The Satellite System Configuration
This is a vast subject, and one that has provided the material for many texts since its inception in 1962. An exhaustive coverage of satellite communications is impossible within a few pages, and in this text an attempt will be made to present only the main principles involved. Some of the basic engineering design aspects will be discussed, and how the laws of physics lead to specific equipment configurations. In many respects the satellite communication link can be viewed as a super-long-distance microwave link, and many of the calculations for terrestrial links can be extended to satellite paths. As with terrestrial telecommunication systems, the satellite industry is gradually becoming digitalized. Interestingly, the satellite is transparent to the passage of analog or digital information. Also, the baseband signals, whether analog or digital, are packaged on an analog radio carrier and only very recent satellites incorporate demodulation or demultiplexing. Some satellites already have onboard processing that is done at the individual channel (subbaseband) level, but the increased level of complexity and cost-effectiveness of future onboard processing is still being debated. Regardless of whether satellite communications are international or domestic, there are several major categories of satellite users:
Satellite systems are very attractive for satisfying the wide area of coverage and the point-to-multipoint nature required for broadcasting. Video coverage of an event at one place on the globe can be sent up to a satellite and redistributed (broadcast) over large areas of the populated world in the form of clear television pictures. A network of satellites can provide global coverage so that telephone conversations can take place between individuals located at any of the remotest places in the world, with only a pocket handset required by the participants. Government applications are mainly to provide surveillance information, and military operations are coordinated and facilitated by satellite communications. Each application requires a significantly different system design approach.
Perhaps the most serious limitation of satellite communications is the total available satellite bandwidth. Although the information-carrying capacity of satellites has been expanding steadily over the years since its inception, the available bandwidth is still very small compared to optical fiber capabilities. For speech communications, there is ample available bandwidth, but for video transmission or high-speed data throughput there are severe limitations. Progress in digital compression techniques is gradually reducing the bandwidth needed for video transmission. Partial-motion videoconferencing is becoming feasible in the kilobit-per-second rather than megabit-per-second realm. Full-motion video, however, still requires several megabits per second, which is fine for broadcasting a few simultaneous TV programs but a problem for the individual requiring multimedia interaction facilities.
On the commercial side, one of the interesting aspects of satellite communications is that the cost of a single-satellite-hop telephone call is almost independent of distance. Whether calling a next-door neighbor or someone on another continent, the amount of equipment involved in the process is almost the same. The application determines the type of satellite system required. Satellites can be placed in three types of orbit:
Equatorial (geostationary)
Polar
Inclined
The type of satellite system determines the altitude at which the satellite is fixed. The geostationary orbit is the style most widely used for broadcasting, where the satellite is in an equatorial orbit and appears to be at a fixed point in the sky relative to an observer on the earth. Simple Newtonian mechanics shows the altitude of geostationary satellites to be an amazing 36,000 to 41,000 km, depending on the earth latitude of the observer. This vast distance makes life difficult for the designer, but good-quality voice and video communications via satellite have been commonplace for many years now. One of the major drawbacks of this large distance is the time taken for a round-trip signal to travel from the earth up to the satellite and back down to earth. Even at the speed of light the signal takes 0.24 to 0.27 s. This time delay can cause an annoying echo unless electronic circuitry is employed to minimize the effect. Another disadvantage of the geostationary orbit is the fact that at latitudes farther away from the equator, the geostationary satellite appears lower and lower toward the horizon. Eventually at about 5° north or south, the satellite is too low on the horizon to receive a clear signal.
Satellites in polar or inclined orbits do not have this problem, but they are no longer geostationary. This would be a problem for broadcasting because an observer on earth would have to track the satellite as the earth moved beneath it, and if permanent transmission were required, several satellites would be needed with some mechanism for making a smooth transition between them before each one sequentially disappeared over the horizon. Polar orbits of about 800-km altitude are convenient for global coverage, so long as several satellites are moving in the same polar orbit and several polar orbits are used. These low-earth-orbit (LEO) satellite systems are the subject of potentially revolutionary constellations of satellites that are effectively global cellular systems. Inclined orbits of about 10,000 km provide global coverage with fewer satellites than LEO systems. These medium-earth-orbit (MEO) systems are consequently in direct competition with the LEO systems.
The satellite was traditionally used in the "bent pipe" mode. As the term suggests, the satellite acts like a slingshot to redirect the incoming signal to different locations on earth.
Radio-Based Systems
Satellite and microwave communications are radio-based systems that have been around for a number of years. At one point, much of the long-distance services in the network were served primarily via microwave, and the international long-distance services via satellite. A good deal of these systems are still in effect serving long-distance networks, rural areas, and international long-distance connectivity. Many organizations have built their own private networks by either launching their own capacity into a satellite orbit, buying into their own rights of way for microwave towers and radios across the country, or just renting the capacity from other suppliers. This is by far, not a dead service or technology.
Much has changed with the introduction of fiberoptics. Yet both of these techniques still have a use for the high and low bandwidth services for:
|
Band |
Uplink frequency |
Downlink frequency |
|
5.925-6.425 GHz |
3.700-4.200 GHz | |
|
14.0-14.5GHz |
11.7-12.2GHz | |
|
27.5-31.0GHz |
17.7-21.2GHz |
Satellite uses a microwave transmission system, with radio connectivity being the primary means of broadcast. This technique has long been considered useful in very long-distance communications. Satellite transmissions were initially begun in the early 1960s, but several enhancements have been made over the years.
Typically, an earth station (uplink and downlink) is used to broadcast information between receivers. Current technology uses a geosynchronous orbiting satellite. Geosynchronous orbit means that a satellite is launched into an orbit above the equator at 22,300 miles. This orbit means that the satellite is orbiting the earth as fast as the earth is rotating. Therefore, it appears to earth stations that the satellite is stationary, thus making communications more reliable and predictable-and earth stations less expensive because they can use fixed antennas.
This communications technique uses a frequency (uplink) broadcast up to the satellite, where a transponder receives the signal, orchestrates a conversion to a different frequency (downlink) and transmits back to the earth. As the communications is transmitted from the satellite toward the earth, a 17° beam is used. This produces a pattern of reception known as a footprint, with the entire U.S. covered by a single satellite. There are obviously many more than just three satellites in orbit, circling the earth, but only three are needed to provide global coverage. The satellites used for commercial applications are in what is called a geosynchronous orbit.
