GTX330 Pirep (longish)

Guest Discussion
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I just brought my 260se/stol back home from Kansas, where it had a new GTX330 transponder with traffic information exchanged for the GTX327. Bevan-Rabell in Wichita performed the GTX330 installation. There was an initial hiccup in that Garmin shipped the units for installation before the FAA had approved their TIS software for the 430/530 units, an atypical gaffe by an otherwise very heads-up company. This resulted in a 2-3 week delay in completing the installation for delivery. Total installed price, including a credit for the GTX327, was just shy of $4100.

As noted in another thread, I do not view mid-air collision as one of the main risks in flying, and statistics back me up on this. Moreover, Aviation Consumer listed collision avoidance equipment 10th on their list of 10 priorities for improving one’s safety (“Setting Priorities,” August 2001 issue). Thus I personally could not justify the $20K+ bill for Skywatch or TCAD. However $4K for the GTX330 was just low enough to win me over.

It works as promised–and doesn’t work as promised, too. Near Wichita it displayed traffic as little white-outlined diamond icons. Each icon has a “trend line” displayed with it, indicating the target’s projected flight path. I like this feature, as it allows one to assess whether a given target is of potential concern. However when a target was making a turn or maneuvering, the software seemed indecisive and the trend line would change erratically with each “refreshing” of the data. Next to each target is also a number such as “+07” or “-10” to indicate its altitude relative to you. I was pleased that I was able to find the aircraft visually in the area suggested by the TIS virtually every time. One exception: ATC called out a plane to me that was close laterally but about 2000 feet below, which the TIS did not detect. I suspect that we were not receiving his transponder due to his antenna position (bottom of the plane?). I suppose this means that a plane could climb into you from below and behind undetected, but I would judge the chances of that happening to be slim.

When a target is closer than 2 nm and within 1000 feet (I believe) of your altitude, the icon changes to yellow and an aural “traffic” alert sounds through your headset. The voice is female, with a British accent, agreeable yet attention-getting at the same time. Aural alerts can be disabled via the audio panel. I might be inclined to do this when nonpilot passengers are in the plane, especially when departing or arriving at a busy airport.

I made a short flight from ICT to EQA in trail behind another 330-equipped 260se, and we were able to track each other on the way back to EQA. At just below pattern altitude, the signal was lost as we sank below Wichita Approach’s radar coverage. The British Lady informed me of this by announcing, “traffic not available,” which was also noted on the 430 display. On the trip home, as predicted I lost coverage about 50 nm southwest of ICT, but surprisingly didn’t get it again until descending into the Phoeni| area. The service map on the Garmin website suggests availability in the Albuquerque area, but it was not active on that day at least. Amarillo is class C airspace, but I guess they don’t have the appropriate radar equipment. Descending towards Phoenix Deer Valley from the northeast, service was intermittent below about 6000 feet, presumably obscured by some mountains in the valley. Once I cleared those mountains, we were in business. Departing DVT, service was available west of the Phoenix area until 40 or 50 nm east of Blythe, and did not pick up again until 30 miles or so west of Palm Springs. It was intermittent while transiting Banning Pass at 4500 feet, and then available until between Santa Barbara and Santa Maria. It picked up again near Salinas and worked well when entering the Bay Area. It was both reassuring and cool to report to ATC that I “had on Skywatch” a Saratoga paralleling my course inbound to PAO even though I didn’t have it in sight.

As I do much of my flying in airspace where coverage is available, I feel that this unit is a good choice. However unlike Skywatch or TCAD it certainly won’t provide coverage at uncontrolled fields that are below approach control radar service. Compared to Skywatch, I will get more than 50% of the utility at less than 20% of the price, and I’m happy with that.

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Kevin: Thanks for the very detailed report. Sounds like you made a good choice.

In reply to:

ATC called out a plane to me that was close laterally but about 2000 feet below, which the TIS did not detect. I suspect that we were not receiving his transponder due to his antenna position (bottom of the plane?).

Just to clarify, the GTX-330 does not receive anybody’s transponder. All the targets you see are sent to you by TRACON based on the targets they see.

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Kevin,

Thanks so much for testing out this new technology and giving a great report on it.

My a/c partner and I have been wrestling with the issue of what to do about traffic awareness gear. Skywatch is just too expensive at this point and I dislike the audio it gives. So your report is most interesting.
Questions:

  1. Are you still given squawk codes, or does ATC now always know your tail number (I understand that the '330 is an S-mode transponder, like used by the airlines, and that it transmits a code unique to each a/c, each of which is registered with the FAA)?

  2. Does the '330 take any more electrical power or generate any more heat (I understand that the reason that is is longer than the '327 is that it contains essentially two transmitters for two different beacon standards, but I’m fuzzy on the details)?

  3. Since S-mode transponders will be required in Europe in GA a/c in a few years (2007?), do you know if Cirrus is going to start offering the '330 as a factory option? (I know you decided not to purchase a Cirrus, but you seem to know a lot about this stuff.)

