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Thursday, February 29, 2024

Doug's Domain

Doug Vetter, ATP/CFI

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Avionics 2004:
Traffic Information Services and the GTX330

(Image: Traffic Information on GNS430
Traffic situation north of Baltimore, MD, level at 5000'

When We Last Saw Our Hero...

Back in early 2003, we decided to perform a major upgrade of the panel. The most significant improvement was the addition of the greatest advance to modern avionics -- the GNS430. At the time we were evaluating our options our avionics shop revealed that a new transponder had just been released by Garmin -- the GTX330. The 330 was billed as an inexpensive Mode S (short for Mode Select Beacon System) unit that utilized a datalink with ATC radar systems to display traffic information on the GNS series of navigators.

Unfortunately, "inexpensive" is a relative term, and the budget did not allow the additional $4200 necessary to install the 330 at that time. We did, however, ask the shop to prewire for the 330, with the hope of reducing the installation time (and cost) later. Fast forward to 2004.

Planning For The Future

The airplane's biennial transponder / encoder correspondence and altimeter checks were due at the end of August, so I figured we'd go ahead with the GTX330 now rather than spend the money to recertify the KT76 installation, only to replace it at some point and have to pay for the certification a second time. In June I gave the avionics shop a heads-up on our plans, and we tentatively scheduled the work for the second week in August. The work would require about three shop days, but I agreed to leave the aircraft with the shop for the week.

After a busy weekend of August flying, I managed to coax a client to follow me in his Cardinal RG to the avionics shop one late Sunday afternoon to drop off the 172. That evening, I wrote the shop an email to confirm the list of things we wanted done, which largely aligned with the quote we'd established several weeks prior:

Monday morning bright and early I received an email from the avionics shop that confirmed work would start that afternoon. I went about my own business and left the job in their capable hands. On Wednesday, I learned that I would not have a ride out there to pick up the airplane on Friday as planned, so I asked if the shop could arrange to pick me up at Old Bridge, and they ultimately agreed. Friday arrived soon enough, and not long after we touched down in the owner's well-equipped Bonanza, we were reviewing the installation.


One of the first cool things I noticed was that the shop had replaced the AUX button in the audio panel with one that read ""TCAD". Very cool. You may be wondering why we chose to route the audio through a switched input on the audio panel. After all, if we give ourselves the option to disable critical audio warnings, aren't we risking the safety margin the GTX330 is designed to create if we neglect to enable them?

(Image: GNS430 Dedicated TIS Display)The shop owner related that he routed his Skywatch audio through an unswitched input. However, because Skywatch works (nicely, I might add) even on the ground, there is always the possibility that the equipment will produce warnings at inopportune times, such as when following someone on a low approach to the airport. He said some of the warnings were annoying, and if doing it again, he would consider using a switched input.

Of course, TIS is different. It works only when your aircraft is within line of sight of a Mode S radar site, and when landing at most non-towered fields like ours that means service only down to about 1000 feet AGL. Under these conditions we'd theoretically lose TIS as we're entering the pattern and thus avoid false traffic alerts. However, the deciding factor was the prospect of landing at an airport with a Mode S radar on the field (in other words most if not all Class B and C airports). This scenario would provide TIS service until touchdown, and create the same distracting warnings as the Skywatch installation. For this reason, I requested the shop use a switched input.

Please check your transponder is ON!

Admit it. You're just like the rest of us. You've taken off without switching on the transponder at LEAST once in your career. You've completed your runup and the takeoff checklist. You've even called "Transponder: To Go", as a verbal reminder to switch it to ALT mode just before you take the active runway. Then, at around 1000 feet, ATC kindly bonks you over the head and asks you to verify your transponder is ON! Doh! Thanks to the 330, this scenario will never happen again in our airplane.

The GTX330 switches itself on and off automatically based on either the position of the landing gear, or simple groundspeed. Since the gear in the 172 is (unfortunately) down and welded, the shop configured the 330 to use groundspeed. This is possible because the GTX330 and GNS430 exchange data over a 429 serial interface. This is the same interface that allows the traffic data from the 330 to be sent to the GNS430 for display on the moving map, and some of the wiring that was completed last year when we installed the 430.

