Satellite Navigation

External communication equipmentSatellite Communication In 1962, the American telecommunications giant AT&T launched the world's first true communications satellite, called Telstar. Since then, countless communications satellites have been placed into earth orbit, and the technology being applied to them is forever growing in sophistication. Communication satellites are orbiting relays which receive, amplify, process, and retransmit signals from one point to another point on the surface of the Earth. Communication satellites have been in use since the 1960s for providing wideband global connectivity and long-range circuits of high quality virtually unencumbered by propagation difficulties. A satellite provides coverage within its footprint that is the area from which it is visible. In the maritime community, satellite communication systems such as Inmarsat provide good communication links to ships at sea. These links use a VSAT type device to connect to geosynchronous satellites, which in turn link the ship to a land based point of presence to the respective nations telecommunications system. With the overcrowding of the hf spectrum, the need for new and advanced long-range communications became apparent. Satellite communications (SATCOM) systems have shown they can provide survivable, reliable, high-capacity, secure, and cost-effective telecommunications for the military.

Diagram- Satellite communications system.Functional block diagram.

Principle of operation. The communications systems use satellites that are either geostationary or orbiting. With a geostationary system the satellite remains in a fixed position relative to a given geographical location (the satellite is actually in a fixed orbit and moves in a consistent relationship to the Earth). With this type of system the satellite can, at all times, receive and transmit messages to any transmitter or transceiver that is within the fixed geographical area visible to the satellite. A communications system based on geostationary satellites may have more than one satellite to cover a greater percentage of the Earths surface.

An orbiting communications satellite moves in an orbit so that it passes above a given geographical location at periodic time intervals. Such a system means that earth bound transmitters or transceivers come into the satellites range at these periodic time intervals and transmit or receive only while the satellite is in range or visible. The transmitter may store messages until the satellite is in range. When messages are transmitted to the satellite, they may also be stored in the satellite until the satellite comes into range of a receiving earth station. Unlike a geostationary system, a single satellite can feasibly cover the whole of the Earths surface. However, there will be time gaps in coverage when the satellite is not in view of given geographical locations. Increasing the number of satellites will increase the coverage of the system by decreasing the time gaps when a satellite is not in view of a given location.

In both types of system a fixed or mobile transmitter can be used. Such a transmitter is mounted on a vessel, aircraft, building etc. and uses a radio signal to send a message to the satellite mounted transponder. The message can be stored in the satellite for later forwarding or immediately forwarded to a receiver or transmitter with a receiving capability (transceiver) mounted on another vessel, aircraft, building etc. In some cases the receiving station will be a large fixed station (an earth station) which will link to the normal terrestrial telephone system.

Purpose of use/Applications.One important aspect of the satellite communications network is that it continues in operation under conditions that sometimes render other methods of communications inoperable. Because of this, satellites make a significant contribution to improved reliability of Navy communications. Satellite communications have unique advantages over conventional long distance transmissions. Satellite links are unaffected by the propagation variations that interfere with hf radio. They are also free from the high attenuation of wire or cable facilities and are capable of spanning long distances. The numerous repeater stations required for line-of-sight are no longer needed. They furnish the reliability and flexibility of service that is needed to support a military operation.TelephonyThe first and still, arguably, most important application for communication satellites is in international telephony. Mobile telephones (to and from ships) must be directly connected to equipment to uplink the signal to the satellite, as well as being able to ensure satellite pointing in the presence of disturbances, such as waves onboard a ship.Television and RadioThere are two types of satellites used for television and radio:Direct Broadcast Satellite (DBS):DBS is a term used to refer to satellite television broadcasts intended for home reception, also referred to as direct-to-home signals. It covers both analogue and digital television and radio reception, and is often extended to other services provided by modern digital television systems, including video-on-demand and interactive features.Fixed Service Satellite (FSS):FSS is the official classification for geostationary communications satellites used chiefly for broadcast feeds for television and radio stations and networks, as well as for telephony, data communications, and also for Direct-To-Home (DTH) cable and satellite TV channels.Mobile Satellite Technology

.