Satellite frequencies currently being used are categorized in bands. These bands are divided into the RF frequency spectrum that allow for different capacities and reception variations. The frequency bands are divided into two separate capacities, called uplinks and downlinks. The service provides for full-duplex operation, so a pair of frequencies is used. The uplink is used for transmission, the downlink is used for receiving .
Satellites are good for broadcast communications; one to many locations. They are also termed distance insensitive, because once the signal is traveling up (22,300 mi) and back down (22,300 mi), the position of the receiver doesn't matter on a land line basis. We do not rent or lease the channel based on distance, but on the capacity of the channel. If two sites are using a satellite to communicate, the price is the same whether they are 10 miles or 3000 miles apart. The round trip from earth to the "bird" and back is essentially the same distance. Go to Top
Obviously, there are some distinct advantages to the use of satellite, or else no one would want to use the capacity. These advantages include some of the following:
Distance insensitive As stated, the distance between the two end points is not a consideration in the pricing.
Single hop With just a single shot up and back from the satellite, most communications coverage should be easily addressed. There is only one repeater function taking place, which helps to eliminate some of the errors in data transmission whenever a signal must be repeated.
Good for remote areas and maritime applications
Broadcast technology
Large amounts of bandwidth
No system offers only great advantages, so the opposite side of the transmission system must be viewed to keep all things fair. Satellites have some distinct and peculiar disadvantages. Many can be overcome, but they do surface whenever a discussion is introduced.
One-way propagation delay This delay is about 1/4 to 1/2 second, and can be disruptive to voice and asynchronous mode data protocols or any protocol that requires an "ack" or a "nak" before transmitting its next block of data. The turnaround on a data stream would be double the 1/4 second, or 1/2 second average turnaround. In using a bisynchronous protocol, a 4800-bits/s transmission speed could be relegated down to 400 bits/s because of the transit delays.
Multihops increase delay, detrimentally impacting voice If one cannot transmit on a single satellite link, for example, around the earth from Boston to the Middle Eastern countries, then a double hop is required. This means that the signal will be transmitted up to one satellite and back down to the earth station, then retransmitted up to a second satellite, then finally back down to the earth station around the horizon. The delay will be at least one second from the time a message is originally sent until it is finally received. The disruptive nature of this for voice makes it virtually unusable. For data, however, protocols can be "spoofed" or faked into thinking that the data got there sooner, and the problem can be overcome.
High path loss in transmission to satellite The transmission path will introduce more loss to the signal while it is making its way to the satellite. Therefore, more power is required and signal-to-noise ratios must be carefully administered or else the information could become unusable.
Rain absorption affects path loss Any radio-based medium (especially in the microwave frequencies) is subject to a certain amount of absorption from water. Rain in a channel path can absorb much of the signal strength, leaving a far weaker signal to find its way to the receiver. Once received, the signal (now weaker and noisier) will have to be amplified.
Congestion building up The satellites were originally designed in a spacing of 4 degrees apart, based on a 360-degree circular orbit. However, as more countries in emerging worlds have sought to use radio rather than wire-based network access, the satellites have been placed in closer orbital slots. They are now being positioned at 2-degree increments. This can lead to congestion on the transmission paths, cause occasional interference and (of course) limit future parking places.
Very Small Aperture Terminals (VSATs)
Very small aperture terminals came into being in the mid 80s, when the size of the satellite terminal shrunk. Since then hundreds of networks have cropped up. The pricing schemes have not yet made it totally feasible for more, but as technology continues to shrink, the use might expand. Costs are continually falling, and benefits derived from VSAT networks are rising, breathing new life into this technology. No longer do users have to own and operate their own hubs, where shared hubs are more realistic. Prices from suppliers have dropped;VSAT units can be as little as $250/month in quantities of 1000 units. Many specific data applications are now being migrated to satellite, via VSAT. In 1991, more than 35,000 VSATs were on record. Newer regional operators are now approaching smaller network users (10-65 nodes). The dishes (VSATs) can be as small as 1.0 meter to 2 meters, thus making it more realistic to place units in remote sites.
However, these smaller terminals are limited in their applications; voice and video (motion) applications aren't generally feasible. VSATs do, however, provide sufficient bandwidth capacities for data transmission (host/host, file transfer LAN/LAN, and some receive-only broadcast video).
V-SATs are located around the country to provide coverage.
The local telephone companies, long-distance carriers, and users alike have all been successful in the use of microwave radio systems. Microwave was the predominant means of handling long-haul transmission here in the U.S. The advantage is that rights of way and natural barriers are more easily overcome. A single radio channel can carry as many as 6000 voice channels in 30 MHz of bandwidth.
Microwave, as any radio technology, is designed around space and frequency spectrum, which is a finite resource. Thus, coordination between paths is extremely important to prevent interference. Microwave radio can use either an analog or a digital modulation method. Each operates differently.
Analog radio uses either amplitude or frequency modulation, usually frequency modulation. In 30 MHz of bandwidth, 2400 voice channels can be carried, equating to a voice channel of 12.5 kHz.
Digital microwave has been available since the mid 70s, with the direct modulation of an RF carrier. Because a modulation technique uses 1 bit per hertz, using a 64-Kbits/s channel would be far too consumptive of radio frequency spectrum. Therefore, a better modulation technique is needed. Phase shift keying or Quadrature amplitude modulation techniques are more common. Using a 16 QAM, a total of 1344 voice channels can be carried on 30 MHz of radio, yielding 90 Mbits/s (3 bits/Hz). Newer 64 QAM supports 2014 channels at 135 Mbits/s (4.5 bits/Hz). Using digital microwave allows for the direct interface of T1/T3 carrier circuits.
Service in the 18- to 23-Hz radio spectrum is called short haul microwave. Typically, this is a point-to-point service operating between 2 and 7 miles and supporting up to 4 T1s. Newer systems support up to 8 T1s.
Digital termination services that operate in the 10-GHz range began in the early 1980s. This short-haul service is designed for metropolitan area digital services, similar in concept to cellular radio. This TDMA technology has access to the system for 24 users at speeds up to Tl. As when dealing with any radio frequency spectrum, the need for licensing exists. Satellite services are usually coordinated by the network suppliers. Microwave can be coordinated by either the network suppliers or user. You will need:
Line of sight
Frequency/licensing
FCC-trained personnel
Time
Microwave can be used for various installations.
Future Use of Microwave and Satellite Systems
Because the radio systems can carry such broadband capacities over radio frequency, we will see several opportunities to use these technologies.
