Garmin’s GTX 330

Five years ago, Mode-S transponders were DOA. This one revives the idea with an affordable anti-collision display.

By Paul Bertorelli

Although we’re not sure they should, some pilots worry about the risk of mid-air collision to the point of obsession. The threat seemingly looms large but, in reality, mid-airs are so far down the list of things that can kill you in an airplane that the numbers are sometimes too small to measure accurately.

On the other hand, if you have a close call or actually swap paint, one-in-a-million odds mean squat. Anyone who’s had the bejesus scared out of them will want an anti-collision device of some kind, thus a lively if not vast market for this equipment has emerged.

The fact that decent anti-collision gear is so expensive, however, has kept it from becoming a common feature in light aircraft cockpits. Twenty grand is just too hard a hit for most owners. Recognizing that, Garmin last year announced a new, ground-based anti-collision system that uses ATC radar data uplinked through a Mode-S transponder. The Garmin GTX330 transponder receives and displays the FAA’s much vaunted traffic information service or TIS through a Mode-S link.

Breakthrough technology? Yes and no. Yes, because it offers capable, sophisticated display of collision threats at a price roughly one quarter that of full-blown anti-collision systems such as Goodrich’s Skywatch or the Ryan TCAD series.

No, because it relies on Mode-S coverage, which, although available in congested terminals, offers far from blanket coverage. What the GTX330 is, then, is an expensive, sophisticated transponder that also delivers effective, relatively reliable anti-collision capability at an affordable price.

What it’s not is a wall-to-wall, see-every-airplane magic box that you can actually put into your panel. The Goodrich Skywatch system comes closest to that, but even it falls short, in our view.

We recently installed a Garmin GTX330 in our company Mooney and test flew it for some 25 hours along the eastern seaboard. Here’s our impression of the technology. But first, a little history.

Mode-S is one of those FAA technologies that seemed like a good idea at the time but was slow to take off, at least in the GA segment. It was developed by Lincoln Labs during the early 1970s and first appeared in certified form in 1980 with the intent of reducing frequency saturation and interference in the so-called ATCRBS or air traffic control radar beacon system, what we know as secondary radar.

Mode-S introduced the concept of a discrete address beacon system that allowed each airplane to be individually addressable by the ATC ground station. The “S” in Mode-S means “select.” With individual addressing, data packets containing useful information could be transmitted to individual aircraft.

Nice idea but even the airlines weren’t interested. The transponders were expensive and there was no practical way to display the even limited data the ATC stations could send. A high-profile mid-air in 1986 between an AeroMexico DC-9 and a Piper Archer at Cerritos, California changed that.

The following year, Congress mandated anti-collision gear aboard commercial airliners, which ultimately led to TCAS I/II. Both systems rely on the data stream from Mode-S transponders to refine bearing and range information to threat aircraft.

Obviously, few GA aircraft have TCAS so Mode-S transponders – still costing three times as much as conventional Mode-A/Mode-C boxes – remained a non-starter for the Cessna and Piper set. To sell Mode-S to GA, the FAA knew it would have to add useful services to the technology and the first one to be fielded is TIS or traffic information service.

Think of TIS as a ground-based version of TCAS that’s a little less accurate but a lot cheaper. It receives transponder information from all aircraft – whether tracked or not – and via the Mode-S addressable link, it displays the position of these aircraft relative to the host airplane sporting the Mode-S datalink.

TIS relies entirely on Mode-S-capable secondary radar. It won’t display primary targets and it only works when the host airplane is in view of a TIS-equipped ATC terminal radar site, of which there are now about 110, with more sites planned. (See the coverage map below.)

TIS won’t see much – if any – deployment on Center radar because the sweep is too slow for the system to deliver useful refresh rates of in-view targets. This is primarily an anti-collision system for terminal areas and not necessarily all of them.

Garmin’s new GTX330 is a direct response to the FAA’s “encouragement” of manufacturers to offer Mode-S products to push what has thus far been nascent technology for GA. (We wouldn’t use the word “blackmail” but we’ve heard others utter it.) Not to suggest that there’s not a commercial market for Mode-S/TIS systems. We think there is.

But first, the transponder itself. The GTX330 is state-of-the-art digital technology. It’s a powerful box – 250 watts output that even a cranky ground system should see. It has all the basic features you expect of any transponder, plus some interesting operational tricks beyond the Mode-S/TIS function. Size wise, it’s standard rack width and height. But it has bunches of buttons – 20 in all – that you don’t find on the typical KT76A. Discrete codes are entered via dedicated keys; just punch them in and they’re seamlessly accepted. A VFR-code button automatically sets a 1200 code and, in the event that you forget to switch the thing on, the transponder has an auto-on feature that brings it alive when the airplane reaches a designated groundspeed – 35 knots. It also automatically reverts to standby after landing, with a user-settable delay. These functions are manually controllable as well.

A key new to the transponder set is one labeled FUNC. This scrolls through a set of auxiliary features that Garmin appears to have added to tart up what is still a $5000 transponder. Toggling the function key calls forth up and down flight timers, elapsed flight time, pressure altitude from the encoder, OAT and density altitude if the airplane has air data and an altitude minder that activates a voice alarm when an altitude limit is exceeded. (Values governing these warnings are user settable.)

