The idea behind traffic awareness systems is pretty simple: Warn the pilot when another aircraft is getting too close. All of the systems I know of rely on the intruder aircraft having an operable altitude-encoding transponder. Traffic warning systems come in several flavors, the first being ATC's radar. When targets begin to get too close together on a controller's screen, an alarm sounds to alert the controller. I don't know if the controller is also given some sort of visual alert, but many a time I've heard the aural alarm sounding in the background while a controller was transmitting. When I hear that alarm, I get my head on a swivel.
Traffic alerting systems for aircraft fall into four broad categories: personal collision avoidance systems (aka PCAS), Traffic Information System (TIS), Traffic Alert System (TAS), and Traffic Alert and Collision Avoidance System (TCAS). There are actually two flavors of TCAS, but I won't go into it since it's rare to find TCAS installed in smaller GA aircraft.
Personal collision avoidance detectors passively listen to transponder replies from other aircraft and, based on signal strength and the altitude encoded in the signal, warn you when an intruder aircraft is nearby. Most of these units let you set the warning envelope - the radius and altitude above and below - so you're not deluged with warnings in a busy airspace. The early versions couldn't tell you where the aircraft was exactly, just that one was near. One newer unit actually gives you the relative location of the traffic. These units are helpful, but remember that if an intruder aircraft doesn't have an operating transponder that is replying to an ATC radar sweep or some other form of transponder interrogation, a PCAS unit won't help you.
TIS depends on ground-based ATC equipment to feed it information about traffic in your area. Not all areas that have radar coverage support TIS, hence the warning you'll hear from many TIS systems: "Traffic unavailable." The Bendix-King systems actually say "TIS not available." Even if TIS is available, there can be interruptions in the uploading of traffic data. In these cases, you'll see a message that indicates a nearby target is in coast mode - it's no longer being observed for some reason. Maybe the pilot of that aircraft turned their transponder off, maybe their transponder has failed, or maybe they just went below radar coverage.
Oh, and the FAA will be phasing out TIS so enjoy it while you can. ADB-S (automatic dependent surveillance-broadcast) is the service that many think will replace TIS and provide even more features and functions. ADB-S has dramatically improved aircraft safety in Alaska, where it was introduced and tested. The skeptic in me suspects that the reason ADB-S has not become widespread is that they haven't figured out how to charge pilots subscriptions fees for this service. For those of us spending hundreds of dollars each year in GPS, terrain, and obstacle database updates, another subscription fee is just what we need.
Active TAS, like the equipment installed in Cirrus aircraft, will actually interrogate transponder-equipped aircraft. This means that you theoretically have traffic detection even when an intruder aircraft is below ATC radar coverage. A couple of years ago, I flew a Cirrus from Texas to California and was fascinated to see a tethered balloon in New Mexico actually show up on the MFD as a target. Seems this balloon is a downward looking radar unit and apparently it has its own transponder, too. I've also seen targets below the Cirrus I was flying come and go on TAS equipment. My guess is that the intruder aircraft's belly-mounted transponder antenna was being shaded.
And if an intruder aircraft has no transponder, then PCAS, TIS, TAS, or TCAS won't be of any use. This was recently illustrated by the mid-air collision of a Hawker jet and a glider at 16,000 feet near Carson City, Nevada.
My first exposure to high-flying gliders as potential collision hazards came when I was flying from the Black Rock Desert (aka Burning Man) to Reno a few years ago. Cruising at 11,500' in a Cessna and talking to Reno approach, the controller called traffic for me that was a primary target only (no transponder) at an unknown altitude. When I called the traffic in sight, ATC asked if I could estimate the glider's altitude. I said it looked to be about 12,000 feet and added that it appeared to be climbing like a stripped monkey.
One of my new pet peeves is how some pilots respond to an ATC traffic advisory.
Norcal: "Cirrus 123, traffic, 12 o'clock, 4 miles, 3000 feet, right to left, is a Diamond Star"What these pilots fail to realize is identifying a target with traffic detection equipment like TIS (Traffic Information System) or TAS (Traffic Advisory System) is not what a controller is concerned with when they issue a traffic advisory. Read Order 7110.65R Air Traffic Control and you'll see that the controller only wants to know whether or not you have the traffic in sight with your eyes. If an aircraft is operating under IFR or in class B airspace, ATC can allow you to maintain visual separation, but again, you must have the other aircraft in sight with your eyes.
Cirrus: "We got him on the fish-finder"
A mistake pilots make when using a traffic detection system is they may be hesitant to react to a target that appears to be a threat. If your traffic detection system warns you of an aircraft that you have not been able to acquire with you eyes, then by all means climb or descend or turn to avoid that target. Except for operations in Class B airspace, ATC will provide traffic advisories on a workload permitting basis. If the controller is busy, there's a lot of aircraft to service, or if the controller is being trained by another controller, they may not warn you of traffic that is near you. Even when you are operating under IFR, you are expected to see and avoid other traffic when you have sufficient visibility to do so. So don't be afraid to take matters into your own hands, if necessary, then tell ATC what you're doing and why.
And don't forget to use your eyes, which are often more effective than the so-called "fish-finder."