Modern Trends in Hf Communication free essay sample

Even today, entire frequency spectrum of VLF, LF, HF, VHF, and UHF, as well as UHF, SHF and soon also EHF SATCOM is used to set up communication with units and forces at sea. Which of these transmission media is selected depends mainly on the availability of the information channel in terms of coverage range, transmission speed and, last but not the least, required data rate. However, the decisive factor is, and will continue to be, the geographical distance to the receiving station. We can distinguish between three such distances as line of sight (LOS), extended line of sight (ELOS), up to approximately 300 NM and beyond line of sight (BLOS) for coverage beyond 300 NM. Two viable options available for communication beyond line of sight and worldwide are following: †¢ High Frequency Communication (2 to 30 MHz) †¢ Satellite Communication HF COMMUNICATION [pic] The key benefit of HF Radio is that it provides communication over very long distances (worldwide with a suitable aerial). It achieves this with sky wave communication, where the HF signal is reflected off the ionosphere (illustrated above) for long distance communication, or using Near Vertical Incidence Skywave (NVIS) for shorter distances. This benefit comes with a number of drawbacks: †¢ Slow speed. Rates vary from 75 to 12,800 bits per second, with 1,200 bits per second typical. This is insufficient to support many desirable applications. †¢ Cannot send and receive at the same time. †¢ The equipment (radios, modems, batteries, and antenna) is large and heavy. A small system with a whip antenna will weigh several kilograms. Transmission can suffer from noise and errors. Transmissions at radio frequencies higher than HF (VHF, UHF, EHF, SHF) overcome all of the drawbacks of HF, and can provide high bandwidth data communications enabling key communication capabilities. The main restriction of all of these frequencies is that they are limited to line of sight. For some technologies, dista nce more than line of sight can be achieved by use of a slightly curved communications path. This gives a little extra distance, but distance is still a key constraint. SATELLITE Satellite communication is becoming the preferred choice for long distance communication, offering relatively high bandwidth. However, there are a number of drawbacks: †¢ Area coverage of a given satellite or satellite system is often constrained, and may not provide what is needed. †¢ Satellites are expensive. †¢ Military satellite terminals are quite a bit larger and heavier than HF radios. Some commercial terminals are smaller †¢ Satellite will not work in all terrain for example it is not suitable in jungle. National control may be an issue for countries that cannot afford their own satellite systems, and need to rely on commercial or foreign systems. †¢ Satellite ground stations are subject to jamming and other threats. †¢ The satellite is vulnerable single point of failure. Chinas January 2007 demonstration of its anti-satellite capabilities is clear evidence of their vulnerability. Satellites are also potentially vulnerable to attack by laser or EMP (electromagnetic pulse). Many countries are not capable of maintenance, fault identification and rectification of satellite space segments Keeping in mind the aforesaid for the military in many nations, if not in all nations, HF is a primary means of communications rather than a backup. In some countries no other communication techniques are contemplated for such services as ship to shore or aircraft to ground over distances beyond line of site. The vulnerability of satellites makes nations consider HF communications as a primary military communication medium even when satellites are part of their military operations. This paper will provide an introduction to HF communications in general and the limitations of traditional HF radio systems. It will then explore the significant improvements in capability and performance that the exploitation of modem technology has made possible. in particular the impact of Adaptive techniques, hardware advancements and signal processing CHAPTER II MILITARY SYSTEM REQUIREMENTS The requirements of a military system range from repetitive broadcasts of simple commands on teletype to secure digital communications. Global operations demand the HF system adapt to propagation conditions in regions varying from equatorial to polar. Each area has its separate propagation problems and there may be a need to tailor systems to propagation requirements of the area. In the architecture of military systems there are operational requirements as follows: †¢ Connectiveness:The requirements may range from point to point or broadcasting †¢ Master station:In almost all cases a master station is contemplated with command headquarters organizing the distribution of the system. In this case multiple networks are needed to allow communications within limited groups. †¢ Relay:Relay may consist of rebroadcasting or rerouting. Path diversity, i. e. moving a signal to a path with minimum propagation outages is an area on which military research and development must concentrate. †¢ Identification:To identify the type, security and range etc of communication The technical requirements from users include many items. While the list may seem to be a wish list, firm needs can be shown in each of the following areas: Voice †¢ Data †¢ Adequate anti-jam margin †¢ Security †¢ Interoperability †¢ Coverage: ground and skywave †¢ Channel optimization †¢ Message error control CHAPTER III HF COMMUNICATION High frequency (HF) radio provides an effective means of communication over long distance oceanic. HF communication is no longer restricted to voice and is undergoing a resurgence of interest due to the need to find a means of long distance data communication that will augment and replace existing VHF and SATCOM data links. HF RANGE AND PROPAGATION In the HF range (2 MHz to 30 MHz) radio waves propagate over long distances due to reflection from the ionized layers in the upper atmosphere. Due to variations in height and intensities of the ionized regions, different frequencies must be used at different times of day and night and for different paths. There is also some seasonal variation (particularly between winter and summer). Propagation may also be disturbed and enhanced during periods of intense solar activity. The upshot of this is that HF propagation has considerable vagaries and is far less predictable than propagation at VHF. SSB MODULATION The spectrum available for HF communication is extremely limited. As a result, steps are taken to restrict the bandwidth of transmitted signals, for both voice and data. Double sideband (DSB) amplitude modulation requires a bandwidth of at least 7 kHz but this can be reduced by transmitting only one of the two sidebands. Note that either the upper sideband (USB) or the lower sideband (LSB) can be used because they both contain the same modulating signal information. In addition, it is possible to reduce (or ‘suppress’) the carrier as this, in itself, does not convey any information. In order to demodulate a signal transmitted without a carrier it is necessary to reinsert the carrier at the receiving end (this is done in the demodulator stage where a beat frequency oscillator or carrier insertion oscillator replaces the missing carrier signal at the final intermediate frequency. The absence of the carrier means that less power is wasted in the transmitter which consequently operates at significantly higher efficiency. Figure 3. 1 shows the frequency spectrum of an RF signal using different types of amplitude modulation, with and without a carrier. In Figure 3. 1(a) the mode of transmission is conventional double sideband (DSB) amplitude modulation with full-carrier. Figure 3. 1(b) shows the effect of suppressing the carrier. This type of modulation is known as double sideband suppressed-carrier (DSB-SC). In practical DSB-SC systems the level of the carrier is typically reduced by 30 dB, or more. The DSB-SC signal has the same overall bandwidth as the DSB full-carrier signal but the reduction in carrier results in improved efficiency as well as reduced susceptibility to heterodyne interference. Figure 3. (c) shows the effect of removing both the carrier and the upper sideband. The resulting signal is referred to as single sideband (SSB), in this case using only the lower sideband (LSB). Note how the overall bandwidth has been reduced to only around 3. 5 kHz, i. e. half that of the comparable DSB AM signal shown in Figure 3. 1(a). IMPACT OF MODERN TECHNOLOGY Due to the accelerated development of systems using satellites, high fr equency (HF) systems were in a recession for almost a decade. Now, we are witnessing a forceful comeback of HF systems in civilian and military sectors. The fundamental problem other than as discussed in chapter I related to the achievement and maintenance of a good quality HF communication circuit is associated with the inherent dynamics of the HF link. Conditions over the link may change rapidly, the communication system must be able to adapt to the prevailing nditions. The dynamic nature of the HF medium necessitates the use of a fast processing capability to optimize the system performance. Digital radio frequency and control system technology together with a cheap and reliable fast processing capability combine to address the HF problem in achievable way. Digital control system provides the nervous system for modern HF system designs. They can orchestrate automatic adaptation, timing, frequency hopping, radiated power level control, information flow control and other control functions required in a modern HF adaptive system. Techniques for real time frequency management are now being implemented in current systems. The impact of Very Large Scale Integration (VLSI) will be evident in all types of future communication system. VLSI technology provide the ability to reduce size, weight and power consumption of existing equipment and to increase the overall system reliability. It is enables more complex systems to be provided by introducing techniques and applications which have not previously been possible because of the lack of necessary technology. Highly reliable HF systems are dependent upon efficient signal processing. Sophisticated digital signal processing has made new modulation modes practicable, whilst at the same time reducing the equipment cost. Such techniques operate on the modulating signal or on the RF carrier in a manner which increase the quality of intelligence transferred from the sender to the receiver. VLSI techniques enable sophisticated modulation and coding to be implemented; high levels of data interleaving can be used to overcome the effect of burst errors. Digital modems are adaptable to different modes by software techniques, avoiding hardware retrofits or conversations. The modulation and