Intro4u2u

Fiber optic cable is composed of one or more transparent fibers enclosed in protective coverings and strength members. Cable types can include simplex, duplex, multifiber, patch cord, and bare fiber.  Simplex cables are fiber optic cable with a single optical fiber.  Duplex cables are fiber optic cable with two optical fibers.  Multifiber cable is a fiber optic cable with several optical fibers.  Patch cords are short lengths of fiber optic cable with connectors.  Bare fiber refers to a fiber optic core and cladding only.  Fiber types can be single mode or multimode.  Single mode describes a fiber with a small core, only a few times the wavelength of light transmitted, that only allows one mode of light to propagate. It is commonly used with laser sources for high speed, long distance links.  Multimode describes a fiber with core diameter much larger than the wavelength of light transmitted that allows many modes of light to propagate. Commonly used with LED sources for lower speed, short distance links.

Common connector types for fiber optic cable include biconic, D4, ESCON, FC, FDDI, LC, loopback, MTP, MT-RJ, MU, SC, SMA, and ST.  Some fiber optic cable comes without connectors.  Important parameters to consider when searching for fiber optic cable include fiber core size, cable diameter, and cable weight.  Fiber core size is the size of the light-conducting central portion of an optical fiber, composed of material with a higher index of refraction than the cladding. The core size is smaller for single mode and larger for multimode fibers.  Cable diameter refers to the diameter of the fiber optic cable including jacket.

Cable performance specifications to consider when searching for fiber optic cable include wavelength, numerical aperture, maximum attenuation, and bending radius.  The wavelength refers to the wavelength that the cable was designed for.  Numerical aperture is the light-gathering ability of a fiber; the maximum angle to the fiber axis at which light will be accepted and propagated through the fiber. The measure of the light-acceptance angle of an optical fiber, NA = sin a, where A is the acceptance angle. NA is also used to describe the angular spread of light from a central axis, as in exiting a fiber, emitting from a source, or entering a detector.  Maximum attenuation is the decrease in signal strength along a fiber optic waveguide caused by absorption and scattering. Attenuation is usually expressed in dB/km.  Bending radius is the smallest radius an optical fiber or fiber cable can bend before increased attenuation or breakage occurs.  Common features for fiber optic cable include polarization maintaining, graded index, and metallized.  A polarization maintaining cable has fiber that maintains the polarization of light that enters it.  A graded index fiber optic cable has optical fiber in which the refractive index of the core is in the form of a parabolic curve, decreasing toward the cladding.  Metallized fibers are coated with metals for increased temperature resistance, soldering, and harsh environments.  An important environmental parameter to consider is the operating temperature.

Fiber optic switches route an optical signal without electro-optical and opto-electrical conversions.  Fiber optic switches can interface with two types of cables, single mode and multimode.  Single mode is an optical fiber that will allow only one mode to propagate. The fiber has a very small core diameter of approximately 8 µm. It permits signal transmission at extremely high bandwidth and allows very long transmission distances.  Multimode describes a fiber optic cable, which supports the propagation of multiple modes. Multimode fiber may have a typical core diameter of 50 to 100 µm with a refractive index that is graded or stepped. It allows the use of inexpensive LED light sources and connector alignment and coupling is less critical than single mode fiber. Distances of transmission and transmission bandwidth are less than with single mode fiber due to dispersion.  Some fiber optic switches can be used for both single mode and multimode cables.

Important switch performance parameters to consider when searching for fiber optic switches include wavelength range, number of input ports, and number of output ports, switching time, insertion loss, polarization dependent loss, cross-talk, data rate, and switching voltage.  The wavelength range specifies the wavelength range the switch is designed to operate in.  The number of input and output ports is important to consider.  The switching time is the amount of time it takes for switching to occur.  The insertion loss is the attenuation caused by the insertion of an optical component.  Polarization dependent loss is the attenuation caused by polarization.  Cross-talk is the ratio of the output power produced by the desired input to the output power produced by undesired inputs.  It is a measure of switching effectiveness.  Data rate is the number of data bits transmitted in bits per second.  Data rate is a way of expressing the speed of the switch.  Switches are active components.  The switching voltage is the voltage needed for switching.

Control signal choices for fiber optic switches include RJ-45, RS232, RS422, and TTL.  Common switch features include rack mountable and LED indicators.  An important environmental parameter to consider for fiber optic switches is the operating temperature.

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Optical communication systems date back two centuries, to the “optical telegraph” that French engineer Claude Chappe invented in the 1790s. His system was a series of semaphores mounted on towers, where human operators relayed messages from one tower to the next. It beat hand-carried messages hands down, but by the mid-19th century was replaced by the electric telegraph, leaving a scattering of “Telegraph Hills” as its most visible legacy.

Alexander Graham Bell patented an optical telephone system, which he called the Photophone, in 1880, but his earlier invention, the telephone, proved far more practical. He dreamed of sending signals through the air, but the atmosphere didn’t transmit light as reliably as wires carried electricity. In the decades that followed, light was used for a few special applications, such as signalling between ships, but otherwise optical communications, like the experimental Photophone Bell donated to the Smithsonian Institution, languished on the shelf.

In the intervening years, a new technology slowly took root that would ultimately solve the problem of optical transmission, although it was a long time before it was adapted for communications. It depended on the phenomenon of total internal reflection, which can confine light in a material surrounded by other materials with lower refractive index, such as glass in air. In the 1840s, Swiss physicist Daniel Collodon and French physicist Jacques Babinet showed that light could be guided along jets of water for fountain displays. British physicist John Tyndall popularized light guiding in a demonstration he first used in 1854, guiding light in a jet of water flowing from a tank. By the turn of the century, inventors realized that bent quartz rods could carry light, and patented them as dental illuminators. By the 1940s, many doctors used illuminated plexiglass tongue depressors.

Optical fibers went a step further. They are essentially transparent rods of glass or plastic stretched so they are long and flexible. During the 1920s, John Logie Baird in England and Clarence W. Hansell in the United States patented the idea of using arrays of hollow pipes or transparent rods to transmit images for television or facsimile systems. However, the first person known to have demonstrated image transmission through a bundle of optical fibers was Heinrich Lamm, than a medical student in Munich. His goal was to look inside inaccessible parts of the body, and in a 1930 paper he reported transmitting the image of a light bulb filament through a short bundle. However, the unclad fibers transmitted images poorly, and the rise of the Nazis forced Lamm, a Jew, to move to America and abandon his dreams of becoming a professor of medicine.

