The Future of Satellite Communications

Prepared by: Joel Klooster

Abstract Since the early days of Sputnik, satellites have been filling the space around earth in both geosynchronous and asynchronous orbits. Satellites must be very precisely launched and placed in orbit to achieve their desired behavior, and the communications requires many complex components. The propagation of electromagnetic waves at radio frequencies is the method of communications, and the free space propagation model gives an understanding of the path loss in power between the transmitter and receiver. Because laser systems provide much higher bandwidth and data rates than RF systems, these optical systems are a significant improvement in satellite communications. Although earth-space optical systems must still be perfected, much research is being done and progress being made, especially in the area of inter-satellite communications.

I. INTRODUCTION

A reporter in Paris is seen live on a television set in Chicago. A tiny dish outside a kitchen window picks up internet information for a computer in the living room faster than any phone line could dream of. A radio signal is maintained on an entire drive from New York to Los Angeles. These are just a few examples of satellites at work. These machines, launched so high into space we cannot see them, are the backbone of the communications industry. Without even realizing the physics of their orbit or the complexity of the their communication, society has become dependent on these man-made machines. Electromagnetic Wave propagation, specifically in the Radio Frequency, is the method by which data is passed from one point on earth to another point in an entirely different hemisphere in almost no time whatsoever. However, society is constantly changing and becoming more complex, and continuously demands more information faster. Because RF communications cannot keep pace with the growing demand, laser communications will soon take over as the method of choice for satellite communications.

II. BACKGROUND

1. History of Satellites

The Russian satellite Sputnik, put into orbit around the earth on October 4, 1957, began a revolution in communications that no one at the time could have fully comprehended. This first satellite, shown in figure 2.1, was simply a metal ball filled with nitrogen gas that carried a thermometer, battery, four whip antennas, and a radio transmitter that emitted beeps whose tone and frequency corresponded to the measured temperature. Although this form of communication was very crude and Sputnik only lasted in orbit 92 days before being burned in earth’s atmosphere, it marked the beginning of an era of satellite communications that has changed the way we live forever.

Figure 2-1: The satellite Sputnik1

Sputnik paved the way for more satellites, and in 1960 the first TV satellite, Echo, was launched to reflect broadcast signals from one station to another. Telstar, the first active TV satellite, was launched in 1962. To reduce transmission interference, Telstar received signals at one frequency and converted them to a different frequency before re-transmitting them to earth. However, Telstar traveled at a different velocity

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than the earth and thus had to be tracked in the sky by special equipment. This problem was solved by Syncon, the first satellite whose velocity was synchronous with the earth, which was launched in 1964. By 1969, a mere 12 years after the launch of Sputnik, a worldwide satellite communications network surrounded the earth in space.

2. Overview of Satellites

Technically, anything that revolves in orbit around another body is a satellite. However, in the remainder of this discussion the term satellite will refer to a man-made object that orbits the earth. This orbit is usually not completely circular, and the satellite does not remain a constant distance from the earth. The farthest point from the earth is the apogee of the orbit, while the nearest point is the perigee.

For satellites orbiting the earth, there are several different types of orbits. The first type is termed Geosynchronous because it orbits at the same rotational velocity as the earth. As a result, the satellite is always in the same position relative to the earth and no tracking is necessary on the part of the receiving stations. This greatly simplifies the communication between the earth and the satellite.

An asynchronous orbit, on the other hand, is much closer to the earth and thus travels at a different velocity than the earth’s rotation. A satellite in asynchronous orbit is constantly changing position relative to the earth. A polar orbit is a type of asynchronous orbit in which the satellite is positioned at a very low altitude over the earth’s poles. The satellite is essentially fixed in space, and the earth actually rotates below the satellite. As a result, a large portion of the earth can be seen by the satellite.

Each type of orbit has advantages and disadvantages that determine the satellites function. Because the atmosphere causes drag on a satellite and slows it down, the higher a satellite is above the earth the longer it can stay in orbit. However, the higher a satellite is put into orbit, the more fuel and thus greater cost is required to get it there. When satellites are first put into orbit they are usually in a fairly elliptical orbit. Small rockets are used to correct the trajectory and create a more circular orbit. Light sensors can also be placed on a satellite to determine the location relative to

the sun. This information can then be transmitted back to the earth. These newer technologies allow satellites to remain in orbit much longer than the first satellites.

If all satellites received and transmitted signals at the same frequencies there would be a great deal of interference between signals and communications would be impossible. As a result, the International Radio Frequencies Board (IRFB) regulates both the position of satellites and the frequencies they use for communicating. Communications satellites transmit on the KU-Band, which is 10 – 17 GHz. However, this full range in not utilized. Field Service Satellites (FSS) and Medium Powered Satellites (MPS) transmit at 10.7 – 11.7 GHz, Direct Broadcast Satellites (DBS) broadcast at 11.7 – 12.5 GHz, and telecommunications FSS satellites operate from 12.5 – 12.75 GHz. Thus, most satellites utilize the band from 10.7 – 12.7 GHz, allowing ample room for other satellites to be added.

