Very helpful, thanks!

So far I've only looked at the data stream being sent to the Avare
application. It's a text stream which includes aircraft call sign (if
avaliable, e.g., UAL178), the ICAO address, lat, lon, and altitude. I'd
look at the source code, but my eyes get bleary pretty quickly.

I don't intend for this to me much more than a toy, though I do plan to
try it in flight just to see how it works. When the time comes, I'll
get proper equipment.

Thanks again.

On 1/10/2016 1:43 PM, Darryl Ramm wrote:
Dan

The ADS-B ground station will be broadcasting ADS-R and TIS-B data for client aircraft that are near (i.e. within the ADS-B service "jockey puck" of +/- 3.500' and 15nm radius around the client aircraft). So it may not be, and does not need to be, client aircraft near *you*. Say a suitable equipped ADS-B Out aircraft was 1,000' directly overhead some other transponder only equipped traffic (within SSR coverage) on the other side of a mountain range, where all those aircraft are out of sight to you. The ADS-B ground you had line of site to the ADS-B ground tower covering that airspace you would still see those ground broadcasts intended really for he client aircraft on the other side of the range.

That's again points to the basic irony looking at TIS-B and ADS-R transmissions, they are made for client aircraft near the target, not necessarily near you. As the other target aircraft moves away from the client aircraft, maybe towards you. You stop seeing it on ADS-B. Oops. And in your case it is more likely than not won't be a ADS-B client aircraft near you that is causing what you see for a remote aircraft on the other side of a mountain, the client aircraft will be over on the other side, closer to the target.

What you see at distances will depend on the signal strength and distance to the aircraft or ground station and you antenna setup. These small software defined radio modules are fun to play with but built at low cost and don't have the worlds best analog RF front ends. It would be fun to compare them to what is used in commercial portable devices. Part of the reason I'm not too excited about folks using these in their aircraft for more than experimenting/playing around. The antenna used and it's location/sky visibility will affect coverage as well. It could well be that you will see lots of distant aircraft via ADS-R and TIS-B but not ADS-B direct, because you can receive transmissions from the ground tower well but not those more distant aircraft even if you have line of site to them.

I've mentioned in the past it is hard to try to reverse engineer overall what is going on, especially you cannot easily tell what is being broadcast for what client aircraft. However it should be relatively easy to technically tell if you are receiving data for a TIS-B target. That information is included in the air broadcast messages and fully exposed in the de-facto standard GDL-90 serial communications these portable (hobby and commercial) ADS-B receivers are using.

The GDL9-90 communication protocol is documented and publicly available. e.g. here https://www.faa.gov/nextgen/programs...c_ICD_RevA.PDF. (that one is old, I'd imagine there may be more recent updates even of he GDL90 has long ago stopped being interesting).

The first question I would have is how much of the GDL-90 protocol does and receiver software really implement and does it software correctly send out the target type field data. I've not played with it so I don't know, hopefully it does. Reading the source code should also make that clear.

So even if most commercial traffic display software won't show on the display what type an ADS-B target is it is possible for somebody a little technical to look at this stuff and tell. Easy for example to just grab the GDL-90 serial steam and process it with a text utility. The software Dan is using does logging and I suspect this is all in those log files or can be turned on, but I've not looked.

it is also clear over the air what a ADS-R or ADS-B direct messages are. If that is simply exposed in the traffic stream form any of these devices should be easy to work out for technical folks.. again just by reading the source code.

Hope that helps.









On Sunday, January 10, 2016 at 11:09:02 AM UTC-8, Dan Marotta wrote:
Hopefully Darryl will respond to this but anyone with the knowledge
will be appreciated.



I have a home-grown ADS-B system consisting of a Raspberry Pi 2 and
a couple of software defined radios feeding Avare on my smart
phone. I have line of sight to the ADS-B tower on top of Sandia
Peak, on the east side of Albuquerque. I see a number of 1090ES
targets around the ABQ area, one at 10,400PA', one at 8,200PA' over
SAF, several in the high 30s to low 40s pressure altitude. But I
don't see any targets very far away from Albuquerque.



Is it that only targets within some volume are being transmitted
through the tower within view or am I simply receiving the 1090ES
transmissions directly from the aircraft? I think this may be the
case as I don't see and aircraft on my display that I couldn't see
with binoculars from my house.



When I first turned on the system I saw some aircraft that appeared
to be in the traffic pattern at ABQ and, from where I live on the
east side of the mountains, their transmissions would be blocked by
the mountains. I speculate that there was an ADS-B Out aircraft
flying relatively low near my house and I was receiving TIS-B
transmissions meant for him. I also saw ADS-B weather that day and
there were a lot of snow storms up and down the Rockies.


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Dan, 5J

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Dan, 5J