In 1951, Holger Møller [or Moeller, the o has a slash through it] Hansen applied for a Danish patent on fiber-optic imaging. However, the Danish patent office denied his application, citing the Baird and Hansell patents, and Møller Hansen was unable to interest companies in his invention. Nothing more was reported on fiber bundles until 1954, when Abraham van Heel of the Technical University of Delft in Holland and Harold. H. Hopkins and Narinder Kapany of Imperial College in London separately announced imaging bundles in the prestigious British journal Nature.

Neither van Heel nor Hopkins and Kapany made bundles that could carry light far, but their reports the fiber optics revolution. The crucial innovation was made by van Heel, stimulated by a conversation with the American optical physicist Brian O’Brien. All earlier fibers were “bare,” with total internal reflection at a glass-air interface. van Heel covered a bare fiber or glass or plastic with a transparent cladding of lower refractive index. This protected the total-reflection surface from contamination, and greatly reduced crosstalk between fibers. The next key step was development of glass-clad fibers, by Lawrence Curtiss, then an undergraduate at the University of Michigan working part-time on a project to develop an endoscope to examine the inside of the stomach with physician Basil Hirschowitz, physicist C. Wilbur Peters. (Will Hicks, then working at the American Optical Co., made glass-clad fibers at about the same time, but his group lost a bitterly contested patent battle.) By 1960, glass-clad fibers had attenuation of about one decibel per meter, fine for medical imaging, but much too high for communications.

Meanwhile, telecommunications engineers were seeking more transmission bandwidth. Radio and microwave frequencies were in heavy use, so they looked to higher frequencies to carry loads they expected to continue increasing with the growth of television and telephone traffic. Telephone companies thought video telephones lurked just around the corner, and would escalate bandwidth demands even further. The cutting edge of communications research were millimeter-wave systems, in which hollow pipes served as waveguides to circumvent poor atmospheric transmission at tens of gigahertz, where wavelengths were in the millimeter range.

Even higher optical frequencies seemed a logical next step in 1958 to Alec Reeves, the forward-looking engineer at Britain’s Standard Telecommunications Laboratories who invented digital pulse-code modulation before World War II. Other people climbed on the optical communications bandwagon when the laser was invented in 1960. The July 22, 1960 issue of Electronics magazine introduced its report on Theodore Maiman’s demonstration of the first laser by saying “Usable communications channels in the electromagnetic spectrum may be extended by development of an experimental optical-frequency amplifier.”

Serious work on optical communications had to wait for the continuouswave helium-neon laser. While air is far more transparent at optical wavelengths than to millimeter waves, researchers soon found that rain, haze, clouds, and atmospheric turbulence limited the reliability of long-distance atmospheric laser links. By 1965, it was clear that major technical barriers remained for both millimeter-wave and laser telecommunications. Millimeter waveguides had low loss, although only if they were kept precisely straight; developers thought the biggest problem was the lack of adequate repeaters. Optical waveguides were proving to be a problem. Stewart Miller’s group at Bell Telephone Laboratories was working on a system of gas lenses to focus laser beams along hollow waveguides for long-distance telecommunications. However, most of the telecommunications industry thought the future belonged to millimeter waveguides.

Optical fibers had attracted some attention because they were analogous in theory to plastic dielectric waveguides used in certain microwave applications. In 1961, Elias Snitzer at American Optical, working with Hicks at Mosaic Fabrications (now Galileo Electro-Optics), demonstrated the similarity by drawing fibers with cores so small they carried light in only one waveguide mode. However virtually everyone considered fibers too lossy for communications; attenuation of a decibel per meter was fine for looking inside the body, but communications operated over much longer distances, and required loss no more than 10 or 20 decibels per kilometer.

One small group did not dismiss fibers so easily — a team at Standard Telecommunications Laboratories initially headed by Antoni E. Karbowiak, which worked under Reeves to study optical waveguides for communications. Karbowiak soon was joined by a young engineer born in Shanghai, Charles K. Kao.

Kao took a long, hard look at fiber attenuation. He collected samples from fiber makers, and carefully investigated the properties of bulk glasses. His research convinced him that the high losses of early fibers were due to impurities, not to silica glass itself. In the midst of this research, in December 1964, Karbowiak left STL to become chair of electrical engineering at the University of New South Wales in Australia, and Kao succeeded him as manager of optical communications research. With George Hockham, another young STL engineer who specialized in antenna theory, Kao worked out a proposal for long-distance communications over single-mode fibers. Convinced that fiber loss should be reducible below 20 decibels per kilometer, they presented a paper at a London meeting of the Institution of Electrical Engineers. The April 1, 1966 issue of Laser Focus noted Kao’s proposal:

“At the IEE meeting in London last month, Dr. C. K. Kao observed that short-distance runs have shown that the experimental optical waveguide developed by Standard Telecommunications Laboratories has an information-carrying capacity … of one gigacycle, or equivalent to about 200 tv channels or more than 200,000 telephone channels. He described STL’s device as consisting of a glass core about three or four microns in diameter, clad with a coaxial layer of another glass having a refractive index about one percent smaller than that of the core. Total diameter of the waveguide is between 300 and 400 microns. Surface optical waves are propagated along the interface between the two types of glass.”

“According to Dr. Kao, the fiber is relatively strong and can be easily supported. Also, the guidance surface is protected from external influences. … the waveguide has a mechanical bending radius low enough to make the fiber almost completely flexible. Despite the fact that the best readily available low-loss material has a loss of about 1000 dB/km, STL believes that materials having losses of only tens of decibels per kilometer will eventually be developed.”

Kao and Hockham’s detailed analysis was published in the July 1966 Proceedings of the Institution of Electrical Engineers. Their daring forecast that fiber loss could be reduced below 20 dB/km attracted the interest of the British Post Office, which then operated the British telephone network. F. F. Roberts, an engineering manager at the Post Office Research Laboratory (then at Dollis Hill in London), saw the possibilities, and persuaded others at the Post Office. His boss, Jack Tillman, tapped a new research fund of 12 million pounds to study ways to decrease fiber loss.

With Kao almost evangelically promoting the prospects of fiber communications, and the Post Office interested in applications, laboratories around the world began trying to reduce fiber loss. It took four years to reach Kao’s goal of 20 dB/km, and the route to success proved different than many had expected. Most groups tried to purify the compound glasses used for standard optics, which are easy to melt and draw into fibers. At the Corning Glass Works (now Corning Inc.), Robert Maurer, Donald Keck and Peter Schultz started with fused silica, a material that can be made extremely pure, but has a high melting point and a low refractive index. They made cylindrical performs by depositing purified materials from the vapor phase, adding carefully controlled levels of dopants to make the refractive index of the core slightly higher than that of the cladding, without raising attenuation dramatically. In September 1970, they announced they had made single-mode fibers with attenuation at the 633-nanometer helium-neon line below 20 dB/km. The fibers were fragile, but tests at the new British Post Office Research Laboratories facility in Martlesham Heath confirmed the low loss.