3. Types of Satellites

There are many different types of satellites and they can be broadly grouped into Non-Communications and Communications Satellites. Non-Communications Satellites include weather satellites, which are used to predict weather patterns and provide current images from space. Scientific satellites, like the Hubble Space Telescope, are used to examine the sun, stars, and other planets. Navigational satellites, like the Global Positioning Satellites, are used to help ships and planes determine their locations. Earth observation satellites watch the earth for changes in temperature, forest patterns, and other environmental factors. Lastly, military satellites are used for radar, early missile warning, and possibly enemy observation. All these satellites perform functions distinct from the commercial communications industry.

Communications Satellites, on the other hand, are used almost exclusively by the commercial communications industry. These satellites are almost always geosynchronous, and are either Field Service Satellites (FSS) or Consumer Satellites. Field Service Satellites usually have a low transmission power of 10 – 20 Watts per channel. Because of the weaker signal, large dish antennas are required to receive the signal on earth.

Direct Broadcast Satellites (DBS), however, are a type of consumer satellite with a much higher

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transmission power of 100 – 200 Watts per channel. Since they require smaller antennas for reception these satellites are ideal for radio or television programming. Medium Powered Satellites (MPS) are another type of consumer satellite. With a 50-Watt transmission power, they transmit a much stronger signal than an FSS but can still broadcast more channels than a DBS, which requires much more power. This makes MPSs a good compromise for power and range of use.

III. PLACEMENT OF SATELLITES

1. Velocities

The placement of satellites is very critical to the correct operation of the satellite, and both velocities and altitudes are associated with the placement. When a satellite is first launched it must have enough energy to escape the earth’s gravitational pull and not fall back to earth. This required escape velocity is 40 320 kilometers per hour (25 039 miles per hour)1. This is a very significant velocity.

Once the satellite has successfully escaped the earth’s gravitational pull, it needs to maintain an orbital velocity to exactly balance the gravitational pull and remain in orbit. The satellite must be in a very precise state of inertia; if the velocity is not great enough the satellite will fall back to earth shortly, and if the satellite has too much velocity it will fly off into space. At an altitude of 242 km (150 miles) this velocity is 27 359 km/h (17000 miles/hr)1. The higher the altitude of the satellite, the lower the velocity must be. So the correct velocity is very important for a satellite to maintain its position.

2. Altitudes

The correct altitude is also very important for a satellite to function correctly. As mentioned before, there are three main types of orbits. A geosynchronous orbit has an altitude of 35 786 km (22 223 miles) and maintains an orbital velocity of 11 300 km/h (7000 m/h)1. This type of orbit is very important for communications satellites because it completes an orbit in 24 hours, remaining in the same position over the earth. The geosynchronous orbit above the equator is termed the Clarke Belt for satellite pioneer Arthur

Clarke. This belt is shown in figure 3-1, and is rapidly becoming filled with satellites.

Figure 3-1: The Clarke Belt2

Asynchronous and Polar orbits, on the other hand, are really useful only for non-communications satellites. An asynchronous orbit is much lower in altitude than a geosynchronous orbit, and thus requires a much higher velocity. Another effect of the lower altitude is that the satellite does not remain fixed over the earth, which, although not suited for communications, can be very useful for other functions. The actual altitude of the asynchronous orbit depends on the function of the satellite.

3. Location of various types of satellites

A low altitude asynchronous orbit is approximately 480 – 970 km (300 – 600 miles) above the surface of the earth. Observation satellites usually orbit at this altitude to perform mapping and make environmental observations such as ice and sand movements. This altitude is also ideal for search and rescue satellites to relay emergency signals from airplanes and ships.

Science satellites, however, orbit at a higher altitude of 4800 – 9700 km (3000 – 6000 miles). These satellites collect data about wildlife, geological activity and other planets, transmitting the data back to earth.

At a high asynchronous orbit of 9700 – 19 400 km (6000 – 12 000 miles) navigation satellites like Global Positioning Satellites (GPS) are used for both civilian and military purposes. Finally, weather and communications satellites operate in a geosynchronous orbit. Thus, the function of the satellite determines its location above the earth.

