113.1 Define each band of the electromagnetic spectrum and describe what signals are commonly used in each. [ref. a, ch 1] <ul><li>Table 1-1.—Radio-Frequency Spectrum </li></ul><ul><li>FREQUENCY DESCRIPTION </li></ul><ul><li>30 GHZ - 300 GHZ extremely high frequency </li></ul><ul><li>3 GHZ - 30 GHZ superhigh frequency </li></ul><ul><li>300 MHZ - 3 GHZ ultrahigh frequency </li></ul><ul><li>30 MHZ - 300 MHZ very high frequency </li></ul><ul><li>3 MHZ - 30 MHZ high frequency </li></ul><ul><li>300 KHZ - 3 MHZ medium frequency </li></ul><ul><li>30 KHZ - 300 KHZ low frequency </li></ul><ul><li>3 KHZ - 30 KHZ very low frequency </li></ul><ul><li>300 HZ - 3 KHZ voice frequency </li></ul><ul><li>Up to 300 HZ extremely low frequency </li></ul>
113.2 Describe the communication capabilities of transmissions in each of the bands below: [ref. a, ch 1] <ul><li>ELF - One-way sub comms. ELF is capable of penetrating ocean depths of several hundred feet with minimal signal loss. </li></ul><ul><li>SLF – 30HZ-300HZ. Provide comm services for subs at a certain depth. </li></ul><ul><li>ULF - Range of electromagnetic waves between 300 HZ - 3 KHZ. Used for comms in mines, as it can penetrate the earth . </li></ul><ul><li>VLF - North Atlantic and Arctic Ocean have atmospheric disturbances that make High-Freq circuits unreliable. “VLF transmissions provide a highly reliable path for communications in these northern latitudes as well as over and under all oceans and seas of the world.” </li></ul><ul><li>LF - Has been used for comms since the advent of radio. Large physical size, high construction and maintenance cost, and seriously hampered by atmospheric noise (at low geographical latitudes). But Low freq waves are not so seriously affected during periods of ionospheric disturbance when comms at high freqs are. </li></ul><ul><li>“ Today LF comms is used to provide eight channels of frequency-division multiplex rtty traffic on each transmission of the fleet multichannel broadcast system.” </li></ul>
113.2 Describe the communication capabilities of transmissions in each of the bands below: [ref. a, ch 1] <ul><li>MF - Includes the international distress frequencies (500 kilohertz and approximately 484 kilohertz). Some ships have MF equipments; distress frequencies may be monitored. Ashore MF transmitter and receiver configuration is usually affiliated with search and rescue organizations. Commercial broadcast band (AM) extending from 535 to 1605 kilohertz. </li></ul><ul><li>HF – Around since WWI. Successful transmission of HF signals over a long distance is dependent upon refraction of radio waves by layers of the ionosphere. </li></ul><ul><li>VHF and Above – Normally only used for LOS (Line of site) due to the fact that this freq will penetrate the atmosphere. Also used for satellite communications. </li></ul><ul><li>VHF – Ideal for ship to shore. ex.. Beach landings of amphibious craft. </li></ul><ul><li>UHF LOS – Tactical Voice. ex.. Moving ships into formation. </li></ul><ul><li>UHF SAT – Tactical Voice/Data </li></ul><ul><li>SHF – Radar and Satellite communications. </li></ul><ul><li>EHF – Satellite communications. </li></ul>
113.3 Define and discuss the following terms: <ul><li>Frequency – Cyclical processes, such as rotation, oscillations, or waves, frequency is defined as a number of cycles, or periods, per unit time. The unit of frequency is hertz (Hz), 1 Hz means that an event repeats once per second. </li></ul><ul><li>Wavelength - is the distance in space occupied by one cycle of a radio wave at any given instant. </li></ul><ul><li>Ducting - Trapping of an RF wave between two layers of the earth’s atmosphere or between an atmospheric layer and the earth. </li></ul><ul><li>Refraction - is the change in direction of a wave due to a change in its speed. This is most commonly observed when a wave passes from one medium to another at an angle </li></ul><ul><li>Multiplexing - is a process where multiple analog message signals or digital data streams are combined into one signal over a shared medium </li></ul><ul><li>Modulation - The ability to impress intelligence upon a transmission medium, such as radio waves. </li></ul>
113.3 Define and discuss the following terms: <ul><li>Demodulation - The removal of intelligence from a transmission medium. </li></ul><ul><li>Bandwidth - The difference between the highest usable frequency of a device (upper frequency limit) and the lowest usable frequency of the device (lower frequency limit)—measured at the half power points. </li></ul><ul><li>Keying - Inserting noise or data onto a frequency </li></ul>
113.4 Discuss the following terms and give a description of what each means in relation to the atmosphere: [ref. b, f] <ul><li>Layers [ref. b, ch 2] The ionosphere is composed of three layers designated D, E, and F, from lowest level to highest level. </li></ul><ul><li>Frequency [ref. b, ch 2] The higher the frequency of a radio wave, the lower the rate of refraction by an ionized layer. </li></ul><ul><li>Radio Waves [ref. b, ch 2] There are two principal ways in which electromagnetic (radio) energy travels from a transmitting antenna to a receiving antenna. One way is by GROUND WAVES and the other is by SKY WAVES. </li></ul><ul><li>Propagation [ref. b, ch 2] Within the atmosphere, radio waves can be reflected, refracted, and diffracted like light and heat waves. </li></ul><ul><li>Azimuth Angle [ref. f, ch 1] The angle measured clockwise from true north in the horizontal plane is called the TRUE BEARING or AZIMUTH angle. </li></ul><ul><li>Elevation Angle [ref. f, ch 1] The angle between the horizontal plane and the Line of Sight is the ELEVATION ANGLE. </li></ul>