The Corning breakthrough was among the most dramatic of many developments that opened the door to fiber-optic communications. In the same year, Bell Labs and a team at the Ioffe Physical Institute in Leningrad (now St. Petersburg) made the first semiconductor diode lasers able to emit continuouswave at room temperature. Over the next several years, fiber losses dropped dramatically, aided both by improved fabrication methods and by the shift to longer wavelengths where fibers have inherently lower attenuation.

Early single-mode fibers had cores several micrometers in diameter, and in the early 1970s that bothered developers. They doubted it would be possible to achieve the micrometer-scale tolerances needed to couple light efficiently into the tiny cores from light sources, or in splices or connectors. Not satisfied with the low bandwidth of step-index multimode fiber, they concentrated on multi-mode fibers with a refractive-index gradient between core and cladding, and core diameters of 50 or 62.5 micrometers. The first generation of telephone field trials in 1977 used such fibers to transmit light at 850 nanometers from gallium-aluminum-arsenide laser diodes.

Those first-generation systems could transmit light several kilometers without repeaters, but were limited by loss of about 2 dB/km in the fiber. A second generation soon appeared, using new InGaAsP lasers which emitted at 1.3 micrometer, where fiber attenuation was as low as 0.5 dB/km, and pulse dispersion was somewhat lower than at 850 nm. Development of hardware for the first transatlantic fiber cable showed that single-mode systems were feasible, so when deregulation opened the long-distance phone market in the early 1980s, the carriers built national backbone systems of single-mode fiber with 1300-nm sources. That technology has spread into other telecommunication applications, and remains the standard for most fiber systems.

However, a new generation of single-mode systems is now beginning to find applications in submarine cables and systems serving large numbers of subscribers. They operate at 1.55 micrometers, where fiber loss is 0.2 to 0.3 dB/km, allowing even longer repeater spacings. More important, erbium-doped optical fibers can serve as optical amplifiers at that wavelength, avoiding the need for electro-optic regenerators. Submarine cables with optical amplifiers can operate at speeds to 5 gigabits per second, and can be upgraded from lower speeds simply to changing terminal electronics. Optical amplifiers also are attractive for fiber systems delivering the same signals to many terminals, because the fiber amplifiers can compensate for losses in dividing the signals among many terminals.

The biggest challenge remaining for fiber optics is economic. Today telephone and cable television companies can cost-justify installing fiber links to remote sites serving tens to a few hundreds of customers. However, terminal equipment remains too expensive to justify installing fibers all the way to homes, at least for present services. Instead, cable and phone companies run twisted wire pairs or coaxial cables from optical network units to individual homes. Time will see how long that lasts.

Attenuation

Attenuation in an optical fiber is caused by absorption, scattering, and bending losses. Attenuation is the loss of optical power as light travels along the fiber. Signal attenuation is defined as the ratio of optical input power (Pi) to the optical output power (Po). Optical input power is the power injected into the fiber from an optical source. Optical output power is the power received at the fiber end or optical detector. The following equation defines signal attenuation as a unit of length:

Signal attenuation is a log relationship. Length (L) is expressed in kilometers. Therefore, the unit of attenuation is decibels/kilometer (dB/km). As previously stated, attenuation is caused by absorption, scattering, and bending losses. Each mechanism of loss is influenced by fiber-material properties and fiber structure. However, loss is also present at fiber connections. Fiber connector, splice, and coupler losses are discussed in chapter 4. The present discussion remains relative to optical fiber attenuation properties.

Q.38 Define attenuation.

ABSORPTION. - Absorption is a major cause of signal loss in an optical fiber. Absorption is defined as the portion of attenuation resulting from the conversion of optical power into another energy form, such as heat. Absorption in optical fibers is explained by three factors:

* Imperfections in the atomic structure of the fiber material
* The intrinsic or basic fiber-material properties
* The extrinsic (presence of impurities) fiber-material properties

Imperfections in the atomic structure induce absorption by the presence of missing molecules or oxygen defects. Absorption is also induced by the diffusion of hydrogen molecules into the glass fiber. Since intrinsic and extrinsic material properties are the main cause of absorption, they are discussed further.

Intrinsic Absorption. - Intrinsic absorption is caused by basic fiber-material properties. If an optical fiber were absolutely pure, with no imperfections or impurities, then all absorption would be intrinsic. Intrinsic absorption sets the minimal level of absorption.

In fiber optics, silica (pure glass) fibers are used predominately. Silica fibers are used because of their low intrinsic material absorption at the wavelengths of operation.

In silica glass, the wavelengths of operation range from 700 nanometers (nm) to 1600 nm. Figure 2-21 shows the level of attenuation at the wavelengths of operation. This wavelength of operation is between two intrinsic absorption regions. The first region is the ultraviolet region (below 400-nm wavelength). The second region is the infrared region (above 2000-nm wavelength).

Figure 2-21. - Fiber losses.

Intrinsic absorption in the ultraviolet region is caused by electronic absorption bands. Basically, absorption occurs when a light particle (photon) interacts with an electron and excites it to a higher energy level. The tail of the ultraviolet absorption band is shown in figure 2-21.

The main cause of intrinsic absorption in the infrared region is the characteristic vibration frequency of atomic bonds. In silica glass, absorption is caused by the vibration of silicon-oxygen (Si-O) bonds. The interaction between the vibrating bond and the electromagnetic field of the optical signal causes intrinsic absorption. Light energy is transferred from the electromagnetic field to the bond. The tail of the infrared absorption band is shown in figure 2-21.

Extrinsic Absorption. - Extrinsic absorption is caused by impurities introduced into the fiber material. Trace metal impurities, such as iron, nickel, and chromium, are introduced into the fiber during fabrication. Extrinsic absorption is caused by the electronic transition of these metal ions from one energy level to another.

Extrinsic absorption also occurs when hydroxyl ions (OH-) are introduced into the fiber. Water in silica glass forms a silicon-hydroxyl (Si-OH) bond. This bond has a fundamental absorption at 2700 nm. However, the harmonics or overtones of the fundamental absorption occur in the region of operation. These harmonics increase extrinsic absorption at 1383 nm, 1250 nm, and 950 nm. Figure 2-21 shows the presence of the three OH- harmonics. The level of the OH- harmonic absorption is also indicated.

