The Case Of The Missing Traffic


The Case Of The Missing Traffic

For the past several weeks I’ve been receiving reports from FlightBox customers of “missing traffic” – traffic observed visually or on some other system (TIS, TCAS) but not seen on their EFB application from ADS-B. In some cases the answer was a simple misunderstanding of the way ADS-B traffic currently works, but a subset of the reports sounded suspiciously like we had a problem – somewhere. Either a bug in the Stratux software that powers FlightBox, or something going on with the ADS-B network.

Yesterday morning I decided to figure out what’s going on – or at least give it my best shot. From my own experience and from customer reports, we have no problem seeing aircraft that are equipped with ADS-B Out. What is missing is the rebroadcast secondary surveillance radar data – the copy of “everything in your hockey puck” that you’re supposed to get if you have ADS-B Out. I figured that one of the following was happening:

A) Stratux was not getting the data because of poor reception
B) Stratux was receiving but ignoring ATC traffic data due to a bug
C) Stratux was not getting the data because the FAA towers are not sending it
D) Secondary surveillance radar was not picking up the traffic

To figure out which of the above is causing the problem I put together a simple, practical test.

I own an Appareo Stratus 2 which I use as the ‘control’ in my experiments. If the Stratus has data flow from a tower but FlightBox does not, it could be failure mode A – poor reception. If both devices connect to and receive the same data from the towers, that essentially rules out condition A. If the Stratus receives and displays traffic that is not visible to the FlightBox, the root cause might be failure mode B – a bug in the Stratux code. But if both devices display the same traffic, that rules out condition B. Failure modes C and D take a bit more effort to test.

My 1976 Grumman Tiger is equipped with FreeFlight Systems RANGR Lite UAT transmitter, and I regularly request status reports from the FAA. As of the last test (a few months ago) I was 100% ADS-B Out compliant. In other words, I should absolutely trigger the towers and receive a “puck” of ATC radar data for aircraft in my vicinity. To test if the ATC radar is making its way to the ADS-B network, I enlisted the help of my friend Larry. Larry flies an Arion Esqual – a precursor to the popular Lightning homebuilt – which has a standard Mode C transponder but no ADS-B Out. His job was to fly in relatively close formation with me to present a controlled traffic target.

Yesterday morning Larry and I took off from Roosterville (our home airport).  I had two iPads running ForeFlight – one connected to the FlightBox and the other connected to the Stratus 2. I took off first and climbed to 1000’ AGL where both systems showed solid connection to two ADS-B towers. When Larry took off, I kept an eye on the iPads looking for him to appear on ForeFlight.

We flew south from Roosterville towards Liberty Landing. I let Larry stay roughly a half mile ahead and 200 feet above me. He did not appear on either iPad at 2,000′ MSL (~1100′ AGL). After roughly five minutes at 2,000′, I asked him to climb up to 3,000′. At roughly 2,500′ MSL, he appeared as an ADS-B traffic target 100’ above me an less than a mile head. Ok, it appeared to be working as expected.

To figure out the “bottom” – the lowest altitude at which it worked – I asked him to descend. He dropped off the screen at roughly 2,200′ MSL. We made a turn out towards the east and decided to see where he re-appeared. He began a steady climb that eventually topped out at 3,500′ MSL, without reappearing on either device. A quick check of the Weather tab showed that FlightBox was continuing to receive ADS-B data from the towers. Both the Stratus and the FlightBox continued to show both 1090-ES and UAT targets, but all appeared to be direct.

We flew for another 30 minutes at various altitudes. Larry only appeared again briefly, at 3,900′ MSL. On the off chance that we were too close – that ATC saw one target instead of two – we diverged to at least a mile separation. No joy. We contacted Kansas City approach to make sure that we being seen by radar. They had us 5-by-5 in exactly the right spot. Larry was at 3,900′ and I was below him at 3,200′. That would seem to eliminate the admittedly far-fetched failure mode D.