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IV. LAUNCHING OF SATELLITES

Launching a satellite into orbit is a very costly and complicated process. The satellite is launched straight up through the atmosphere to reduce drag, and is attached to a rocket or shuttle. When the satellite has passed through the atmosphere its nose is tilted onto the correct course. This tilting is controlled by an Inertial Guidance System (IGS) that makes use of gyroscopes and accelerometers to determine the desired path of the rocket. Since the earth rotates east, the satellite is usually launched eastward to make use of the additional velocity supplied by the earth’s rotation. This velocity is greatest at the equator, so satellites are usually launched as close to the equator as possible to save money and weight by reducing the necessary fuel. Once the satellite has reached its desired altitude (as determined by the IGS) small rockets are fired to turned the satellite to the appropriate horizontal angle, and the satellite is released from the rocket.

The timing of the launch is also critical when planning a satellite launch. The “launch window” is the easiest time to launch a satellite into the necessary orbit, and depends on many factors. It is desired to obtain the most efficient course to the appropriate orbit. So there are many factors to consider when launching a satellite into orbit.

V. OPERATION OF SATELLITES

1. Transmission

Satellites have grown significantly in complexity and functionality since the early days of Sputnik. The main components on the satellite itself to enable transmission are:

the body, which serves as the bus for the system;

the power, usually stored in a battery and captured from the sun by solar photovoltaic cells mounted on the body;

an onboard computer to control the satellite and monitor various subsystems;

a radio system with antenna to receive and transmit data;

and Altitude Control System (ACS) to keep the satellite oriented correctly.

All these parts work together so the satellite can receive and transmit information to and from other satellites and the earth.

Data originates on earth, and the signals are sent from the earth to the satellite via a transmission station; this process is known as uplink. The means of sending these signals is usually frequency modulation (FM). This eliminates any problems with frequency and dynamic range, and is less susceptible to interference than amplitude modulation (AM). The uplink signal to the satellites is at a frequency of 14 GHz, and the downlink signal from the satellite to the earth is sent in the KU-Band of 10.7 – 12.7 GHz. A transponder is used to receive, convert, and transmit the signals at different frequencies. Thus, a communications satellite’s main function is to receive data at one frequency and transmit it to a different part of earth at another frequency.

2. Reception

A receiving antenna on the earth must then pick up the signal sent from the satellite. The more concentrated to beam is the stronger the signal is; however, this also results in a smaller area on the earth being able to receive the signal. The area of land that is actually able to receive the signal is called the footprint, and footprint diagrams show the coverage area and the antenna size required to receive the signal in both central and outlying areas of the footprint. A footprint diagram is shown in figure 5-1.

Figure 5-1: Footprint Diagram2

3. Low Noise Converter

When the signal has been sent from the satellite to the earth, there must be a terrestrial downlink station to receive the signals. For communications satellites, the station is usually a dish antenna. One of the key parts of an antenna is a Low Noise Converter (LNC), also

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known as a Low Noise Block Converter (LNB). This frequency converter, shown in figure 5-2, is placed at the focal point of a dish antenna to convert the signals that come in at a high frequency to signals in the range of 950 – 2150 MHz. The signal is then amplified before being sent to the tuner. The name of the device comes from the fact that this conversion must occur with a very small amount of noise or the noise would also be amplified and the relatively weak signal would become distorted. Thus, the LNC is a very precise instrument that plays an important role in translating the incoming signal from the satellite to something useable.

Figure 5-2: Low Noise Convertor2

4. Polarization

Polarizers are another important part of any dish antenna. A satellite polarizes outgoing signals to concentrate the beam and give it a specific direction. Signals can be polarized linearly (either horizontal or vertical) or circularly (either right-hand or left-hand). These four types of polarization are shown in figure 5-3. When an electromagnetic wave is polarized it has instantaneous electric field components that are orthogonal to each other. These components can be vertical and horizontal linear or left- and right-hand circular. A polarizer is contained within the LNC to select on of the types of polarized signals.

Figure 5-3: Types of Polarizers2

There are three common types of polarizers. A mechanical polarizer consists of a small motor that operates on pulses and has a metal probe that rotates between the horizontal and vertical directions. The polarizer must be very precise when switching to avoid signal loss. If a circular polarizer is used, it can also receive circularly polarized signals.

Unlike a mechanical polarizer, a ferrite-magnetic polarizer contains no moving parts. Instantaneous switching allows for little signal loss, but all channels on different frequencies must be pre-programmed into the polarizer. A circular depolarizer can also be implemented with a ferrite-magnetic polarizer.

The last type of polarizer, a 14 / 18 V electrically controlled polarizer, is contained within the LNC and requires no additional connections.

5. Tuner

Once the signal has been depolarized and converted to a lower frequency by the LNC, it is then passed to the tuner. The tuner is used to process audio signals and once again convert the frequency, this time from 950 – 2150 MHz to the 47 – 850 MHz required by TV tuners.

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A signal can be scrambled intentionally to allow for restricted access. This is done so that the signal can only be viewed by a subscription or for copyright protection. If the signal is scrambled then a decoder is required to decode the signal.

