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Air to Ground Communications


SwiftwaterRescueEplett.jpgAir to air communication is critical for safe aircraft operations. And, because of standards in training, equipment and frequency allocation, air to air communication is generally reliable. Air-to-ground communication, on the other hand, is not quite as reliable and can lead to serious problems during normal aircraft operations.

There are over 17,000 state and local law enforcement agencies in the United States. For very practical reasons, each police agency has its own radio frequency or frequencies. Moreover, each state and local agency is influenced by a political parent organization, such as a municipal police force, and is ultimately managed by a city. When making budgetary choices, organizations spend money for public safety-related technologies at vastly different rates.

The combination of local choices and the need to have independent police radio frequencies has created technological fragmentation in public safety. Efforts to solve fragmentation are often focused on increasing interagency interoperability. Aviation is particularly susceptible to interoperability issues. For instance, a vehicle pursuit may cross jurisdictional lines, and aircrews may be forced to communicate with different agencies. One jurisdiction (like a state or country) may have aviation assets that a smaller agency within the larger jurisdiction may not.

A first step in understanding how and why communication systems may not be compatible is a brief explanation of the concept of frequency allocation. Recall that there are thousands of state and local law enforcement agencies who need radio frequencies. In addition to the myriad of law enforcement agencies using radio frequency communications, there are commercial, military and amateur users as well, to name a few. Furthermore, practical radio frequencies are a finite resource. There are only so many frequencies that can be used for police radio communications.

Further complicating the competition between users of the finite resource is the fact that radio frequencies have characteristics that define their usefulness. This means that when decisions about allocation are made, there are trades-offs. For instance, lower frequencies tend to go farther than higher frequencies. This means that higher frequency "channels" can be re-used by geographically distant agencies more often than lower frequencies. Think of driving across country you leave home listening to your favorite radio station. Soon, your station fades, is replaced by static, ultimately disappears and then is slowly replaced by another radio station. Effectively, the frequency of your original station can be reused because it is distant enough from another broadcaster.

Many public safety organizations, like police departments, find themselves allocated to 800MHz (a relatively high frequency). This means they probably have more repeater antennas to obtain coverage over a large area. In addition to using 800MHz, newer radio frequency configurations use digital technology. Driving across country, your commercial radio station was likely broadcasting an analog signal. A primary difference between analog and digital signals is how the signals reception reacts on distance. As you drove away from your commercial station, you heard increasing static and interference, but you could continue to understand the broadcast.

On the other hand, the usefulness of digital radio frequency communications tends to suddenly drop off, not fade away. Think of it this way; with the fading commercial station, you are able to listen intently, filter the static and interference and probably infer some of what you miss you know the lyrics to the song, so if a few words are garbled you continue singing along. With digital communications, voices and sounds are converted into the same binary code that computers use. At the point where interference causes enough of the ones and zeros to drop, the signal cannot be converted back into words. So, whereas analog signals tend to gradually decrease in usability, digital signals tend to cease all at once.

The characteristics of 800MHZ digital radio are not all negative. Indeed, many of them are quite beneficial. For instance, the distance factor has meant that more frequencies are open to use by public safety. Also, high frequency transmissions can have better building penetration than lower frequencies. However, the addition of more frequencies to law enforcement, and particularly aviation, is not necessarily a benefit. Having to keep track of scores of frequencies can be unsafe and counterproductive. In the past, if aviation assets arrived over the scene of an incident after the ground units, at least one of the ground units would have to switch to a frequency that the aircraft maintained. However, switching frequencies in the middle of a tactical operation or emergency is something that should be avoided.

When an 800 MHZ digital radio frequency scheme is combined with a trunked radio system, both air and ground units are provided potential answers to communication problems. Even with the increased use of high frequencies, there are still only so many to be used. Lets presume that a mid-sized municipal government is provided with 10 frequencies. In the past, they would have to divide these 10 between police, fire and other government services. If they provided police and fire with one uplink and one downlink each and one tactical each, they would have used 60 percent of their frequency allocation. However, anyone who has worked graveyard shifts knows that the air can be dead for long periods of time. Indeed, even during peak times there is a lot of dead space between transmissions.

A trunked radio system takes advantage of this space between transmissions by using Frequency Division Multiple Access (FDMA). As an analogy, lets compare railroad and highway transportation. When frequencies are assigned for a specific group of users, it is like a railroad. A municipal agency with 10 frequencies has 10 tracks and only one train, or group of users, can be on a track. FDMA is much more like a highway, wherein each group of users is a like a bus and each frequency is a lane. The users who want to communicate with each other are on the same bus. As the bus drives down the highway, it can change to an open lane. The users are grouped together on the same bus and referred to as a talk group.

As the name implies, the frequencies are divided so that multiple users can access them. Although a medium size city might only have ten frequencies, if it is using FDMA technology, it can have many more "talk groups" that share the frequencies simultaneously. The reference to a "trunked system" goes back to the beginning days of telephone when an operator physically plugged cables into a panel in order to connect users. Now, a computer constantly tracks the communications of the various talk groups and the open frequencies. When one member of a talk group broadcasts (base, ground or air), the computer selects an open frequency and shifts all users to that frequency. This means that the computer is constantly in contact with all system users, coordinating frequency use.

The State of Delaware has approximately 1,982 square miles with about 850,000 residents. In addition to the state police, who provide primary, full-service policing, there are 29 local police agencies within the three counties that comprise Delaware. The state police provide aviation law enforcement, fire support and medical services to the entire state, and in some instances, surrounding states such as Pennsylvania and Maryland. During the mid 1990s, Delaware undertook a state-wide deployment of an 800MHZ trunked radio system. Eventually, the system would encompass all government services including the different police agencies, volunteer fire departments, hospitals and other emergency services.

For Delaware, the first task of the project was to develop the talk groups. Because it is a statewide, coordinated system, Delaware was able to designate different police, volunteer fire, hospital and other public entities as specific talk groups. Now, aircrews are provided a template which lists each of the different talk groups. Responding to public safety emergencies throughout the state, Delaware aircrews are able to punch in the number of the appropriate talk group and establish immediate communications. In addition, aircrews often respond to emergencies within one of the surrounding states. Some of those states have provided their FDMA talk groups to Delaware, and the aircraft radios are pre-programmed. Other states have decided (for technical reasons) to rely on radio communications patches via their central dispatch centers. However, pre-programmed talk groups can be a more reliable, quick way of establishing communications.

Because the trunked system relies on repeating antennas, an aircraft that might normally be out of transmission range can now easily communicate with ground units because the signal for the talk group is passed along the system. In those instances when the aircraft is responding from a distant location, instant and reliable communications can be easily established.

With a trunked radio system, communication can be improved between all public safety entities and aircrews. Moreover, in large communications projects like the one in Delaware (and in other states such as Colorado), although agencies share the same infrastructure, each is able to communicate privately with its own personnel and with each other during emergencies. Through the use of technology, limited RF resources can not only be multiplied by intra and interagency communications, they can be greatly enhanced.

About the Author:
Lieutenant Raymond E. Foster, LAPD (ret.) is the author of several books including Police Technology (Prentice Hall, July 2004) and Leadership: Texas Hold 'em Style; as well as numerous articles about law enforcement and technology.