These absorption peaks define three regions or windows of preferred operation. The first window is centered at 850 nm. The second window is centered at 1300 nm. The third window is centered at 1550 nm. Fiber optic systems operate at wavelengths defined by one of these windows.

The amount of water (OH-) impurities present in a fiber should be less than a few parts per billion. Fiber attenuation caused by extrinsic absorption is affected by the level of impurities (OH-) present in the fiber. If the amount of impurities in a fiber is reduced, then fiber attenuation is reduced.

Q.39 What are the main causes of absorption in optical fiber?
Q.40 Silica (pure glass) fibers are used because of their low intrinsic material absorption at the wavelengths of operation. This wavelength of operation is between two intrinsic absorption regions. What are these two regions called? What are the wavelengths of operation for these two regions?
Q.41 Extrinsic (OH-) absorption peaks define three regions or windows of preferred operation. List the three windows of operation.

INO has developed and manufactured high attenuation optical fibers for connector-type and patch cord-type attenuator assembly.

Singlemode features:

* Attenuation levels: 0.002 to 20 dB/cm @ 1310 nm and 1550 nm
* Compatible with smf 28TM
* Cladding mode suppression design for very low modal interference effects
* The unit length of each connector-type attenuator can be adjusted from 10 mm to 23 mm

Multimode features:

* Attenuation levels: 0.05 to 5dB/cm@ 850 nm
* Attenuation uniformity: +/-10% over 100 nm bandwidth
* Attenuation levels: 1 to 5 dB/cm@ 850 and 1310 nm
* Compatible with conventional 50/125 µm fibers
* The unit length is specified per meter for both singlemode and multimode patch cord-types

Wavelength
Light that can be seen by the unaided human eye is said to be in the visible spectrum. In the visible spectrum, wavelength can be described as the color of light.

To put this into perspective, take a look at Figure 8. Notice that the colors of the rainbow - red, orange, yellow, green, blue, (indigo, not shown), and violet ­ fall within the visible spectrum.

Optical fiber transmission uses wavelengths which are above the visible light spectrum, and thus undetectable to the unaided eye. Typical optical transmission wavelengths are 850 nanometers (nm), 1310 nm, and 1550 nm.

Both lasers and LEDs (light-emitting diodes) are used to transmit light through optical fiber. Lasers are usually used for 1310 or 1550 nanometer, single-mode applications.  * LEDs are used for 850 or 1300 nanometer multimode applications.

*Safety note: Never look into the end of a fiber which may have a laser coupled to it. Laser light is invisible and can damage the eyes. Viewing it directly does not cause pain. The iris of the eye will not close involuntarily as when viewing a bright light; consequently, serious damage to the retina of the eye is possible. Should accidental exposure to laser light be suspected, an eye examination should be arranged immediately.

Window
There are ranges of wavelengths at which the fiber operates best. Each range is known as an operating window. Each window is centered around the typical operational wavelength.

These wavelengths were chosen because they best match the transmission properties of available light sources with the transmission qualities of optical fiber.

Frequency
The frequency of a system is the speed of modulation of the digital or analog output of the light source; in other words, the number of pulses per second emitted from the light source. Frequency is measured in units of hertz (Hz), where 1 hertz is equal to 1 pulse or cycle per second (Figure 10). A more practical measurement for optical communications is megahertz (MHz) or millions of pulses per second.

Attenuation
Attenuation is the loss of optical power as light travels down a fiber. It is measured in decibels (dB/km). Over a set distance, a fiber with a lower attenuation will allow more power to reach its receiver than a fiber with higher attenuation.

While low-loss optical systems are always desirable, it is possible to lose a large portion of the initial signal power without significant problems. A loss of 50% of initial power is equal to a 3.0 dB loss. Any time fibers are joined together there will be some loss. Losses for fusion splicing and for mechanical splicing are typically 0.2 dB or less.

Attenuation can be caused by several factors, but is generally placed in one of two categories: intrinsic or extrinsic.

Intrinsic Attenuation
Intrinsic attenuation occurs due to something inside or inherent to the fiber. It is caused by impurities in the glass during the manufacturing process. As precise as manufacturing is, there is no way to eliminate all impurities, though technological advances have caused attenuation to decrease dramatically since the first optical fiber in 1970.

When a light signal hits an impurity in the fiber, one of two things will occur: it will scatter or it will be absorbed.

Scattering
Rayleigh scattering accounts for the majority (about 96%) of attenuation in optical fiber. Light travels in the core and interacts with the atoms in the glass. The light waves elastically collide with the atoms, and light is scattered as a result.

Rayleigh scattering is the result of these elastic collisions between the light wave and the atoms in the fiber. If the scattered light maintains an angle that supports forward travel within the core, no attenuation occurs. If the light is scattered at an angle that does not support continued forward travel, the light is diverted out of the core and attenuation occurs.

Some scattered light is reflected back toward the light source (input end). This is a property that is used in an Optical Time Domain Reflectometer (OTDR) to test fibers. This same principle applies to analyzing loss associated with localized events in the fiber, such as splices.

Absorption
The second type of intrinsic attenuation in fiber is absorption. Absorption accounts for 3-5% of fiber attenuation. This phenomenon causes a light signal to be absorbed by natural impurities in the glass, and converted to vibrational energy or some other form of energy. (Figure 12)

Unlike scattering, absorption can be limited by controlling the amount of impurities during the manufacturing process.

Extrinsic Attenuation>
The second category of attenuation is extrinsic attenuation. Extrinsic attenuation can be caused by two external mechanisms: macrobending or microbending. Both cause a reduction of optical power.

Macrobending
If a bend is imposed on an optical fiber, strain is placed on the fiber along the region that is bent. The bending strain will affect the refractive index and the critical angle of the light ray in that specific area. As a result, light traveling in the core can refract out, and loss occurs. (Figure 13)

A macrobend is a large-scale bend that is visible; for example, a fiber wrapped around a person’s finger. This loss is generally reversible once bends are corrected.

To prevent macrobends, all optical fiber (and optical fiber cable) has a minimum bend radius specification that should not be exceeded. This is a restriction on how much bend a fiber can withstand before experiencing problems in optical performance or mechanical reliability. The rule of thumb for minimum bend radius is 1 1/2″ for bare, single-mode fiber; 10 times the cable’s outside diameter (O.D.) for non-armored cable; and 15 times the cable’s O.D. for armored cable.

Microbending
The second extrinsic cause of attenuation is a microbend. This is a small-scale distortion, generally indicative of pressure on the fiber. (See Figure 14 below.) Microbending may be related to temperature, tensile stress, or crushing force. Like macrobending, microbending will cause a reduction of optical power in the glass.