To review the facts:

  • Other than a few momentary differences, both devices showed the same traffic.
  • Both devices held a connection to at least one and most often two towers throughout the flight.
  • The few times that Larry appeared, he appeared on both devices at the same time.
  • When he disappeared, he did so from both devices within 30 seconds of each other.
  • We flew to within 10 miles of the closest ADS-B tower and had excellent reception on both devices.
  • Kansas City approach had both of us on their scopes.

Later in the day I went back up alone and spent some time in the airspace below the Kansas City class B directly along the approach path for runway 18 at MCI. I was within 10 miles of the ADS-B tower at Dearborn, Missouri and maintained a solid connection to it. I spent twenty minutes or so visually observing airliners arriving and departing above me. None of them appeared on either ADS-B system.

All of this seems to indicate that the cause of the missing traffic is failure mode C – the ADS-B network simply is not sending the data. The next question is, of course, why? My ADS-B Out transmitter was running. I should have had a puck that included Larry and the MCI traffic. Why wasn’t I able to see either?

Hard to say. It might be that the only radar that feeds into the ADS-B system is ARTCC (“Center”) and the altitude range we tested (2,000′ – 4,000’ MSL) was below the visibility “cone” for their radar. But they did have Larry as low as 2,500′ MSL for a while. It seems more likely that whatever gateway feeds ATC secondary surveillance radar data into the ADS-B network is flaky. Or that the software in the towers that calculate the “pucks” has a problem.

Whatever the cause, never assume that ADS-B provides a complete picture of traffic – even if you’re equipped with ADS-B Out. We’re still very much in a “see and avoid” world, and probably will be until at least 2020.

If you have two ADS-B In systems and a friend who’s willing to play along, please try to replicate my experiment. I would love to have enough evidence to take this to the FAA and get some kind of response.

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23 Comments so far

Matt PoffPosted on  2:42 pm - May 23, 2016

Steve, I want to thank you for looking into this issue. I’m one of the folks that sent you a note about missing traffic. I suspected that I was only receiving air-to-air traffic on either frequency. Even while I was at 3500 feet with visible traffic (non-ADS-b out) nearby. Very disturbing that the FAA is having these issues. It will get better as more aircraft are equipped with ADS-b out but I think the FAA has dropped the ball. Since my plane has been updated with a Garmin GTX 330ES I’m done. Also, consider sending your finding to the AOPA to reach a large audience and perhaps they can bend the ears at the FAA and then report in their magazine. Thank you again, Matt

Steven HaskettPosted on  2:52 pm - May 23, 2016

I’m new to Stratux. Yesterday I was investigating how the unit worked using the web interface. With my single receiver, I could get traffic on 1090 MHz (heavy iron) and the occasional UAT traffic on 978 MHz. I was parked a few hundred yards from the tower which is located on the airport, excellent signal. I was expecting to see traffic re-broadcast from one band to the other, but as near as I could tell the only way to get all the traffic available was to have two receivers and merge the output. Is that the case, or was I seeing the same issue of the FAA not re-broadcasting the traffic?

    ssokolPosted on  4:48 pm - May 23, 2016


    It’s possible – not necessarily likely – that there were no aircraft in the area with ADS-B Out (and with the “Send ADS-B In” setting enabled on their transmitter) to trigger the retransmission of traffic data. The heavy iron is no help – some of them do have 1090-ES enabled but few if any have the “Send ADS-B In” setting active, and even if they did there’s not much likelihood of anything interesting being inside their “puck” most of the time.

    What you suggest about mixing the two traffic sources is already happening – we feed both 1090-ES direct and UAT direct to your EFB app. What’s missing isn’t the direct traffic but the copy of all the non-ADS aircraft that are seen by ATC radar.

    Most likely, you’re seeing the same problem that I was – the towers continuously broadcast a stream of FIS-B data (weather, TFRs, etc.) but rarely send out traffic, even with a “client” aircraft in the area. I may fly down to New Century (KIXD) some time this week, park on the ground and use their tower for some tests. There’s all sorts of activity there between IXD, OJC, K81, K34, LXT, and LRY all within 20 miles.



