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FAQ Frequently Asked Questions Page 5

34.  Why is it so hard to locate an alarmed PASS Device?
Think of sitting in a room full of people, and someone's cell phone goes off: everyone reaches for their phone because they are not quite sure which one is riging. Finding a PASS Device is quite similar. Most firefighters have experienced the difficulty of locating an alarmed PASS Device. Remember that one of the purposes of a PASS Device is to let other firefighters know that a firefighter is in trouble. Since it is so loud, the PASS Device is well suited to letting others know of the alarm, but it is that loud volume, however, that creates the problem.

A person's ear is designed to listen everywhere. This is a trait that has evolved over the years probably to warn of the presence of a predator. Luckily, humans have two ears, and that helps alleviate the problem. There are two tricks that we can play with our ears: 1) they can tell us which ear hears a sound first, and 2) which ear hears a louder volume.

We can distinguish between the volume each ear hears, and get a rough idea of the direction of a sound. However, our ears can only hear to certain volumes before this ability disappears. Unfortunately, the volume of a PASS Device is well above that level. This is why it is easier to know where a PASS Device is when you are far away, because the volume is low. As you get closer to the unit, your ability to determine the direction of the PASS device decreases.

We can determine which ear hears the sound of the PASS Device first and go in that direction. However, the sound echoes off of most surfaces. Think about trying to find out where a person was standing when they yelled into the Grand Canyon. You will hear them coming from different directions based on which echo you listen to. In this case, the sounds will arrive at different times that are easy to sort out. Trying to sort out an echo from the source when in close proximity becomes a challenge, primarily because the sounds arrive so close to one another (in fractions of a second). This is why you can focus on a sound, only to hear it coming from another direction as soon as you turn your head.

A separate problem has to do with the shape of the ear. The next time you see a dog, notice how it will "cup" its ears and point them in a specific direction. They do this to make it easier to hear in only that direction. When you pull a Nomex hood over your head, you do the opposite. You actually flatten your ears making them even more omni-directional.

35.  Can GPS be used to locate firefighters?
GPS devices use a system of 31 (as of December 2008) earth-orbiting satellites, known as the Global Positioning System, which transmit radio signals at frequencies of 1227 and 1575 MHz. These radio signals are picked up by a hand-held unit that calculates its location based on the timing of the signals. If a unit receives signals from six or more satellites, it can determine its position to about 15 feet horizontally and about 150 feet vertically. If signals from fewer than six satellites are received, the unit is less accurate.

GPS has a very hard time working inside a building, in fact it rarely does. The problem is that the materials used to build most any structure do a wonderful job of blocking the signals. The problem has to do with the frequency of the signal: a higher frequency signal will be blocked more than a lower frequency.

Think of how current cell phones work inside of buildings. How often do you have to walk to the window or even outside to get a good signal? Many of these phones operate in the 400-800 MHz range and are also affected by the building. Now think of a 100 MHz FM radio station. It works pretty well, but a 1 MHz AM radio station will work even better. In fact, AM stations will work almost anywhere in a building.

The bottom line is that GPS systems work well as long as there is an unobstructed view of the sky. The radio signals will not penetrate most building materials, so GPS cannot be used successfully indoors.

36.  Could Pathfinder's Pulse technology be added to thermal imaging cameras?
Yes. Adding the Pulse™ technology to a TIC would give the rescuer vital information on which direction to go, and more importantly, which directions not to go both in searching for firefighters and in locating the exit. The TIC allows the firefighter to move more quickly and to stay better oriented in a smoky environment. The Tracker tells the rescuer which way to go to find the victim. The combination of the two technologies would be more powerful than either one separately.

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