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VHF Digital Communication

VHF Digital Communication


Introduction

Communication has gone a long way from the first radio broadcast for public audience was transmitted on January 1910 from New York's Metropolitan Opera with the use of a 500-w transmitter. Lee De Forest broadcasted using two microphones and a 500-w transmitter. (Mishkind et al, 2006). The subsequent progress to public radio and broadcasts was made possible through the utilization of technologies that allowed for greater areas of coverage. One of the most important technologies that contributed to the field is transmission through very high frequency or VHF.

Information and telecommunications technology is one of the most important elements of modern societies. The utilization of VHF revolutionized major developments in its field as well as the military, entertainment and commerce. Though there are more advanced communication systems today, VHF remains to be a backbone of major communication systems. As a testament to its significance, most emergency broadcast and communications are still programmed to utilize VHF (Federal Communications Commission [FCC], 2006).

As the world becomes even more globalized, the importance of communication becomes even more highlighted. The International Standards Organization (ISO) in its efforts in the field of communication is concerned not just in creating universal systems but also in ensuring that though there maybe slight differences in communication systems in most countries, they must not be to the degree that there is conflict or grounds for miscommunication. VHF plays an important role in this effort since it provides a common ground for all stakeholders in the communications industry. Therefore, it is important not lonely to have an understanding of its current state and applications but its future implications as well.


Understanding VHF

To be able to appreciate VHF technology, it is essential to understand its portions among other broadcast frequencies, its structure and uses. VHF has been a popular and convenient means of communication whose applications today are being further studied for other technologies such as digital radio and television broadcasting (Office of Technology Assessment [OTA], 2006).

VHF has proven itself to be a fundamental channel of communication as well as an adaptable one. Its core strength is its accessibility not only to businesses or professionals but to ordinary people as well. It has survived newer technologies because of its ability to integrate itself into developments.


The Technology

VHF is a radio frequency that ranges between 30 – 300 MHz and lies between the HF (high frequency) and UHF (ultra high frequencies). Its foremost application is in radio telecommunications and together with UHF in broadcast television. Aside from its broadcast uses, it is also a major component of terrestrial navigation systems such as VHF omni-directional range of VOR and military communications ("Very high frequency", 2006).
Thus, VHF is the most appropriated communications for short-distance terrestrial communications. Its main vulnerability is that it is dependent on the line of sight to transmit information and therefore ahs significant geographic constraints. However, it less affected by atmospheric disturbances that cause interference in transmission as well as interference from the low frequencies generated by electrical equipment (Stimac, 2006). Another key feature is that it is not reflected as readily by the ionosphere unlike higher frequencies and thus do not interfere with other broadcasts on the same frequency in other areas. If long-distance communications over bodies of water, the amplitude modulated voice communications on VHF frequencies is the main option used in the world's civil aviation industry traffic controls or ATC.


System Architecture

The main consideration in the structure of VHF broadcast is the power of transmitters and transmission, the distance of the horizon, the sensitivity of receivers and line-of-sight considerations. In VHF Digital Communication Systems, the tradition VHF system is augmented by digital and satellite technology as seen inn Figure 1. The key to the merging of VHF and digital technology are network and communication switching centers.
VHF transmitters are regulated by government closely because of the ease by which the transmissions can be received as well as to regulate the traffic along the frequencies for legitimate users. On the other hand regulation for the receivers is done mainly to protect the fidelity of other communication on other bands and to optimize communication channels.
The line-of-sight horizon distance can be computed following these formulas ("Very high frequency", 2006):

VHF propagation can be enhanced by tropospheric ducting, a phenomenon that occurs when a cold weather front develops. Troposphere ducts allow VHF to travel farther distances. Another phenomenon that can enhance VHF is the occurrence of Sporadic-E as a result of sunspots that form ionized patches that become dense enough to reflect VHF signals ("Very high frequency", 2006).


