ads-b out and Transponder ??

[RotorDude]

I agree with your analysis.

What I am not sure about is when and how Excelis will resolve the issue. One option would be for them to generate a TISB for a target that has a SIL or SDA of 0. A second question is whether or not the GBT will treat an aircraft as a non client, and therefore not generate any TISB or ADSR services on its behalf. Currently it treats the portable as a client and as you described, that is why it doesn't generate a TISB for it.

OK, but then at the very least, until this is done, there should be a very clear and well publicized warning to users of uncertified OUT units that by turning their unit on, even their mode C becomes invisible to certified users.

 

[Jim Logajan]

A Mode-C target that matches your an ADS-B out is not transmitted through TIS-B.

The only time a Mode-C signal and an ADS-B message can be correlated as coming from the same aircraft is when they use or know the unique ATC squawk code. Otherwise there is no way to unambiguously "match" the two signals - certainly not if current squawk code is 1200. Remember that the only info that ATC gets from a Mode-C radar return besides the squawk code is the pressure altitude, bearing, and distance, but that ADS-B data message contains GPS coordinates, 24 bit transponder code, and registration number, among other bits.

In my reading on the subject, I don't recall seeing anything about which system - TIS-B surveillance processing (ground) or in the aircraft - is expected to perform any heuristic analysis that attempts to figure out which Mode-C target belongs to the ADS-B client that is being responded to.

 

[tmyers]

Since the flight I was referring to my friend has upgraded his transponder to broadcast a certified ADS-B out signal. Next time we get a chance to do so is there a way we can check to see if in fact he is getting only the air to air and not ground to air?

 

[John Collins]

T
In my reading on the subject, I don't recall seeing anything about which system - TIS-B surveillance processing (ground) or in the aircraft - is expected to perform any heuristic analysis that attempts to figure out which Mode-C target belongs to the ADS-B client that is being responded to.

I agree, I don't think that a TISB will be reconciled to be own ship and it will just show as a ghost. The GBT should be able to filter out a mode S radar return with an ADS-B Out position as they would share the same 24 bit address as well as the pressure altitude and squawk code, or at least they should. With a portable, even this may not work because the portable doesn't broadcast a pressure altitude or a transponder code unless the pilot enters the data into the unit. It is my understanding that the Skyguard has provisions for the pilot to enter the transponder code, but I suspect it uses GPS altitude and not pressure altitude. At least this is what I saw in a compliance test on one of these units.

 

[John Collins]

Since the flight I was referring to my friend has upgraded his transponder to broadcast a certified ADS-B out signal. Next time we get a chance to do so is there a way we can check to see if in fact he is getting only the air to air and not ground to air?

If the other aircraft has a compliant transponder, it is an ADS-B Out device only and does not have ADS-B In. Assuming he doesn't have a certified ADS-B device, then the only way he can see ADS-B via air to air is with a portable receiver. If he sees your aircraft and its N number on his portable, that is via air to air. Also assuming his transponder is setup correctly,and you turn your unit off, it should reappear as a ground station generated TISB without an N number. This might take some time before you show up as a TISB target, maybe 20 or 30+ seconds after you turn your unit off.

If he is getting both air to air and ground to air targets at roughly the same position and altitude, it will appear as if it is a ghost target.

 

[Jim Logajan]

I agree, I don't think that a TISB will be reconciled to be own ship and it will just show as a ghost. The GBT should be able to filter out a mode S radar return with an ADS-B Out position as they would share the same 24 bit address as well as the pressure altitude and squawk code, or at least they should. With a portable, even this may not work because the portable doesn't broadcast a pressure altitude or a transponder code unless the pilot enters the data into the unit. It is my understanding that the Skyguard has provisions for the pilot to enter the transponder code, but I suspect it uses GPS altitude and not pressure altitude. At least this is what I saw in a compliance test on one of these units.

I have the SkyGuardTWX and it does have provision for entering the transponder code. So the squawk code has to be entered both on the installed Mode-C transponder and into the SkyGuard unit via the tablet control.