Initially available for broadcast to stationary TV receivers, popular mobile direct broadcast applications made their appearance with that arrival of two satellite radio systems: Sirius and XM Satellite Radio Holdings. Some manufacturers have also introduced special antennas for mobile reception of DBS television. Using GPS technology as a reference, these antennas automatically re-aim to the satellite no matter where or how the vehicle (that the antenna is mounted on) is situated.

Amateur radio

.

Amateur operators have access to the OSCAR satellites that have been designed specifically to carry amateur radio traffic. Most such satellites operate as space borne repeaters, and are generally accessed by amateurs equipped with UHF or VHF radio equipment and highly directional antennas such as Yagis or dish antennas. Due to the limitations of ground-based amateur equipment, most amateur satellites are launched into fairly low Earth orbits, and are designed to deal with only a limited number of brief contacts at any given time.

Satellite Broadband

.

In recent years, satellite communication technology has been used as a means to connect to the internet via broadband data connections. This is very useful for users to test who are located in very remote areas, and can't access a wireline broadband or dialup connection.Role of satellite communicationsSATCOM links, one of several kinds of long distance communications links, interconnect communications centres located strategically throughout the world. These SATCOM links are part of the Defence Satellite Communications System (DSCS) and Fleet Satellite Communications. Satellite communications systems are very important to the worldwide military communications network for two primary reasons. First, they continue to operate under conditions that cause problems for other methods of communication. Second, they provide reliable and secure communications to previously inaccessible areas. In many cases, these communications requirements can only be satisfied by sophisticated satellite communications systems. By satisfying such needs, SATCOM makes a significant contribution to the improved reliability of naval communications.

Advantages of satellite communicationsSome of the unique advantages SATCOM has over conventional long-distance communications are as follows:SATCOM links are unaffected by the propagation problems associated with hf radio communications.SATCOM links are free from the high attenuation problems of facilities that use wire or cable for routing communications.SATCOM links span long distances.The numerous repeater stations required for line-of-sight and troposcatter systems are not needed.Satellite links provide the required flexibility and reliability needed to support military operationsCapacityCurrently, military SATCOM systems can provide communications between backpack, shore, airborne, and shipboard terminals. These SATCOM systems can handle thousands of communications channels at the same time.ReliabilitySATCOM frequencies are only slightly affected by atmospheric phenomena and do not depend on reflection or refraction. Reliability is based on the skill of operators and maintenance personnel and the condition of the satellite communications equipment.VulnerabilityCommunications satellites are relatively safe from threats of harm. Because these satellites are in such high orbits, any attempt to disable or destroy them from the Earth would be difficult and expensive. However, Earth terminals are a different story. They offer a more attractive target for destruction by conventional methods. But these terminals can be protected by the same methods taken to protect other vital installations. So overall, the satellite system is nearly free from harm by an enemy. Operationally, highly directional earth terminal antennas provide a high degree of freedom from jamming. The wideband system can use ant jamming techniques, which also reduces vulnerability.

FlexibilityMobile military satellite earth terminals with trained crews can be deployed and put into operation anywhere in the world within hours.LimitationsThe technical characteristics of the satellite and its orbital parameters are the main limitations to a satellite communications system. Two additional limiting factors for active satellites are transmitter power and receiver sensitivity. Energy for electricity is limited to whatever can be produced by the solar cells, which limits the satellites output power. This problem is made worse by users who increase their output power to the satellite, causing the satellite to try to retransmit at the new power level, at the expense of reducing signals to other users.References

wikipedia.commarineinsight.comRole of Satellite communications- electriciantraining.tpub.comSatellite applications- Encyclopedia BritannicaNaval Shipboard Communications Systems- John C. Kim, Eugen I. Muehldorf

VHF RadioMarine VHF radiorefers to the radio frequency range between 156.0 and 162.025MHz, inclusive. In the official language of theITUthe band is called theVHF maritime mobile band.It's installed on all large ships and most seagoing small craft. It is also used, with slightly different regulation, on rivers and lakes. It is used for a wide variety of purposes, including summoningrescueservices and communicating withharbours,locks,bridgesandmarinas, and operates in thevery high frequency(VHF) range, between 156 and 162.025MHz. Although it is widely used for collision avoidance, its use for that purpose is contentious and is strongly discouraged by some countries, including the UK.Diagram

vhf set and vhf channel. standard handheld maritime vhf.

