 |
|
Radiation of electrical energy
Every electrical circuit carrying alternating current radiates a certain amount of electrical energy in the form of electromagnetic waves, but the amount of energy thus radiated is extremely small unless all the dimensions of the circuit approach the order of magnitude of a wave length. Thus, a power line carrying 60-cycle current with a 20-ft spacing between conductors will radiate practically no energy because a wave length at 60 cycles is more than 3000 miles, and 20ft. is negligible in comparison.
On the other hand, a coil 20ft. in diameter and carrying a 2000-kc current will radiate a considerable amount of energy because 20ft. is comparable with the 150-meter wave length of the radio wave.
From these considerations it is apparent that the size of radiator required is inversely proportional to the frequency.
High-frequency waves can therefore be produced by a small radiator, while low-frequency waves require a large antenna system for effective radiation.
Every radiator has directional characteristics as a result of which it sends out stronger waves in certain directions than in others.
Directional characteristics of antennas are used to concentrate the radiation toward the point to which it is desired to transmit, or to favor reception of energy arriving from a particular direction.
General and control of radio –frequency power
The radio-frequency power required by a radio transmitter is practically always obtained from a vacuum-tube oscillator or amplifier.
Vacuum tubes can convert direct-current power into alternating-current energy, for all frequencies from the very lowest up to 30000mc, or even greater.
Under most conditions the efficiency with which this transformation takes place is in the neighborhood of 50 per cent or higher. At frequencies up to well over 1,000 mc, the amount of power that can be generated con tenuously by vacuum tubes is of the order kilowatts.
Modulation
The transmission of information by radio waves requires that some means be employed to control the radio waves by the desired intelligence. One way to do this, termed amplitude modulation is to vary the amplitude of the radiated wave in accordance with the intelligence to be transmitted. In radio telegraphy, this involves turning the radio transmitter on and off in accordance with the dots and dashes of the telegraph code. In radio-telephone transmission by amplitude modulation the radio-frequency wave is varied in accordance with the pressure of the sound wave being transmitted.
Similarly in picture transmission, the amplitude of the wave radiated at any one time is made proportional to the light intensity of the part, of the picture that is being transmitted at that instant.
Intelligence may be transmitted by other means than by varying the amplitude.
For example, one may maintain the amplitude constant and vary the frequency that is radiated in accordance with the intelligence, thus obtaining frequency modulation. Frequency modulation has many advantages and is widely used in very high-frequency communication systems.
Geodesy
Geodesy, also called geodetics, is the scientific discipline that deals with the measurement and representation of the earth, its gravitational field and geodynamic (polar motion earth tides, and crustal motion) in three-dimensional time varying space. The shape of the earth is to a large extent the result of its gravity field. This applies to the solid surface (orogeny: few mountains are higher than 10 km, few deep sea trenches deeper than that). It affects similarly the liquid surface (dynamic sea surface topography) and the earth’s atmosphere. For this reason, the study of the Earth’s gravity field is seen as a part of geodesy, called physical geodesy.
In geometric geodesy we formulate two standard problems: the geodetic principal problem and the geodetic inverse problem. In the case of plane geometry (valid for small areas on the Earth’s surface) the solutions to both problems reduce to simple trigonometry. On the sphere, the solution is significantly more complex, e.g., in the inverse problem the azimuths will differ between the two end points of the connecting great circle are. On the ellipsoid of revolution, closed solutions do not exist; series expansions have been traditionally used that converge rapidly. Alternatively, the differential equations for the surveying can be solved numerically, e.g., in MatLab (TM).
A geodetic network is a network of triangles which are measured exactly by techniques of terrestrial surveying or by satellite geodesy. In “classical geodesy” (up to the sixties) this is done by triangulation, based on measurements of angels and of some spare distances; the precise orientation to geographic North is done by methods of geodetic astronomy. The mainly used instruments are theodolites and tacheometers to be equipped by infrared range finders, data bases, communication systems and partly by satellite links. Beginning with ca 1960, the electronic distance measurement (EDM) was introduced, when the first prototypes became small enough to work in the field. EDM increased the network accuracies up to 1:1 million (1 cm per 10 km; today at least 10 times better). And also the economy of surveying. At the same time the geodetic use of satellites began, e.g., the bright satellites of Echo I and II and Pageos. By means of these space probes, global networks were determined, which later proved the theory of plate tectonics.