demodulation process can now be performing digitally. Wideband modulation schemes and frequency hopping techniques are becoming more common; digital control synthesizers are capable of changing frequencies many times a seconds. Voice processing and speech recognition techniques enable limited vocabulary speech to be transmitted over a narrow bandwidth. Applications that rely upon fast processing of voice signals demand the use of filters that can provide high stability and performance. This can be achieved through the use of VLSI technology in the implementation of digital filters. Progress in VLSI techniques is such that integrated receivers front ends, incorporating frequency changers and IF amplifiers, are already feasible. Future development may lead to direct analog to digital conversion of RF signals with high rate conversion. Problems of dynamic range, inter-modulation and conversion speeds remain to be overcome. The use of electronically steered receiving antennas arrays is also made feasible by the use of VLSI techniques which are able to provide the processing power necessary to perform efficient wave front analysis. Evolving equipment designed to meet continuing needs has yielded significant improvement of receivers, trans receivers, and input/output devices. Equipment that relieves onerous tasks such as radio tuning and radio monitoring is now popular. The technical challenge has been to obtain electronic control without losing the performance available from the best manual equipment. While digital technology will assume some of the hardware burden, high power and high frequency analog circuitry will still be required. The key thrusts are in tuning speed, spectral purity and system bandwidth. Fault detection and diagnosis can be improved using processor waste monitoring, control system and built in test (BITE). These enable online systems tests to be automatic and repeated on a frequent basis to indicate potential problems before they become serious. Even greater capabilities will result form a signal processing power of very high speed integrated circuitry (VHSIC) technology. The associated speed and processing power will replace additional circuit boards of analog components in the processing of still more sophisticated waveforms. The trend will continue with digital embedding of present day function such as artificial intelligence based control and cryptographic facilities. The scope for technology improves HF communication systems appeared to be considerable. In the following chapters some of the principal trends of future HF communication systems will be discussed. These advanced techniques, as already evident from the aforesaid brief overview, cover a wide range of technical innovation. CHAPTER IV REAL TIME CHANNEL EVALUATION THE CONCEPTS OF REAL TIME CHANNEL EVALUATION (RTCE) Certainly the cornerstone of advanced HF communications is the concept of evaluation in real time of the interference and propagation problems of the circuit and dealing with them, automatically. The basic systems are of the open and closed loop varieties. The open loop has no feedback path and the closed loop does. For a broadcast an open loop might do; for two way communications it will not. Probably the most advanced form of RTCE is that of an adaptive system whose parameters can change in response to changes in available capacity. Even if constant rate transmission is mandatory the source and channel encoding procedures must be made adaptive to counter changes in the received SNR. Added redundancy for example when there are losses in propagation must form part of the system. From this concept there must be system control strategies. The provision of a high integrity engineering order wire facility is vital. Continuous checking of traffic quality must be made with automatic repeat request techniques. The source encoder must be able to compress or modify the basic source data. ASPECTS OF REAL TIME CHANNEL EVALUATION The general principles of RTCE are in place; what are the possible means of implementing it? , the real time oblique ionospheric sounding could be a portion of the channel evaluation to be performed, similar to that made by an ionospheric oblique sounder. A waveform could be generated for channel evaluation. During the pause of the emissions, measurements of noise and interference could be performed at both the master and the slave station of the link. After processing the small group of available frequencies could be chosen. The initial band of frequencies is to be probed for a particular time and circuit could be performed by simplified algorithms or tables for certain sites and times. Within the possible bands the precise frequencies would be sampled to determine optimum transmission parameters. The properties of the path which are monitored include: †¢ Noise and interference spectral density †¢ (Signal + Noise) level †¢ Multipath spread †¢ Doppler spread The measurement of the properties of a communication channel is particularly important in digital communications because high-speed data transmission critically depends upon them. The measurements in levels of increasing complexity are: †¢ Measurement of multipath spread and Doppler spread; Doppler shift and spectral skewness. †¢ Measurement of second-order channel functions. †¢ Measurement of instantaneous channel functions. For the parameters above, measurement techniques used are based upon differentiation, level crossing and correlation. For