Not always seeing ADS-B rebroadcast traffic

[bkspero]

My plane is equipped with 1090es ADS-B out and a Dual XGPS170 UAT frequency receiver on the glareshield for ADS-B in. The 1090es transponder is configured to indicate that the plane is equipped with 978 ADS-B in. I am based between New York City and Philadelphia so I am almost always flying in airspace with radar coverage, and frequently within the 30 mile veil of Class B airspace. It is rare to not be receiving TIS-B and FIS-B from at least 2 ground stations, and not uncommon to see 7 at once. Traffic is displayed on any of 3 Android tablets running either Fltplan Go, Avare, or Naviator all with the same symptoms. Those apps are not set to filter traffic...they should display everything within the field of view. I often fly in trail with airplanes having UAT for ADS-B out.

The problem I am having is that I am often not seeing those aircraft when they are flying behind and/or below mine. In those locations they are in a blind spot for direct reception by the XGPS170, but it should, I thought, still be receiving traffic information from ground stations and displaying targets that are in such blind spots. That doesn't seem to be happening, though. I can turn such that the XGPS170 can receive their direct signals and the traffic reappears. So the breakdown seems to be with TIS-B rebroadcast.

Do I misunderstand how ADS-B should operate in this situation? Should I not expect to be getting their traffic information via TIS-B rebroadcast? If my understanding is correct, what could be causing the symptoms that I see?

 

[denverpilot]

Basics first. Do you receive weather data correctly on UAT? I’ll assume so since you say you can see up to seven transmitter sites. Are they showing you’re receiving good data from them on your devices?

Most forum posts here that have had folks not receiving on UAT have been because they’re really not transmitting the correct bits to notify that they’re UAT for the IN side.

What does the FAA test page say for one of your flights?

 

[Axtel4]

As @denverpilot said, check or ADS-B In equipage bits in the transponder. Your FAA Operation Summary will show the ADS-B emitter's reported ADS-B receive capability.

With a single band ADS-B receiver you will need the bit set for that band. (UAT In for a 978 MHz receiver.) If no bits are set the ground station will not transmit any TIS-B /ADS-R data.

If both bits are set, the ground station will suppress (not send) the ADS-R data thereby not contributing to spectrum congestion. The reasoning is because "requesting" aircraft is capable of receiving the ADS-B data air-to-air so the ground station won't duplicate the data for the aircraft.

 

[bkspero]

Thanks for the responses.


The XGPS170 receives weather to the limits of the apps being run. All receive METARs and TAFs, and NexRad radar.

I've gotten the FAA ADS-B Performance Monitor Report done twice. The most recent being in April of 2017. Both passed no problems. The Operations Summary pages of both reports say, "In Capability: UAT". I think that means that my 1090es transponder is properly configured to indicate that the plane is equipped with UAT ADS-B in only (I've also gone through the settings and that is the way they are set). So I think I should be getting rebroadcast on 978 MHz.

That said, Axtel4 described how rebroadcast data is not sent if both the 1090es and UAT receive bits are set (to reduce congestion). Does that mean that rebroadcast of traffic having UAT out is suppressed (to me) because my plane has its UAT in bit set? If that happens, then it would explain what I am experiencing. Recall that both planes that I lose track of when they are below and behind mine have UAT ADS-B out. So if the rebroadcast system thinks that I should be receiving their UAT signal directly, then it would not be rebroadcasting their traffic information to me. I would only be seeing them when the XGPS170 is receiving their direct UAT signal.

 

[denverpilot]

Thanks for the responses.


The XGPS170 receives weather to the limits of the apps being run. All receive METARs and TAFs, and NexRad radar.

I've gotten the FAA ADS-B Performance Monitor Report done twice. The most recent being in April of 2017. Both passed no problems. The Operations Summary pages of both reports say, "In Capability: UAT". I think that means that my 1090es transponder is properly configured to indicate that the plane is equipped with UAT ADS-B in only (I've also gone through the settings and that is the way they are set). So I think I should be getting rebroadcast on 978 MHz.

