COMMUNICATION TECHNOLOGIES

Archive for the ‘Satellite Communication’ Category

Essentially a satellite system consists of three basic sections and uplink a satellite transponder and a downlink.

Uplink Model:

The primary component within the uplink section of a satellite system is the earth station transmitter .A typical earth station transmitter consists of an IF modulator an IF –to-RF microwave up-converter, a high-power amplifier (HPA), and so means of bandlimiting the final output spectrum IF modulator converts the input baseband signals to either an FM, a PSK at QAM modulator intermediate frequency.

Transponder:

A typical satellite transponder consists of an input bandlimiting device, an input low-noise amplifier, a frequency translator, a low-level power amplifier, and an output bandpass filter. This transponder is an RF-to-RF reapter. Other transponder configurations are IF and baseband repeaters similar to those used in microwave repeaters. The output of the LNA is fed to a frequency translator, which converts the high-band uplink frequency to the low-band downlink frequency. The lo-level power amplifier, which is commonly a traveling wave tube, amplifies the RF signal for transmission through the downlink to earth station receivers. Each RF satellite channel requires a separate transponder.

Downlink Model:

An earth receiver includes an input BPF, an LNA, and an RF-to-IF down-converter. Again, the BPF limits the input noise power to the LNA. The LNA is a highly sensitive, low-noise device, such as a tunnel diode amplifier of a parametric amplifier. The FR-to-IF down-converter is a mixer\bandpass filter combination that converts the received RF signal to an IF frequency.

Cross-Links:

Occasionally, there is an application where it is necessary to communicate between satellites. This is done using satellite cross-links or inter-satellite links (ISLs). A disadvantage of using an ISL is that both the transmitter and the receiver are space bound. Consequently, both the transmitter’s output power and the receiver’s input sensitivity are limited.

Back-off loss:

High-power amplifiers used in earth station transmitters and the traveling-wave tubes typically used in satellite transponders are nonlinear devices; their gain is dependent on input signal level. It can be seen that as the input power is reduced by 4 db, the output power is reduced by only 1 db. There is an obvious power compression. To reduce the amount of intermodulation distortion caused by the nonlinear amplification of the HPA, the input power must be reduced by several db. This allows the HPA to operate in a more linear region.

Multiple Accessing:

Satellite multiple accessing implies that more than one user has access to one or more radio channels within a satellite communications channel. Transponders are typically leased by a company or a common carrier for the purpose of providing voice data transmission to a multitude of users. The method by which a satellite transponder’s bandwidth is used or accessed depends on the multiple-accessing method utilized. Most commonly used multiple-accessing method arrangements frequency-division multiple accessing(FDMA) , time-division multiple accessing (TDMA), each earth station’s transmissions are assigned specific uplink and downlink frequency bands within an allotted satellite bandwidth; they may be preassigned or demand assigned consequently, FDMA transmissions are separated in the frequency domain and, therefore , must share the total available transponder bandwidth as well as the total transponder power. With TDMA, each earth transmits s short burst of information during a specific time slot within a TDMA frame. The bursts mean be synchronized so that each station’s burst arrives at the satellite at a different time. With CDMA, all earth stations transmit within the same frequency band and, for all practical purposes, have no limitations on when they may transmit or on which carrier frequency.

Frequency  Division Multiple Accesses:

Frequency-division multiple accesses (FDMA) is a method of multiple accessing where a given RF band width is divided into smaller frequency bands called subdivisions. Each subdivision has its own IF carrier frequency. A control mechanism is used to ensure that two or more earth stations do not transmit in the same subdivision at the same time. Essentially, the control mechanism designates a receive stations for each of the subdivision. Carrier frequency and bandwidth for FDM/FM satellite systems using multiple-channel-per-carrier formats are generally assigned and remain fixed for a long period of time. This is referred to as fixed-assignment, multiple accesses (FDM/FM/FAMA).an alternate channel allocation scheme is demand – assignment, multiple access (DAMA). Demand assignment allows all users continuous and equal access of the entire transponder bandwidth by assigning carrier frequency on a temporary basis using a statistical assignment process. The first FDMA demand-assignment system for satellites was developed by combat for use on the Intel sat series IVA and V satellites.