the second item, the techniques used are correlation techniques, multitone approach, pulse pair method and chirp technique. For the measurement of the parameters in item 3, the methods used are cross correlation, multitone, and pulsepair. CHAPTER V CODING With the digitization of HF came the need for coding for both error correction and security. With its heavy burden of propagation problems, coding represented for HF a means of facing problems in a particular environment and for distinctive conditions. However coding also provides a means for boosting the data rate in a channel towards the theoretical limit established by Shannon for that channel. Linear block codes (of which cyclic codes are a subclass) and convolutional codes are the main categories of codes of interest to HF communications. They are capable of correcting random errors due to white Gaussian noise as well as burst errors due to impulse noise. In block codes, a block of information bits is followed by a group of check bits. The latter verify the presence of errors in the former. At the receiving terminal the check bits are used to verify the information bits in the block immediately preceding the check bits. There are various classes of block codes including hamming codes which require a minimum number of bits for correction of single errors, cyclic codes which have ease of implementation, ability to correct large numbers of random errors, long bursts of errors and loss of synchronization. In convolutional codes, check bits are continuously interleaved with information bits and they check the presence of errors not only in the block immediately preceding them, but in other blocks as well. It has become feasible to mechanize coders and decoders that are known from theoretical work to be optimum and units have entered the field of HF communications. In practice convolutional codes are used mostly for error correction. Most of the coding applications to HF communications have consisted of error correction through the use of parity checks and of automatic error correction by retransmission. One of the main purposes of error correction is to reduce the effects of long deep fades. Very long recurrent codes or short codes with a considerable number of interleaved blocks can be used. Time delays of 1 to 2 seconds are experienced with decoding. Code rates between 1/2 and 4/5 are used if what is required is only error detection. The use of coding is expanding with the availability of inexpensive off the shelf computers. Its extensive use is vital to combat the problems of propagation and interference, and to allow for secure communications. CHAPTER VI EQUIPMENT ADVANCES Significant improvement has been made through electronic control. In addition to the obvious advantages of digitization, this reduces the use of highly trained operators and allows for automatic equipment testing. RECEIVERS The current dominant performance improvement in receiver design has involved decreasing the unwanted products from the front-end converter. LO synthesizer phase noise is now the dominant deterrent to discriminating against a strong adjacent signal. A strong signal acts as a local oscillator. LO phase-lock-loop synthesizers are an economic compromise between tuning speed and phase noise. The industry now perceives the economic and performance advantages to digital implementation of selective filters, demodulators, and automatic gain control with many parameters such as amplitude and phase characteristics as stable as the logic clock drive. There are now available for example commercial digital signal processing chips and boards. Receivers are more frequency agile than transmitters; surveillance receivers at control centers can automatically monitor channel occupancy and potential interference. Operational convenience of the new receivers includes function control by microprocessors at remote locations and built in test equipment. TRANSCEIVERS AND ANTENNA COUPLERS New transceivers are following the lead of digitally controlled receivers; they are designed at the present primarily for the SSB modulation voice bandwidth channel. Remote controlled receivers can solve the problem of limited cockpit space, manpack use without removing the pack, and remote placing of equipment in tactical ground vehicles. AUDIO AND DATA CHANNEL PERIPHERALS While the audio modem is a de facto standard, two notable input/output devices are now being developed. They are the small data terminal and the secure voice terminal. There is no concensus on modem design. There are unique HF low data rate modems but without interoperability. There are indications of strong competition for 2400-bps modems offering enhanced performance. Serial modems are vying with parallel modems; national extensive tests are needed to adopt a standard unit. In the field, handheld terminals substitute for teletype equipment and computer terminals. They allow off-line composition and editing before transmission at teletype or burst rates. Internal simple FSK models are typical. Demodulation, storage and display complete their capabilities. Most use typewriter keyboards. HF modems for high speed data such as 2400bps digitized voice have evolved toward multipletone signaling carrying differential phase shift keying. Parallel tones having durations much longer than the expected multipath delays are favored, even though parallel channels have been an expensive implementation. Digital data processing has now minimized