That said, Axtel4 described how rebroadcast data is not sent if both the 1090es and UAT receive bits are set (to reduce congestion). Does that mean that rebroadcast of traffic having UAT out is suppressed (to me) because my plane has its UAT in bit set? If that happens, then it would explain what I am experiencing. Recall that both planes that I lose track of when they are below and behind mine have UAT ADS-B out. So if the rebroadcast system thinks that I should be receiving their UAT signal directly, then it would not be rebroadcasting their traffic information to me. I would only be seeing them when the XGPS170 is receiving their direct UAT signal.

You’ve got it. If you announce you want both you’ll only get air to air. But you’re set to UAT so you’ll only get UAT which is what they want you to do.

I missed it. Do you have an external antenna for UAT? If not, yeah, you could miss stuff on UAT and only get air to air from the transponder. And aircraft blockage would be the most often culprit in reception problems in that case.

I don’t believe they filter assuming you’d receive air to air directly on UAT but not every transmitter site sends up traffic on UAT and I believe only one is going to send you yours.

I’d have to go check to see if there’s a spec on duplication when UAT ground transmitters overlap coverage but I suspect with all the silly filtering they’re doing with the dangerous hockey puck, it wouldn’t surprise me at all if traffic is routed only to a single uplink transmitter based on your last reported location. Whatever’s closest. And if that’s blocked by the aircraft from a wimpy antenna inside the aircraft with a questionable quality receiver... well, it’s gone.

Let’s just say in every other modern RF based data system you’d never ever let a safety related packet go without an acknowledgement of reception in the protocol. You’d never see any other system use “fire and forget” into RF for safety items. You’d have the receiver ACK the reception of said data or you’d retransmit it.

The over the air protocol design of ADS-B has a lot of holes in it. Bad ones. For a safety system anyway.

Think about this scenario... UAT uplink transmitter goes bad. Are they monitored at any distance to prove they’re putting out rated power and all transmissions aren’t corrupted? I don’t believe they are. If they are it’s because someone figured out that the protocol is incredibly dumb and weak for a modern RF data system carrying critical safety of flight data.

ADS-B is great. I don’t trust it any further than I can throw it, though. All it takes is a bit of on board RF interference and the entire thing stops working without warning or any way to see it. If it were a proper RF data transport protocol you could at least set an error warning rate and toss an email to the aircraft owner that their ACK/NAK ratio was way out of whack from the norm for the system post-flight. As it’s built now, you’d just go on your merry way not hearing jack squat and nothing would tell you other than the traffic screen not showing everything it should.

Thus, it’s “advisory” only to me, once you analyze how it actually works. Relying on thousands of variable receiver systems to properly receive packets without asking the receiver to say “got it!” is incredibly stupid RF data design work.

Most apologists for it will whine that it was destined in the 80s or 90s. We had plenty of ACK/NAK going on in data systems back then. This wasn’t new.

 

[Axtel4]

I don’t believe they filter assuming you’d receive air to air directly on UAT but not every transmitter site sends up traffic on UAT and I believe only one is going to send you yours.

From Advisory Circular 90-114:

2-3.

ADS-B BROADCAST SERVICES.

ADS-B implementation includes three broadcast services: Automatic Dependent Surveillance-Rebroadcast (ADS-R), TIS-B, and FIS-B.

ADS-R. Because the ADS-B system operates on two separate frequencies (1090 MHz and 978 MHz), there is a need to translate, reformat, and rebroadcast the information from each frequency to enable aircraft operating on the alternate frequency to process and use the other’s information. This process is referred to as ADS-R and occurs within the ADS-B ground station.

NOTE: Aircraft operating on the same ADS-B frequency exchange information directly and do not require ADS-R translation. Aircraft with ADS-B IN capability on both UAT and 1090ES do not require ADS-R service.