Navigation can be defied as the art of science of plotting, ascertaining, or dir4ecting the course of movements, in other words, knowing where you are and being able to find your way around. The most ancient and rudimentary method of navigation is wandering. Wandering is simple continuing to travel about until you reach your destination, assuming of course that you have one. With radio navigation, position is determined by measuring the travel time of an electromagnetic wave as it moves from a transmitter to a receiver. There approximately 100 different types of domestic radio navigation systems currently being used. Some use terrestrial (land-based) broadcast transmitters and others use satellite (space-based) broadcast transmitters.

Loran Navigation:

Until recently (long range navigation) was the most effective, reliable, and accurate means of radio navigation. Loran-A was developed. The most recent version, loran-C surfaced in 1980.today, loran is used primarily for recreational aircrafts and ships. With loan, receivers acquire specially coded signals from two pairs of high-powered, land based transmitters whose locations are precisely known. Loran is only accurate as the preciseness of the transmissions times of the coded signals, systems errors are due primarily to propagation problems, and such as the fact that earth’s surface is not smooth or perfect round

Navistar GPS:

Navistar is an acronym for navigation system with time and ranging, and gps is an abbreviation of global positioning system, Navistar GPS is the newest and most accurate system of radio navigation available. Navistar GPS is a satellite – based open navigation system, which simply means that is it available to anyone equipped with a GPS receiver. The Navistar satellite system was completed in 1994 and is maintained by the United States air force.

GPS Services:

GPS provides two levels of service or accuracy standard positioning service and precise positioning service. The standard positioning service is a positioning and timing service that is available to all gps service users. GPS will provide a predictable positioning accuracy that 95% of the time is to within 100m horizontally, 156 vertically. for security reasons the accuracy of the sps service is intentionally degraded by the DOD through the use of a technique called selective availability .the precise positioning is a highly accurate military positioning ,velocity and timing service that is available on a continuous, world wide basis authorizes by the DOD.

Navistar Segments:

Navistar segments of the three segments a space segment a ground segment, and a user segment. Satellite segments us air force command formally declared the Navistar gps satellite system.

Iridium LLC is an international consortium owned by a host of prominent companies, agencies, and government, the iridium project, which even sounds like something out of star wars is undoubtedly the largest commercial venture undertaken in the history of the world. It is the system with the most satellites, the highest price tag, the largest public relations terms and the most peculiar design. Iridium uses a GSM based telephony architecture to provide a digitally switched telephone networking and global dial tone to call and receive from any place in the world. This global roaming feature is designed into the system. Each subscriber is assigned a phone number and will receive only one bill no matter in what country or area they use the telephone.

System Layout:

Iridium system subscriber telephone sets used in the iridium system transmit and receive L band frequencies and utilize both frequency and time division multiplexing to make the most efficient use of a limited frequency spectrum. Other communications links used in Iridium include EHF and SHF bands between satellites for telemetry, command and control as well as routing digital voice packets to and from gate ways. An iridium telephone enables the subscribers to connect either to the local cellular telephone infrastructure or to the space constellation using its dual-mode feature. Iridium gateways are prime examples of the advances in satellite infrastructures that are responsible for the delivery of a host of new satellite services. The purpose of the gateways is to support and manage roaming subscribers as well as to interconnect iridium switched telephone network.

Satellite Constellation:

Providing full earth coverage is the underlying basis of the Iridium satellite system. This allows Iridium to cover the entire surface area of earth and whenever one satellite goes out of view of subscribers a different one replaces it. The satellites are phased appropriately in north-south necklaces forming co rotating planes up one side of earth, across the poles and down the other side. The first and last planes rotate in opposite directions, creating a virtual seam. Each satellite is equipped with three L- band antennas forming a honeycomb pattern that consists. As the satellite moves in its orbit, the footprints move across earth surface spectrum managements one-way communications links between cellular mobile units and base stations.