(Image: GTX330 Showing OAT and PA)When slowing below 35 knots groundspeed, the transponder will switch to GND (ground) or STBY (standby) mode, depending on what type of radar site last interrogated it. If in a Mode S coverage area, the unit will go into GND mode. If in a traditional radar coverage area such as center radar, the transponder will enter STBY mode. The time at which the 330 enters STBY or GND mode can be provisioned from 0 (instantly) to 99 seconds. We have it set to the default of 24 seconds, which seems to nicely coincide with our turn off of the runway.

STBY mode is familiar to all pilots, and means that the transponder will not respond to ANY interrogations. So, what's GND mode for? A Garmin tech rep told me that the Mode S Beacon System is designed to report the following data:

In GND mode, he went on to say, the 330 would respond to both "all call" and selective interrogations, and return only the information denoted with asterisks.

GND mode is apparently used by ground surveillance radar equipment, now in testing at certain large airports, to more accurately locate and predict the movement of aircraft on the airport surface.

In practice, when in GND mode on the surface of an airport outside the coverage of a radar site (towered or non-towered), the unit does reply to interrogations from the only other possible sources: active traffic detection equipment (TCAS, TCAD or Skywatch) or ADS-B equipped aircraft. I originally thought this would mean I'd have to squawk VFR after turning off the runway, but since the unit responds only with one's tail number, rather than the squawk code, I know it's safe to leave the squawk code from the last flight in the unit until I receive the new code upon startup for the next leg.

Oh, and speaking of switching to VFR, it's well known that the VFR button will conveniently and quickly change the code to 1200, but what's not obvious is that pressing the VFR button a second time the unit will return to the prior squawk code. This isn't a "feature" per se, but it makes sense when you think about it. If you accidentally hit the VFR button during a turbulence encounter and clear your code, just hit it again and it will be dutifully restored. Also, if you're wondering, once entered, the current squawk code is configured in non-volatile memory, so an interruption in power to the GTX330 will not reset the code to 1200. Well done, Garmin.


A few days last winter I had trouble getting our 10 year old Narco encoder to warm up and communicate altitude to the KT76 transponder. Although the transponder reply light blinked and otherwise seemed happy, I knew the encoder wasn't healthy because the GNS430 displayed the message "No altitude input". The problem, it seemed, was the heater used in the encoder to keep its temperature controlled crystal oscillator happy. Given its age and how much Narco would likely charge to repair or replace it, we considered a new encoder.

There are a few options when it comes to encoders, but the most important feature to look for in an encoder today is support for multiple serial outputs as well as a gray-code output. Many components of a modern avionics stack such as multi-function displays and terrain warning systems require altitude input, so the more outputs, the better. And while having gray code in addition to serial interfaces isn't strictly required (and on a fully modern stack, may be unnecessary), it can provide the installer with greater installation flexibility when integrating new and old equipment.

Additionally, while most encoders support a 100' resolution, the shop and I discussed options for an encoder with 10' resolution, or more specifically, one compatible with the MX20 MFD and CNX80 NAV/COM/GPS units. While I couldn't be sure whether we would install either unit, I applied the same logic used to justify a bootstrap heading gyro when it was time to replace our ailing DG. It seemed foolish to me to not spend the few extra dollars to get an encoder that would support us now and in the future. When all was said and done the shop installed the top-shelf Sandia SAE5-35 unit.

I have to admit it was VERY cool to see the transponder power up and report a pressure altitude of exactly 220 feet. I immediately realized the benefits of a 10' resolution encoder. First, ATC would get a more accurate picture of our altitude because the encoder was that more accurate and the Mode S system is capable of specifying altitude in 25' increments. Second, it would make the GTX330's altitude monitor function extremely accurate and far more useful.

(Image: Sample GTX330 Screens)Altitude Monitor

This aircraft will likely never justify an autopilot with altitude preselect and altitude alerter, so having a altitude monitor function in the GTX330 is a great addition to the panel -- and it doesn't take up any additional space either!