  4. Are you still happy that you got the 260se vs. the SR20? (I’ll never forget the demo ride you gave me in the 260se, most amazing aerodynamics).

  5. Do you know if the '330 will display on the ICDS-2000 or the Avidyne MFD? (I know you don’t have them, but maybe you know). Or the Sandel?

Thanks again!

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Kevin, thanks much for a great post.

Does anyone know, can this be installed (currently) in an SR20 or 22? I’ve seemed to read varying accounts of this.

Andy

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Thanks for the pirep.

Any thoughts on the acceptability of the update delay especially relative to
nearby aircraft? Did the nearby aircraft appear to be roughly where they
were shown on the display?

thanks
Michael

6

Seems to me that the utility of the TIS system could be increased, with little to no additional cost, by the FAA’s doing two things:

  1. Including primary targets in the TIS display. My understanding is that this is possible but just disabled in the current TIS implementation.

  2. Transmitting TIS information from Center radars. I understand the limitation that the information would be delayed a little more than TRACON information – IIRC, Center radar sweeps every 12 seconds as opposed to 5 seconds at the TRACONs.

But even having 15-20 second old traffic information is better than nothing. Consider that it often takes close to that long for a controller to spit out “Cirrus 96706, traffic 3 o’clock, 2 miles, southwest bound at 3000 ft is a Beech Bonanza” (for example). So even verbal traffic indications realistically have a 10-20 second delay anyway. Also, for light GA aircraft (i.e. those most likely to be using TIS and not have TCAS), closure rates at a max of, say, 200 knots would mean that in 20 seconds, the distance between two head-on planes would change by 2 miles. Considerably less than 2 miles if the closure angle is not head-on, or if both planes are not travelling at 200 knots. While not great, having an extreme worst case of 2 mile inaccuracy in the TIS display is still much better than nothing.

Anyone have any ideas whether there are any other reasons why these 2 features are not enabled, or whether appealing to the FAA or AOPA on our behalf would do any good?

Just curious!
Steve

7

Thanks for pointing this out. In this case, I don’t know why I didn’t get him on TIS, as ATC did call him out to me.

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  1. Are you still given squawk codes, or does ATC now always know your tail number (I understand that the '330 isan S-mode transponder, like used by the airlines, and that it transmits a code unique to each a/c, each of which is registered with the FAA)?

They gave me squawk codes as always. Moreover FWIW none of the controllers commented on the mode S aspect.

  1. Does the '330 take any more electrical power or generate any more heat (I understand that the reason that it is longer than the '327 is that it contains essentially two transmitters for two different beacon standards, but I’m fuzzy on the details)?

I’m even fuzzier on these details.

  1. Since S-mode transponders will be required in Europe in GA a/c in a few years (2007?), do you know if Cirrus is going to start offering the '330 as a factory option? (I know you decided not to purchase a Cirrus, but you seem to know a lot about this stuff.)

I do not know what Cirrus has in mind re the 330.

  1. Are you still happy that you got the 260se vs. the SR20? (I’ll never forget the demo ride you gave me in the 260se, most amazing aerodynamics).

I’m delighted with the 260se and will be flying it for the foreseeable future. I’m sure I would have been delighted with the Cirrus also.

  1. Do you know if the '330 will display on the ICDS-2000 or the Avidyne MFD? (I know you don’t have them, but maybe you know). Or the Sandel?

I have an MX20 in my 260se and the 330 will not display on that unit. I don’t know about the others. I’m sure Mr. Stone the Garmin tech rep could answer the questions that I could not.

One more thing: the avionics shop said they could not configure the 330 to automatically turn on ALT and the flight timer at GS >30 kt as was the set-up with the 327. According to them it would only work with a squat switch. This seems odd to me and I may seek a second opinion.

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In reply to:

Thanks for pointing this out. In this case, I don’t know why I didn’t get him on TIS, as ATC did call him out to me.

Maybe the ATC TIS software excludes targets that are very close to your position but highly displaced vertically. Do you see targets that are thousands of feet higher or lower or just targets that are close to your altitude?

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A Google search on “mode s datalink” turned up all kinds of interesting (and very arcane) information.

One of the articles says that TIS (the traffic advisory system that the GTX330 displays) is designed to display traffic within a five mile radius and within 1200’ in altitude according to mode C data, which explains the traffic that the controller saw but the 330 didn’t.

It would be nice to see more, but the datalink runs at an effective throughput of 200-300 bps (that’s 0.3 kbps) or about 1/200 the speed of a typical modem. This severely limits the amount of information that can be transmitted.

There’s a somewhat petulant paper at MIT describing how Lincoln Labs delivered the mode S spec in 1975 but it wasn’t deployed; finally the midair over Cerritos CA in 1986 spurred FAA regs requiring TCAS-II in airliners, which runs on top of Mode S datalink.