And important improvement was the introduction of radio and electronic satellites like Geos A and B (1965-70), of the Transit system (Doppler effect) 1967-1990 – which was the predecessor of the GPS – and of laser techniques, small networks for cadastre and technical projects are mainly measured terrestrially, but in many cases closed together to national and global networks by satellite geodesy.
In meantime, several hundred geodetic satellites are orbiting, supplemented by a huge number of remote sensing satellites – and last but not least by the navigation systems of GPS and Glonass, which will be followed by the European Galileo satellites in 2008. Nowadays the space based geodetic networks are more flexible and economic than terrestrial ones; the further existence of fixed point networks is already discussed, but will survive at least for administrational and legal demands on local and regional scales. Whereas the worldwide networks can not be defined to be fixed, because Geodynamics is chancing the position of all continents by amounts of 2 cm up to 20 cm per year. Therefore modern global networks like ETRF or ITRF (International Terrestrial Reference Frame) show not only coordinates of their “fixed points”, but also their annual velocities.
Map Design Process
The map design process begins with consideration of the purpose of the map and collection of relevant data. Often secondary sources such as census data can be used, but data from airphoto interpretation, analysis of satellite imagery, field surveys, or questionnaires may also provide the basis for map. However, data collection is only the first step. The map design process includes making decisions about selection of geographic features and attributes to be represented on the map, selection of an appropriate map projection given the purpose of the map and the geographic region of interest, choice of an appropriate scale taking into account the needs of the map user, generalization of features and attributes represented on the map, selection of appropriate symbolization, and design of overall layout of the map.
The map design process is especially important because maps are a synoptic form of communication. The map reader sees the entire graphic image at once. Unlike verbal communication in which the speaker or writer can control the sequence in which information is transmitted and received, the map maker has little control over how the map user will view and interpret the map but nevertheless has a responsibility to ensure that map is designed in such way as to ensure that the map user understands the map’s intended message. Good design that visually emphasizes the most important information is only means that the Cartographer has to accomplish this.
Who makes the design decisions depend greatly on the type of map (s) being produced. In case of national or provincial topographic maps or hydrographic charts, cartographic conventions are well established and difficult to change. Design decisions are made by committees who have the responsibility to ensure that any changes in feature representation or symbolization are consistent with required accuracy standards and acceptable to the user community. Introducing revised symbolization becomes a tedious process, especially in the case of hydrographic and aeronautical charts used for navigation. In these instances, modification of cartographic conventions must take into account the safety and ultimately legal ramifications of any proposed changes. At the opposite extreme, maps created by an individual researcher allow for more creative approaches. Several goals of map design can be identified. These include: Clarity, Order, Balance, Visual Contract, Unity and harmony, Visual Hierarchy. A well –designed map should achieve all of these goals.
Balance. Balance refers to the overall layout of the map elements. While the subject area or body of the map is the main focus of interest and should occupy as large a space on the page as possible, a map has several other components that can be manipulated to achieve a balanced design. These elements include the title, legend, scale, north arrow or graticule to indicated orientation, insert maps and border. The objective is to keep the map reader’s attention focused on the map and allow the eye to wander off the page.
Balance is achieved by positioning map elements relative to the visual center of gravity of the map which tends to be slightly above the actual center of the map. In positioning elements on the page, it must be recognized that darker, stronger colors or shading patterns and larger objects have greater visual weight. In attempting to balance these components of the map we want to keep the map body as large as possible while avoiding crowding of other map elements around the edges of the map. There are no hard and fast rules for achieving a balanced design. Usually, it is necessary to experiment with alternative map layouts in the early stages of the design process until an acceptable layout is obtained.