CATV Uses coaxial broadband cables to delivery compressed video to users. These systems use digital modulation at 90 Mbits/s. However, as high-definition TV becomes less expensive and more widespread, the 135 Mbits/s of bandwidth throughput can be achieved on microwave/satellite systems to a coax or fiber interface.
DBS Direct broadcast satellite systems, a receive-only (one way) transmission for individual reception. Using a very small aperture antenna (1 meter or less), a viewer will receive TV signals through a personal earth station (or some small component). DBS will be available to CATV, MATV, and broadcast users alike. This is perceived as a supplementary service, rather than a replacement.
Interconnection already exists between cellular systems and microwave However, as Iridium comes about in the late 90s, satellites in low polar orbit will be used for worldwide cellular connections. This will go further, into the personal communications networks (PCN), using a personal communications server (PCS) system to deliver calls to/from the cellular arena.
VSATs will continue to get smaller, and newer modulation/time sharing techniques will provide greater access to bandwidth Prices will continue to drop as these services continue to compete with fiber and microwave alternatives.
Geostationary Satellite Communications
GSO satellites orbit in the same rotational direction as the earth at an altitude of about 36,000 km above the equator as calculated by simple Newtonian mechanics. Interestingly, that altitude is the same regardless of the mass of the satellite. Because of its high altitude, a GSO satellite is visible from about 42 percent of the earth's surface. A satellite that orbits at an altitude less than 36,000 km above the equator has to move at a higher velocity to remain in orbit which means the satellite moves relative to the earth. For the orbiting altitude of 36,000 km the velocity is 3.1 km/s, and for a 780-km altitude (LEO) the velocity is 7.5 km/s. Clearly, any nonequatorial orbit regardless of altitude will cause the satellite to move relative to the earth; that is, the satellite will be in a nongeostationary orbit.
The geostationary satellite is composed of several subsystems, and a view of INTELSAT VIII It has:
An antenna subsystem for receiving and transmitting signals.
Transponders, which receive signals, amplify them, translate their frequency, and retransmit them back to earth. In some cases demodulation to baseband, switching, rerouting, and remodulation functions are also included.
A power generating and conditioning subsystem to provide power for the satellite and to supply all of the different voltages and currents the electronic circuits require.
Antenna subsystem. New types of antennas are being introduced with each new generation of satellite. In addition to parabolic reflectors, metallic lens antennas and dielectric lens antennas are being used. Array antennas are also used. These antennas are made up of several individual dipoles, helical elements, or open-ended waveguides. Many of these radiating elements are combined to form the desired pattern. Beam shaping with array antennas is much easier than with other types, and they are less prone to interference than other antennas.
Sometimes it is necessary to concentrate the transmitted energy into a narrow beam and direct it to a much smaller area than over the full earth. This is known as a spot beam antenna. This area is often called a footprint. Spot beam antennas clearly have a higher gain and therefore a larger aperture than global coverage antennas. A large, single-antenna dish can produce many spot beams. Many small feed horns are positioned so that their signals are reflected into narrow beams by the dish. Multiple spot beams have the following advantages:
Transponders. A transponder is the equipment in the satellite that receives, amplifies, frequency translates, and retransmits the RF signals back to earth. In the C-band, the antenna arrangements provide a sixfold frequency reuse using two spatially isolated hemispherical beams and four zonal beams. In addition, there are two C-band global beams. At the Ku-band there are spot beams on vertical and horizontal polarizations.
Antenna beam shaping
Intelsat VIII transponder frequency plan
The satellite is powered by silicon solar cells. Cylindrical satellites such as INTELSAT VI have their bodies covered with silicon solar cells. The cells are fixed to the whole cylindrical surface. Because the satellite is spinning on its axis, at any time only half the cells are exposed to sunlight. Also, most of the cells are at an angle, which grossly reduces amount of light energy that can be converted to electrical energy.
A more efficient arrangement is to use flat solar panels often call "sails." INTELSATs VII and VIII have this configuration. If solar panels are used for power generation, the satellite must be three-axis stabilized. This is achieved by using momentum wheels. The solar panels must also be directed toward the sun, and this is done by using a sun sensor and stepping motor.
Eclipses are an unavoidable nuisance for geostationary satellite communications. The situation would be worse than it is now if the earth's rotational axis were at right angles to the plane of orbit around the sun, because in that case there would be eclipses every day when the satellite is in the earth's shadow. In reality, the situation is worst at each of the two equinoxes, at which time satellite is in shadow for 69 min. On days to either side of the equinoxes time spent in shadow gets progressively smaller, but it takes 21 days on 'r side to be completely eclipse-free. That means that a total of 84 days per yea are affected. If batteries were not used on the satellites to keep the system alive during those periods when the sun does not provide power via the solar panels, the system would be unavailable for service.
Batteries are a limiting element in the life of the satellite. Their length service depends on factors such as the number of charging and discharging, cycles, and depth of discharge. Because batteries are so heavy, their quantity (and therefore the total charge capacity) must be minimized, but a maximum allowable depth of discharge (typically 70 percent) defines the capacity INTELSATs I to IV used nickel-cadmium cell batteries, whereas nickel hydrogen ones were used in the V to VIII series.
A satellite contains instruments that relay information concerning the condition of the subsystems on board. Special earth stations monitor this information and they are equipped to:
Perform orbit and course corrections
Switch transponders into and out of service
Steer spot beam antennas
Activate feeds to shape the beam
Control the charging of storage batteries
Thrust and stabilization.
A satellite in orbit must always have its antennas pointing toward the earth in order to maintain continuous communication, for this reason, the satellite must be stabilized. A satellite can rotate about its three axes without moving from its orbital position. The three motions are referred to as yaw, pitch, and roll. There are three methods of stabilizing a satellite:
If a satellite is spin stabilized (as is INTELSAT VI), the antenna assembly must be despun, so that it always faces the earth. A servo mechanism controls the speed at which the antennas are despun relative to the satellite body. Also, satellites tend to wobble, like a spinning top. This motion, called nutation, is minimized by nutation dampers. INTELSAT satellites have earth and sun sensors that position a satellite and maintain that position throughout its life.
The satellite carries a comprehensive thrust system because from time to time the orbit needs to be corrected, the orbital velocity needs to be increased, and the spin velocity needs to be increased. Hydrazine gas is the propulsion fuel that is used to provide the thrust to make the adjustment. The release of gas from the respective thrusters is controlled by solenoid valves, operated by commands from ground telemetry, tracking, and command stations. The apogee motor gives the final orbit velocity.