the parallel channel cost penalty. MODEMS Both theoretical and experimental aspects of modems were considered by Monsen. He found that combined modulation and coding techniques can remove much of the fading effect even in high data applications where intersymbol interference is a serious problem. Experimental results for various models were compared. Successful communication was achieved using equalizer modems in a high data rate application. Monsen emphasized the importance of delay in producing high quality communications-if delay can be tolerated the capabilities of HF can be enormously increased. CHAPTER VII PROPAGATION In HF communications, the ionosphere must be listed as a black box along with transmitter, encoder, antenna, and receiver. The abandonment of HF as a vital portion of modern communication resources is due in part to the difficulties of dealing with the ionosphere. HFs most vital problem as a modern communication system will remain propagation. THE PROBLEMS †¢ Multipath propagation with different time delays, amplitudes and polarizations will distort the signal, producing fading which might not be correlated even within a 3kHz bandwidth. Digital transmission is particularly affected by fading even for time delay differences as short as 50 us (path length difference of 15 km). †¢ Ionospheric tilts. Horizontal gradients both large and small scale make it difficult to model a propagation path because the ionosphere changes with time and with both latitude and longitude. †¢ The effects of irregularities. Irregularities acting as scatters produce rapid and slow fading. Sporadic E or highly ionized regions at about 100 km with relatively small extent produce unusual propagation. For HF the fluctuations are primarily from small scale irregularities in the F layer. F layer irregularities produce rapid fading of HF signals both in the equatorial, auroral and polar areas. Mid-latitude irregularities can on occasion produce serious fading on HF signals. These, however, are problems which can be dealt with by means of coding and error correcting codes. THE LASTING PROBLEMS The problems which any HF system must deal with are problems resulting from geophysical disturbances. Solar flares with particular characteristics and magnetic sector changes on the sun produce ionospheric disturbances. †¢ A direct solar flare effect is the sudden ionospheric disturbance-blackout of HF due to increased absorption at -60 km. These are short lived (up to an hour) but frequently the suddenness causes the operator to look for trouble in his equipment. †¢ A magnetic storm decreases the frequencies capable of being reflected at auroral latitudes, produces absorption, and creates irregularities at F layer heights which cause fading. Absorption is the problem most difficult to deal with in modern systems. †¢ The worst problem for reliability is that of polar cap absorption (PCA) where very high energy protons produce very high level absorption events predominantly over the polar cap. Overhead (without the problem of slant range increases) absorption has been recorded up to 10-20 dB at 30 MHz. In this case the increase of power to overcome the absorption would be enormous. PCAs are rare but their effects can last for several days. This essentially reduces the reliability of HF with the only solution path diversity with the reflecting points chosen outside the polar cap. DEALING WITH PROPAGATION †¢ Path diversity, rerouting of communications to seek the least troubled path has been tried. Multiple frequencies and multiple paths has lead to improvements from 37% reliability (four frequencies-short path) to 62% reliability in a disturbed ionosphere at high latitudes (no error correction or coding). †¢ Dynamic models of solar-terrestrial relationships. In at least two groups real time data from solar, interplanetary, and terrestrial measurements are fed into an elaborate program to forecast effects at high latitudes. These are at beginning stages but they promise much more than the simplistic means of forecasting magnetic storms now in use. HAARP The High Frequency Active Auroral Research Program is an ionospheric research program jointly funded by the US Air Force, the US Navy, the University of Alaska and the Defense Advanced Research Projects Agency (DARPA). Its purpose is to analyze the ionosphere and investigate the potential for developing ionospheric enhancement technology for radio communications and surveillance purposes (such as missile detection). The mos t prominent instrument at the HAARP Station is the Ionospheric Research Instrument (IRI), a high power radio frequency transmitter facility operating in the high frequency band. The HAARP project aims to direct a 3. 6 MW signal, in the 2. 