Applications

The most common application of VHF technologies is in broadcast and navigation. To accommodate the number of users on the frequencies, bands have been reserved for specific purposes and are monitored by appropriate regulatory bodies such as the FCC. The purpose of the monitoring efforts is not just to regulate but also for assessment of developments in the application of the technologies utilizing VHF (OTA, 2000)For example, in the United States, VHF is divided as for specific channels for television and radio stations while others are divided according to the class of devices that use them or by the people who can operate within the frequencies ("Very high frequency", 2006):
30–46 MHz: Licensed 2-way land mobile communication
30–88 MHz: Military VHF-FM, SINCGARS
43–50 MHz: Cordless telephones, 49 MHz FM walkie-talkies, mixed 2-way mobile communication
50–54 MHz: 6 meter band Amateur radio
72–74 MHz: Remote Control devices
74–80 MHz: TV channel 5
81.5–87.5 MHz: TV channel 6
87.5–108 MHz: FM radio broadcasting (88–92 non-commercial, 92–108 commercial)
108–118 MHz: VOR Air navigation beacons
118–132 MHz: Airband for air traffic control, AM, 121.5 MHz is emergency frequency
132–144 MHz: Auxiliary civil services, satellite, space research, and other miscellaneous services
144–148 MHz: Amateur band 2 Meters
148–174 MHz: VHF Business band, unlicensed Multi-Use Radio Service (MURS), and other 2-way land mobile, FM
156–174 MHz VHF Marine Radio; narrow band FM, 156.8 MHz
162.40–162.55: NOAA Weather Stations, narrowband FM
174–216 MHz: TV channels 7 through 13, and professional wireless microphones
216–222 MHz: reserved for future use
222–225 MHz: Amateur "¼ Meter band
above 225 MHz: Federal services, notably military aircraft radio (225–400 MHz) AM, HAVE QUICK, dGPS RTCM-104

Emerging applications that are now being accommodated by the technology includes digital television, Free-to-Air TV as well as digital audio broadcasting.


ACARS Data Transmission

Aircraft Communication Addressing and Reporting System (ACARS) serves as digital data system for the transmission of short messages between aircraft and ground stations via radio or satellite (see Figure 1). An integral element of the technology is its VHF Subnetwork which ensures the communication link to ground communication centers. The ACARS system was developed in the late 1980's and easily have been adapted by the aviation industry. They are made up of ACARS Management Units (MUs) and Control Display Units (CDUs). Between these two, it is the MUs that send and receive the digitalized VHF messages from ground communication center that mange and route information to respective airlines.

ACARS utilize VHF through radio stations around the world directly providing a cost-effective means of keeping linked with ground transceivers called Remote Ground Stations (RGSs). The range is defined by the altitude of the aircraft transmission and typically transmission rages fall close to 200 miles. However, the geographic constrictions of VHF still apply during ACARS data transmission. The typical ground system end points of the communication system are government regulated institutions like the FAA and the CAA or airline operations headquarters. The purpose of is to ensure that the system is regulated and is treated as a national concern. For example, the CAA functions as a traffic and communication hub to provide flight clearances. On the other hand, airline headquarters provide the necessary information to facilitate airline operations that include gate assignments, airline routing, flight information of machines, personnel and passengers (Sagnier, 1990)

The information provided via VHF by a RGS maybe limited by it strange but it can provide direct information about localities and conditions in the area. Most government bodies prescribe the installation of RSGs whenever possible to enhance air traffic communications and to provide basic communication in case of equipment malfunction of interference (FCC, 2006).

A limitation of transmitting messages via ACARS is the restriction on the length of the messages that can be dispatch: it can relay: ACARS can not be more than 200 characters in length. Another factor is the delay in the transmission and reception of the message which may become critical at times of emergency or difficulty for aircrafts.