FREQUENCY BANDS FOR SATELLITE COMMUNICATIONS SYSTEMS
|
Frequency Bands (GHz) |
Up-Link (GHz) |
Down-Link (GHz) |
Bandwidth (GHz) |
|
6/4 |
5.925-6.425 |
3.7-4.2 |
500 |
|
14/12 |
14.0-14.5 |
11.7-12.2 |
500 |
|
29/19 |
27.5-31.0 |
17.7-21.2 |
3500 |
Geometry of sun transit outage.
Most of today's satellite systems use the 6 / 4-GHz band pair, which is commonly used for long-haul terrestrial systems. The higher frequency bands permit more gain with the same size antennas and do not interfere with terrestrial systems; however, propagation loss due to rain is increased. The 14/12-GHz band pair is now being developed, while the 29/19-GHz band pair is more experimental.
The earth station antenna is very large (about 30 meters in diameter), thereby providing high signal gain and narrow-beam radio propagation. Although the satellite is in a geostationary orbit, automatic tracking is provided by means of a telemetry and control system to account for any small position deviations and to maximize the received signal. This system also performs a number of "station keeping" functions such as keeping the satellite at its assigned longitude and inclination by the use of small gas jets and adjusting the gain of the satellite receiver to balance up/down-link transmission.
The satellite is stabilized to maintain a fixed relation to the earth's axis and to eliminate tumbling. This permits a moderate-gain satellite antenna to be used. Solar cells and nickel-cadmium batteries are the primary power sources for the satellite equipment. The batteries are needed during solar eclipses.
Because satellite power is limited, FM/FDM modulation is used, since this provides a favorable tradeoff of power for bandwidth. In addition, an FM signal is not subject to amplitude nonlinearities. Therefore, the satellite amplifiers can be operated very close to saturation and thereby provide maximum power output.
Of the many design considerations that are unique to satellite systems, only six are mentioned below: (1) sun transit outage, (2) satellite eclipse, (3) rain effects, (4) transmission delay, (5) noise, and (6) interference coordination.
As shown, the sun appears directly behind the satellite, and emissions from the sun fall upon the earth station antenna, causing a large increase in satellite circuit noise. This phenomenon takes place during the spring and fall equinoxes and persists a few minutes each day for a period of about six days. The resultant increase in noise is avoided by temporarily switching to a protection satellite located at a different longitude, using a separate earth station tracking antenna.
During two 46-day periods each year, the earth's shadow causes the satellite to experience an eclipse of the sun The duration of the daily eclipse varies, lasting up to about 1 hour. During this time, the satellite is deprived of solar energy and must rely or battery power. In addition, this exposes the satellite and its circuitry to large temperature changes.
Rain causes three major impairments to a microwave satellite signal; attenuation, thermal noise, and depolarization. Raindrops scatter and absorb microwave energy, resulting in rain fades that, although small in the 6/4-GHz band pair, increase with frequency and are very serious in the higher 14/12 and 29/19-GHz band pairs. Rain is also a dissipative medium at microwave frequencies and radiates thermal noise. The combined effect is a decrease in system signal-to-noise ratio. With earth station site diversity protection, system outages due to rain fades are held to a minimum. Modern 6/4-GHz satellite systems, such as COMSTAR and Telstar 3, increase their channel capacity by using the same frequency with different polarizations for two channels. These systems are degraded by rain depolarization of the signal. The nonspherical shape of the raindrops converts the signal-linear polarization to elliptical polarization and thereby impairs the ability of the antenna to discriminate between the two channels. This impairment is more dominant than rain attenuation in the 6/4-GHz band pair, and unlike attenuation, its effect diminishes with higher frequency.
The 250-millisecond, 1-way transmission delay from one earth station up to the satellite and down to another earth station inhibits voice communications slightly, but it is a serious problem for data transmission. Although data sets are now available with modified protocols to operate over satellite circuits, many earlier data terminals would experience difficulties. The round-trip delay of 500 milliseconds on a satellite circuit would cause very annoying talker echo were it not for the use of echo cancelers. Prior to the invention of these devices, the practice was to use a satellite facility in one direction and a terrestrial facility in the other direction of transmission.
Satellite transmission is impaired by noise from the earth itself. Thermal radiation from the warm earth extends across the entire beamwidth (main lobe) of the satellite antenna and is the dominant source of satellite antenna noise. The earth station is also significantly affected by noise from the earth. Since the earth station antenna is pointed at least several degrees above the horizontal, this noise is not in the main lobe of the earth station antenna but in the back and side lobes.
Interference sources include other communications satellites and terrestrial microwave systems. To control intersatellite interference, satellite's currently operating in the 6 / 4-GHz band pair are placed no closer together than 4 to 5 degrees.Since the 4-GHz down-link signal can interfere with a terrestrial system as well as other satellite systems, the FCC restricts the transmitted satellite signal power to no more than 5 watts. To control 4-GHz terrestrial interference into earth station antennas, site interference studies are conducted prior to locating an earth station. A large percentage of potential sites are rejected based on these ground exposure studies. There is also possible up-link interference into a 6-GHz terrestrial system. This is controlled by limiting earth station antenna elevation angles to no less than 5 degrees. Go to Top
TV signals transmitted over satellite using analog techniques (FM) are limited to one, or possibly two, TV channels per transponder. The remarkable advances in video compression techniques over the past few years have now enabled five to ten digital TV signals to be transmitted, depending on the level of quality required. Digital video signals for home use are typically 2 to 8 Mb/s depending on the program content. Obviously a football game contains more rapid motion than a talk show. Only a few years ago data rates of 30 to 100 Mb/s were required to achieve similar subjective quality.
Direct-to-the-home video has certainly benefited from the advances made in satellite EIRPs, earth station G/Ts, and concatenated coding, but the most significant advances have been made by video compression. In 1990 it was dim-cult to place one video signal on a 36-MHz transponder. By 1996, five to ten video channels could be accommodated in a 36-MHz transponder.
Broadcasting TV via satellite is a huge business. At the receiving end, individuals have their own dishes and receivers. Hotels put a dish on the roof and then distribute the channels to every room in the hotel. Cable companies can use the broadcast from several satellites and package the large number of channels for distribution along their coaxial cables.