I did a bit more net searching regarding the ghosting ("ownship") issue with aircraft having both a Mode-C and Mode-S or UAT installed and found the following information:

An RTCA document from 2007 that is on the FAA web site and concerns performance standards (worth snagging a copy unless one has a newer version):
http://adsb.tc.faa.gov/WG4_Meetings...P-WP21-07_MOPS ASAS master PHX 06Dec07 dw.pdf

As of this revision of the ASAS MOPS, ASSAP requirements and test scenarios do not specifically address correlation between TIS-B and other sources (ADS-B, ADS-R) or ownship for the airport surface. The geometric filters used for determining correlation would be unable to function properly in the dense target environment that could exist on the surface, potentially resulting in a higher rate of ghosting between ADS-B and TIS-B targets and possibly ghosting own-ship as well.

At airports where TIS-B is supported on the airport surface, the following factors should be considered:

Reliable correlation of a detection from a surface surveillance system (i.e., Multilat) may only be possible via the 24 bit ICAO address available in ADS-B and from the Mode S transponder. Likewise, reliable TIS-B/ADS-B correlation may not be possible on ADS-B equipped aircraft with ATCRBS transponders. Users should expect ghost targets or shadows of ownship for aircraft equipped this way. On the surface, ASSAP will only suppress a TIS-B track (from the CDTI) if it has the same track ID as ownship.

The TIS-B transition between the surface surveillance and airborne surveillance may not be seamless due to the fact that the service provider may use independent trackers for each. As a result, spurious targets may appear for one or two update intervals during the transition (e.g., in the region between liftoff/touchdown and 500 AGL)
​

The FAA ADS-B TSO that includes some errata for a 2009 version of the above document is at http://adsbforgeneralaviation.com/wp-content/uploads/2011/12/TSO-C195.pdf though the latest is http://www.faa.gov/nextgen/programs/adsb/media/TSO-C195a.pdf

Also found this on http://www.flyadsb.com/techoverview/techoverview.htm:

There may be, on occasion, “shadowing” or “ghosting” observed of either one’s own ship symbol or another depicted target aircraft on the CDTI. This may be caused by the ADS-B-to-SSR target association process within the ground system. This can result in duplicate targets for equipped or receiving aircraft. This phenomenon may be minimized by standardized filtering software implemented into the aircraft avionics. Some pilots are making productive use of this capability to confirm that their encoder/transponder combination is transmitting their correct altitude.
​

I haven't had time to really review any of the above material to see what "geometric filter" processing involves, among other details. I suspect the technical details on the ground systems is not publicly available at any price.

 

[Pedals2Paddles]

In other words... Yes you might see a ghost of your own aircraft if it can't positively link them by code, or if your xpder is so far out of whack, it looks like a different plane.

 

[RotorDude]

In other words... Yes you might see a ghost of your own aircraft if it can't positively link them by code, or if your xpder is so far out of whack, it looks like a different plane.

I am wondering if a user with non-certified portable ADS-B OUT, driven by safety concerns, would be motivated to "squawk" 1200 in the ADS-B transmitter, to prevent the suppression of his mode C signal in the ground station's ADS-B signal to other traffic?
IOW, as I now understand it, if he were to squawk the same code in both boxes, it would cause the ground station to suppress his signal in the transmitted TIS-B signal to other traffic in the area, but if the codes were different, the ground station would very likely not assume that the targets are the same. Am I missing something?

 

[John Collins]

I am wondering if a user with non-certified portable ADS-B OUT, driven by safety concerns, would be motivated to "squawk" 1200 in the ADS-B transmitter, to prevent the suppression of his mode C signal in the ground station's ADS-B signal to other traffic?
IOW, as I now understand it, if he were to squawk the same code in both boxes, it would cause the ground station to suppress his signal in the transmitted TIS-B signal to other traffic in the area, but if the codes were different, the ground station would very likely not assume that the targets are the same. Am I missing something?

I don't think this changes anything. It is the SDA value that must be broadcast to indicate that the ADS-B Out unit is not a certified unit that suppresses the certified unit to display the target. At this point the GBT treats it as a legitimate client, even though it isn't one. The only compliance report I saw with this device did not have a transponder code being broadcast. There were other issues such as not having a pressure altitude and the NIC and NACp values were not believable.