Something that I never realized about TCAS-II that shows up in one article is that the Resolution Advisory that the two planes’ TCAS systems tell the pilots are decided by some form of negotiation across the mode S datalink, but directly between the two planes. I had figured that there was some sort of static formula to decide who would climb and who would dive, but it seems a bit more clever than that.

For the true networking geeks, there is a series of technical training slides, describing the mode S datalink communications protocols in fairly gruesome detail. The short version is that it appears to be X.25 running in 28 byte frames. That’s as '70s as orange shag carpeting.

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In reply to:

The short version is that it appears to be X.25 running in 28 byte frames.

X.25? Aaarghhhh! I had finally forgotten that… now I’ve remembered.

698×549 73.2 KB

  • Mike.
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In reply to:

The short version is that it appears to be X.25 running in 28 byte frames. That’s as '70s as orange shag carpeting.

Might as well be using two tin cans and some string!Just so everyone’s clear, the GTX-330 does not implement TCAS I or II, and there is no threat resolution or plane-to-plane communication going on.

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Ah, someone equally scarred. I implemented CCITT Yellow Book X.25 sometime during the first Reagan administration, if memory serves. Bleah.

There is some information on potential schemes for improving the data throughput, involving protocol changes and directional antennas and such, claiming to be able to achieve 30+Kbps, but it seems unlikely that it’ll ever be done. One of the schemes involves doing CSMA (think of a giant Ethernet with no collision detection.) Whee.

Years ago when I was doing international standards work, we were defining the OSI interdomain routing protocol, and a rep for one of the government contractors kept trying to keep the packets small because they wanted to run it over mode S datalink (this must have been late 80s sometime.) The scheme was stillborn, happily, as was the protocol.

I have half a mind to write a spec for running TCP/IP over mode S datalink, but I would have to publish it on 4/1 (the traditional date for publication of such things as “The use of IP over Avian Carriers.”)

14

When TIS service was available, I found the aircraft where the display indicated I should look. Thus it did not seem that the update delay was a significant factor.

Just yesterday I was practicing instrument approaches at Sacramento Mather (KMHR). As we were being vectored on downwind for the first approach, we noticed a target on the display that changed direction, position and altitude amazingly quickly. I wondered whether it was a system bug of some kind (or a UFO?) until we changed to tower frequency and found out that a T38 was also practicing approaches to MHR!

15

Steve is right on the mark with his analysis. In addition, having the availability of primary targets would make this system somewhat better than Skywatch in that virtually ALL traffic, not just transponder equipped traffic, would show with the 330. The only limitation Vs Skywatch would be the availbility of radar coverage.
But let’s face it: the biggest chance od a collision is in the denser traffic areas where ther IS radar coverage.

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In reply to:

Including primary targets in the TIS display.

One thing to keep in mind here is that there is no altitude information on primary targets, so no traffic alerts would be available. Posting a “traffic alert” for every primary target would probably result in too much distraction. I believe the displays can only track a limited number of targets simultaneously. Tracking primaries might saturate these units. Currently, they will display the n closest targets, ignoring targets far above or below you, but with primaries it would not be able to draw the distinction.

You make a good point about jets using active systems and thus not being subject to the problem with the long update period and inherent position uncertainty due to Center’s 12-second antenna rotation rate. But keep in mind that the large positional inaccuracy would be felt the most when you really need the accuracy to be good - when an aircraft is close to you. A couple miles of uncertainty is acceptable when you’re tracking a target 10 miles away, but it looms large when someone gets close, especially if they are converging from the side. It may cause you to be concentrating your gaze at the wrong clock position.

I’m not saying I’m opposed to implementing these ideas, but I don’t find it as compelling.

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Don’t forget the " High Risk " area — from my perspective ---- is during take - off and traffic pattern … higher density of aircraft - higher workload for all pilots in the area…and many times you will be below effective radar coverage - leaving the 330 ineffective.

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In reply to:

But let’s face it: the biggest chance od a collision is in the denser traffic areas where ther IS radar coverage.

Brian, I have skywatch and the best use for me is to help identify traffic in the pattern at uncontrolled fields. Many of these fields are below radar coverage. In more dense rada controlled traffic areas you normally have the ability to get separation services.

Mason

19

That totally depends on where you are. In many high density areas, radar coverage is quite good for satellite airports and you really do not need coverage below pattern altitude as you would not be below that altitude until base and fibal approach. At that point the traffic pattern is preety orderly anyway. When I am on final I would NOT want the distraction of a system warning me of a plane a mile ahead that just took off. That traffic is “no factor”. The problem with many of these TCAS systems is too mauch alarming around the airport; the palce where you need false alarms the least. Converging traffic approaching the airport until pattern entry is the greatest threat area. Must areas will have good radar coverage there.

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In reply to:

One thing to keep in mind here is that there is no altitude information on primary targets, so no traffic alerts would be available.

Gordon,

Is this something that is documented somewhere? I’m puzzled because the FAA does have 3-D radars that would give both range and height. There are three possible scenarios.

  1. Primary target
  2. Beacon target
  3. Primary and beacon target (the best scenario)