6. Antennas

The most important part of the dish antenna, of course, is the dish itself. There are several different types of dishes, each with distinct features that have unique advantages and disadvantages. The Prime Feed Focus (PFF) dish, shown in figure 5-4, is a concave dish with the LNC positioned in the center of the dish. Since most incoming signals are blocked by the LNC, it has a low efficiency of only about 50%. PFF dishes usually have large diameters greater than 1.4 meters, making it less sensitive to small changes in direction of the signal. AS a result, PFF dishes are relatively good at picking up signals outside their normal footprint. However, rain and snow can easily accumulate in the dish, negatively affecting performance. Although a PFF dish is better than others outside a normal footprint, its low efficiency prevents it from being a very widely used antenna.

Figure 5-4: Prime Feed Focus Dish2

Figure 5-5: Offset Dish2

The dish shown in figure 5-5 is an offset dish antenna, and has the LNC positioned on the side of the dish rather than the central focal point. Since the LNC does not block the signals, the antenna has a much better efficiency than a PFF dish and can use a much smaller diameter. Also, unlike the PFF dish, which must be positioned at a slanted angle, the offset dish can be positioned almost vertically and rain and snow accumulation is not as much of a problem. For these reasons, an offset dish antenna is becoming more and more widely used.

A dual-offset dish antenna has an even higher efficiency than a single offset dish. This antenna, pictured in figure 5-6, makes use of two dishes to obtain an efficiency of about 80%, which is very good. The two receiving dishes are different sizes and face opposite directions. The larger dish is directed toward the satellite to receive the signals, while the smaller dish is used to collect the signals from the larger one and send them to the LNC. Although the dual offset dish antenna has a very good efficiency, it is more complicated and costly to set up than the other antennas.

Figure 5-6: Dual-Offset Dish2

The last type of antenna dish, a flat antenna, is shown in figure 5-7. This type of antenna has a LNC build in and is very compact. However, its main use is limited to receiving signals from a DBS satellite in the central part of a footprint.

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Figure 5-7: Flat Dish Antenna2

Not only are there different types of satellites, but also there are also different sizes depending on where the antenna is within the footprint. A small antenna is only about 60 – 70 cm in diameter, and has a wide opening angle to allow easy installation and positioning. However, this size antenna in not very selective or sensitive to small changes in direction, so the possibility of interference is higher than with larger sizes.

A small antenna is most useful for receiving signals from a MPS in the central part of a footprint. A DBS sends an even higher-powered beam, so antennas for this type of satellite can be even smaller, with diameters of about 45cm. Small antennas are relatively cheap and can be mounted in a wide variety of places, although they are not overly sensitive.

Medium sized antennas, on the other hand, are able to select between a wide variety of satellite signals with minimal interference. With diameters of about 90cm, these antennas have a very good cost-to-performance ratio, although they do require a rotor to properly position. Large satellites, with diameters from 1.2 – 1.5 meters, are extremely selective and virtually eliminate interference. However, the very small opening angle can make them somewhat complicated to install and tune. A large dish antenna also requires a rotor to operate correctly. The high quality and sensitivity means that the antenna can receive weaker signals in more outlying areas of a footprint. However, these antennas must be quite strong to resist wind, and can be quite costly.

The orientation of antennas is mostly the same, regardless of size or type. The dish should be

positioned toward the southern sky, and as much obstruction as possible should be eliminated. The dish should be aligned in both the horizontal and vertical directions. The upright angle (in the vertical direction) should be 30 degrees; the azimuth angle (in the horizontal direction) should be adjusted east or west to point toward the desired satellite. These angles can be seen in figure 5-8.

Figure 5-8: Antenna Angles2

The polar mount principle, seen in figure 5-9, is used to receive signals from multiple satellites. The antenna rotates to the position selected by the tuner to receive the signal from the desired satellite. So the positioning of the antenna must be done quite precisely to receive the correct signal; the polar mount principle also allows different signals from different satellites to be selected.

Figure 5-9: Polar Mount Principle2

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VI. RADIO SIGNAL PROPOGATION

1. Background

Satellites communicate with each other and with the stations on the earth via electromagnetic radio signals. The data the satellite must send is converted to an electromagnetic wave and transmitted through an antenna. The electromagnetic wave travels through free space until it is received by another antenna and can be converted back to the desired signal form.

The propagation of electromagnetic waves occurs primarily through three mechanisms: reflection, diffraction, and scattering. Reflection is when the electromagnetic wave strikes a very large object relative to the wavelength of the signal. The propagating wave bounces off these objects and continues through space. When an object with sharp edges obstructs the path of the propagating wave, diffraction occurs. Finally, scattering is similar to reflection except that the objects obstructing the wave are small relative to the wavelength, but the number of objects is high. These three methods combine to effect electromagnetic wave propagation.