 

[RotorDude]

I don't think this changes anything. It is the SDA value that must be broadcast to indicate that the ADS-B Out unit is not a certified unit that suppresses the certified unit to display the target. At this point the GBT treats it as a legitimate client, even though it isn't one. The only compliance report I saw with this device did not have a transponder code being broadcast. There were other issues such as not having a pressure altitude and the NIC and NACp values were not believable.

I may be missing something, so please bear with me. I understand that the ADS-B ground station considers the plane to be "legitimate client", and therefore does not include its "own ship" position in the TIS-B broadcast back to that plane. But since it's still getting a completely independent mode C signal from it (via a ground radar station), and since it has no way of positively "reconciling" that mode C signal with the non-certified client (for all it knows they could be two separate aircraft flying loose formation), why wouldn't the GBT include at least the radar-based mode C signal in the TIS-B info?
IOW, I can understand it would suppress the ADS-B info, since that could be picked up by other traffic directly via air-to-air, but why suppress a mode C signal that could potentially be coming from another aircraft with no ADS-B?

 

[John Collins]

I may be missing something, so please bear with me. I understand that the ADS-B ground station considers the plane to be "legitimate client", and therefore does not include its "own ship" position in the TIS-B broadcast back to that plane. But since it's still getting a completely independent mode C signal from it (via a ground radar station), and since it has no way of positively "reconciling" that mode C signal with the non-certified client (for all it knows they could be two separate aircraft flying loose formation), why wouldn't the GBT include at least the radar-based mode C signal in the TIS-B info?
IOW, I can understand it would suppress the ADS-B info, since that could be picked up by other traffic directly via air-to-air, but why suppress a mode C signal that could potentially be coming from another aircraft with no ADS-B?

The scenario you present is true for a certified or non certified unit. The mode A/C based radar position and data has to be reconciled with the ADS-B GPS derived position. When they are not reconciled, that is when ghosting will appear from the erroneous TISB, IOW two targets at essentially the same position and altitude. Even with a mode S transponder and a UAT ADS-B Out system that share the identical information and unique aircraft ID, occasionally there will be ghosts. I have such a configuration and see ghosts on rare occasions. I attribute them to radar systems that are not mode S capable and at long range have sufficient inaccuracy in their position tracking to place my target far enough from my GPS position that the ghost appears. When I see a ghost, it is usually more than a half mile from my GPS position.

So the GBT sees two targets, one ADS-B the other radar ATCRBS, same speed, same direction, same altitude, real close to each other, it should resolve them to be a single target. Remember, for the most part, there should be two sources for a target with ADS-B and a transponder. Two transponder targets and one ADS-B target in the same location is troubling, but one of each is normal.

 

[Pedals2Paddles]

I was thinking exactly what you are. Your Mode C target will be broadcast on TIS-B if it is not positively linked to your ADS-B out target. If you transponder and Skyguard ADS-B are both set to squawk 1234, then you have the problem we've been discussing. Certified receivers will ignore the skyguard air-to-air, and TIS-B won't broadcast your mode C target because it's linked to the ADS-B target by your squawk code.

So if you set your Mode C correctly for the assigned 1234 code, and your ADS-B for an intentionally incorrect 1200, that would in theory do what you're asking. In theory, the ground towers will see your ADS-B target and your Mode C target as two different aircraft. The towers will therefore broadcast your Mode C target with TIS-B. Certified receivers will see that. The problem though becomes everyone else, including yourself, gets your ghost target. You very well may have your own ADS-B in application screaming traffic alert because it thinks you're about to crash into yourself. Some applications have some kind of ghost suppression. But how accurate, I don't know.

In theory. And I'm not suggesting doing this either.

 

[RotorDude]

I was thinking exactly what you are. Your Mode C target will be broadcast on TIS-B if it is not positively linked to your ADS-B out target. If you transponder and Skyguard ADS-B are both set to squawk 1234, then you have the problem we've been discussing. Certified receivers will ignore the skyguard air-to-air, and TIS-B won't broadcast your mode C target because it's linked to the ADS-B target by your squawk code.