113.5 Describe the following modulation types and their common usage: [ref. c] <ul><li>USB/LSB - When a carrier is modulated by voice or music signals, a large number of sum and difference frequencies are produced. All of the sum frequencies above the carrier are spoken of collectively as the UPPER SIDEBAND. All the difference frequencies below the carrier, also considered as a group, are called the LOWER SIDEBAND. </li></ul><ul><li>CONTINUOUS WAVE (CW) The "on-off" KEYING of a continuous wave (cw) carrier frequency was the principal method of modulating a carrier in the early days of electrical communications. The intervals of time when a carrier either was present or absent conveyed the desired intelligence. This is still used in modern communications. When applied to a continuously oscillating radio-frequency source, on-off keying is referred to as cw signaling. This type of communication is sometimes referred to as an interrupted continuous wave (icw). Morse Code. </li></ul>
113.5 Describe the following modulation types and their common usage: [ref. c] <ul><li>Amplitude Modulation (AM) is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent. For example, changes in the signal strength can be used to reflect the sounds to be reproduced by a speaker, or to specify the light intensity of television pixels. </li></ul><ul><li>Frequency Modulation (FM) conveys information over a carrier wave by varying its frequency. In analog applications, the instantaneous frequency of the carrier is directly proportional to the instantaneous value of the input signal. </li></ul>
113.6 Define the following terms as they apply to RADAR: [ref. f] <ul><li>PRI - Pulse Repetition Interval, The reciprocal of PRF is called the Pulse Repetition Time (PRT), Pulse Repetition Interval (PRI), or Inter-Pulse Period (IPP), which is the elapsed time from the beginning of one pulse to the beginning of the next pulse. </li></ul><ul><li>PRF – Pulse Repetition Freq. is the number of pulses per time unit (e.g. Seconds). </li></ul><ul><li>RF – Radio Frequency. Freq at which radar operates </li></ul><ul><li>BW – Bandwidth is the difference between the upper and lower cut-off frequencies of a radar receiver, and is typically measured in hertz. </li></ul><ul><li>CW – Continuous-wave radar; non-pulsing. Sending out a very pure signal of a known freq. Determines velocity, can’t determine range. </li></ul>
113.6 Define the following terms as they apply to RADAR: [ref. f] <ul><li>PW – Pulse Width. PW has units of time and is commonly expressed in ms. PW is the duration of the pulse. </li></ul><ul><li>Scan Rate - When a radar is scanning, it is necessary to control the scan rate so that a sufficient number of pulses will be transmitted in any particular direction in order to guarantee reliable detection. If too few pulses are used, then it will more difficult to distinguish false targets from actual ones. False targets may be present in one or two pulses but certainly not in ten or twenty in a row. Therefore to maintain a low false detection rate, the number of pulses transmitted in each direction should be kept high, usually above ten. </li></ul><ul><li>Bearing/Azimuth/Elevation: </li></ul><ul><li>Bearing - The bearing to a target may be referenced to true north or to your own ship. </li></ul><ul><li>Azimuth - Angular measurement in the horizontal plane in a clockwise direction. </li></ul><ul><li>Elevation - is the angle between the horizontal plane and the line of sight, measured in the vertical plane. </li></ul>
113.7 Identify the following types of RADAR, their functions, and give examples of each: [ref. f] <ul><li>Air Search - Air-search radar systems initially detect and determine the position, course, and speed of air targets in a relatively large area. </li></ul><ul><li>Surface Search - A surface-search radar system has two primary functions: (1) the detection and determination of accurate ranges and bearings of surface objects and low-flying aircraft and (2) the maintenance of a 360- degree search pattern for all objects within line-of-sight distance from the radar antenna. </li></ul><ul><li>Fire Control - is a radar which is designed specifically to provide information (mainly target azimuth, elevation, range and velocity) to a fire-control system in order to calculate a firing solution (i.e. information on how to direct weapons such that they hit the target (s)). </li></ul>
113.8 Explain the difference between OPELINT and TECHELINT. [ref. e] <ul><li>OPELINT - is concerned with operationally relevant information such as the location, movement, employment, tactics, and activity of foreign non-communications emitters and their associated weapon systems. </li></ul><ul><li>TECHELINT - is concerned with the technical aspects of foreign non-communications emitters such as signal characteristics, modes, functions, associations, capabilities, limitations, vulnerabilities, and technology levels. </li></ul>
113.9 Define the following: [ref. a] <ul><li>Half Duplex- one-way flow of information between terminals. Technical arrangements often permit transmission in either direction, but not simultaneously. This term must be qualified to show s/o (send only), r/o (receive only), or s/r (send or receive). </li></ul><ul><li>Full Duplex- The full-duplex (fdx) mode is a method of operation in which telecommunications between stations takes place simultaneously in both directions using two separate frequencies. In other words, a ship may send and receive different messages at the same time. The term "full duplex" is synonymous with "duplex.“ </li></ul><ul><li>Simplex- mode uses a single channel or frequency to exchange information between two or more terminals. Communications is in one direction only. </li></ul>
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