John CollinsPosted on  6:54 pm - May 23, 2016

I would assume that you should be receiving at least 3 towers at all times and occasionally up to 5. If you don’t have an external antenna for your test systems, the fuselage can block your reception of one or more of the towers. Unlike weather, TISB is only uplinked on a single tower. The uplink tower is based on the one with the best reception from your ADS-B Out. Since your ADS-B Out has a belly mounted antenna, the tower that gets the best signal can be a different one than the ones that able to be seen inside your cockpit. If so, then you won’t see the uplink.

Also, your comment about the airlines, although true, understates the situation. The compliant version is version 2, most of the airlines that do have ADS-B Out don’t have version 2, they have version 0 or 1. These earlier systems do not have the bits defined that inform the ground station that they can receive TISB, so the ground station blindly broadcasts the TISB on 1090 MHz. If you only have a single frequency UAT receiver, it will be awfully quiet unless your aircraft also has ADS-B Out.

    ssokolPosted on  7:12 pm - May 23, 2016

    I have a belly-mounted L-band antenna that I used for the FlightBox during the test. The Stratus 2 has an internal patch antenna. There are two towers in the Kansas City area that are line-of-sight above 2000′ MSL – the low power tower at Dearborn Missouri that serves MCI and the medium power tower in Olathe that serves IXD (and Garmin HQ). Go up to 4,000 MSL and you can usually pick up the tower out of Topeka KS. At 5,500 you pick up Columbia MO. During the test I had two – Dearborn and IXD – throughout the flight. Given my location, I would assume that the stronger reception was from the Dearborn tower.

    The test systems were both dual-band and running the latest available firmware. My aircraft is ADS-B Out equipped and should have triggered both TIS-B and ADS-R broadcast for my puck. It might turn out to be something odd, but I’m still pretty sure the data was never sent.

Jeff VanderMeulenPosted on  11:18 pm - May 23, 2016

I recently acquired my FlightBox and it performs as advertised. Prior to the FllightBox, I was using a Dual blue tooth antenna for my iPad GPS feature for ForeFlight. Should I continue using the Dual antenna or does it interfere with the operation of the FlightBox? The Dual antenna is located about 12″ from the FlightBox on the glare shield of my plane.
Thanks for a good, affordable product.

    ssokolPosted on  12:23 pm - May 24, 2016

    Hi Jeff,

    If the Dual is providing solid GPS, by all means continue using it. There shouldn’t be any problem as the Dual uses Bluetooth and FlightBox uses wifi.



BlakePosted on  12:05 am - May 24, 2016

Just to point out a little irony.
The Flightbox is clearly into supporting “Open” systems, yet in your testing you are using a closed system for EFB software (iPads running ForeFlight).

Perhaps it would be better for you to be more consistent in philosophy if you supported Ifly (not locked into Apple IP), or some other Open EFB.

You did some good testing, to bad none of the more Open EFB SW solutions could have taken advantage of it.

    ssokolPosted on  12:28 pm - May 24, 2016

    Hi Blake,

    I used ForeFlight because it’s the EFB with which I am the most familiar, because their license policy allows you to us it on two tablets, and because it works with the Stratus 2 which I needed to use as a “control” in the experiment. I actually use iFly, Aerovie, DroidEFB, WingX Pro, FltPlan Go, and Avare for testing FlightBox. The testing I did over the weekend has implications for all users of the ADS-B system, not just ForeFlight users. The test procedures could be reproduced using any number of different devices and EFB or MFD platforms.



DavePosted on  12:40 am - May 24, 2016


I too have the FreeFlight Systems RANGR Lite UAT transmitter (ADS-B Out only) that was installed in October. I immediately discovered that I was not triggering the ground stations even though the FAA “certified” my ADS-B Out is compliant. After a lot of back and forth between my Avionics shop, FreeFlight and Garmin, FreeFlight finally ‘fessed up that the RANGR Lite is broadcasting the wrong header in the stream to trigger the ground stations to broadcast TIS-B traffic. This is only an issue in the RANGR Out-only product.