The power of either the transmitted signal, the received signal, or the ratio of the two signals is a very important consideration in analyzing electromagnetic wave propagation. The power can be measured in either Watts or dBm, where

PdBm = 10 x Log(Pwatts x 103)

Models of electromagnetic wave propagation usually attempt to determine the power of the signal at a receiver placed a certain distance from the transmitter. The variation of this power at certain nearby locations is also examined3. Large-scale propagation models that attempt to determine the average strength of a signal at arbitrary distances from the transmitter are used to determine the coverage area, or footprint, of various satellites.

2. Free Space Propagation Model

The free space propagation model is one such model that is used for satellite communication systems. This model assumes a line-of-sight between the satellite and receiver with no obstructions. It is intuitive that as the distance between the transmitter

and receiver grows, the received power decreases, and the free space model predicts that this is a power law function.

First, we can determine the power of the received signal as a function of the distance, Pr(d), between the receiving and transmitting antennas3:

where Pt = transmitted powerGt = transmitter antenna gainGr = receiver antenna gaind = transmitter – receiver separation (in meters)L = system loss factor not from propagation

= signal wavelength (in meters)

This equation is known as the Friis free space equation. We can also find the gain of an antenna, G r

or Gt, which is related the antenna’s effective aperture, Ae. The effective aperture is related to the size of the antenna, and the gain is found by3:

and

or

where c is the speed of light in meters per second, f is the frequency of the signal is Hertz, and wc is the frequency in radians per second.

As can be seen by the Friis free space equation, the power of the received signal decays as the square of the distance between the transmitter and the receiver. The gain is a dimensionless quantity, and is really the ratio of the radiation intensity in the direction of the received signal to the radiation intensity if an isotropic antenna transmitted the signal. Such an antenna does not exist, but the theoretical antenna would radiate in all directions at an equal intensity.

The radiation from an antenna can be examined through the use of a radiation pattern. This is a graphical depiction of the radiated energy from an antenna. There are both main lobes, in the primary direction of radiation, and side lobes in any other direction from the antenna.

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The radiation pattern of an omni-directional antenna is pictured in Figures 6.1a) and 6.1b). This type of antenna radiates (and receives) electromagnetic waves equally in all directions. A directional antenna, on the other hand, radiates (and receives) most of its power in one direction. This type of antenna is shown in figure 6-2.

Figure 6-1a): Side View Radiation Pattern of Omni-Directional Antenna4

Figure 6-1b): Top View Radiation Pattern of Omni-Directional Antenna4

Figure 6-2: Radiation Pattern for Directional Antenna4

The beam width of a directional antenna can be measured as the angle between the half-power points on either side of the main lobe. The points have a power 3 dB below the value of the main power.

The Effective Isotropic Radiated Power (EIRP) is another important factor dealing with free space electromagnetic propagation. The EIRP is the maximum power that can be transmitted from an antenna, and can be found as4:

where: Pout = output power of transmitter (in dBm)Ct = cable attenuation of transmitter (in dB)Gt = transmitter antenna gain (in dBi)

The power at the receiving antenna, Pr, can also be determined4:

where: Pl = path loss (in dB)Gr = receiving antenna gain (in dBi)Cr = cable attenuation of receiver (in dB)

These equations can be used in conjunction with the Friis free space equation to determine if the power of the received signal is above the receiver sensitivity limit. Since this analysis will reveal if the link between transmitter and receiver will work or not, the calculations are referred to as a link budget calculation.

3. Electromagnetic Wave Path Loss

The path loss is a very important concept to understand when dealing with electromagnetic wave propagation. The path loss is the fading of the electromagnetic signal, and is defined as the difference between the received power and the transmitted power. This is a positive quantity and is measured in dB. The decrease in power of the transmitted signal occurs for many different reasons, and thus is hard to predict. However, if only distance is considered, the path loss (PL) is3

which simplifies to

If the antenna gains are also considered, the path loss becomes

There are other factors that could decrease path loss. If the received signal comes from multiple directions with varying path lengths and delays the summed signal may be attenuated. Such a multipath reception is illustrated in figure 6-3.

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Figure 6-3: Multi-path Reception4

Weather conditions, such as heavy rain or wind, can also reduce the power of a signal.

A bad line-of-sight is the other cause of signal fading. The line-of-sight is the straight line between the transmitting and receiving antennas. A clear line-of-sight exists if no objects obstruct this line within the Fresnel Zone. This is the area of a circle around the line of sight, and is pictured in figure 6-4.