So how's the monitor work? Very simply. Use the FUNC button until you see "Alt Monitor" on the screen and, when the aircraft's altitude precisely matches the assigned altitude, hit the START/STOP button once. That's all there is to it. While enabled, the transponder displays your height above or below the selected altitude -- in 10 foot increments if equipped with a suitable encoder, of course. Should you stray off altitude more than 200 feet or just start a normal climb or descent, "Bitchin' Betty" will pipe up and say "Leaving altitude!" in a pretty sexy British accent (no surprise, considering the 330 was originally designed to meet the EuroControl's mandate for Mode S transponders). Let me tell you, the first time I heard that voice in a 172, I had to smile. Never thought I'd see the day when we had this kind of technology in our bug smasher. :-)

The one issue I have with the altitude monitor feature is that firmware version 3.06 does not provide the ability to set the altitude deviation warning to less than 200 feet. Considering the ATCRBS "snitch patch" is set to this altitude, I questioned the practicality of the feature. However, since it takes one or more radar sweeps for the snitch patch to have its way with you, it's likely that you'll receive the warning fast enough to correct your oversight before ATC really knows anything -- particularly so if you have a 10 foot resolution encoder.

When I queried the Garmin rep about this, he said the present software design assumed a 100ft resolution gray code encoder, but plans are in the works to support provisioning of a deviation altitude less than 200 feet. Unfortunately, he could not elaborate on availability.

OAT and Density Altitude

Another unique feature of the GTX330 transponder is support for the connection of an outside air temperature probe. The compatible probe is a unit made for the Davtron 655 indicator. I ordered mine directly from Davtron for $44 well in advance of the installation because they were backordered for 6-8 weeks and I didn't know exactly when I would schedule the installation.

The good news is that the OAT readout and density altitude calculations work as advertised. The bad news is that the OAT probe and the "Orange Juice Can" OEM OAT probe tend to disagree up to 10 degrees celsius in flight. I happen to know from experience that the OEM probe reads low by a few degrees so perhaps the error is not as high as I think, but the problem I see is the inability of the GTX330 firmware to provide a means to calibrate the probe. I'm not looking for perfection here, but I WOULD like to get the error down to 1 or 2 degrees over the normal (-20 to +40 C) operating range.

First Impressions

Perkiomen Valley airport is about 25 miles northwest of Philadelphia, PA, which just so happens to be home to the first fully-deployed STARS radar system. Throughout the takeoff roll the words "TRFC UNAVAIL" were prominently displayed on the lower right hand portion of the GNS430's display to let us know that we were not within reception range of the TIS datalink.

At approximately 900 feet AGL, however, the warning message disappeared and the display came alive with traffic. The day of the pickup I had only a short time to play with the various features of both the transponder and the manner in which the data is displayed on the 430, but needless to say, I was impressed. It became immediately clear that the GTX330 was not just a transponder, installed primarily for the benefit of ATC (and now the needlessly paranoid men in black from Washington) -- but a true safety-enhancing device.

Traffic Display Highlights

(Image: TIS Coverage Volume)The chief advantage of the depiction of TIS traffic data on the 430 has over Skywatch-derived traffic is the presentation of flight path trend vectors. A line extends out in front of the target aircraft to show its direction with 45 degree resolution. Incidentally, you may notice that on some screenshots the direction line appears to be displayed in less than 45 degree increments relative to the host aircraft, but remember that the targets are plotted on a moving map display and will rotate with the display as the host aircraft turns. This gives the false appearance of more accurate resolution.

While traffic displays derived from ADS-B equipment vary the length of the line to depict where the target will be in one minute, on TIS, the length of the line is fixed, and there is no way to determine how fast the aircraft is traveling. While the predictive algorithms used in the TIS ground systems take into account the time lag of the radar sweep to produce the most accurate position possible, and the 430 will warn if any traffic will enter the protected zone around the aircraft within 34 seconds, I sure would like to see the relative speed of the targets at a glance.

Selecting the target with the map cursor displays the distance to the target to within one-tenth of a mile resolution, as well as its altitude separation in hundreds of feet from the host aircraft. This is effectively the same data you can infer from the map once you get used to it. I had expected this data to include the target aircraft's squawk code, but that is not available. It sure would be nice to be able to respond to a traffic alert from ATC in a busy environment with "are you referring to the guy squawking 1344?" That would let the controller know you were at least looking in the right direction. But, I guess we'll have to install Garmin's ADS-B solution if we want that information.