Microbending is very localized, and the bend may not be clearly visible upon inspection. With bare fiber, microbending may be reversible; in the cabling process, it may not.

Dispersion
Dispersion is the “spreading” of a light pulse as it travels down a fiber. (See Figure 15.) As the pulses spread, or broaden, they tend to overlap, and are no longer distinguishable by the receiver as 0s and 1s. Light pulses launched close together (high data rates) that spread too much (high dispersion) result in errors and loss of information.

Chromatic dispersion occurs as a result of the range of wavelengths in the light source. Light from lasers and LEDs consists of a range of wavelengths. Each of these wavelengths travels at a slightly different speed. Over distance, the varying wavelength speeds cause the light pulse to spread in time. This is of most importance in single-mode applications.

Modal dispersion is significant in multimode applications, where the various modes of light traveling down the fiber arrive at the receiver at different times, causing a spreading effect.

Pulse width is measured at full width-half maximum; in other words, the full width of the pulse, at half the maximum pulse height. Dispersion limits how fast, or how much, information can be sent over an optical fiber.

There are other effects of dispersion, but further discussion is beyond the scope of this text. For more in-depth reading, please consult the bibliography at the end of this publication.

Bandwidth
In simplest terms, bandwidth is the amount of information a fiber can carry so that every pulse is distinguishable by the receiver at the end. (Figure 16)

As discussed in the previous section, dispersion causes light pulses to spread. The spreading of these light pulses causes them to merge together. At a certain distance and frequency, the pulses become unreadable by the receiver. The multiple pathways of a multimode fiber cause this overlap to be much greater than for single-mode fiber. These different paths have different lengths, which cause each mode of light to arrive at a different time.

System bandwidth is measured in megahertz (MHz) at one km. In general, when a system’s bandwidth is 200 MHz·km, it means that 200 million pulses of light per second will travel down 1 km (1000 meters) of fiber, and each pulse will be distinguishable by the receiver.

Check Your Understanding
Would you like to see how much you’ve learned?

1. In optical fiber systems, what are the typical wavelengths of operation?
400 nm 455 nm 490 nm
620 nm 750 nm 800 nm
850 nm 1310 nm 1550 nm

2. What is an optical window?
A glass object on the side of a building
A range of wavelengths at which fiber best operates
The moment in time when fiber became a commercially-viable enterprise

3. Which of the following is not true of attenuation?
It is the loss of optical power as light travels down a fiber
It may be induced by scattering absorbing macrobending and microbending
It is measured in nanometers

4. What is bandwidth?
A measure of the information-carrying capacity of an optical fiber
The side-to-side measurement of a wedding ring
A term used to express the total loss of an optical system

Attenuation is a general term that refers to any reduction in the strength of a signal. Attenuation occurs with any type of signal, whether digital or analog. Sometimes called loss, attenuation is a natural consequence of signal transmission over long distances. The extent of attenuation is usually expressed in units called decibels (dBs).

If P_s is the signal power at the transmitting end (source) of a communications circuit and P_d is the signal power at the receiving end (destination), then P_s > P_d. The power attenuation A_p in decibels is given by the formula:

A_p = 10 log₁₀(P_s/P_d)

Attenuation can also be expressed in terms of voltage. If A_v is the voltage attenuation in decibels, V_s is the source signal voltage, and V_d is the destination signal voltage, then:

A_v = 20 log₁₀(V_s/V_d)

In conventional and fiber optic cables, attenuation is specified in terms of the number of decibels per foot, 1,000 feet, kilometer, or mile. The less the attenuation per unit distance, the more efficient the cable. When it is necessary to transmit signals over long distances via cable, one or more repeaters can be inserted along the length of the cable. The repeaters boost the signal strength to overcome attenuation. This greatly increases the maximum attainable range of communication.

The world of PBX development is one of constant change and innovation. New versions of business telephony switching software and hardware are released continually by major PBX vendors. Because DataVoice Recording Controllers are developed to integrate with specific PBXs, we have to stay abreast of what organisations expect from their telephony solutions, before they even think about recording. And we do. DataVoice have dedicated Recording Controllers for a complete range of PBX vendor platforms.

Recording Controllers (RCs) are PBX-specific software solutions that provide additional recording management functionality in the DataVoice Recording environment. In general, RCs provide:

·         Intelligent Control – agents or devices are known to the system, which provides the ability to record only specific important extensions, according to either extension number and/or agent name. On some PBX systems the RC can provide comprehensive telephonic recording and even telephonic playback control. With an additional module the RC provides fully-fledged statistical and rules-based recording control.

·         Information – the RC can automatically add call-related information such as agent name, calling numbers, custom descriptions and call priorities to the recording database. The RC can also provide powerful and comprehensive follow-the-call and scenario recreation functionality. This even includes information such as call segment start/stop reason codes.

·         Flexibility – agent free-seating and quick configuration changes to recorded devices result in less administration

·         Efficiency – for trunk and extension recording, recording capacity needs to match the number of telephony lines recorded, but when selective recording is an option, the RC can provide integrated recording resulting in vast savings in recording infrastructure

·         Integration – with additional modules the RC provides tightly-integrated screen recording with audio-synchronised recording and playback control. The RC Toolkit can be used to develop and implement integration between business applications and the DataVoice Recording solution (e.g. call tagging, transaction linking, and user desktop call control). The RC SDK in turn can be used for business process integration, where the business process itself triggers recording control decisions according to configured business rules.

·         High Availability – the RC provides maximum recording system availability through 1:1 or N+1 recorder back-up configurations. In addition, dual redundant RCs and recording systems may also be deployed.

·         Health monitoring – when used in conjunction with Nexus Enterprise, the RC can provide recording system health status warnings via SMS and/or e-mail.
Recording options

DataVoice Recorders can connect to telephony sources in a number of ways:

·         Trunk-side recording – recorders are connected in parallel to the E1 or T1 trunks entering the PBX

·         Extension side recording – recorders are connected to the analogue, proprietary digital, or generic digital extensions via the extension connector blocks of the PBX, or to VoIP extensions via standard network connections

·         Integrated recording – a direct connection between the PBX and the recorder provides audio of all monitored calls

Each of these recording implementations has different real-world advantages, and the option most suited to customer requirements depends on a variety of factors. For instance, does an organisation want to record internal calls between some or all of its employees? Does it have many extensions in multiple locations? Does it have more than one PBX? Does it have a multitude of trunks coming in from the telephone network, but only wants to record a 50 seat Contact Centre? The answers to these and a few other important questions determine the solution we would offer

The invention relates to controlling the discharge of used-up air out of an enclosed space, particularly an aircraft body. Redundant control paths must be provided to assure or at least increase the probability of a proper operation even when there are faults in the system.