The function key also controls the display brightness and contrast. The display itself is of the DSTN liquid crystal variety similar to that used in the Garmin 150/250/300 series. It has dense gray characters against a greenish background and reverse video for night ops. The discrete code is in large text on the left, auxiliaries such as timers and the altitude nag are on the right.We don’t think of transponders as having a learning curve, exactly, but this one does, especially when the TIS functions are considered. These functions are controlled entirely via menu – albeit minimally – from the Garmin 430/530 which does traffic display duty. We suspect that many owners won’t fool with the transponder’s higher functions but we found them useful, especially the timers.

Traffic, Traffic
Unless you’re a loyal supporter of FAA boondoggle technology, the only real reason for buying a Mode-S transponder is the traffic function, leading to the pointed question: does the damn thing work? It does, but one shouldn’t expect too much of the system nor be lulled into complacency in the mistaken belief that it sees all.

In a nutshell, the GTX330 merely serves as a conduit between the data crunching ATC ground station and the GNS 430/530 display. TIS works like this: when a Mode-S-equipped transponder enters a coverage area, it receives what’s called an “all-call” signal from the ground station, an electronic handshake follows and the airborne Mode-S system enters a “roll-call” of airplanes being addressed.

If TIS is available – and it isn’t available everywhere and even in advertised coverage areas, it isn’t always available – the ground station transmits data packets containing radar-derived relative positions of nearby traffic.

To minimize clutter, the service volume is a hockey-puck with dimensions shown in the graphic below. Within the service volume, the system is capable of displaying up to eight targets and if there are more, it will drop out the least threatening ones.

For now, only Garmin equipment can display the GTX330’s output, although this may change going forward. This is a touchy subject, for competitive reasons. The targets appear in standard TCAS symbology, with a hollow white round diamond representing a non-threat aircraft and a solid yellow circle representing a threat aircraft.

Each target symbol has a calculated vector line indicating direction of flight and relative altitude with a plus or minus indicating above or below the host aircraft’s altitude. If the target is climbing or descending at 500 FPM or more, an up-down arrow will indicate that.

The targets appear on the Garmin’s map pages, along with the standard map and nav symbology. However, the display automatically reverts to a traffic-only page – called traffic advisory mode – if a threat aircraft is calculated to approach on a course within .5 NM laterally and plus/minus 500 feet vertically within 34 seconds. That’s another way of saying you’ve got 34 seconds to start looking seriously, regardless of closure speed. The system also issues an aural “traffic, traffic” than can be piped through the audio system.

Flight Performance
We found that the system performed essentially as advertised. But perfection eludes it. Plying the east coast snowbird route, we found that even in TIS coverage areas, the ground segment was occasionally unavailable for inexplicable reasons. When queried, not a single controller even knew what TIS was, let alone whether it was turned on. (The FAA says training and familiarization are planned.) Nonetheless, we would estimate that availability was we’ll over 90 percent in the coverage areas.

Azimuth-wise, targets appear about where TIS says they are. The system is certainly accurate enough to see and avoid the intruder. We noticed some track jump, where a target would shift slightly when its position was updated on the next sweep and due to antenna shadowing in turns, the tracking data occasionally drops out and comes back. In one instance, we saw what we suspect was a phantom target directly under our own position. This lasted for a few seconds and then vanished. In a couple of instances, traffic that we saw visually wasn’t displayed on one sweep, then suddenly appeared on the next.

Overall, we would characterize these shortcomings as acceptable limitations of the technology. They aren’t dealbreakers, in our view. But it’s critical to understand that they exist. It’s also critical to know that in areas where TIS isn’t available, the system is blind.

Further, even within coverage areas, low-altitude Mode-S/TIS reception may be spotty. That means if you’re operating into a busy outlying field near a major terminal where you want all the traffic help you can get, it might not be available. We encountered this at our former homebase, Oxford, Connecticut. Due to distance from the radar site, we had trouble seeing airplanes in the pattern, even though the entire state of Connecticut has overlapping Mode-S/TIS coverage.

What to make of this system? We think the right way to look at it is to value it by subtracting the cost of a high-end transponder, say Garmin’s digital GTX327, from the price of the GTX330. The GTX327 sells for about $2100, the GTX 330 for $4995. So you’re getting a capable traffic function for under $3000, assuming you already have a GNS430 or 530.

Measured against systems such as Skywatch and Ryan’s azimuth-capable 9900BX TCAD, both of which sell in the $20,000 range, variable with display interface, Garmin’s GTX 330 looks like a good value to us. Because Skywatch is an active system that queries nearby transponders on its own and will thus see airplanes in areas where TIS won’t work, the comparison is apples to oranges. But on a purely functional basis, you’ll pay four times more for Skywatch for that additional coverage. From the bottom looking up, TIS offers better capability than rudimentary transponder listeners such as Monroy’s ATD-200 and the TPAS RX-100 from Sure Check, both of which sell for under $1000.

We think owners will have to decide if the more complete coverage from Skywatch is worth the extra dollars. If you can’t afford a $20,000 anti-collision system, the GTX330 makes at least minimal traffic coverage available at reasonable cost.

Also With This Article
Click here to view the Garmin GTX 330 Checklist.
Click here to view “TIS Service Volume/Symbology.”
Click here to view “Ooops, You’re Busted.”

Contact Garmin at Garmin International at 1200 E. 151st Street,Olathe, KS 66062, 800-800-1020 and on the Web at