(Image: TIS Traffic Alert Criteria)When the map is set below range of 20NM, the altitude data disappears from the target display, in which case I can still see a high-level picture of the targets within the TIS coverage volume of +3500ft, -3000ft and a 7.5NM radius of my aircraft. Since I usually cruise with the map in the 50NM range, I have to zoom in to figure out their altitude and potential threat status. This was obviously done to reduce clutter, and I couldn't really fault the designers for this. Most people won't bother to look at the display as closely as I was. The point is, after all, to let the equipment do its job while you continue to look outside. When it determines a target conflict, it will very clearly annunciate"Traffic, Traffic!" and pop up a very obvious warning display to aid you in your surveillance efforts.

One of the first questions I asked when I looked into the GTX330 is "Will TIS traffic show primary targets, or those aircraft legitimately operating without a transponder (gliders, vintage aircraft, etc.) but otherwise visible by the ground radar system?" The answer is NO. This is particularly unfortunate, because this is precisely the case where TIS could distinguish itself from every other traffic detection system, most of which rely on active interrogation of the target aircraft. The unit will, however, show Mode-A targets, which result from people flying aircraft with no (operable) encoder, or the transponder in ON, rather than ALT mode. And yes, even in the busy NYC/NJ airspace just outside of the Mode C veil, there are guys flying around like this. It took me less than an hour of flying to find one.

Big Brother

Another popular question has been "I understand that the 330 is programmed with my tail number or Mode S ID assigned by the FAA. Does that mean ATC will see my tail number on their scope even if I'm not talking to them?"

The answer is again, NO. Radar facilities collect the flight data sent by Mode S transponders, but I've been told by a few controllers that the data can't be pulled up in real time (even by the latest STARS equipment). From the controller's perspective at least, Mode A and S targets look alike. If a Mode S equipped aircraft busts some airspace while NORDO, ATC will use the raw data to track the aircraft. On the other hand, consider how this would benefit search and rescue efforts, or prevent a case of "mistaken identity" like several recently publicized events. If you're falsely accused of busting the Washington ADIZ with a Mode A transponder aboard, you look like everyone else and have no way to prove your innocence -- but with the 330 you do.

As a matter of practice the whole "Big Brother" issue simply isn't one. Your GTX330 will give you all the privacy you had with your old transponder, which is to say, next to nothing. If the men in black want to find you, they will -- even if you turn your transponder off entirely. Just don't do anything stupid, and get over it. Call Mode S a small price to pay for admission to one hell of a party.

A Different Kind Of Target Practice

I also learned another aspect of the GNS430 on the first IFR trip down to Hagerstown with the GTX330. Departing Hagerstown eastbound, Potomac usually clears us to the south toward Martinsburg and then to the east to Baltimore. This time, however, they cleared us from Hagerstown VOR direct to Westminster. It didn't take me (and the GNS430) long to figure out that this track took us over P-40 at 5000 feet, which is technically part of P-40 or the overlying restricted area, depending on how you read the chart (it says both include 5000 feet...go figure). Anyway, at the risk of losing my certificates for a mandatory 180+ days for a controller screw-up, I naturally confirmed with Potomac that this was an acceptable route and altitude, and after a short delay, he came back with "that's approved at 5000".

As we transitioned the area normally off limits to mere mortals, I confirmed that it's not possible to disable airspace warnings for prohibited areas. The GNS430 bitched incessantly that we were about to enter prohibited airspace and the "MSG" indicator remained illuminated even after we'd acknowledged the warnings. This convinced me that if every airplane were equipped with a GNS430, accidental penetrations of SUA would be a thing of the past. Think $10K is a lot to spend on a GPS? How much are your certificates worth to you? How much would you pay for an attorney to defend the undefendable? To borrow an old adage, "$10K worth of prevention..."

After we'd exited the lateral boundaries of P-40 still in one piece, I had to wonder whether our Mode S transponder and the data it was relaying was perceived differently by the men in black (who may very well have a different kind of radar), whether we were being used for target acquisition practice, or simply because Baby Bush was in Texas at the time. Maybe all of the above. No matter, I guess. The men in black didn't show up at my house afterwards, and I managed to get a good look at the "mountain hideaway" for the first time. And before you ask for pictures, let me tell you that there's not much to see, but this is definitely not an area in which you would want to sightsee without a clearance. :-)


The one thing I can say without question, given the relatively few hours I've flown behind the unit, is that there is a LOT of traffic within 3 miles of the aircraft that I'd never see without the GTX330 on board. The unit does its job well, and given the price, it's probably second only to the GNS series of navigators in the "bang for the buck" department.

Well, enough writing. I have have to go traffic hunting!