BACKGROUND INFORMATION

Conventionally, outflow valves for controlling the volume of used air to be discharged out of a passenger aircraft body are controlled by closed loop controllers through unidirectional databuses. Each closed loop controller supplies a control signal or valve adjustment value through a separate databus to the respective outflow valve or valves. These valves are distributed along the length of the aircraft body and provide an air passage from the inside of the aircraft body to the atmosphere. In response to such a control the valves discharge regulated volumes of used air to the atmosphere. A further feedback databus is provided for each valve to provide feedback information regarding the current status of the respective valve to the closed loop controller.

Such a conventional system requires a total of eight databuses for each valve in the system. Four of one-way databuses connect two master controllers to two slave valve controllers and four one-way return databuses connect the slave valve controllers with master controllers, for supplying feedback information to the master controllers. In spite of this number of databuses, the conventional systems are not constructed to increase the reliability of the system. Moreover, each master controller can control only the outflow or air discharge valve to the slave controllers of which it is connected. Intercommunication between any master controller and any one of a plurality of slave valve controllers is conventionally not possible. Another drawback of such conventional systems is seen in that a two-way intercommunication between all components of the system is either not possible at all or at least economically not feasible.

OBJECTS OF THE INVENTION

In view of the above it is the aim of the invention to achieve the following objects singly or in combination:

to construct a used air discharge system for an aircraft in such a way that a variable and two-way information exchange is possible between any component of the system with any other component of the system;

such a system must be capable of responding flexibly to system faults and emergencies;

the system shall be economical, yet sufficiently reliable to meet official aircraft construction standards;

such a system must also be lightweight with due regard to the ever present demand for increasing the payload of passenger aircraft;

the system must be able to respond to currently prevailing operating conditions in the aircraft especially the pressure within the aircraft cab in; and

to make sure that any system component can communicate with any other system components at least to the extent of providing status information to the master controllers, whereby any information transmission shall take place along the shortest possible connection between the respective system components.

SUMMARY OF THE INVENTION

The above objects have been achieved according to the invention by a closed loop control system for controlling an air discharge from an aircraft body having a body wall through which at least one air outflow valve extends. The present closed loop control system in its simplest construction with but one valve is characterized in that the valve is controllable by any one of two slave valve controllers which in turn are controllable by any one of two master controllers. The slave controllers and the master controllers a re interconnected by a signal or information transmission loop for transmitting control information from the master controllers to the slave controllers and for transmitting feedback or other information for example representing the current cabin pressure, from the slave valve controllers and other system components such as air pressure gages, to the master controllers so that each of the master controllers can control any one of the slave controllers and so that return information can reach any one of the master controllers.

Such a system has the advantage that it can be easily extended to virtually any number of valves, whereby the respective signal and information transmission loop is extended by additional conductor sections sufficient to provide the described interconnection between any master controller and any slave controller of the system while simultaneously satisfying the redundancy required by aircraft manufacturing regulations.

Another important advantage of the invention is seen in that the information transmission loop can be provided with an intermeshed conductor system through the simple device of conductor junction boxes or fiber optical couplers depending on whether insulated electrical conductor wires or optical lightwave conductors are used. In both instances virtually any number of parallel and series connected conductor sections may be used to form an intermeshed loop conductor system.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be clearly understood, it will now be described, by way of example, with reference to the accompanying drawings, wherein:

FIG. 1 shows an arrangement with one air outflow valve responsive to two slave valve controllers which in turn are connected through the present transmission loop to two master controllers; and

FIG. 2 shows a block diagram similar to that of FIG. 1, however illustrating an expanded system with N-number of valves each having two slave controllers and two master controllers interconnected by an intermeshed conductor loop.

DETAILED DESCRIPTION OF PREFERRED EXAMPLE EMBODIMENTS AND OF THE BEST MODE OF THE INVENTION

FIG. 1 shows an embodiment with one air discharge valve V which is controllable by two master controllers 1 and 2 controlling in turn two slave valve controllers 1D and 2D. The master controllers 1 and 2 that may be combined to form a unit 4 shown in FIG. 2, are connected to the slave controllers 1D and 2D through a conductor loop 3 and loop entering and exiting conductor sections. More specifically, the master controller 1 which is equipped with a logic signal or information evaluating circuit 1A is connected to the loop 3 by a column conductor section 1B at an interface 3A. The loop 3 in turn is connected to the slave controller 1D by a column conductor section 1C at an interface 3D. Similarly, the master controller 2 also equipped with a logic signal or information evaluating circuit 2A is connected to the loop 3 by a column conductor section 2B at an interface 3B. The loop in turn is connected by a column conductor section 2C to the slave controller 2D at an interface 3C.

The loop is formed by the above mentioned interfaces 3A, 3B, 3C and 3D forming junctions which are interconnected by conductor sections as follows. The interface or junction 3A is connected to the junction or interface 3B by a row conductor section 3F. The interface or junction 3D is connected to the junction or interface 3C by a row connector section 3H. The junction or interface 3B is connected to the junction or interface 3C by a column conductor section 3G. The junction or interface 3A is connected by a column conductor 3E to the junction or interface 3D. Thus, the circular conductor loop 3 is formed.

The interfaces or junctions are, for example, junction boxes where the conductor sections are insulated electrical wires or conductors or these junctions are optical fiber couplers where the conductor sections are lightwave conductors. The arrow heads at each end of the conductor sections indicate that the conductors can transmit signals in either direction.

There are two types of operations possible, namely when the system is free of any fault and when there is one or are more faults in the system. In both instances the system will automatically use the shortest possible information transmission conductor path available under the circumstances or rather under all operating conditions. These “circumstances” are monitored and evaluated by the logic circuits 1A and 2A which provide respective control signals. These “circumstances” may involve, for example, any fault in the system, a current air pressure in the aircraft cabin, other air control parameters, e.g. temperatures and similar considerations that are taken into account in order to make the breathing air as comfortable for the passengers as possible. Each of the master controllers 1 or 2 is intended to be able to control through any one of the slave valve controllers 1D and 2D the respective valve V. The controllers are connected to a power supply input merely symbolically indicated by an arrow PI. These power inputs PI may include a normal power supply and an emergency power supply.

Under a normal operating condition the valve V is operated by the master controller 1 through the following shortest conductor path: master controller 1, logic circuit 1A, conductor section 1B, junction 3A, conductor section 3E, junction 3D, conductor section 1C, slave valve controller 1D.