Block diagram-Vhf transmit and receive system.

Principle of operationA marine VHF set is a combined transmitter and receiver and only operates on standard, international frequencies known aschannels. Channel 16(156.8MHz) is the international calling and distress channel. Transmission power ranges between 1 and 25 watts, giving a maximum rangeof up to about 60nautical miles(111km) betweenaerialsmounted on tall ships and hills, and 5 nautical miles (9km; 6mi) between aerials mounted on smallboatsat sea level.[1]Frequency modulation(FM) is used, with vertical polarization, meaning that antennas have to be vertical in order to have good reception. Marine VHF mostly uses "simplex" transmission, where communication can only take place in one direction at a time. A transmit button on the set or microphone determines whether it is operating as a transmitter or a receiver. The majority of channels, however, are set aside as "semi-duplex" transmission channels where communication can take place in both directions simultaneously.[1]Each semi-duplex channel has two frequency assignments. Semi-Duplex channels can be used to place calls on the publictelephonesystem for a fee via a marine operator. Sets can be fixed or portable. A fixed set generally has the advantages of a more reliable power source, higher transmit power, a larger and more effective aerial and a bigger display and buttons. A portable set (often essentially a waterproof, VHFwalkie-talkiein design) can be carried on a kayak, or to alifeboatin an emergency, has its own power source and is waterproof ifGMDSS-approved. A few portable VHFs are even approved to be used as emergency radios in environments requiring intrinsically safe equipment (e.g. gas tankers, oil rigs, etc).Vhf TransmitA typical vhf transmit and receive system is shown in the block diagram. On the transmit side, the operator, at a remote location, talks into the handset. The handset is connected to radio set control, C-1138. The radio set control output is fed to transmitter transfer switchboard, SB-988/SRT. The switchboard performs the same function as it does in the lf and hf systems. The output of the switchboard is connected to the transmit side of the transmitter/receiver (transceiver), AN/VRC-46 or AN/VRC-80. The transceiver converts the input signal to an rf signal for transmission and the rf is radiated into the atmosphere by the antenna.Vhf ReceiveIn the diagram, the incoming signal is picked up by the antenna. This signal is fed to the receive side of the transceiver. The transceiver output is fed to the receiver transfer switchboard. The switchboard output is connected to either radio set control or to a speaker amplifier, AM-3729, or both, depending on the users preference. The output of the radio set control is fed to the handset and the speaker amplifier output is routed to the speaker.

Purpose of useMarine VHF Radios are two-way communicators which are used to transfer and receive messages. However, the most important function of a Marine VHF Radio is that it is very helpful when it comes to sending distress signals across the channels to coast guards and other ships and boats in the periphery. Also, certain Marine VHF Radios can be used for the purposes of making calls through a marine operator for a certain sum making it double up as a telephonic communicator too. Another important aspect of Marine VHF Radios is the fact that they come in two main categories: portable ones and non-portable ones. The portable ones have waterproof coverings and are battery operated in order to facilitate the power transmission. The fixed or the non-portable Marine VHF Radios cover a lot of aerial ground, their source of power transmission and energy, huger in comparison to their portable counterparts and are therefore far more feasible in terms of their operational facilities.It is used for a wide variety of purposes, including summoningrescueservices and communicating withharbours,locks,bridgesandmarinas, and operates in thevery high frequency(VHF) range, between 156 and 162.025MHz. Although it is widely used for collision avoidance, its use for that purpose is contentious and is strongly discouraged by some countries.

ReferencesMarine VHF Radio- Wikipedia.marineinsight.comCommunications system pdf- US Navy education and training center.