8–10 MHz region of the HF [High Frequency] band, into the ionosphere. The signal may be pulsed or continuous. Then, effects of the transmission and any recovery period can be examined using associated instrumentation, including VHF and UHF radars, HF receivers, and optical cameras. According to the HAARP team, this will advance the study of basic natural processes that occur in the ionosphere under the natural but much stronger influence of solar interaction, as well as how the natural ionosphere affects radio signals. This will enable scientists to develop techniques to mitigate these effects in order to improve the reliability and/or performance of communication and navigation systems, which would have a wide range of applications in both the civilian and military sectors, such as an increased worldwide HF communications, accuracy of GPS navigation, and advancements in underwater and underground research and applications. This may lead to improved methods for submarine communication and the ability to remotely sense the mineral content of the terrestrial subsurface, among other things. HAARP is the subject of numerous conspiracy theories, with individuals ascribing various hidden motives and capabilities to the project. Journalist Skeptic computer scientist David Naiditch called HAARP a magnet for conspiracy theorists, saying the project has been blamed for triggering catastrophes such as floods, droughts, hurricanes, thunderstorms, and devastating earthquakes in Pakistan and the Philippines aimed to shake up terrorists. HAARP has been blamed for diverse events including major power outages, the downing of TWA Flight 800, Gulf War syndrome, and chronic fatigue syndrome. Conspiracy theorists have also suggested links between HAARP and the work of Nikola Tesla (particularly potential combinations of HAARP energy with Teslas work on pneumatic small-scale earthquake generation) and physicist Bernard Eastlund. According to Naiditch, HAARP is an attractive target for conspiracy theorists because its purpose seems deeply mysterious to the scientifically uninformed. Conspiracy theorists have linked HAARP to numerous earthquakes. An opinion piece on a Venezuelan state-run television channels website named HAARP as a cause of the 2010 Haiti earthquake CHAPTER VIII NEW SYSTEMS Several systems which use some of the advanced techniques are operational. Among these are the following: RACE (Radio Telephone with Automatic Channel Evaluation). It was developed to improve the quality of telephone services provided by HF radio to remote areas. Naval HF systems aim towards frequency agility on shipboard platforms where collocation interference is a dominant concern. The UK ICS-3 stresses the radical lowering of all self generated components of the noise floor. Primarily ICS-3 enhances groundwave and skywave transmissions through rapid propagation selection and data handling. SELCAL, scanning and channel evaluation, are now part of operating systems with link quality, selective paths and frequencies, and general channel evaluation available. SELCAL HF networks using scanning are being automated. SNOTEL meteor burst system is run by the U. S. Department of Agriculture to relay snow cover data from over 500 remote sites in the West. There are now vendors and equipment specifically for single-frequency meteor burst networks. The impact of SNOTEL is the economy of a simple data terminal that uses many propagation modes in the relatively unused HF/VHF spectrum. A digital sounder has been applied to network frequency management. The unit, developed at the University of Lowell, allows for frequency agility, numerical processing, system clocks, etc. Equipment development is moving along at a rapid pace using off-the-shelf items such as keyboards, modems, and other digital equipment to meet demands of the new modern HFcommunications. HIGH FREQUENCY INTERNET PROTOCOL (HFIP) It is usually associated with Automatic Link Establishment and HF radio data communications. HFIP provides protocol layers enabling internet file transfer, chat, web, or email. HFIP commonly uses ionosphere propagation of radio waves to form a wide area network that can span thousands of kilometers. HF transceivers in HFIP service typically run 20 to 150 Watts for portable or mobile units, up to approximately 2000 Watts transmitter output for high power base stations with HFIP servers. STANAG 5066 is a common HFIP standard. STANAG 5066. STANAG 5066 Profile for High Frequency (HF) Radio Data Communication is a NATO specification to enable applications to communicate efficiently over HF Radio. STANAG 5066 provides peer protocols that operate above an HF Modem and below the application level. STANAG 5066 includes the (mandatory) SIS (Subnet Interface Service) protocol that enables an application to connect to an HF Modem through a STANAG 5066 server over TCP/IP. This enables a clean separation between application and modem. [pic] The diagram above shows a configuration of three sites communicating by HF Radio, using STANAG 5066 to provide end to end communication between a set of applications. The site shown in detail illustrates how STANAG 5066 fits with applications and hardware. It comprises: †¢ An HF Radio, which is an analogue device. An HF Modem, which converts from analogue to digital. †¢ Encryption (optional). Data Encryption will generally be used with HF Radio, and this will be achieved by an encryption unit (COMSEC) that sits between the HF Modem and STANAG 5066 Server. †¢ A STANAG 5066 server. There will be one STANAG 5066 server associated with the modem. The STANAG 5066 Servers communicate with each other over the HF Modem, using protocols spe cified by STANAG 5066. †¢ One or more data applications communicating with the STANAG 5066 server using the SIS protocol. CHAPTER IX HF RADIO NETWORK CENTRIC WARFARE NETWORK CENTRIC WARFARE Network Centric Warfare, also commonly referred as Network Enhanced Capability, is widely documented and described. Key features include: †¢ Everything is connected, to enable all players to communicate and share information. †¢ A wide mix of technologies and components are involved. †¢ High speed datalinks are utilized where possible. †¢ Many