The good news is that FreeFlight finally released a fix for this last month, which I had installed (under warranty). I then tried to run the same test you tried using Garmin Pilot and a GDL 39 receiver compared to Stratux (978-only) running using WingX. I was seeing traffic in Pilot that I wasn’t seeing on WingX but then there were times that I didn’t see any traffic on WingX and that’s when I discovered the Disconnect problem that you and I have been exchanging emails about the last few days. So, my tests are still pending. There is a flag in G Pilot that tells you whether you are getting TIS-B, and that’s on all the time since the fix was installed, so I know I am always triggering the ground stations to send traffic now.

To further complicate matters, the RANGR must be configured to tell ground stations whether you only have a 978 receiver on board, only a 1090 receiver, both or neither. Prior to the software update, the RANGR was hard coded to tell the ground stations that you have neither a 978 or 1090 receiver, so the ground stations did not send traffic when it got “pinged” by the RANGR. Depending on whether you have a 978 or 1090 receiver on board, or both, configuring the RANGR correctly is important, otherwise you may get duplicate 1090 traffic or no 1090 traffic at all.

So, unless your RANGR is In/out, or has last month’s software update, you will need to rerun your tests after you get the update…which is covered under warranty by FreeFlight. This probably explains your unexpected, inconsistent observations.



    ssokolPosted on  12:31 pm - May 24, 2016


    Interesting – and a bit depressing. I was down for nearly a month during the initial installation. Did you have to send the box back to FreeFlight, or is there a way to perform a field upgrade (i.e. firmware update) on the system?

    I’ll be curious to have someone using a different ADS-B Out solution reproduce the experiment. I’ve had people with 1090-ES transponders also report missing traffic.



DavePosted on  2:32 pm - May 24, 2016

Hi Steve,
It’s a field firmware upgrade that can be performed without removing the unit from the plane; it takes 1-2 hours.
My avionics shop removed the unit when they did the upgrade since they were worried about draining my battery and they felt they have better control doing the upgrade on the bench. But that was their decision; FreeFlight says it’s an in the plane firmware upgrade.
Your avionics shop can get the upgrade off the FreeFlight website now; it’s available.

ssokolPosted on  2:47 pm - May 24, 2016

Ok, mia culpa – there was one additional failure mode I didn’t think of, which may be part (or all) of the problem. It didn’t occur to me that the RANGR Lite could be the problem – it passes the FAA test with flying colors. But at the suggestion of Dave (see above) I contacted FreeFlight and guess what – they had a very subtle bug in the system.

It turns out that the configuration utility allows you to enable an option to request traffic from the towers. However, the setting is ignored by the device. So the installer thinks everything is configured correctly, but in reality the system *does not* transmit the request flag, causing the towers to ignore you. They have a fix that my avionics shop will (weather permitting) install later this week.

So this makes me think there may be another use for the Stratux software: testing ADS-B installations. Apparently the test process provided by the manufacturer didn’t catch this issue, nor did the test set used by the installer. (So much for the infallibility of of certified, TSO approved system.)

So that may be the issue – at least “my” issue. But I’ve had reports of missing data from users with 1090-ES Out systems, so I’m still wondering if there’s a deeper problem. I suppose it could be that the issue with the RANGR Lite – not sending the traffic request flag – is common, or that the avionics techs that installed those systems simply didn’t enable it as the owner did not buy and install a certified ADS-B In system. Time will tell.

DavePosted on  11:06 pm - May 24, 2016

Hi Steve,

You are onto something with your suggestion of using Stratux to test ADS-B installations, but I would generalize that to testing ADS-B as part of the installation and afterwards as we, i.e., all the timel. Failures of on board or ground equipment can occur at any time. So, if Stratux can detect that TIS-B coverage is unavailable, that may be a normal state for someone without ADS-B Out, but is a red flag for someone with ADS-B Out.