Figure 6-4: Fresnel Zone4

The radius of the first Fresnel Zone is

When 80% of this first Fresnel Zone is clear, the electromagnetic propagation can be modeled as free space. This is almost always the case with satellite communications.

A last limitation of the free space model is that the distance between the receiver and transmitter, d, must be in the far-field, or Fraunhofer, region. This is the area past the far-field distance, df. This distance is related to the largest linear dimension of the transmitting antenna, D, and wavelength of the signal by:

where df >> D and df >> lamda.

The separation between the satellite and ground terminal is generally great enough that d is well within the Fraunhofer region, and the free space model is an

adequate predictor of path loss in satellite communications.

VII. LASER COMMUNICATIONS

1. Advantages

Because society’s demands are always growing, and Radio Frequency (RF) satellite communication systems have about reached their limit, alternative forms of communications are constantly being explored. One extremely hopefully alternative is laser or optical communication systems. Lasers operate at frequencies seven or eight times higher than typical RF systems, providing higher bandwidth, less beam divergence, and smaller antennas5. Moreover, the very high-peak power pulses typical of laser transmitters allows for high data rates. These are significant advantages for satellite communications.

2. Overview of Different Lasers

There are many different types of lasers available, and they are usually classified according the material used to achieve the optical gain. The three most common types for communications are:

gas lasers solid-state lasers semi-conductor lasers

However, all lasers are grounded on the same basic operating principles.

The fundamental elements of a laser are shown in figure 7-1. Essentially the laser is a very high-Q cavity resonator surrounding some type of amplifier. The amplifier operates by either exciting or pumping the gain medium to a higher energy level.

Figure 7-1: Fundamental Components of a Laser5

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In a gas laser the excitation occurs via an electric discharge through the gas. Although a helium-neon (HeNe) laser is the most common laser for everyday consumer purposes, a carbon-dioxide (CO2) laser is used much more frequently in communication systems. A CO2 laser emits infrared radiation with a wavelength of 10.6 um, can produce several kilowatts of output power, and is extremely efficient.

An electric discharge is used to excite the CO2

molecules to a much higher vibrational energy state. When the molecules fall back to their base state, they release energy at 10.6 um wavelength. Because this is a relatively long wavelength, the reflecting surfaces must have a finish of only about 1 um tolerance, which is easily manufactured.

Although the CO2 laser has its advantages, the solid state Neodymium: yttrium, aluminum, garnet (Nd:YAG) laser is the most used laser for large-scale space communications. The amplifier is a rod of crystalline YAG lightly doped with Nd. Optical energy excites the Nd atoms to a higher energy state, and their return emits energy with a wavelength of 1.664 um. The source of the optical energy is either high intensity tungsten filaments or an ion-arc lamp. Arrays of these semi-conductor diode-pumps are required to raise the efficiency above seven percent5.

The modulation system of a laser is what drives the actual communication. There are many different methods, the simplest being to simply vary the pump or exciter of the amplifier. However, this method can only be utilized when the exciting device will respond to different frequencies and where the laser cavities can be controlled. Since this is only possible with a semi-conductor diode laser, this method has limited use.

Lasers other than the semi-conductor diode have a separate optical device inside or outside the cavity that can be used to modulate the light. This element is usually a birefringence modulator, shown in figure 7-2.

Figure 7-2: Birefringence Modulator5

This modulator uses the electric-field induced birefringence of the crystal to rotate the polarized incident light, thereby modulating the laser output.

When no voltage is applied across the modulator, the linearly polarized light (really the sum of two circularly polarized vectors rotating in opposite directions) exits the modulator in the same plane in which it entered. When a voltage is applied across the modulator, and induced birefringence is created that slows the rotation of one of the circularly polarized light vectors, but not the other. The reduction in velocity of one of the vectors is called the retardation. The effect of this retardation is that the emerging light is no longer vertically polarized, but will have been rotated by some angle. This angle is a function of both the width and length of the crystal, as well as the applied voltage.

Cavity dumping and “Q-switching” are the primary methods for implementing this modulation control in lasers. In cavity dumping the cavity port is opened and closed, typically by polarizing the radiation within the cavity. The field is stored up inside the cavity at the desired polarization, and electro-optically rotating the polarization plane sends out this plane from the cavity. The time that these pulses are generated can be carefully controlled, and frequencies up to hundreds of kilohertz are possible. This makes cavity dumping quite useful for communications.

Q-switching modulation operates by varying the Q of the resonant cavity, greatly increasing the peak

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power of the pulses. The Q can be changed in one of two ways: electro-optical modulation, or by rotating one of the reflecting mirrors. Q-switching allows pulse position modulation on the order of Gbps.

