Signals are attenuated to a much greater degree with short waves than with medium waves, i.e., there is a random change in field strength at the reception point due to interference from radio beams. There are also areas of the earth's surface or water area where signals from a transmitter cannot be received, either on surface or space waves (blind spots or zones of silence). During ionospheric disturbances and magnetic storms, considerable short-wave energy is absorbed, hampering radio communication and sometimes even making it impossible. Ionospheric disturbances are unusually frequent in polar zones. The short-wave range is overloaded with radio stations operating simultaneously. Therefore, mutual interference (especially at night) is frequently the prime factor in determining the operating range of short-wave stations. Short-wave radio communication with a space wave depends in large measure on the time of day and year. Therefore, in designating frequencies for short-wave communication, account must be taken of special radio forecasts and to select these frequencies in accordance with daily and seasonal propagation of waves on specific communication lines. Because of their large propagation range, radio communication on short waves is easier to intercept and jam than communication in other ranges. Ultra-short radio waves are propagated along the surface of the earth within the visible horizon, as well as through scattered reflection from irregularities in the troposphere and ionosphere beyond the limits of direct visibility. The direct radio communication range with ultra-short waves can be approximately determined using a special table (see Section 85) or the equation D = 3.55(√h2 + √h2 [km], (179) where h1 and h2 represent the heights to which the transmitting and receiving antennas are raised, expressed in meters. The range and reliability of ultra-short wave radio communication depends largely on the type of antenna, the scanning point of the radio station, the terrain and conductivity of the medium along which the radio waves are being propagated. Radio communication on ultra-short waves is stable at various times of the day and year and is subjected to less interference than communication on short waves. Radio communication on ultra-short tropospheric and ionospheric radio stations has a range of up to 20-25 miles when a surface wave is used, and up to several hundred miles when there is tropospheric scattering of radio waves and reflection from ionized traces of meteors. The coverage of a surface wave overlaps the coverage of a wave reflected from a nonhomogeneous troposphere, and consequently blind spots are not observed. 4. Use of Radio Communication in Special Cases Under International Rules In maritime radio communication there are signals of particular importance, such as alarm, distress, urgent and safety signals. These are international signals. An alarm signal is used to alert all radio stations to an impending signal transmission and to notify them of distress. A distress signal is sent only by order of the Commanding Officer when ship and crew are threatened with unavoidable disaster and he requests assistance.* A distress signal must indicate the name of the ship in distress, its coordinates, nature of the catastrophe, nature of the assistance required, and other information which might facilitate the rendering of assistance. In receiving signals from a ship in distress, the Commanding Officer makes the decision to render assistance according to the situation. Having received a distress signal from a Soviet merchant ship, the captain must transmit to a Soviet shore station the message received and, if possible, take a bearing on the ship in distress. A ship proceeding to render assistance must inform the ship in distress of its name, position and speed. If the ship sending the alarm or distress signal no longer requires assistance, it must so state. An urgent signal is transmitted whenever a ship is not threatened with immediate catastrophe, but some accident has occurred aboard the ship and cannot be repaired by the crew using its own resources. An urgent signal can be transmitted whenever there is an urgent message affecting the security of another ship, aircraft, harbor craft or personnel visible from the ship. Examples of this are an accident aboard a ship transmitting a message, an accident aboard a ship or aircraft visible from the vessel, serious illness of a crew member and the need to obtain medical advice, man overboard and the impossibility of rendering assistance to him. A safety signal indicates that the given radio station will transmit a message concerning the safety of navigation. The master of each merchant vessel encountering dangerous ice, dangerous floating objects or any other immediate danger to navigation, or a tropical storm, or who is under a gale wind at an atmospheric temperature below zero degrees, resulting in icing up of the superstructures, or under winds of 10 or more, concerning which he did not receive a storm warning, must convey this information to all nearby vessels with all the resources at his command, as well as to the shore radio station with which he may be in communication. 5. Procedure for Transmitting Signals of Special Importance A radiotelegraphic alarm signal consists of 12 4-second dashes at 1-sec intervals. The alarm signal is transmitted using automatic alarm signal transmitters or telegraph key controlled by a stop watch. A radio telephone alarm signal consists of two sinusoidal tones with frequencies of 2200 and 1300 cycles, transmitted alternately for one minute. The length of each signal is 250 microseconds. *Warships use international distress signals only when operating alone in peacetime, when assistance cannot be rendered by one's own warships and merchant vessels. A radiotelegraph distress signal consists of an SOS group transmitted in the form of a single sign. As a rule, a distress signal is transmitted immediately after an alarm signal, so that if the situation permits, the distress signal is separated from the alarm signal by two minutes of silence. A radiotelegraph distress signal consists of an SOS distress signal repeated three times, the letters DE and the call sign of the radio station of the ship or name of the ship in distress repeated three times. A radiotelephone distress signal consists of the word MAYDAY, the word "calling" and, in an international exchange, “ici" and the call sign or name of the ship in distress repeated three times. The distress signal is transmitted for general attention and is not addressed to any specific radio station. A radiotelegraph urgent signal consists of three repetitions of the letters XXX, transmitted with sufficient time intervals between letters and groups. A radiotelephone urgent signal consists of three repetitions of the word PAN. An urgent signal is addressed to a specific radio station and is transmitted before the call-up. A radiotelegraph safety signal consists of three repetitions of the letters TTT, transmitted with sufficient time intervals between letters and groups. A radiotelephone safety signal consists of three repetitions of the word SECURITE. A safety signal is not addressed to any particular radio station, but is applicable to all shipboard radio stations and is