For individual users receiving satellite to the home, the equipment is very simple and getting cheaper every year. All that is needed is an antenna, low-noise block downconverter (LNB), satellite receiver, and TV set. The dish size, as usual, depends on the frequency band, satellite EIRP, LNB noise temperature, and the latitude of the receiver location. The satellite EIRP is designed so that, for example, the C-band dishes are typically 2 to 2.5 m in diameter in the tropical zone while at high latitudes, say 35° or more, larger diameters of 3 to 3.5 m are needed. The noise temperature of the LNB for the 2-m dishes at about 30° latitude must be about 25 K, which is achievable with a cheap, mass-produced HEMT amplifier within the LNB. A 35-K LNB would cause noise that would appear on the TV picture as "snow," and a 45-K LNB would be almost unwatchable.
Ku-band satellite systems are designed for use with very small antenna sizes, typically 0.5 to 1 min diameter.
For a small number of users (say four) sharing the same C-band dish and LNB, a 4-way splitter will allow the downconverted IF signal (about 950 to 1050 MHz) to feed four satellite receivers and four televisions, so that each receiver is tuned to a specific channel on each TV and each user can tune to a chosen satellite program by tuning the receiver to a specific program frequency.
|
Uplink (2.0 GHz) | ||
|
1 |
Earth station (handset) transmit power |
-1.0 dBW |
|
2 |
Earth station (handset) antenna gain |
0.0 dB |
|
3 |
Uplink EIRP [1+2] |
-1.0 dBW |
|
4 |
Uplink path loss |
178.7 dB |
|
5 |
Air absorption loss |
0.2 dB |
|
6 |
Satellite received power [3-4-5] |
-179.9 dBW |
|
7 |
Satellite G/T per user |
2.0 dB/K |
|
8 |
Boltzmann's constant |
-228.6 dBW/Hz/K |
|
9 |
C/No [6+7-8] |
50.7 dB |
|
10 |
45.6 dB | |
|
11 |
E b/N 0[9-10] |
5.1dB |
|
12 |
Required E b/N 0 |
2.5 dB |
|
Margin [11-12] |
2.6 dB | |
|
Downlink (2.2 GHz) | ||
|
1 |
Satellite EIRP per beam |
24.2 dBW |
|
2 |
Downlink path loss |
179.6 dB |
|
3 |
Air absorption loss |
0.2 dB |
|
4 |
Handset received power [1-2-3] |
-155.6 dBW |
|
5 |
Handset G/T |
-22.2 dB/K |
|
6 |
Boltzmann's constant |
|
|
7 |
C/No [4+5-6] |
50.8 dB |
|
8 |
Bit rate B (36 kb/s) |
45.6 dB |
|
9 |
E b N 0 [7-8] |
5.3 dB |
|
10 |
Required E b/N 0 |
2.5 dB |
|
Margin [9-10] |
2.8 dB | |
Path length (distance from handset to satellite) = 10355 km.
A typical VSAT earth station configuration is where the antenna in the 6/4-GHz C-band is on average about 2.4 m, and 1.2 m for Ku-band. Spread spectrum allows 1 m or less even in the C-band. Offset parabolic antennas are popular because they allow wall mounting as well as rooftop or ground installation.
The outdoor unit contains the circuitry of the low-noise-block downconverter, the upconverter, and solid-state RF power amplifier in a weatherproof en that is attached to the back of the antenna feed. The PSK modulator can also be included in this housing. So, the IF or baseband signal is carried by a 100-m to 300-m coaxial cable to the indoor unit, which is placed close to the data terminal equipment (DTE). The indoor unit usually contains the modulator/demodulator, and the baseband processor that is connected to the DTE by a standard interface. An optional TV receiver can usually be fed from the indoor unit to receive a TV channel, if it is offered by the satellite signal being received.
Nongeostationary Satellite Systems
LEO and MEO satellite capacity is very low compared to terrestrial cellular, which should lead to a peaceful coexistence because the systems complement each other. Indeed, cellular service providers see satellite systems as an opportunity to enhance their business. The claim of global operation is a grand statement, but a closer view shows that only limited coverage is possible in urban areas and indoors because of radio path shadowing and building attenuation. A key market exists in very remote areas where the telecommunications infrastructure is sparse or nonexistent.
The most important reason for using polar orbits is to enable communication from and to any point on earth. Clearly, a single polar orbiting satellite can cover only a small area at any one time, even though it is over much of the globe during the months and years that the earth rotates beneath it. A constellation of several satellites in each of several orbits is needed for full earth coverage at all times.
The relatively low altitude of LEOs means that the path loss is considerably smaller than for geostationary orbits. Consequently, the overall system gain for LEOs favors the use of small mobile handset "earth stations" instead of the large GSO earth stations. The economic potential of a LEO system is very attractive. During the next few years the extent of the success of nongeostationary orbit systems will become clear when two or more competing systems are fully deployed. As with all telecommunications services, success or failure depends on offering the service at an attractive price and the subsequent market share acquired by that price. The sources of market share in this case have two major components: (1) customers already using a terrestrial mobile system, and (2) potential customers who presently have no access to any mobile system, or at least not at an affordable price. Capturing long-term market share from existing terrestrial mobile operators will not be easy, especially because the performance of systems such as GSM, IS-54/136, and IS-95 is hard to beat. It is recognized that a major customer base for the LEO and medium earth orbit (MEO) systems will be from developing countries that have large rural populations and relatively modest telecommunications infrastructures. The per minute cost and, equally important, the handset cost will determine the extent of their usage.
From the technical viewpoint, there is already a battle among LEO and MEO participants. First, the 1992 World Administrative Radio Conference (WARC-92) allocated for worldwide services the use of frequencies 2483.5 to 2500 MHz in the S-band for the downlink, and the frequencies 1610 to 1626.5 MHz in the L-band for the uplink. This is a very small bandwidth and no doubt there will be serious lobbying from both proposed and potential operators for these bandwidths to be increased. If no further bands are allocated, several LEO and MEO systems must use the same frequency bands. Band sharing poses some technical questions that are difficult to answer. While multiple satellite-based CDMA systems can coexist in the same frequency band, the same is not true for multiple TDMA systems. Four major competing consortia have emerged. Iridium and INMARSAT-P are TDMA-based systems, whereas Globalstar and Odyssey are CDMA systems. Furthermore, the coexistence of a CDMA system on the same band as a TDMA system is possible, but the percentage drop in total capacity due to interference between the signals from the two systems is still being evaluated.
