually all antennas in use today radiate energy preferentially in some direction. An antenna used by a commercial terrestrial radio station, for example, is trying to reach people to the north, south, east, and west. However, the commercial station will use an antenna that radiates very little power straight up or straight down. Since they have very few listeners in those directions (except maybe for coal miners and passing airplanes) power sent out in those directions would be totally wasted.The communications satellite carries this principle even further. All of its listeners are located in an even smaller area, and a properly designed antenna will concentrate most of the transmitter power within that area, wasting none in directions where there are no listeners. The easiest way to do this is simply to make the antenna larger. Doubling the diameter of a reflector antenna (a big "dish") will reduce the area of the beam spot to one fourth of what it would be with a smaller reflector. We describe this in terms of the gain of the antenna. Gain simply tells us how much more power will fall on 1 square centimeter (or square meter or square mile) with this antenna than would fall on that same square centimeter (or square meter or square mile) if the transmitter power were spread uniformly (isotropically) over all directions. The larger antenna described above would have four times the gain of the smaller one. This is one of the primary ways that the geosynchronous satellite makes up for the apparently larger transmitter power which it requires.

One other big difference between the geosynchronous antenna and the low earth antenna is the difficulty of meeting the requirement that the satellite antennas always be "pointed" at the earth. For the geosynchronous satellite, of course, it is relatively easy. As seen from the earth station, the satellite never appears to move any significant distance. As seen from the satellite, the earth station never appears to move. We only need to maintain the orientation of the satellite. The low earth orbiting satellite, on the other hand, as seen from the ground is continuously moving. It zooms across our field of view in 5 or 10 minutes.Likewise, the earth station, as seen from the satellite is a moving target. As a result, both the earth station and the satellite need some sort of tracking capability which will allow its antennas to follow the target during the time that it is visible. The only alternative is to make that antenna beam so wide that the intended receiver (or transmitter) is always within it. Of course, making the beam spot larger decreases the antenna gain as the available power is spread over a larger area , which in turn increases the amount of power which the transmitter must provide.
Power Generation
You might wonder why we don't actually use transmitters with thousands of watts of power, like your favorite radio station does. You might also have figured out the answer already. There simply isn't that much power available on the spacecraft. There is no line from the power company to the satellite. The satellite must generate all of its own power. For a communications satellite, that power usually is generated by large solar panels covered with solars cells - just like the ones in your solar-powered calculator. These convert sunlight into electricity. Since there is a practical limit to the how big a solar panel can be, there is also a practical limit to the amount of power which can generated. In addition, unfortunately, transmitters are not very good at converting input power to radiated power so that 1000 watts of power into the transmitter will probably result in only 100 or 150 watts of power being radiated. We say that transmitters are only 10 or 15% efficient. In practice the solar cells on the most "powerful" satellites generate only a few thousand watts of electrical power.Satellites must also be prepared for those periods when the sun is not visible, usually because the earth is passing between the satellite and the sun. This requires that the satellite have batteries on board which can supply the required power for the necessary time and then recharge by the time of the next period of eclipse.