Ships service telephone system The Ships Service Telephone Circuit (J-Dial) requires an external electrical power source and has a keypad just like a home phone. When installed, it may be used as an alternate means of communication. The J-Dial system shouldnt be relied upon because it is not as rugged as the sound-powered phone system and may go out of commission early in action. Some of the newer systems have a limited battery backup in the event of a loss of electrical power. On some ships, the J-Dial phones are available for damage control communications when located near or at repair stations and may be used when other damage control communication systems are inoperative. Although the ships service telephones are not part of the battle communication system, they can prove invaluable if the regular systems fail. They are standard telephones powered by the ships generators and are normally used in carrying out the administrative routine aboard ship. Two features expedite the telephone-calling process: the executive cut-in telephone and the hunt-the-not-busy-line feature. Executive cut-in telephones, clearly marked, are for emergency calls and for the use of persons in authority. Operationally, these telephones are the same as a standard telephone but are limited in number and can be used to call a station that is in use. Instead of a busy signal being returned, the cut-in phone breaks into the circuit. The caller then can interrupt the conversation in progress to deliver an important message. The hunt-the-not-busy-line feature can be used when a call is made to an area that has a group of consecutively numbered telephone stations. After the lowest numbered station has been dialed, the switchboard connects the calling station to the lowest numbered idle telephone. When all the circuits of the group called are in use, a busy signal is returned as with a standard telephoneImage- Ships service telephone

Radiotelephone Radiotelephone is one of the most useful military communications methods. Radiotelephone equipment for tactical use usually is operated on frequencies that are high enough to have line-of-sight characteristics; that is, the waves do not follow the curvature of the earth. As you know, these characteristics limit the usual range of radiotelephone from 20 to 25 miles. This is important because it reduces the chances of the enemy intercepting the message. Radiotelephone procedures can be learned easily by persons with no other training in communications. TheMarine Radiotelephone ServiceorHF ship-to-shoreoperates onshortwaveradio frequencies, usingsingle-sideband modulation. The usual method is that a ship calls a shore station, and the shore station's marine operator connects the caller to thepublic switched telephone network. This service is retained for safety reasons, but in practice has been made obsolete by satellite telephones (particularlyINMARSAT) andVoIPtelephone and email viasatellite internet.Short wave radio is used because it bounces between theionosphereand the ground, giving a modest 1,000 watt transmitter (the standard power) a world-wide range. Most shore stations monitor several frequencies. The frequencies with the longest range are usually near 20MHz, but the ionospheric weather (propagation) can dramatically change which frequencies work best. Single-sideband (SSB) is used because the short wave bands are crowded with many users, and SSB permits a single voice channel to use a narrower range of radio frequencies (bandwidth), about 3.5kHz. In comparison,AM radiouses about 8kHz, andnarrowband(voice or communication-quality)FMuses 9kHz. Marine radiotelephony first became common in the 1930s, and was used extensively for communications to ships and aircraft over water. In that time, most long-range aircraft had long-wire antennas that would be let out during a call, and reeled-in afterward. Radiotelephone has some disadvantages. You may find transmissions unreadable because of static, enemy interference, or high local noise level caused by shouts, gunfire, and bomb or shell bursts. Wave propagation characteristics of radiotelephone frequencies sometimes are unpredictable, and tactical transmissions may be heard from great distances. Most radiotelephone messages are in plain language, and if information is to be kept from the enemy, users must keep their messages short, stick to the proper procedures, and be careful of what they say.Purpose of use The ships service telephones are not part of the battle communication system, they can prove invaluable if the regular systems fail. The radiotelephone, because of its directness, convenience, and ease of operation, is used by ships, aircraft, and shore stations. It has many applications and is used for ship-to-shore, shore-to-ship, ship-to-ship, air-to-ship, ship-to-air, air-to-ground, and ground-to-air communications. Modern means of operation make it possible to communicate around the world by radiotelephone. One of the most important uses of radiotelephone is short-range tactical communications. This method permits tactical commanders to communicate directly with other ships. Little delay results while a message is prepared for transmission, and acknowledgments can be returned instantly. One of the most important uses of marine radiotelephony has been to change ships' itineraries, and to perform other business at sea.

ReferencesPdf on Introduction to radio-frequency communications by US Navy.