C CristPosted on  8:04 am - May 26, 2016

Steve I like this process and can hardly wait to receive my own FlightBox

Matt PoffPosted on  3:19 pm - May 28, 2016

Hello Steve, I believed that I was missing traffic as well, although my Archer has a GTX330ES. Based on some of the above posts I contacted my avionic installer and asked them to check with Garmin about possible settings issues. Sure enough, when mine was installed the understanding by the installer was to set the In feature based on the unit it’s self. So the 330ES was set to NO for both frequencies. Yesterday I took the plane back and in about 90 seconds the technician had reset both receive parameters to YES since my Stratux is dual frequency. Now I should be pinging the ground stations to include traffic. As I was departing for home I quickly noticed two aircraft entering the pattern on my iPad running FlyQ. I radioed the Cessna on downwind to inquire whether or not he had ADS-b Out. He stated he did not, meaning I was receiving his target track from an FAA station. Finally. In addition, I began to see many more traffic symbols on my 20 minute flight back than I had noticed before. It appears that the idea of setting these parameters correctly is not a subject that is covered in any description of the ADS-b system. I had purchased the 1/2 wave antennas in an attempt to improve performance but yesterday I just used the regular 1/4 wave antennas and they seemed to perform fine, once the settings were corrected. Thank you for your assistance in sorting out this confusing subject. Best wishes to you and your company. Matt

    Joe O’BoylePosted on  1:01 am - Jun 22, 2016


    Can you explain to me how the flightbox unit is communicating with the Garmin GTX330 ES unit that the frequencies set to “in feature”/ “receive parameters” to “yes” instead of “no” would matter. Does the GTX330 receive the flightbox WiFi frequency?

      ssokolPosted on  3:21 pm - Jun 22, 2016


      The 330ES doesn’t connect with the FlightBox, but what it sends out in its squit (the Mode S output) has a direct impact on what the FlightBox receives from the ADS-B towers. By setting the “ADS-B In Equipped” flags for both frequencies, the 330ES in Matt’s airplane is telling the towers to broadcast traffic data that is relevant to his flight. This causes the software in the towers (or somewhere in the ADS-B network) to create and manage a “hockey puck” for Matt’s plane. The system will broadcast traffic reports for any aircraft within that hockey puck space that is seen by ATC secondary surveillance radar (“transponder radar”) but which is not sending ADS-B Out data. There’s not reason for the towers to send position info for ADS-B out equipped targets as Matt’s FlightBox will get that information directly from the target.

      If Matt had a Single Band FlightBox, he would set up the 330ES to indicate that he can receive UAT directly but not 1090-ES. In that case the towers would add “ADS-R” or “rebroadcast” to Matt’s hockey puck. Position data for any 1090-ES targets in Matt’s puck would be broadcast by the towers on UAT, allowing him to see both non-equipped aircraft (i.e. aircraft with no ADS-B Out) and also aircraft equipped with 1090-ES.

        JoePosted on  11:38 pm - Jun 22, 2016

        So Steve, I have a Garmin GTX330 ES transponder and the dual band flight box. I should ask the avionics’ installer to set the ADS-B “in flags” to “yes/on” to maximize my ability to see other traffic (both those equipped and not equipped with ADS-B out0 on my iPad running Foreflight. Is that correct terminology?


          ssokolPosted on  12:54 pm - Jun 23, 2016

          Hi Joe,

          Yes – your tech should understand that. I’m not sure what the fields are called in the 330ES configuration. Just tell him that you have ADS-B in on both UAT and 1090-ES and need the system configured to request ADS-B In traffic.



AllanPosted on  11:09 pm - Jun 2, 2016

I fly frequently in the San Francisco Bay area with a 978 MHz UAT transceiver connected to an iFly GPS. Very heavy traffic at times. I frequently see aircraft visually, and get traffic alerts from ATC, that I don’t see on my GPS. I have checked my transmissions online with the FAA and they are fine. It appears that the ground stations are not broadcasting some of the traffic that they see on radar. Not sure why, but for now I still use flight following.

Tony YaconoPosted on  6:51 pm - Aug 4, 2016

I have The Program True Flight and the FlightBox will not connect to it. The Flight Box will work with many other programs, so we know it is doing its job.
Can you tell me how to make the connection to True Flight?
Thank You/God Bless

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