A nongeostationary earth orbit satellite constellation combines with the terrestrial network to form what is often referred to as a universal mobile telecommunications system (UMTS). Fixed satellite earth stations are necessary to control the traffic within a geographic area known as the guaranteed coverage area (GCA). This is a relatively simple task for the geostationary satellite case because the area of the earth covered by each satellite does not change. The situation becomes considerably more complex when one or more satellites are in motion above the earth. Based on the satellite orbital details, each fixed earth station has information that allows it to predict the position of all satellites and spot beams passing over its area at any time in the future. Clearly, a satellite or spot beam does not have to pass directly overhead to enable communication but, as a passing LEO satellite moves farther away from the direct overhead pass, its visibility time (time it remains above the horizon) becomes shorter and the path loss correspondingly increases, becoming closer to the worst-case value. Satellites passing directly overhead are above the horizon for the longest time, but high population concentrations enforce the use of off-overhead satellite path usage because the capacity of just one satellite is insufficient to accommodate all the traffic. Fortunately, in most cases, the speed of ground mobility even in fast cars is small relative to that of the satellite; otherwise the systems become even more complex. Communication to aircraft customers using LEO systems requires only slightly more handoff actions. The worst-case situation is when the aircraft is moving latitudinally (east-west or west-east) and not, as one might intuitively imagine, when moving longitudinally in the opposite direction to that of the satellite (north to south or south to north). In addition, the Doppler effect is a concern for high-speed mobile units. Go to Top
Low-earth-orbit satellite systems. These satellite constellations form the basis of a new category of global cellular radio systems. There is a definite sense of elegance that makes them very attractive. Even so, the financial risks involved are significant, because there are some technical difficulties that are not easy to surmount, as well as the serious competition that will evolve when several constellations are in service. The variation in propagation attenuation caused by the varying path length as a satellite makes a pass from the horizon (worst case of, say, 8° elevation) to overhead (best case) is about 10 dB of free-space loss plus rain attenuation. That attenuation difference is not small, and a power control mechanism is needed to cope with the variation. The shadow problem caused by a satellite moving behind a high building during a conversation is not trivial, and can lead to dropped calls in downtown areas. Some constellation designs allow for this. For example, Iridium has a 20-dB variable satellite EIRP to cope with shadow and path loss variations. The additional attenuation experienced by a mobile unit operating within a building is high. Mobile satellite systems are primarily designed for outside use. Direct mobile-to-satellite connections within buildings are possible, but not in high-rise locations, where reasonable quality might be obtained only on the top one or two floors. Many factors can affect the call quality; for instance, being close to a window might provide unobstructed sight of a satellite for the duration of a call. In conditions that create poor signal quality, arrangements therefore have to be made to use the terrestrial cellular or PCS network. LEO service providers are already linking up with cellular counterparts. Initially, the cost of LEO connection per-minute charges will be higher than that of terrestrial operators, so a dual-mode handset is essential with the terrestrial mode as the default setting. The LEO has the advantage of negligible time delay between earth and the satellites compared to the geostationary satellite systems. This relatively small link distance also allows communications via a low RF power output handset.
The first low earth
orbit (LEO) satellite system is being pioneered by Motorola, providing partial
operational status in 1998, and full operation two or three years later. The
concept has six low-earth polar-orbital planes, each containing 11 satellites, totaling 66
satellites. These form a fixed "cage" around the earth, which rotates within it.
The altitude of each satellite is 780 km above the earth's surface. The original
design had seven satellites per orbit, totaling 77 satellites. The atom
containing 77 electrons rotating about the nucleus is called iridium, which lends its name to
the system. Because cost reduction efforts have reduced the total number of
satellites to 66, perhaps the system is now more akin to the atom dysprosium.
The orbits are separated by 31.6° for the corotating satellite planes and 22° for the counterrotating pair
(1 and
6).
LEO satellites move at an amazing 27,000 km/h, and the antenna spot beam moves at 6.6 km/s relative to the earth as the satellite approaches the horizon. The complete orbital period is only 100 min and 28 s. Each satellite appears above the horizon for only 5 to 10 min, so a sophisticated interconnection between satellites is necessary to ensure continuity for a long call during which one satellite hands over the call to another overflying satellite either in the same or an adjacent orbit.
The system uses both FDMA and TDMA to make the most use of the limited spectrum available. The satellite has two modes of communication. First, a very remote or mobile user can communicate directly with a satellite moving overhead by an L-band frequency radio connection. An individual subscriber lit is a small hand-held, pocket-sized phone similar to present cellular style telephones. A fixed user accesses one of the satellites via a gateway. Intersatellite crosslink communications are at Ka-band, as are the satellite-to-ground system control links. Because the satellites orbit above the stratosphere, the path losses for the crosslinks do not include the rain attenuation losses that uplinks or downlinks suffer. The crosslinks operate in the 22.55- to 23.55-GHz frequency band at a transmission rate of 25 Mb/s. Each satellite can have four crosslinks. One is between each of the two satellites in front and hind within the same orbit, and the other two are to satellites on either side adjacent orbits.
The large number of satellites necessitates a large number of launches, and the finite lifetime of each satellite will require almost a permanent, ongoing production and replacement of satellites. These systems are obviously very expensive. The charges, however, need to be competitive with other call charges, with a premium for the unique feature of global roaming. Iridium targets business travelers as its major customer base. Iridium satellites all use onboard baseband processing to receive, route, and switch traffic between any of the satellites within the constellation. All satellites in adjacent orbits rotate in the same direction except for one pair adjacent orbits, approximating a hexagonal cell structure rotating about the earth. The cell overlap increases toward the poles, and some beams are switched off reduce interference and maintain effective frequency reuse. Each satellite has 48 circular spot beams generated by phased-array antennas, which project a total earth coverage footprint of 4600 km in diameter. For earth traffic each satellite has three phased-array panel antennas oriented at 0° to each other. Each antenna forms 16 spot beams totaling beams per satellite; there are consequently 48 X 66 = 3168 beams for the global constellation. Because the satellites converge toward the poles, only out 2150 spot beams need to be on for global coverage. The rest are turned ', which also conserves power.