As stated, geosynchronous satellites orbit earth above the equator with the same angular velocity as earth. Hence, geosynchronous satellites appear to remain in affixed location above one spot on earth’s surface. Since a geosynchronous satellite appears to remain in a fixed location, no special antenna tracking equipment is necessary-earth station antennas are simply pointed at the satellite. A single high-altitude geosynchronous satellite can provide reliable communications to approximately 40% of the earth’s surface. Satellite remains in orbit as a result of a balance between centrifugal and gravitational forces.

If a satellite is traveling at too high a velocity, its centrifugal force will overcome earth’s gravitational pull, and the satellite will break out of orbit and escape into space. At lower velocities, the satellite’s centrifugal force is insufficient; gravity tends to pull the vehicle toward earth. Obviously, there is a delicate balance between acceleration, speed, and distance that will exactly balance the effects of centrifugal and gravitational forces. The closer to earth a satellite rotates, the greater the gravitational pull and the greater the velocity required keeping it from being pulled to earth. Low-altitude satellites orbiting 100 miles above earth travel at approximately 17,500 MPH.

Geosynchronous orbits are circular; therefore, the speed of rotation is constant throughout the orbit. There is only one geosynchronous earth orbit; however, it is occupied by a large number of satellites. In fact, the geosynchronous earth orbit is the mostly widely used earth orbit for the obvious reason that the satellites in a geosynchronous orbit remain in a fixed position relative to earth and, therefore, do not have to be tracked by earth station antennas. Ideally, geosynchronous satellites should remain stationary above a chosen location over the equator in an equatorial orbit however the sun and the moon exert gravitational forces solar winds sweep past earth and earth is not perfectly spherical.

Therefore these unbalanced forces cause geosynchronous satellites to drift slowly away from their assigned locations in a figure eight excursion with a 24 hour period that follows a wandering path slightly above and below the equatorial plane. In essence it occurs in a special type of inclined orbit sometimes called a stationary inclined orbit. Ground controllers must periodically adjust satellite position to counteract these forces. If not the excision above and below the equator would build up at a rate of between 0.6 and 0.9 per year. In addition geosynchronous satellites in a elliptical orbit also rift in an east or west direction as viewed from Earth The process of maneuvering a satellite within a pre-assigned window is called station keeping.

FDM/FM system using a single satellite transponder .with earth coverage antennas and for full full-duplex operation, each link required two RF satellite channels. To avoid interfering with earth station 1 earth station 2 must transmit and receive different RF carrier frequencies. The RF carrier frequencies are fixed and the satellite transponder is simply an RF to RF repeater provides the uplink/downlink frequencies translation. This arrangement is economically impractical and also extremely inefficient. The RF satellite channels are fixed between any two earth stations thus the voice-band channels from each station are committed to a single destination.

In systems where three or more earth stations wish to communicate with each other, fixed frequency or dedicated-channel systems such as those are inadequate a method pf multiple accessing is required .that is each earth station using the satellite has a mean of communicating with each of the other earth stations in the systems through a common satellite transponder .multiple accessing is sometimes called multiple destination. Because the like RS449 standard the RS 530 interface standard does not specify electrical specifications for the RS 530 are outlined by either the RS442A standard.

The principal use of the RS336 is for dial backup of private up data circuits and for automatic dialing of remote terminals. Satellite multiple accessing implies that more than that one user has access to one or more radio channels within satellite communication channel. Transponders are typically leased by a company or o common carrier for the purpose of providing voice or data transmissions to a multiple of users. The method by which a satellite transponder bandwidth is used or accessed depends on the multiple-accessing method utilized.

The three most commonly used multiple accessing arrangements frequency –division multiple accessing (FDMA), time – division multiple accessing (TDMA), and code –division multiple accessing (CDMA). With FDMA, each earth station transmissions are assigned uplink and down link frequency bands within an allotted satellite bandwidth they may be reassigned or demand assigned. Consequently, FDMA transmissions are separated in the frequency domain and therefore must share the total available transponder bandwidth as well as the total transponder power. Signal separation is accomplished with envelope encryption/decryption techniques.