ATN and OSI

ATN which stands for Aeronautical Telecommunication Network and OSI which stands for Open System Interconnection are two supporting technologies that also utilize VHF. VHF digital communication is among the data links that can be utilized to communicate with aircrafts via ATN together with satellite transmissions and Mode S radars. At the time that ATN was developed, aircrafts were already using ACARS for communications (Sagnier, 1990). OSI was developed in an effort for international standardization of aircraft communications. Though all airlines were utilizing the same system of technology, variances on frequencies and language were apparent (OTA, 2000).

Today ATN on OSI networks function following universal standards for all aircraft transmissions. However ATN has had some conflicts with OSI: "The main difficulty for ATN is to know to which host - that is to say to which physical machine - transported data are to be brought" (Sagnier, 1990). An example given is the ATN's difficulty in identifying the access point of the OSI or the Network Service Access Point (NSAP) which is essential in the identification of the transportation and services layers. Also, because NSAP role is one as an intermediary gateway, it only exists in the hosts and not in the systems themselves.

Thus, VHF communication between the aircrafts and ground communications may have more difficulty that ACARS however the universality of ATN on OSI networks have a significant nearing considering the nature of today’s airline traffic. The key factor for OSI networks is that linking protocol is standardized even at the level of sub-networks. However, Sagnier points out that "An air-ground sub-network is an ephemeral entity created by appropriate contact of ground and board elements which each make up only half of the transmission chain".

More than anything else communication on OSI has emphasized the complexity and intricacy of creating universal systems particularly those that involve communication exchange. Aside from contemplating the technologies necessary for the system, it also has to take into factor social, political and commercial considerations.


Conclusion

VHF Digital Communication promises to be fundamental element in developing communications. Though it an old technology, it has been able to adapt itself to the changes in the information and telecommunications industry well. Developing communication technologies still use its technology and structure to build new innovations. As seen in ACARS and ATN, VHF digital communications still function in important roles to facilitate and enhance of data and communication.

It is undeniable that a lot has still to be done to refine current communication systems in response to the changing needs of industries, trade and international standards. But this is a challenge that not only VHF is facing as globalization increases. As long as VHF communications remain to be reliable and convenient means of communication, it can be expected to be included in new standardization or universal communication initiatives.

The continued development of VHF technologies may not be as cutting edge as the development of totally new communication technologies but it will remain to have a great impact on communications and society. VHF communications has defined the way we perceive communication should be: universal, accessible and practical. And these standards will remain to be the same standards by which we will view the effectivity and the value of all other communications systems that are to be developed in the future.

In conclusion, VHF role is not as a basic element of communication systems but in essence is a fundamental one. Some time in the future, there will be better technologies that VHF and there will be developments that will totally make it obsolete. However, today it still remains to be a reliable, widespread and cost-effective means of communication that has proven the test of time and human communication demands.








References

Federal Communications Commission (2006).Technology Full Report: Current FCC Regulations. Washington, D.C.:U.S. Government Printing Office
Mishkind, Barry, Halper, Donna L., Fybush, Scott, White, Thomas, Bjorkman, Bruce, Roberts, Mark, Plummer, Rick, Hilliker, Jim, Harvey, Terry, Absher, Frank, Weiskopf, Myke, Denbow, Carl and Rolf, Robert (2006). A Chronology of Radio Broadcasting 1900-1960 . Retrieved on December 22, 2006 from http://members.aol.com/jeff560/chrono1.html
Office of Technology Assessment (2000). Congress of the United States. Critical Connections: Communication for the Future, OTA-CIT-407, Washington, D.C.:U.S. Government Printing Office
Sagnier Y. (1990). The Aeronautical Telecommunication Network. Retrieved on December 22, 2006 from http://www.cena.fr/~sagnier/datalink/atn/atn_a.htm
Stimac, Tomislav (2006). Definition of frequency bands. Retrieved on December 22, 2006 from http://www.vlf.it/frequency/bands.html
Very high frequency (2006). Wikipedia Free Encyclopedia. Retrieved on December 22, 2006 from http://en.wikipedia.org/wiki/Very_high_frequency
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Posted by: Christie Ingram

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