Frequency reuse is on a 12-beam pattern basis. Only one satellite is accessed a user at a given time and so no satellite diversity is used. The 12-dB shadowing margin allows some building penetration. A satellite variable antenna gain from 7 to 15 dBi equalizes the signal into the satellite to account variable path loss. The level of onboard processing in Iridium is higher than all other previous satellite systems. Switching is done at the individual voice band level, so each satellite has demodulation-demultiplexing-switching-multiplexing-modulation capability. Satellite-to-earth-station gateway and system control links use Ka-band (19.4 to 19.6 GHz downlink; 29.1 to 29.3 GHz uplink) and earth stations use 6.25-Mb/s backhaul links for terrestrial interconnects. The overall throughput of each satellite is about 100 Mb/s.
The subscriber links use the 1616- to 1626.5-MHz frequency band (10.5 MHz) for both up- and downlinks. The 12-frequency reuse (FDMA) is enhanced by each frequency containing a 90-ms TDMA frame that provides four 50-kb/s user channels. Voice channels use 4.8 kb/s, and data 2.4 kb/s. By using voice activation, each beam contains 80 channels, so there are 12 X 80 = 960 channels per 12-beam frequency reuse cluster. Because there are 2150 beams globally, the total channel capacity is 80 X 2150 = 172,000 channels available globally. If one considers the global surface to be about 70 percent water, and perhaps another 15 to 20 percent dense forest, desert, or mountains, where population is also very sparse, it would appear that some spot beams could be underutilized.
GPS (Global Positioning System)
GPS, which stands for Global Positioning System, is the only system today able to show you your exact position on the Earth anytime, in any weather, anywhere. GPS satellites, 24 in all, orbit at 11,000 nautical miles above the Earth. They are continuously monitored by ground stations located worldwide. The satellites transmit signals that can be detected by anyone with a GPS receiver. Using the receiver, you can determine your location with great precision.
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| The first GPS satellite was called GPS Block I. Launched in 1978, it was a developmental satellite. Another nine Block I satellites were launched through 1988. |
Since of history's most exciting and revolutionary developments, and new uses for it are constantly being discovered. But before we learn more about GPS, it's important to understand a bit more about navigation.
Since prehistoric times, people have been trying to figure out a reliable way to tell where they are, to help guide them to where they are going, and to get them back home again. Cavemen probably used stones and twigs to mark a trail when they set out hunting for food. The earliest mariners followed the coast closely to keep from getting lost. When navigators first sailed into the open ocean, they discovered they could chart their course by following the stars. The ancient Phoenicians used the North Star to journey from Egypt and Crete. According to Homer, the goddess Athena told Odysseus to "keep the Great Bear on his left" during his travels from Calypso's Island. Unfortunately for Odysseus and all the other mariners, the stars are only visible at night - and only on clear nights.
The next major developments in the quest for the perfect method of navigation were the magnetic compass and the sextant. The needle of a compass always points north, so it is always possible to know in what direction you are going. The sextant uses adjustable mirrors to measure the exact angle of the stars, moon, and sun above the horizon. However, in the early days of its use, it was only possible to determine latitude (the location on the Earth measured north or south from the equator) from the sextant observations. Sailors were still unable to determine their longitude (the location on the Earth measured east or west). This was such a serious problem that in the 17th century, the British formed a special Board of Longitude consisting of well-known scientists. This group offered £20,000, equal to about a million of today's dollars, to anybody who could find a way to determine a ship's longitude within 30 nautical miles.
The generous offer paid off. In 1761, a cabinetmaker named John Harrison developed a shipboard timepiece called a chronometer, which lost or gained only about one second a day - incredibly accurate for the time. For the next two centuries, sextants and chronometers were used in combination to provide latitude and longitude information.
In the early 20th century several radio-based navigation systems were developed, which were used widely during World War II. Both allied and enemy ships and airplanes used ground-based radio-navigation systems as the technology advanced.
A few ground-based radio-navigation systems are still in use today. One drawback of using radio waves generated on the ground is that you must choose between a system that is very accurate but doesn't cover a wide area, or one that covers a wide area but is not very accurate. High-frequency radio waves (like UHF TV) can provide accurate position location but can only be picked up in a small, localized area. Lower frequency radio waves (like AM radio) can cover a larger area, but are not a good yardstick to tell you exactly where you are.
Scientists, therefore, decided that the only way to provide coverage for the entire world was to place high-frequency radio transmitters in space. A transmitter high above the Earth sending a high-frequency radio wave with a special coded signal can cover a large area and still overcome much of the "noise" encountered on the way to the ground. This is one of the main principles behind the GPS system.
So you can more easily understand some of the scientific principles that make GPS work, let's discuss the basic features of the system. The principle behind GPS is the measurement of distance (or "range") between the receiver and the satellites. The satellites also tell us exactly where they are in their orbits above the Earth. It works something like this: If we know our exact distance from a satellite in space, we know we are somewhere on the surface of an imaginary sphere with radius equal to the distance to the satellite radius. If we know our exact distance from two satellites, we know that we are located somewhere on the line where the two spheres intersect. And, if we take a third measurement, there are only two possible points where we could be located. One of these is usually impossible, and the GPS receivers have mathematical methods of eliminating the impossible location.
Global Positioning System (GPS)
Global Positioning System, GPS was developed by the US Department of Defense to provide all-weather round-the-clock navigation capabilities for military ground, sea, and air forces. Since its implementation, GPS has also become an integral asset in numerous civilian applications and industries around the globe, including recreational uses (e.g. boating, aircraft, hiking), corporate vehicle fleet tracking, and surveying.
GPS employs 24 spacecraft in 20,200 km circular orbits inclined at 55 degrees. These spacecraft are placed in 6 orbit planes with four operational satellites in each plane. All launches have been successful except for one launch failure in 1981. The full 24-satellite constellation was completed on March 9, 1994.
GPS receivers use triangulation of the GPS satellites' navigational signals to determine their location. The satellites provide two different signals that provide different accuracies. Coarse-acquisition (C/A) code is intended for civilian use, and is deliberately degraded. The accuracy using a typical civilian GPS receiver with C/A code is typically about 100 meters. The military's Precision (P) code is not corrupted, and provides positional accuracy to within approximately 20 meters. Numerous on-line tutorials on how GPS works and its applications are available, including those at the . GPS satellites are controlled at the GPS Master Control Station (MCS) located at Falcon Air Force Base outside Colorado Springs, Colorado. The ground segment also includes four active-tracking ground antennas and five passive-tracking monitor stations.
