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  #21  
Old 05-14-2018, 04:25 PM
BobTurner BobTurner is offline
 
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Originally Posted by snopercod View Post
May I revive this old thread? I just installed a Garmin GDL-82 and it seems to be working as advertised, but my "TIS-B Client Percentage" has been in the 20% to 50% range. On my latest flight, I was out of radar contact for a few minutes, so that probably explains it. (I thought the whole idea of ADS-B was that you no longer needed ground-based radar, but that doesn't seem to be the case.) In my area, there is only a single low-powered ground station, surrounded by mountains, so that is probably another factor. Steve's explanation (above) was very helpful in understanding how the system works, but I still have some questions: Is my GDL-82 transmitting to the radar facility, a ground station, other UAT-equipped aircraft, or all of the above? Steve wrote earlier that "the ground stations send out a list of ICAO addresses that are receiving TIS-B client services every 20 seconds". How does an aircraft get on that list? When I'm taxiing for takeoff at KAVL, I seem to be receiving traffic from somewhere, although I'm out of range of the nearest ground station. Does ATC radar at KAVL put me on "the list", or do I have to wait until I'm airborne and in range of the ground station? If my transponder antenna was weak or poorly placed, could that cause a low Client percentage? Speaking of transponder antennas, mine was cut for the original 1090 MHz output of my Garmin GTX-327. Should I have replaced it with a 978 MHz antenna which is around 10% longer?
TIS-B client: low percentage indicates you were often out of contact with a ground station.
Your -82 signal is designed to be received by other aircraft who are listening on 978MHz, and ground stations.
Imbedded in your “out” data stream is your ICAO address. I don’t know what the system does if you’re in anonymous mode. If you change your squawk to anything other than 1200 (e.g., getting radar advisories) anonymous mode is automativally turned off.
If you have a weak or poorly placed antenna you most likely will fail the airborn test, and the biennial transponder test.
If you are using the -82 as designed (sharing one antenna with the ADSB and transponder) keep the transponder antenna. The 82 is designed to live with the small mis-match.

Edit. You don’t say what you’re using for ADSB-in. The -82 must be configured to tell the ground station if you have receive capability on 978, 1090, or both.

Last edited by BobTurner : 05-14-2018 at 07:04 PM.
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  #22  
Old 05-15-2018, 06:41 AM
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snopercod snopercod is offline
 
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TIS-B client: low percentage indicates you were often out of contact with a ground station.
I'm using Stratus 2 / Foreflight / iPad as my "in" setup. I have the 978 and 1090 "receive" parameters in the GDL-82 both set to "capable" so I think I'm set up correctly. My iPad shows that I'm receiving data on both channels. The only ground station in the entire area is 12 NM S of my home airport and it's "Low-tier" so reception on the ground is "marginal".

I guess one thing I don't understand is how my outgoing sqawks get into "the ADS-B system". I had it in my head that there were three paths:

1. Via ATC radar
2. Via a ground station [GBT]
3. Indirectly via air-to-air with another UAT-equipped aircraft.

Our G3Xpert (Steve) posted earlier that I wasn't a TIS-B "client" unless I was in radar contact. I'm having difficulty with that concept because the whole idea of "NextGen" was "free flight" whereby ATC would know where aircraft were in areas of no radar coverage. Of course the FAA computers might know where I am but that doesn't mean they will send traffic info back to me. So is the system set up to cut me off from traffic data when I'm out of radar contact - the very time that I need the traffic information the most?

On a recent 20 min. flight GMU==>AVL at 4,500', I was out of radar contact for 5 minutes because I was low. But FlightAware showed my total ground track so the "ADS-B system" knew my position and altitude all along. I'm pretty sure, though, that ATC doesn't yet use a readout of our ADS-B data. I asked one of the controllers at AVL if he could tell if my ADS-B (out) was working. He said "we have that on a separate screen but I'll go check". Apparently he had to get up out of his station and go look.

It seems that my (out) system is working fine, but I just need to understand how much I can trust the traffic (or lack of traffic) I'm seeing on my iPad. Next time I fly (which may be over a week due to rain), I'll investigate further.
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Last edited by snopercod : 05-15-2018 at 06:45 AM.
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  #23  
Old 05-15-2018, 12:40 PM
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I captured some screen shots today. I pulled my plane out of the hangar so the GPS had a view of the sky. My hangar is probably 200' from the rotating radar antenna so I suppose I was in "radar contact". I was 12 miles away from the ADS-B ground station and my signal strength was showing "Marginal". My iPad was showing several targets, and I tapped on each one to reveal the source. Two of the targets reported the source was "1090 ADS-B" and one reported "978 TIS-B". After Citation N975MD landed, I went up to the Captain and asked him what type of transponder he had. He told me it was all integrated and probably a Collins. He knew it was 1090ES and thought it might also broadcast on 978. My iPad said he was on 1090ES. These screens probably bring up more questions than they answer, but here they are:







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  #24  
Old 05-15-2018, 01:55 PM
BobTurner BobTurner is offline
 
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Originally Posted by snopercod View Post
had it in my head that there were three paths:

1. Via ATC radar
2. Via a ground station [GBT]
3. Indirectly via air-to-air with another UAT-equipped aircraft

r.
1. There are a small number of ATC radar sites that send traffic to those with a mode S transponder. But that’s not you, you get nothing directly from atc radar.
3. I would say directly, not indirectly, you get traffic info from other aircraft with either a UAT (978 MHz) or an S-ES transponder (1090 MHz).
2. The ground stations are fed traffic that ATC radar is seeing (whatever they see, regardless of formal radar contact. Many vfr targets will have no N number data because the radar does not know it.), as well as adsb aircraft that they are receiving signals from. The ground station will filter out all the targets that have adsb-out, both 978 and 1090, because you told it that you could receive those directly. The remaining radar targets will be sent to you if they are within 15 miles horizontally and 3000’ vertically.
You may frequently get data outside that area, if you intercept a ground broadcast intended for another airplane.

You still need to look. When you are going into a small airport below radar coverage, ADSB will be unable to show you any aircraft who do not have adsb-out.

Your Citation captain doesn’t know his equipment. Per the FARs one plane can listen on both frequencies, but may not transmit on both. Too much confusion. And since jets almost always go above 18,000’, they will have 1090SES for adsb out.

Last edited by BobTurner : 05-15-2018 at 02:11 PM.
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  #25  
Old 05-15-2018, 04:00 PM
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Originally Posted by BobTurner View Post
1. There are a small number of ATC radar sites that send traffic to those with a mode S transponder. But that’s not you, you get nothing directly from atc radar
I'm mulling that over. While sitting on the apron in front of my hangar today, somehow ATC knew I was there as indicated by the "Ownership: Detected N10CF" in the screen shot above. So you're saying they didn't recognize me through the ATC Radar tower 200' away. How, then?

Aside: Did you know Ed Williams at Livermore?
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  #26  
Old 05-15-2018, 05:26 PM
BobTurner BobTurner is offline
 
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Originally Posted by snopercod View Post
I'm mulling that over. While sitting on the apron in front of my hangar today, somehow ATC knew I was there as indicated by the "Ownership: Detected N10CF" in the screen shot above. So you're saying they didn't recognize me through the ATC Radar tower 200' away. How, then?

Aside: Did you know Ed Williams at Livermore?
Unless you have a mode S transponder, all radar knows is your squawk code and altitude (and location). It looks to me like you have received info from a ground station.
I worked with Ed at Livermore Lab for many years. He and I, and a third person, were partners in a 182 for 20 years. Heís retired now, living in Maui, HI. Last December he had a mild heart attack, I donít think heís been flying for some years now.
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  #27  
Old 05-15-2018, 08:08 PM
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snopercod snopercod is offline
 
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It looks to me like you have received info from a ground station.
That's possible. I'm just trying to figure how this all works. I'm pleased that you worked with Ed. Small world. He and Marlene had me over to their house in Berkeley for supper once. I haven't spoken to him for many years, but use his Formulary quite often.
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  #28  
Old 09-25-2018, 09:21 AM
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I just installed a DeltaPop blade-type transponder antenna to replace my Archer SA-005 and am very pleased so far. My "radar contact" was much better over the mountains and my ADS-B Client % was 46% - not bad considering that there were no ground stations in the area where I was flying.
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  #29  
Old 10-03-2018, 04:39 PM
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snopercod snopercod is offline
 
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I'm still trying to understand how all this works. I went up today and stayed in the "practice area" just outside the AVL Class C. I was with Approach the whole time at 4,500', just circling around and admiring the scenery. There was a non-equipped C-172 in the same general area at approximately the same altitude, but nothing showed on my "Target" screen (iPad). I was very close to an ADS-B ground station, and Foreflight showed "Good Signal" from the ground station. After about ten minutes of Approach de-conflicting the two of us, the Skyhawk reported climbing to 6,000' so Approach wouldn't have to work so hard. About that time, the C-172 showed up as a target on my screen. I tapped on the target and it said "TIS-B" with no N-Number (of course, since he wasn't equipped). We were both in radar coverage the whole time, so why the delay in TIS-B uplinking him to me?
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  #30  
Old 10-03-2018, 04:48 PM
greghughespdx greghughespdx is offline
 
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There was a non-equipped C-172 in the same general area at approximately the same altitude, but nothing showed on my "Target" screen (iPad) . . . We were both in radar coverage the whole time, so why the delay in TIS-B uplinking him to me?
How close was he to you when he was not showing on the screen? Within 15 miles? And just to be sure, do you have ADS-B Out working in your aircraft?
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