So if you set your Mode C correctly for the assigned 1234 code, and your ADS-B for an intentionally incorrect 1200, that would in theory do what you're asking. In theory, the ground towers will see your ADS-B target and your Mode C target as two different aircraft. The towers will therefore broadcast your Mode C target with TIS-B. Certified receivers will see that. The problem though becomes everyone else, including yourself, gets your ghost target. You very well may have your own ADS-B in application screaming traffic alert because it thinks you're about to crash into yourself. Some applications have some kind of ghost suppression. But how accurate, I don't know.

In theory. And I'm not suggesting doing this either.

Thanks, that makes sense. I don't have an OUT myself at the moment, certified or not, just contemplating options. I do have an IN, and do see some ghosting from time to time. I also have Zaon XRX and even though it's supposed to have ghost suppression, it too sometimes has it. (Good for this time of year anyway. :wink2
As for the Skyguard and similar uncertified OUT units, I can see that their owners might want to squawk 1200 in the unit and use the ghost as an indicator that others are seeing them. Might be a slight nuisance, but probably safer that way.

 

[tmyers]

He is getting TIS-B on his 430 and 400 but I don't remember if the TIS-B broadcasts a tail number or not. He also has FF with an ads-b in device.

If the other aircraft has a compliant transponder, it is an ADS-B Out device only and does not have ADS-B In. Assuming he doesn't have a certified ADS-B device, then the only way he can see ADS-B via air to air is with a portable receiver. If he sees your aircraft and its N number on his portable, that is via air to air. Also assuming his transponder is setup correctly,and you turn your unit off, it should reappear as a ground station generated TISB without an N number. This might take some time before you show up as a TISB target, maybe 20 or 30+ seconds after you turn your unit off.

If he is getting both air to air and ground to air targets at roughly the same position and altitude, it will appear as if it is a ghost target.

 

[John Collins]

He is getting TIS-B on his 430 and 400 but I don't remember if the TIS-B broadcasts a tail number or not. He also has FF with an ads-b in device.

Unless he has a GDL88, there is no TISB on the GNS 430 and it needs to be a WAAS GNS430W. I assume you are referring to Mode S TIS, which is a diferent system and not based on ADS-B, but is supported in some areas with a GTX330 and a GNS430.

 

[Indiana_Pilot]

So just an update.. I received the Sky Guard uat transmitter and it seems to be working so far.. I plugged it in and wanted to see if I could pick up the signal on my gdl39.. It seems that I can.. It shows as a brown triangle with the tail number right on top of my location for about 30 seconds then goes away.. I assume this is due to the Garmin Pilot app assuming it is my own aircraft? I'm guessing it's brown because I'm on the ground? I'm taking it flying tomorrow to test it out. I will be flying around KUWL.. This is farm country in between ground stations.. If it works here then it should work decent.. I usually start receiving weather from the gdl about 1500' agl.. I am planning on mounting the transmit antenna in one of the rear windows.. (out of room up front for transmitter and it will sit nicely behind the rear seats. I will let you guys know how it works out tomorrow evening.

 

[Indiana_Pilot]

I just returned from a weekend flight to Tennessee and brought along my recently acquired UAT Transmitter from Sky Guard.

Here is what I found.. Using my Garmin GDL-39 and iPad I was able to receive Full TIS-B traffic coverage about 90-95% of the time (This was with the antenna suction cupped in the back window. I lost full TIS-B over the mountains off and on. I flew between 4500-9500' MSL on my trip and it was surprising how solid it was.

The antenna also seemed to work best when slanted at a 45 degree angle on the window.

It also continued to receive UAT TIS B traffic in between "outages" If I lost signal for more than 5 minutes then I would lose the traffic targets. (verified by selecting the target and seeing it was being received by ground stations or not)

The spottiest coverage actually seems to be over my home airport (KUWL) regardless of altitude.

Another thing to note that in some areas I would get the "traffic alerts" for my own aircraft. This happened a few times while descending to land. It would show a target right on top of me. at +000 altitude. I am guessing this is coming from the ground stations.

I think it was totally worth the money...

 

[Pedals2Paddles]

Thanks for the great review on the Skyguard. It's on my "sometime soon" list of gadgets for sure.