 

[dans2992]

Interesting. So, in order to best ensure you receive all traffic, it’s probably best to specify _only_ 978 in even if you have a dual band receiver....

Thanks, FAA.

 

[denverpilot]

Interesting. So, in order to best ensure you receive all traffic, it’s probably best to specify _only_ 978 in even if you have a dual band receiver....

Thanks, FAA.

No guarantee you’ll receive all traffic even then.

 

[dans2992]

No guarantee.... but better odds.

 

[denverpilot]

No guarantee.... but better odds.

Most “safety systems” have a minimum standard to engineer to for number of lost messages and attempted retries, based upon urgency of the message.

ADS-B as a properly engineered RF data safety system, is a complete joke. It only gets praise because the alternative is nothing and most people were blissfully unaware of the traffic around them until they saw some.

The lack of known delivery of messages in this system is going to get someone killed someday. Then we’ll be told there will be a multi-billion dollar modernization project and ADS-B Version II.

 

[bkspero]

I don’t believe they filter assuming you’d receive air to air directly on UAT but not every transmitter site sends up traffic on UAT and I believe only one is going to send you yours.

Denverpilot, thanks. Please clarify this for me, as the only way I can explain what I am seeing is if they are filtering out traffic having UAT ADS-B out. I've flown 100 nm having a plane that I know is below me and directly behind not showing on my ADS-B in display. In my area that probably means that my receiver has been in contact with at least 10 and maybe as many as 30 ground rebroadcast stations. I could accept that I might miss that trailing traffic intermittently if I was only getting rebroadcast information on its position from a single ground station at a time, but under that circumstance I doubt that this problem would be continuous for the whole flight as I was handed from from one ground station to another. The odds of the key ground station always being in my blind spot over 100 nm are, I think, near zero. The trailing plane, however, is continuously in my blind spot, so the experience would make sense if the ground stations were filtering out traffic with UAT ADS-B out (causing me to rely on UAT air to air), and I was not receiving UAT air to air from the trailing airplane due to its position.

BTW, I am only using the OEM antenna on the XGPS170 positioned on my glareshield. Hence the blind spot below and behind the plane. Interestingly, it seems to have no issues either with planes directly below or below and ahead (or any other direction). Only below and directly behind.

 

[denverpilot]

Not sure. Don’t think it’s supposed to work that way. The problem is, who would one contact to see what the other end is sending and why on the FAA side of things in a scenario like yours? Maybe the email for the group that was analyzing the rebate tests would get you closest to an expert? Or an email (not a phone call, it’s too technical and detailed to survive that intact) to the FSDO that’ll be bounced around a bit before finding the technical folks? Not sure.

 

[bkspero]

Thanks again, D. I think I will try attending the next webinar/seminar that comes up on NexGen and ask the question there. If that doesn't work, then next stop the FSDO.

 

[Axtel4]

So the breakdown seems to be with TIS-B rebroadcast.

The “breakdown” isn’t with the TIS-B rebroadcast (or rather broadcast), but the breakdown of the air-to-air reception of the 978 UAT ADS-B Out transmission. Since you are UAT IN ADS-B equipped, and have broadcast that status to the ground station via the UAT IN capability status bit, the ground station will not rebroadcast any 978 UAT ADS-B Out (UAT ADS-R) traffic to your ADS-R client “request.” The ground station will broadcast the Mode C and Mode S Version 0/1 traffic as TIS-B data and Mode S Version 2 1090ES ADS-B traffic as ADS-R data to your aircraft.

The problem I am having is that I am often not seeing those aircraft when they are flying behind and/or below mine.

What you are describing is antenna shielding of our ADS-B In device. In the scenario you described, your receiver is setting on your glare shield and your air frame is blocking reception behind and below your aircraft. Also, compounding the problem, the UAT transmitting aircraft probably has their ADS-B antenna installed on the belly of the aircraft and their air frame is blocking their signal to you. This attenuates (blocks) the ADS-B signal above the aircraft. Given the location of the transmitting and receive antennas and the path loss distance between the two aircraft one could see in excess of 90 dB of attenuation between the TX and RX antennas. This attenuation would put the air-to-air transmitted signal below the receive threshold of your portable ADS-B receiver. This is part of the reason the FAA recommends antenna diversity installations in ADS-B Out installations.