3. Antennas

Like their RF counterparts, laser communication systems transmit data via electromagnetic propagation. Thus, the antenna is an important aspect of the laser system to transmit and receive this electromagnetic energy as well. However, the antennas in laser systems are much simpler than RF antennas, and their size and orientation are determined mainly by the wavelength and other system properties5. Also, the radiation emitted from the antenna depends mostly on diffraction, and reflection and scattering do not play as large a role as they do in RF systems.

Since the lasers used for communications are circularly symmetric and have Gaussian distributed electric fields, the far-field pattern of the laser can be described by5:

where: G = far field gain at off-axis angle thetaL = range distancek = 2pi/lamdaE_(r1) = electric vector amplitude in plane of transmit aperturer1 = radius of transmit aperture

A typical Cassegrainian, or Gimbal, telescope is shown in figure 7-3. For this type of telescope antenna, the gain becomes5

where: r0 = 1/e2 beam radius at the telescope aperturea = obstruction radiusb = truncation radius

Since the gain of the antenna varies inversely with the square of the wavelength, very high gains are possible even for quite small antenna diameters.

Figure 7-3: Cassegrainian Telescope6

As in RF systems, the gain of the receiving antenna in a laser system is an important system parameter. In a laser system, the optical detector is usually much large the received energy once it has been focused, resulting in low sensitivity to slight changes in the received wave. For a satellite system with large transmitter-receiver separation, the receiver gain is a function of the antenna radius, ra, and the wavelength:

The free space path loss for a laser communications system is the same as that previously discussed for an RF system.

4. Optical Detectors

Just as in an RF system, the electromagnetic energy received by a laser communications receiver must somehow be converted to an electric current that can be interpreted by the system. However, because of the much higher frequencies in a laser system, the conversion is a square law function, whereas an RF system produces a current proportional to the amplitude of the electromagnetic energy field.

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This conversion can be understood by the photon-wave theory of light, since these photons have some momentum capable of exerting a force. This force frees electrons from the atoms of the cathode of the detector, and these photoelectrons are attracted to the anode, creating a current.

This process of converting optical energy to current can itself cause noise in the system, since each wave contains a different number of photons. However, this process follows conventional statistics, and the signal-to-noise ratio (SNR) is simply equal to the average number of photons received5. This provides a useful parameter for defining laser communications systems.

5. Optical Modulation Formats

The modulation techniques of laser communication systems is the area that differentiates them most from RF systems. This is due mainly to quantum effects of optical signals, coupled with the fact that thermal noise is not the primary source of noise as it is with RF detectors. Most modulation formats for high data-rate laser systems are based on short energy pulses with a high peak power and low duty cycle.

The Pulse Gated Binary Modulation (PGBM) format in one modulation technique commonly used. This format is quite simple, where a pulse occurring at a set period can be turned either “on” or “off”. Thus, a binary 1 corresponds to a laser pulse, and the absence of this pulse symbolizes a 0. Although this makes use of the low duty cycle (i.e., pulse width is much smaller than the pulse period), it is relatively inefficient in power output since the energy of about half the pulses are gated out.

This inefficiency is solved by the Pulse Polarization Binary Modulation (PPBM) technique. Rather than suppressing the energy for a binary 0, the laser pulse is simply rotated 90 degrees. This also reduces noise in the reception, since a pulse occurs every bit period. However, requiring an additional channel for the rotated polarization increases receiver complexity.

Pulse Interval Modulation (PIM) is probably the most efficient modulation technique, since it transmits multiple bits per pulse. The normal interval between pulses is divided into N separate time “slots,” and the

pulse is sent during only one of these time slots. The binary data to be transmitted is grouped into log2(N) bit words. Systems with a value of N up to 128 are common, allowing log2(128) = 7 bits to be transmitted per pulse. Whichever time slot contains the transmitted pulse is the value of the word, and this word can be decoded into individual bits.

Figure 7-4 shows a typical waveform for this modulation format. In this example, N = 32 and 5 bits per pulse are transmitted. The initial pulse is sent during time slot 23, effectively transmitting 1 0 1 1 1.

Figure 7-4: Pulse Interval Modulation5

A PIM system has a very low duty cycle, since the pulse period is increased to N discrete time segments. This format also reduces noise by sampling each individual time slot separately. For these reasons, PIM format is a popular modulation technique in laser communications systems.

VIII. LASER COMMUNICATIONS DEVELOPMENT

Optical and laser communication certainly have many advantages, but of course bringing the theory to reality is not easily accomplished. However, in recent years there have been significant strides to bring optical communications for inter-satellite links (ISLs) to fruition. Atmosphere and weather still cause propagation effects that have slowed the development of laser earth-satellite communications.

1. Applications

The main use of ISLs is to network a group of satellites together to relay messages at very high data rates (up to Gbps). These links can occur between

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satellites in the same orbit of between satellites at very different altitudes. Although most current ISLs use RF communications, most new satellites being launched will be making use of optical communications.