When there is a fault in the system, for example, the conductor section 3E does not work, the shortest possible conductor path will again be established as follows: 2, 2A, 2B, 3B, 3G, 3C, 2C, 2D. If the controller 2 and the conductor 3E do not work the control can still be accomplished, for example along: 1B, 3A, 3F, 3B, 3G, 3C, 2C.

FIG. 2 illustrates an embodiment in which a plurality of master control units 1, 2, . . . , N-1, and N form control units 4 which are connected to slave valve control units 5 also arranged in pairs so that each valve V1 to VN has two slave valve controllers as described above with reference to FIG. 1.

However, in FIG. 2 the loop is constructed as an interlinked loop 3′ wherein a circular or rather an endless loop includes additional conductor sections arranged in rows RCS and columns CCS with further junctions JP to accommodate a larger number of master control units 4 and a correspondingly larger number of controlled or slave controller units 5. Thus, the loop 3′ has, for example eight major junctions 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8 and a number of secondary junctions 3.9 and 3.10 interconnected in an endless loop by conductor sections 3.11, 3.12, 3.13, 3.14, 3.15, 3.16, 3.17, 3.18, 3.19, and 3.20 As mentioned, additional junctions may be provided at the crossing or junction points JP between the row conductor sections RCS and column conductor sections CCS. The conductor sections 1B, 2B, N-1B and NB, as well as the conductor sections 1C, 2C, N-1C and NC form part of column conductor links and are simultaneously entrance and exit conductor sections for the endless conductor loop 3′.

The logic signal evaluation circuits 1A, 2A, . . . , N-1A, NA assure that under normal operating conditions and under emergency operating conditions always the shortest interconnection is established. Moreover, the logic circuits make sure that any of the master controllers can control any one of the slave valve controllers, whereby again the shortest available communication link has preference over any longer possible communication link. The logic circuits also assure that any available master controller will control any available slave valve controller for safely operating the respective valve under all operating conditions. If one valve is inoperable for whatever reason, any of the other valves can be operated for the intended purpose of discharging used air from an aircraft body.

Although the invention has been described with reference to specific example embodiments, it will be appreciated that it is intended to cover all modifications and equivalents within the scope of the appended claims. It should also be understood that the present disclosure includes all possible combinations of any individual features recited in any of the appended claims.

For the probe to carry out its mission it is essential that it be able to point itself accurately. Numerous phases of the mission, from the trajectory correction manoeuvres to the flyby and data downlink to Earth involve the probe orienting itself to a new attitude and maintaining it to a fraction of a degree. Indeed, if the encounter requires a slew manoeuvre then the probe must accurately be able to control a steady attitude rate. The probe’s attitude control system must also be able to carry out the trajectory correction manoeuvres required to ensure an accurate flyby of the target.

An attitude control system consists of a feedback loop with 3 elements; a sensor (to determine attitude error), a control system to determine the required response to any error and an actuator to implement the response. The control system is provided by the spacecraft computer (see Section 13) so we must consider sensors and actuators.

Sensors. For a spacecraft in deep space the usual attitude sensors are star trackers. These track a star, or for more modern trackers a pattern of stars, and from the position of the image(s) determine the orientation of the spacecraft [Piscane94]. Modern star trackers use sensitive CCD arrays to image many stars simultaneously and can provide very accurate pointing data. The star sensors on Clementine, together with special star-pattern matching software, allowed it to determine its attitude to within 0.03° within 100 ms [Wertz96]. Two such star trackers - one facing along the probe’s axis and one radially outward - would provide the necessary attitude control data, with a combined mass of 0.6 kg. The star trackers would have to cope with the thermal-control roll during cruise flight, but for the roll rate envisaged (circa 0.5 rad/s) this would take the form of a slow image rotation for the axially-pointing tracker and should be acceptable.

Other sensors often used on spacecraft include Sun sensors and rate gyrometers. The use of these would add redundancy to the attitude control system and would also allow easier measurement and control of quantities such as slew rate. For this initial design though it was assumed that star trackers would suffice for general attitude monitoring whist the main imaging payload would be used as an additional sensor during the encounter phase.

Actuators. Two principle types of actuator are available for spacecraft in deep space: reaction wheels and thrusters. A spacecraft using one or more reaction wheels requires the use of thrusters anyway to perform off-loading, but the use of reaction wheels makes fine attitude control easier and, if they are run as momentum wheels (i.e. with a non-zero nominal speed) they provide dynamic rigidity via momentum bias.

For this mission it was decided that in accordance with the policy of avoiding mechanisms wherever possible reaction or momentum wheels would not be used. The mission’s short duration means, though, that fuel supply is not a major issue.

The attitude control requirements of the probe’s mission can be broken down into the following tasks:

• Despin, repoint and spin up to cruise spin after boost motor separation.
• Despin, point for trajectory correction manoeuvre, repoint and spin up, repeated for each TCM.
• Despin and repoint prior to flyby.
• Slew control during the flyby.
• Repoint and spin up with antenna Earth-pointing for data download.

An initial estimate of the mass moment of inertia of the probe, based on a rough mass budget and the approximate structural design, was 20 kgm2. As regards manoeuvres, the following assumptions were made:

* Spin during boost motor firing would be 60 rpm (typical for apogee boost motor firings on communications satellites).
* Cruise spin would be 5 rpm, or approx 0.5 rad/s. This figure is fairly arbitrary but should provide for reasonably even thermal control.
* Repointings for TCMs are at most 2 rad and are carried out in about 100s.
* The mass moment of inertia I used for all repointing calculations would be that around the spin axis, as this will be the largest I for the probe.
* The slew comprises a slew up to the peak slew rate and then down again. For a 20 km/s flyby at a range of 100 km this will be 0.2 rad/s. In fact the peak rate will be less than this as the probe does not track the target all the way to closest approach so this is an over-estimate.

The torque increment delta-T for changing the angular speed of an object of mass moment of inertia I by delta-omega rad/s is

(11-1)

whilst the torque required to repoint the same object by angle theta in time t is

(11-2)

For the manoeuvres described above the torque increments are as in Table 11.1.
Despin from 60 rpm (6 rad/s)     120 Nms
Spin up to or down from 5 rpm (0.5 rad/s)     10 Nms
Slew rate up to and down from 0.2 rad/s     8 Nms
Repoint by 2 rad in 100 s     0.016 Nms

Table 11.1. Torque increment for various attitude control tasks

It was estimated in Section 7 that 5 TCMs would be required. Each TCM requires a spin down from and then up to 5 rpm and a repointing manoeuvre for a combined delta-T of 20.016 Nms. The delta-T budget can thus be calculated as per Table 11.2, giving a total of 248.816 Nms.