Does that mean that rebroadcast of traffic having UAT out is suppressed (to me) because my plane has its UAT in bit set?

Yes. When you have the UAT IN bit set, the rebroadcast of the 978 UAT data to your aircraft is suppressed because the bit indicates your aircraft has onboard capability to receive the 978 UAT ADS-B data air-to-air. The 1090ES ADS-B aircraft will be rebroadcast to your aircraft on the 978 UAT link. In addition, TIS-B will also be broadcast to your aircraft on the UAT link.

There are three components to the ADS-B traffic picture. These are air-to-air reception, TIS-B, and ADS-R.

Air-to-air: This is the ability to receive the 1090 ES or 978 UAT ADS-B squitters of properly equipped ADS-B Out aircraft directly from those aircraft. No interaction with or processing of the ADS-B data is required by the ground stations. This is the quickest and most efficient method of receiving ADS-B data for target aircraft.

ADS-R: This data form requires compliant ADS-B Out transmissions for the target aircraft. It also requires processing by an ADS-B ground station. It is broadcast on either the 1090ES or 978 UAT ADS-B link. The data consists of the aircraft ID, position, altitude, and performance data of the target aircraft. For example, since your aircraft (ADS-R client) is equipped with a 978 UAT receiver and provides the UAT IN bit to the ground station the ground station will repackage the 1090ES ADS-B data for the 1090ES aircraft in your service volume and rebroadcast the data to your 978 UAT IN equipped aircraft on the 978 UAT link allowing you to see the 1090ES aircraft. Also, since you have broadcast you can receive 978 UAT directly the 978 UAT is not rebroadcast from the ground station. This also works the same way for 1090ES equipped aircraft.

Further, “ADS-R service is not offered in all service volumes, but if the service is provided the ADS-R Client must be ADS-B Out equipped, have broadcast a valid position report within the last 30 seconds and received by a ground (Radio Station) RS and must be ADS-B In on only one link. Aircraft that are dual link technology equipped will not receive an ADS-R uplink message, they will receive a single ADS-B target.” – FAA presentation “What We Have Learned About ADS-B and How Do We Stay Under the RADAR” at IFIS2014

TIS-B: This service allows ADS-B In equipped aircraft to see Mode A/C and Mode S Ver. 0 & 1 equipped aircraft. The Mode C / Mode S aircraft targets received by ATC are repackaged and rebroadcast as TIS-B data. TIS-B data does not include ADS-B Out or ADS-R target aircraft.

 

[denverpilot]

@Axtel4 great description. I was also under the impression that ground retransmitted air on UAT. I see it doesn’t now.

The “hidden node” problem has been well known for decades in RF data systems. This system sure can suffer from it.

In this design, stifling the ground station and assuming air to air reception will always work — with the ground stations being the only “Nodes” that literally can and must hear everyone at all times in their service volume — any hidden node issues caused by aircraft shielding/attenuation are amplified.

Arghhhhhh.

Spray and pray relying on two aircraft mounted antennas to hear each other in three dimensions with no serialization of packets or acknowledgements or retries? With hidden nodes and collisions at the receiver too? Ick.

I already knew about the dual RF networks of course. That’s a whole different engineering mess.

The mandatory power level of the 1090 stuff is MUCH higher than the UAT stuff on 900 megs. I can *almost* see relying on that since that’s a lot of power at close range.

I honestly don’t want to go look. I don’t know the power level on UAT. M

Okay I did. And it’s better than I thought. A little.

UAT Low Power is minimum 7 watts measured at the coax connector and an assumed 3db I’d coax loss which they claim will cover 20 miles.

Hmmmm. Maybe. Maybe. It should. It can.