Most of the satellites mentioned thus far are called orbital satellites, which are Nonsynchronous. Nonsynchronous satellites rotate around earth in elliptical or circular pattern. In a circular orbit, the speed or rotation is constant; however, in elliptical orbits the speed depends on the height the satellite is above earth. The speed of the satellite is greater when it is close to earth than it is farther away.

If the satellite is orbiting in the same direction as earth’s rotation and at an angular velocity greater than that of earth the orbit is called a prograde or posigrade orbit. If the satellite is orbiting in the opposite direction as earth’s rotation or in the same direction with an angular velocity less than that of earth the orbit is called a retrograde orbit.

Satellite Elevation Categories:

Satellites are generally classified as having either low earth, medium earth orbit or geosynchronous earth orbit. Most satellites operate in the 1.0 GHz to 2.5 GHz frequency range. Motorola’s satellite based mobile-telephone system, iridium, is a Leo system utilizing a 66-satellite constellation orbiting approximately 480 miles above earth’s surface. The main advantage of LEO satellites is that the path loss between earth stations and space vehicles is much lower than for satellites revolving in medium-or high-altitude orbits. Less path loss equates to lower transmit powers, small antennas, and less weight.

Satellite Orbital Patterns:

Although there is an infinite number of an orbital path only three are useful for communication satellites. A satellite can follow as it rotates around earth: inclined, equatorial, or polar. All satellites rotate around earth in an orbit that forms a plane that passes through the center of gravity of earth called the geocenter. Inclined orbits are virtually all orbits except those that travel directly above the equator or directly over the north and south poles. The angle of inclination is the angle between the earth’s equatorial plane and the orbital plane of a satellite measured counter clock wise at the point in the orbit where it crosses the equatorial plane traveling from south to north. To provide coverage to regions of high latitudes, inclined orbits are generally elliptical.

Details:

Mobile satellite systems provide the vehicle for a new generation of wireless telephone services called personal communications satellite systems. Universal wireless telephone coverage is a developing MSS Service that promises to deliver mobile subscribers both traditional and enhanced telephone features while wide-area global coverage. MSS satellite are in essence, radio repeaters in the sky, and their usefulness for mobile communications depends on several factors, such as the space-vehicle altitude, orbital pattern, transmit power, receiver sensitivity, modulation technique, antenna radiation pattern, and number of satellites in its constellation.

Satellite communications systems have traditionally provided both narrowband and wideband voice, data, video, facsimile, and networking services using large and very expensive, high-powered earth station transmitters communicating via high-altitude, geosynchronous earth-orbit satellites. Personal communications satellite services, however, use low earth-orbit and medium earth earth-orbit satellites that communicate directly with small, Low-power mobile telephone units. The intension of PCSS mobile telephone is to provide the same features and services offered by traditional, terrestrial cellular telephone providers.

Advantage and Disadvantages:

The primary and probably most obvious advantage of PCSS mobile telephone is that it provides mobile telephone coverage and a host of other integrated services virtually anywhere in the world to a truly global customer base. PCSS can fill the vacancies between land-based cellular and PCS telephone systems and provide wide-area coverage on a regional or global basis. PCSS is ideally suited to fixed cellular telephone applications, as it can provide a full complement of telephone services to places where cables can never go because of economical, technical, or physical constraints. PCSS can also provide complementary and backup telephone services to large companies and organizations with multiple operations in diverse locations, such as retail, manufacturing, finance, transportation, government, military, and insurance.

Most of the disadvantages of PCSS are closely related to economics, with the primary disadvantage being the risk associated with the high costs of designing, building, and launching satellites. There is also a high cost for the terrestrial-based networking and interface infrastructure necessary to maintain, coordinate, and manage the network once it is in operation. In addition, the intricate low-power, dual-mode transceivers are more cumbersome and expensive than most mobile telephone units used with terrestrial cellular and PCS system.