The amount of fuel that this will require can be calculated from the impulse requirements to achieve this torque. The radius of the final configuration of the probe was 0.5 m, so a total impulse of 493.632 Ns is required for attitude control tasks. Impulse is simply the product of fuel mass and specific impulse, so the fuel masses required to achieve this for the 2 main liquid propellant thruster types are:

* Monopropellant hydrazine (Isp = 220 s): 2.24 kg
* Hydrazine / N2O4 (Isp = 300 s): 1.65 kg

To this must be added the fuel required for trajectory correction. In Section 7 a total delta-V requirement of 170 m/s was estimated. Eqn 10.2 gives the propellant to dry mass ratio required for a given delta-V; for the maximum probe mass of 140 kg the resulting propellant requirements are:

* Monopropellant hydrazine: 10.90 kg
* Hydrazine / N2O4: 8.08 kg

Adding these together gives a total thruster fuel budget as follows:

* Monopropellant hydrazine: 13.14 kg
* Hydrazine / N2O4: 9.73 kg

Reaction Control System Design. The thrusters, propellant tanks and associated lines and ancillary equipment form the reaction control system (RCS). In order mainly to estimate its contribution to the probe’s mass budget a basic design for the probes RCS was carried out. This covered propellant selection, overall system design and mass budgeting.

Propellant Selection. Both monopropellant and bipropellant systems have their advantages and disadvantages. As was seen earlier, a bipropellant system, because of its higher Isp, requires a smaller propellant mass. However it also requires additional propellant lines and valves whilst the differing masses of hydrazine and N2O4 required make it more awkward to maintain the position of the centre of mass of the RCS. A study by the University of Surrey into propulsion systems for small satellites concluded that the advantages of bipropellant systems outweighed their problems even for small spacecraft [Wertz96]. For this mission study though, where design simplicity was a major concern, the lower performance of a monopropellant hydrazine system was accepted to allow the use of a less complex RCS.

RCS Configuration. For full attitude control the RCS must provide for positive and negative sense thrusters in all three axes, for a total of 12 thrusters. In theory opposed thruster pairs could be used for TCMs but given the large size of these manoeuvres it was decided to use a separate TCM thruster. A number of monopropellant hydrazine thrusters are available with 20 N nominal thrust [Larson92, Jane's]; such a thruster could complete the worst-case initial TCM of 85 m/s in under 10 minutes. The attitude control thrusters would be smaller, e.g. 0.5 N. Two such thrusters mounted on the probe’s circumference, i.e. 0.5 m from the axis of rotation, would take 2 minutes of continuous operation to despin the probe from 60 rpm, or longer if used in pulsed mode for more accuracy.

For the sake of redundancy two separate attitude thruster strings should be provided. It was decided not to provide a redundant TCM thruster as if necessary pairs of attitude control thrusters could be used in this role, although as mentioned above this should only be a fallback measure. A baseline design for the RCS meeting these requirements is shown at Fig 11.1. Note that 2 cross-connected tanks are used so as to allow the position of the centre of mass of the system to be maintained without having to keep a single fuel tank centrally-located.

Figure 11.1. Baseline RCS configuration

This configuration follows the safety guidelines quoted by the University of Surrey study and provides for redundancy for each thruster and string of thrusters.

Mass Budgeting. The mass of the fuel tanks can be calculated from their volume and wall thickness, the latter being driven by the operating pressure. As this is a direct blowdown system the tank pressure will be the operating thruster pressure; this is typically 6-20 bar [Larson92].

The fuel requirement calculated earlier was 13.14 kg of hydrazine. Although this calculation was itself quite conservative it was decided to add a further margin of around 20% and carry 16 kg of fuel, i.e. 8 kg per tank. In order to avoid too much fuel thruster performance variation it was also decided to keep the tank pressure variation to no more than a factor of 3, which required an initial gas pressurant volume equal to the half the fuel volume. Hydrazine has a density of 1.0 kg/m3 so each tank has a volume of 12 litres. For spherical tanks this gives a diameter of 28.5 cm. The mass of 4 litres of nitrogen pressurant at a nominal temperature of 275K can be found from the gas law:

(11-3)

where R is the gas constant for nitrogen, 296.8 J/kgK. This gives a nitrogen mass of 98 g per tank, of 196 g in total.

The tank wall thickness is limited by the stress imposed by the internal pressure. For a spherical tank of radius r and thickness t with internal pressure p the stress sigma is given by

(11-4)

For an aluminium tank designed such that is kept 100% below the tensile strength limit the wall thickness is found to be 0.61 mm, giving a mass of 0.467 kg per tank. Allowing for tank fittings and supports a tank dry mass of 1 kg was thus assumed.

The total mass budget for the RCS is given at Table 11.3. Masses for components such as valves are taken from the University of Surrey study, whilst thruster masses are based on commercially available units [Jane's].
Component

Summary and Design Issues. The attitude control system as presented here uses star trackers and, during the flyby, the payload imaging system as sensors and a monopropellant hydrazine reaction control system as actuator. The RCS is a dual-string system with a separate 20 N thruster for trajectory correction. It provides 170 m/s delta-V and the attitude control requirements for the mission.

This particular attitude control configuration has sufficient propellant budget and control authority to carry out the attitude control tasks required of it. However, a more detailed design study should involve modelling of the dynamics of the system (an area which has been neglected in this analysis) to ensure that such manoeuvres can be carried out with the required degree of accuracy. In particular there are two issues which such further work should consider:

Sensors. As mentioned, the attitude control system specified uses star trackers and the main imaging sensor to provide attitude data. As an important part of the mission profile (the target encounter slew) involves accurate measurement of spacecraft angular rates further study should assess whether specific rate sensors such as gyroscopes should be used. A number of compact solid-state gyroscope systems (e.g. ring laser gyros) have been developed that are potentially suitable for use on small spacecraft [Fortescue95].

Actuators. The encounter slew in particular also requires very fine attitude and angle rate control. There is a risk in using thrusters that successive impulses may result in the probe’s attitude ‘hunting’ around the required slew. Although the RCS contains ample fuel to allow for this such behaviour would be undesirable and could compromise imaging. As such a more detailed study should address the dynamics of fine attitude control. One approach could be to use extremely small thrusters operating on cold gas. These can operate down to thrust levels of circa 10-2 N [Fortescue95] and would have the added advantage of avoiding possible sensor contamination at the critical phase of the mission. Given the short duration of the encounter slew the required gas supply could be provided from the main RCS pressurant. An alternative approach would be to evaluate fully the use of a reaction wheel. Although not considered in this study because of the desire to avoid mechanisms where possible such a system may prove attractive on further analysis for providing fine slew control.