ads-b out and Transponder ??

[Indiana_Pilot]

I have been looking at the Skyguardtwx portable ads-b out transmitter. I am only wanting to do this so I can obtain all the traffic data using GDL-39 and Garmin Pilot.

My question is that I am able set a squawk code for it along with the ICAO and N Number.

Does this mean if I am on flight following I will need to change both the squawk on this ads-b out and the transponder too? Will they see 2 targets on their screen ?

 

[Jim Logajan]

Has Skyguard obtained FAA approval for their portable unit to meet the 91.225 requirement for ADS-B-out? I didn't think the FAA was going along with that.

There is no need for FAA approval at this time. After 2020 the FAA says such units will be prohibited from airspace where ADS-B is required.

 

[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.

 

[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.