1. Current Technology

Japan is one of the leaders in developing laser communications applications. The government is funding development of laser technologies, and ultimately hopes to have to system shown in Figure 8-1, consisting of both inter-satellite links and adaptive optics on earth to permit earth-satellite communications that could be relayed terrestrially via fiber optics. A 10-Gbps system is planned to be functional by 2010.

Figure 8-1: Japanese Laser Communication System Plan7

Europe is another major developer of optical systems, mostly through the European Space Agency (ESA). The Advanced Relay and Technology Mission Satellite (ARTEMIS) was launched on a Japanese H-2 rocket in 2001. It provides a data relay from Low Earth Orbit (LEO) satellites to Geosynchronous Earth Orbit (GEO) satellites. The ISL link is optical. The ARTEMIS satellite is shown in Figure 8-2.

The ESA has developed a fair number of optical terminals for ISL, and is striving for LEO to GEO, GEO to GEO, and other applications. Some of these applications are shown in Figure 8-3. The Small Optical User Terminal (SOUT) is a prototype LEO to GEO optical terminal developed in 1995; although SOUT was not space qualified, SILEX has been developed from the prototype and has a maximum range of 45 000 km.

Figure 8-2: ARTEMIS test satellite7

Figure 8-3: European Developed Optical Terminals7

Small Optical Telecommunications Terminal (SOTT) was completed in 1996 and is capable of data of rates of 10 Gbps. SOTT is pictured in Figure 8-4. Solid State Laser Communications in Space (SOLACOS) is a 1.604 um Nd:YAG laser prototype. Finished in 1997, SOLACOS is capable of 650 Mbps in GEO-GEO applications. Short Range Optical Inter-satellite Link (SROIL) is being developed for LEO applications with a range of 6000 km and data rates of 1.2 Gbps.

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Figure 8-4: SOTT7

The United States has also been a large player in the laser communications satellite scene. However, until recently the major developments have been for military purposes. Laser Communications International built a 1.55 um prototype terminal, similar to Figure 8-5.

Figure 8-5: Laser Communications International Optical Terminal Prototype7

Raytheon is also building a 1.55 um terminal for ISLs of at least 1 Gbps. MIT and NASA have also been developing high-speed optical communications applications. Moreover, MIT is developing a prototype

to test earth-satellite communications to determine the atmospheric effects. The Optical Communications Demonstration shown in Figure 8-6 is currently being developed for 1 Gbps near-earth testing. Although the focus has traditionally been classified military applications, the US commercial laser satellite industry is catching up to the Japanese and Europeans.

Figure 8-6: Optical Communications Demonstration7

IX. CONCLUSION

Satellites have become one of the most important methods of communication, and more and more homes and businesses are taking advantage of the flexibility and range satellite communications offer. From television to space exploration, satellites are becoming indispensable to society. Since the early days of Sputnik, satellites have been constantly improving to keep pace with technology and its ever-increasing demands. Satellites are now extremely complex, employing many different components to receive and transmit information from one part of the globe to another.

Radio Frequency communication has been used quite successfully to relay this information quickly. However, society’s demands are constantly growing,

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and satellites must keep pace with the rest of the changing technology. Because laser communications systems have much higher bandwidth and data rates, along with smaller beam divergence, these optical systems appear to be the way to go for satellite communications.

Although space-earth optical communications are not yet a reality, considerable progress has been made in laser communications systems, and optical inter-satellite links are already being used. Due to the considerable research being performed and the advantages of laser communications of RF communications, optical laser systems are the future of satellite communications.

References

[1] “Howstuffworks: How Satellites Work”. accessed Feb. 19, 2002. http://www.howstuffworks.com/satellite.htm

[2] “Satellites.com: Introduction to Satellites”. accessed Feb. 11, 2002. http://www.satellitedish.com/2_About.htm

[3] “Wireless Communication: Principles and Practice”. Theodore S. Rappaport. Prentice Hall, Inc. New Jersey. 1996.

[4] “Radio Signal Propagation.” Breeze.com Wireless Communications. Accessed Feb 20, 2002. http://www. Scs.careleton.ca/~barbeau/courses/490_542/802.11/ signal_propagation.pdf

[5] Latzman, Morris. Editor. “Laser Satellite Communications.” Prentice Hall Inc. New Jersey, 1987.

[6] Marshelek, Robert G. “Laser Communications Requirements Drive Cost Effective Solutions”. accessed May 10, 2002. http://www.its.bldrdoc.gov/meetings/art/ art99/slides99/mar/mar_s.pdf

[7] “Optical Communications and Inter-satellite Links.” Accessed March 04, 2002. http://itri.loyola.edu/satcom2/03_06.htm

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