GPS has 3 parts: the space segment, the user segment, and the control segment. The space segment consists of 24 satellites, each in its own orbit 11,000 nautical miles above the Earth. The user segment consists of receivers, which you can hold in your hand or mount in your car. The control segment consists of ground stations (five of them, located around the world) that make sure the satellites are working properly.
One trip around the Earth in space equals one orbit. The GPS satellites each take 12 hours to orbit the Earth. Each satellite is equipped with an accurate clock to let it broadcast signals coupled with a precise time message. The ground unit receives the satellite signal, which travels at the speed of light. Even at this speed, the signal takes a measurable amount of time to reach the receiver. The difference between the time the signal is sent and the time it is received, multiplied by the speed of light, enables the receiver to calculate the distance to the satellite. To measure precise latitude, longitude, and altitude, the receiver measures the time it took for the signals from four separate satellites to get to the receiver.
The GPS system can tell you your location anywhere on or above the Earth to within about 300 feet. Even greater accuracy, usually within less than three feet, can be obtained with corrections calculated by a GPS receiver at a known fixed location.
To help you understand the GPS system, let's take the three parts of the system - the satellites, the receivers, and the ground control - and discuss them in more detail. Then we'll look more closely at how GPS works.
The first eleven spacecraft (GPS Block 1) were used to demonstrate the feasibility of the GPS system. The orbit inclination used for these satellites was 63 degrees, differing from the 55 degrees used for the operational system. The Block 2 spacecraft began the operational system. The Block 2A spacecraft (A = Advanced) were a slight improvement over the Block 2.
GPS Block 1 satellites formed the GPS Demonstration system and were followed by
the Block 2 operational system.
GPS Block 1
Spacecraft
3-Axis stabilized, nadir pointing using reaction wheels. Dual solar arrays
supply over 400 watts (EOL). NiCd batteries. S-Band (SGLS) communications for
control and telemetry. UHF cross-link between spacecraft. Hydrazine propulsion
system.
Payload
Two L-Band navigation signals at 1575.42 MHz (L1) and 1227.60 MHz (L2)
GPS Block 2
GPS Block 2 is the Operational system, following the Demonstration system comprised of Block 1 spacecraft. The Block 2A are "Advanced" versions of this spacecraft. The complete constellation has 24 spacecraft in 6 high-altitude orbit planes.
Spacecraft
3-Axis stabilized, nadir pointing using reaction wheels. Dual solar arrays
supply 710 watts (EOL). S-Band (SGLS) communications for control and telemetry.
UHF cross-link between spacecraft. Hydrazine propulsion system.
Payload
Two L-Band navigation signals at 1575.42 MHz (L1) and 1227.60 MHz (L2). Each
spacecraft carries 2 rubidium and 2 cesium clocks. Also carry nuclear detonation
detection sensors.
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| GPS Block II is a
production satellite first launched in 1989. Block II consists of 24
satellites, the last one launched in 1994.
|
As we've said, the complete GPS space system includes 24 satellites, 11,000 nautical miles above the Earth, which take 12 hours each to go around the Earth once (one orbit). They are positioned so that we can receive signals from six of them nearly 100 percent of the time at any point on Earth. You need that many signals to get the best position information. Satellites are equipped with very precise clocks that keep accurate time to within three nanoseconds - that's 0.000000003, or three billionths, of a second. This precision timing is important because the receiver must determine exactly how long it takes for signals to travel from each GPS satellite. The receiver uses this information to calculate its position.
The first GPS satellite was launched in 1978. The first 10 satellites were developmental satellites, called Block I. From 1989 to 1993, 23 production satellites, called Block II, were launched. The launch of the 24th satellite in 1994 completed the system.
The GPS control, or ground, segment consists of unmanned monitor stations located around the world (Hawaii and Kwajalein in the Pacific Ocean; Diego Garcia in the Indian Ocean; Ascension Island in the Atlantic Ocean; and Colorado Springs, Colorado); a master ground station at Schriever (Falcon) Air Force Base in Colorado Springs, Colorado; and four large ground antenna stations that broadcast signals to the satellites. The stations also track and monitor the GPS satellites.
GPS receivers can be hand carried or installed on aircraft, ships, tanks, submarines, cars, and trucks. These receivers detect, decode, and process GPS satellite signals. More than 100 different receiver models are already in use. The typical hand-held receiver is about the size of a cellular telephone, and the newer models are even smaller. The hand-held units distributed to U.S. armed forces personnel during the Persian Gulf war weighed only 28 ounces.
GPS Techniques and Project Costs
The GPS system was developed to meet military needs of the Department of Defense, but new ways to use its capabilities are continually being found. As you have read, the system has been used in aircraft and ships, but there are many other ways to benefit from GPS. We'll mention just a few.
During construction of the tunnel under the English Channel, British and French crews started digging from opposite ends: one from Dover, England, one from Calais, France. They relied on GPS receivers outside the tunnel to check their positions along the way and to make sure they met exactly in the middle. Otherwise, the tunnel might have been crooked.
Remember the example of the car with a video display in the dashboard? Vehicle tracking is one of the fastest-growing GPS applications. GPS-equipped fleet vehicles, public transportation systems, delivery trucks, and courier services use receivers to monitor their locations at all times.
GPS is also helping to save lives. Many police, fire, and emergency medical service units are using GPS receivers to determine the police car, fire truck, or ambulance nearest to an emergency, enabling the quickest possible response in life-or-death situations.
Automobile manufacturers are offering moving-map displays guided by GPS receivers as an option on new vehicles. The displays can be removed and taken into a home to plan a trip. Several Florida rental car companies are demonstrating GPS-equipped vehicles that give directions to drivers on display screens and through synthesized voice instructions. No more getting lost on the way to Disney World!
Mapping and surveying companies use GPS extensively. In the field of wildlife management, threatened species such as the Mojave Desert tortoise are being fitted with GPS receivers and tiny transmitters to help determine population distribution patterns and possible sources of disease. GPS-equipped balloons are monitoring holes in the ozone layer over the polar regions, and air quality is being monitored using GPS receivers. Buoys tracking major oil spills transmit data using GPS.
Archaeologists and explorers are using the system. Anyone equipped with a GPS receiver can use it as a reference point to find another location. With a basic knowledge of math and science, plus a hand-held GPS receiver, you could be an instant hero if you and friends got lost on a camping trip.
The future of GPS is as unlimited as your imagination. New applications will continue to be created as the technology evolves. The GPS satellites, like handmade stars in the sky, will be guiding you well into the 21st century.