transmitted before the call-up. 6. International Radio Frequencies for Transmission A frequency of 500 kc-the international frequency for radiotelegraphic call-up and transmission of distress signals. It may be used to transmit: 1) distress and alarm signals and an exchange of radiotelegrams concerning distress; 2) urgent and safety signals and reports; 3) call-ups, answers to call-ups for shipboard radio stations) and for signals warning of a shift to working frequencies before an exchange is begun. In order to assure greater safety of life at sea, and above the sea, all shipboard radio stations, as well as shore stations guarding medium waves must maintain acoustic observation at the call-up and distress frequency (500 kc) from the 15th to the 18th and from the 45th to the 48th minute of each hour of their watch (periods of radio silence), discontinuing all transmission, with the exception of exchanges related to distress, urgent and safety signals. All transmissions in the 490-510 kc frequency range are prohibited during these time intervals. Shipboard radio stations exchanging messages in the shortwave range during this period must have their receiver turned on at 500 kc or have their automatic alarm signal receiver turned on. A frequency of 2182 kc is the international radiotelephone distress signal call-up and transmission frequency. It is used for call-up, answer to call-up, notification of the transition to working frequency, an exchange in event of distress, and for urgent and safety signals and messages. All shipboard radiotelephone stations operating in the intermediate frequency range must maintain acoustic observation at a frequency of 2182 kc from zero minutes to the 3rd and from the 30th to the 33rd minute of each hour of their watch. All transmissions, with the exception of distress, urgent and safety signals, in the 2170-2194 kc frequency range are prohibited during these time intervals. A frequency of 156.8 mc is the international frequency for call-up, answer and, if necessary, for transmission of distress messages. All other transmissions in the 156.725-156.875 mc frequency range are prohibited. A frequency of 8364 kc has been designated for use by rescue vessels and other floating or airborne rescue equipment in order to establish communication with mobile marine stations involved in search and rescue operations. 7. Special Radio Transmissions Meteorological radio transmissions consist of synoptic forecasts (of storms and cyclones), ice warnings and observations, weather observations and weather messages. Warnings concerning cyclones and other extreme hazards are transmitted from ship to ship and to the nearest shore radio stations, preceded by a safety signal at distress frequencies (500 and 2182 kc), or one of the frequencies which may be used in case of distress. The Commanding Officer of a ship should indicate to a shipboard radio station which weather reports should be obtained in the area of navigation. Warnings, weather forecasts and weather messages are usually transmitted by radio stations on a fixed schedule at specific frequencies indicated in the international list of radio stations, two-way radio nets and special services, and designated for general use or only for the use of special services of the navy, air force, etc. They are warned by a CQ call. Radio stations of the Ministry of the Merchant Marine of the USSR transmit weather messages in accordance with instructions governing the procedure for transmission of weather reports by radio stations of the Ministry of the Merchant Marine. Transmission of warnings and notices to mariners. Warnings concerning floating objects, icebergs, reefs, changes in the navigational situation, etc. (navigation notices) are transmitted to ships in order to assure safety of navigation. Navigation notices are either routine or special. Routine notices are transmitted in accordance with instructions governing the procedure for transmission of navigation notices to mariners by radio stations of the Ministry of the Merchant Marine, according to a schedule attached to the instructions. Special notices are transmitted by radio stations at a frequency of 500 kc, preceded by transmission of a safety signal. General information is transmitted by shore radio stations on an established schedule and on assigned frequencies indicated in the international list of radio stations, two-way radio nets and special services. A shipboard radio station should receive navigation notices, with the Commanding Officer of the ship issuing instructions governing receipt of routine notices from certain shore radio stations. Time signals are transmitted by radio stations in order to correct ship chronometers and to check clocks. These signals are transmitted on frequencies indicated in the international list of radio stations, two-way radio nets and special services. Shipboard and shore radio stations must receive time signal transmissions at least twice a day in order to check their clocks. Radio stations which, for a variety of reasons, are not able to receive time signal transmissions from radio stations indicated in the international list of radio stations, two-way radio nets and special services may pick up time signals transmitted by radio broadcasting stations. SECTION 51. VISUAL COMMUNICATION, SIGNALING AND 1. General Aspects Ships of the Soviet Navy are fitted with first-class communications equipment, and yet the presence of modern radio communications equipment has not decreased the need, role and significance of visual communication. Joint operations by a large number of warships and merchant vessels of various types, as well as the increasing demand for communication security, has produced a need for visual communication together with electronic equipment. Visual communication and signaling are achieved using objects (signal flags, signal figures, flag semaphores), lights (signal lights, searchlights and yardarm blinkers), and pyrotechnic (rockets and signal flares) means of communication. As a rule, a ship answers a call-up with the means of visual communication with which it was called. The texts of orders, reports and announcements selected from codes for transmission using visual communication means are called signals, whereas those transmitted using an open text are called semaphores. Semaphores (signals) are classified as outgoing, incoming, and intermediate, depending upon passage. A semaphore (signal) transmitted from a ship (station) is called an outgoing semaphore, whereas one received by a ship (station) is called an incoming semaphore and one received by a ship (station) for transmission to another ship (station) is called an intermediate semaphore. Each semaphore is recorded on prescribed forms and signed by the sender. After the signature a four-digit number is sent indicating the time (hour and minutes) the semaphore was signed. All semaphores (signals) to be transmitted visually are received from the senders aboard ship both under way and at anchor to the officer of the watch, and to the Officer of the Deck when the ship is secured to a pier. Depending upon the address, signals are either: 1) general, i.e., addressed to all ships of a formation or all ships and lookout and communication stations in sight of a hoisted (transmitted) signal; |