applications are used, from core traditional components such as formal messaging and situational awareness, to new applications such as Video Teleconferencing, Voice over IP, Instant Messaging and Presence. IP (Internet Protocol) is used everywhere. IP as the single network technology is a central technical approach. THE ROLE OF HF RADIO IN NCW In terms of planning an overall architecture the technology constraints lead to several possible views as to how HF Radio fits into the overall communications picture: †¢ A legacy component that will be replaced by newer technologies. †¢ A component f or use in special and selected situations. †¢ A component that will be widely used. †¢ A strategic backup communication mechanism, in the event of satellite failure or destruction. If options 2, 3 and/or 4 are chosen, it is critical that HF Radio is well integrated into the Network Centric Warfare architecture, and that effective application functionality is provided to support a full set of mission critical applications operating over HF Radio. KEY APPLICATIONS OVER HF RADIO A goal of Network Centric Warfare is to maximize information sharing. While not all applications are suitable for HF Radio, a number of applications are. This section sets out a list of applications that may be mission critical, are suitable for HF Radio, and could reasonably co-exist and share an HF link: †¢ Voice. Situational awareness. Situational awareness is a key application for tactical military deployments. †¢ Formal Messaging. Military formal messaging. †¢ Internet Mail. Internet mail over HF is defined as a part of STANAG 5066, using the HMTP (HF Message Transfer Protocol). †¢ Instant Messaging and Presence. XMPP. Presence status reporting and Instant Messagin g can be useful applications for tactical deployments. †¢ Directory. Replication of directory data over HF is important to support messaging applications (address book and security) and configuration for other applications. Web. Limited Web browsing may be useful in some situations in support of mission critical operation. This is quite viable over faster HF links. This is not intended as an exhaustive list, but to give a sense of what could be sensibly achieved over an HF link. HIGH FREQUENCY GLOBAL COMMUNICATIONS SYSTEM (HFGCS) The USAF High Frequency Global Communications System is a worldwide network of 15 high-power HF stations providing command and control communications between ground agencies and US military aircraft and ships. Allied military and other aircraft are also provided support. IAW agreements and international protocols as appropriate. The HFGCS is not dedicated to any service or command, but supports all DoD authorized users on a traffic precedence/priority basis. General services provided by the HFGCS are: †¢ General Phone Patch and Message Relay Services †¢ Automatic Link Establishment (ALE) †¢ HF Data Support †¢ Command and Control Mission Following †¢ Emergency Assistance †¢ Broadcasts †¢ HF Direction Finding Assistance †¢ ATC Support †¢ E-Mail connectivity CHAPTER X CONCLUSION In systems development, Real Time Channel Evaluation is made feasible by both equipment and conceptual advances. The conceptual advances include coding and error correction techniques for security and in order to minimize propagation and interference problems. The availability of digital equipment allows RTCE to select a frequency for a particular path, propagation mode, and modulation selection. Propagation advances include better statistical models as well as advances in short term forecasting methods responsive to changes in solar-geophysical parameters. Forerunners of adaptive HF systems are operational in meteor scatter systems used for data collection as well as other operations. The advances both in equipment and in conceptual system development have been many. HF will continue to be a used portion of the spectrum. Various diversity systems ranging from time to space diversity should be made available to the system operator. Geographical diversity is one of the least used but most important methods of handling problems. New approaches include frequency band extension to higher frequencies in order to use meteor burst transmissions above 30 MHz. BIBLIOGRAPHY 1. Introduction to HF Radio Propagation by Australian Government, IPS Radio and Space service. 2. HF Communications, A System Approach by Nicholas Maslin 1. High Frequency Active Auroral Research Program Wikipedia 2. High Frequency Global Communications System – Wikipedia 3. High Frequency Internet Protocol – Wikipedia 4. HF Radio Network Centric Warfare, research paper published in 09 April 2008, www. isode. com. 5. STANAG 5066: The Standard for Data Applications over HF Radio, www. isode. com. 6. Introduction to HF Radio Propagation by Australian Government, IPS Radio and Space service. 7. Current Trends in Tactical Naval Communications by Stephen Metzger director of sales naval communications at Rhodes and Schwarz. . General Concepts of Modern HF Communications by Jules Aarons Boston University. 9. Modern HF Mission Planning Combining Propagation Modeling AND Real-Time Environment Monitoring by D. Brant, G. K. Lott, S. E. Paluszek, BE. Skimmons Applied Research Laboratories, The University of Texas at Austin, USA Naval Postgraduate School. 10. Modern Aircraft H F Communications into the 21 Century by N C Davies, M J Maundrell, P C Arthur, P S Cannon, R C Bagwell, J Cox, UK Defence Evaluation and Research Agency. 11.