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Traffic Alert Buyers GuideTraffic alert systems (also called Portable Collision Avoidance Systems, or PCAS) are a great way to improve situational awareness by making your traffic scan more efficient. Until recently, portable units were prohibitively expensive for many pilots, and there was a big gap in features between those units and the full-blown TCAS (Traffic Alert and Collision Avoidance System) found in business jets and airliners. Coupled with a portable GPS you may already own, some portable systems can give you a real-time traffic display on a moving map, without the massive cost you'd incur upgrading your avionics stack to take advantage of emerging technology like ADS-B.PCAS Strengths and Weaknesses Portable traffic systems aren't a substitute for using see-and-avoid. Used effectively, they will make it easier for you to spot traffic out the window sooner than you otherwise might, giving you more time to decide whether and how to deviate. The current generation of portable traffic systems have some important shortcomings worth noting. Two of the dominant systems on the market are Zaon's MRX and XRX. There is a big difference in price, size and features between the two. First, here's what they have in common. Both units sit on top of your aircraft's panel inside the cockpit and have attached antennas to process traffic information. As traffic comes near, both units will sound audible alerts and alarms, and both units include cables that let you hear those alerts through your intercom or headset. Both units can be powered with a cigarette lighter adapter. Both units are passive devices, meaning that they listen for Mode C transponder broadcasts transmitted by other aircraft around you. The units determine how close another aircraft is to you by analyzing the Mode C packets from each aircraft and the strength of the signals received. This is one of the biggest differences between portable units and full-blown TCAS units. TCAS sends out active interrogation signals from an aircraft to all other aircraft nearby. The transponders in those aircraft respond with their Mode C information. Passive systems like the MRX and XRX save on cost by omitting the hardware to broadcast interrogation signals. That's fine most of the time, since odds are you'll be flying within view of ground radar or an aircraft with TCAS - both of which will cause aircraft to broadcast Mode C signals that your portable traffic unit can detect. Be aware that the portable systems may not detect other aircraft if you're in an area with poor radar coverage, like at lower altitudes in the intermountain West. And the portable systems, like TCAS, won't detect aircraft without transponders, such as many gliders. TCAS units have an added level of complexity you won't find in these portable units. TCAS units in airliners are constantly communicating with one another so that if two planes come too close, the TCAS units will give instructions to their respective pilots that don't cause a mid-air collision. One unit will tell one pilot to pitch up, while the other unit, coordinating with the first, may tell the other pilot to pitch down, for example. With portable units, you get no such guidance on how to avoid traffic. It's up to you to spot the traffic the unit points out, then decide the best course of action to avoid a midair collision. MRX: Small package, small price The MRX is the more compact and feature-limited of the two units. It's about the size of a deck of cards, with a small, stubby antenna near the back. Its eight-character display will provide the range and relative altitude of aircraft near you - but not which direction to look out the window. The device will keep track of up to 10 nearby aircraft in the background, cycling through the three closest ones on the 8-character display. The lack of bearing information means that if the unit tells you another plane is a mile away and 100 feet above you, you may have to scan all the way around you to locate the other plane. While that can be a lot of needless head-turning, it may be just what you need to get your head out of the cockpit if you're fixated on the instruments. The MRX allows you to specify the range and altitude limits to scan: anywhere between 1 and 5 nautical miles away, and from 500 to 2,500 feet above and blow. In other words, its maximum scanning volume is 10 miles across and 5,000 feet high, with your aircraft at the center. XRX: More features at added cost The XRX costs about three times as much as the MRX, but adds crucial bearing information to get you looking out the window in the correct direction. The XRX has a much larger profile than the MRX, since the unit contains directional antennas, a compass and a bank sensor to help it determine where other traffic is relative to you. The LCD display shows several lines of information, including position and relative altitude information for up to three other aircraft at the same time. For directional guidance, the XRX uses a simple set of four arrows that point at traffic ahead, behind or to your left or right. If two arrows are highlighted at the same time (say, ahead and to the right), that means the traffic is at your 1- to 2-o'clock position. The system breaks the compass into 8 segments, each 45 degrees wide, helping you narrow your scan when spotting nearby traffic. XRX and GPS connections The real value of the XRX comes when you buy a $75-$80 cable and connect it to one of several compatible portable GPS units. GPS units that can display TIS (Traffic Information System) data can take the signal from the XRX and plot traffic near you right on your moving map, using color-coded symbols to show proximity, direction and relative altitude. Normally, TIS-capable portable GPS units must be connected to the aircraft's Mode S transponder to receive traffic information. Those transponders can cost $2,500 or more, plus installation. But you can use the XRX with a compatible GPS unit without needing a Mode S transponder. While the XRX's built-in display can only show three targets at a time, a connected GPS may be able to show several more at once. The data resolution is also slightly more accurate when the XRX is connected to a GPS. Zaon says its four-arrow display limits accuracy to 45-degree segments. When connected to a GPS, the XRX will show another plane's bearing accurate to about 10 degrees. Targets may still bounce around on the GPS display and won't necessarily follow the smooth path you'd see if your GPS was receiving uplinked TIS data.
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