But... antenna gain and directionality and nulls combined with aircraft blockage... mmmmmm it’s questionable to me.

RF systems can be pretty cranky with a simple bad connector at 978 MHz. And aircraft have a lot of vibration to screw up coax, connectors, and antennas.

Medium looks a lot saner to me as a minimum standard for air-to-air and a spray and pray system with collisions present.

https://www.icao.int/safety/acp/acp...01-report on a.i.1-appendix b - uat sarps.pdf

(And that was the first from a Google search. I didn’t see if it had been amended since 2005.)

Ah well. It’s a mess. At least it has some FEC in it. That helps.

How far away are the trailing aircraft you don’t see? If they’re further out than 20nm, and your UAT system is a “Low” system, that’s all it’s designed to do. But I assume these aircraft are a lot closer than 20nm.

 

[denverpilot]

To continue that thought. If you’re not seeing stuff directly behind and below you for about 20nm, the entire system on receive may need to be checked. Those power numbers should work most of the time at that range at 978 MHz.

If the antenna is top mounted on the aircraft that helps explain the loss of traffic behind and below. Antenna null in gain there plus the aircraft shielding the reception from the other aircraft.

Antenna, coax, connectors, and maybe even the receiver itself, to see if it’s meting the spec for signal strength and error rate on receive at the lowest power level with test gear.

And it should be tested both on a bench and then on the aircraft with nothing on injected down the coax or received by the antenna even, if that’s possible. That eliminates first the receiver only and then the entire receive system incline the the antenna system.

And then if that works a final test with every electronic device being used, even portables and cheap chargers for those, if the bench test works. You *could* be fighting noise from other devices on board.

Just thoughts. Hopefully helpful.

Also think about this. If the OTHER aircraft has a bad coax, connector, or transmitter... it’ll look like your receiver is deaf.

It’d make a lot more sense for the ground stations to rebroadcast on UAT. It has a lot more data speed than 1090 and shouldn’t be overloaded to the breaking point in high traffic density areas. The ground stations are going to have a lot easier time of receiving a broken/weak aircraft than other aircraft will.

Anyway... maybe your avionics shop has the proper test gear to test your receiver in bench, installed, and installed with everything turned on configurations. Might be worth a look at the numbers if they do.

 

[John Collins]

Without an external antenna mounted to the bottom of your aircraft, you will also miss TISB and ADSR messages from the one and only ground station assigned to broadcast these messages for your aircraft's benefit. So just like your aircraft can shield you from receiving air to air if the ADS-B system is below you, your aircraft can shield you from the one and only ground station broadcasting your ADS-R and TIS-B messages. It doea you no good that you are receiving from seven other ground stations. If you really care about the traffic from below you or from a ground station, install a $30 bottom mounted antenna.

 

[dans2992]

What determines which ground station sends you the ADS-R and TIS-B?

 

[denverpilot]

What determines which ground station sends you the ADS-R and TIS-B?

A computer.

Hahahah. Just messing with you man.

I don’t think FAA has published how the backend works. We probably don’t want to see that sausage getting made, either. Could be ugly.

I’ve always wondered where the processing happens because there would be built in delay in nearly any design that could handle multiple sites in a particular area.

And lord only knows what happens when the processing stuff goes down. And it will. Haven’t seen any “our hockey puck machine isn’t working today” NOTAMs yet.

Wonder what those will look like.

 

[dans2992]

I’ve been observing which towers send which TIS-B / ADS-R traffic here in Phoenix. One of them sends about 99%.

 

[John Collins]

What determines which ground station sends you the ADS-R and TIS-B?

The station receiving the best signal from your ADS-B Out. Since your ADS-B Out has an antenna on the underside of the aircraft, the closest to your aircraft is likely to have the best reception. So with an antenna mounted inside an aluminum fuselage, there is good likelihood of a portable being blocked. So a small antenna mounted with the same view of the ground as your ADS-B Out antenna will be best. These antennas are cheap.