JasonC wrote:
Which is not relevant to Europe where there is no TIS-B.
Agree, but it is relevant to the reference reddit post.
Peter wrote:
Anyway this is getting off topic for ADS-B. Am I right there is no justification for a late warning with ADS-B unless at least one of the two installations is bodged?
TISB, not ADS-B. TISB is once every so often, ADSB is once per second. The Reddit dude is not dealing with direct ADS-B. With once per second, even missing a few here and there, I bet you would be able to reconstruct a loop with the data in the ether. Radar on the other hand, not so much. Remember TISB is generated from radar returns.
Edit: Radar will also not be able to maintain a track for an aerobatic maneuvering aircraft.
Peter wrote:
Would ADS-B be any better?
For targets that are equipped with ADS-B Out, sure. I normally see targets on my GNS530W/GDL88 dual frequency ADS-B In out to 40 NM and further on the portable ADS-B with ForeFlight. So only ghosts and rapidly maneuvering non equipped aircraft sneak up on me.
Here’s the latest from FAA on ADS-B Out exemption for aircraft with no engine driven electrical system,. As with Mode C transponders, these aircraft will be exempted from the requirement.
I bought my first plane in 2002 IIRC and learned how to fly in it, negative electrical system & transponder, almost entirely in US airspace that would otherwise require Mode C. I plan to keep flying it in its present ultra-simple lightweight configuration and the FAA exemption for ADS-B being a mirror image of the existing exemption for Mode C will mean I can continue to base both my planes from the same Class D ATC airport and hangar, located within a ‘mandatory’ transponder area due to close proximity of Class B airspace. If that were not the case I’d have to sell it to a quieter area. My second plane is Mode C equipped and in due course will have to be ADS-B Out equipped to fly in my area, a pain in the neck, a significant cost, and tracking by N-number.
Maybe I will sell one anyway… If I sell the one that needs ADS-B Out then that’s the end of my worry. If I sell the other it’ll be in greater demand given explicit FAA clarification on the non-requirement for that plane, plus the sale will fund the ADS-B Project and the money left over will help pay down a rental propery mortgage 
Garmin have received EASA validation of their FAA Part 23 and 27 STC for the GTX335/345 for certified ADS-B OUT. The STC also includes the legacy GTX33/330 ES models including diversity variants.
Can you ask Shaun to switch it on in my 345?
Is it possible to have ADS-B IN without a GPS source?
I was told today by Avidyne that the Avidyne TAS605 “A” upgrade, with the (soon to be available, apparently – discussed elsewhere) ADS-B IN feature, will not need any GPS input to work.
How can this be done?
It is easy to see how you do with with Mode C targets. The relative height comes from the TAS6xx getting pressure altitude via Gray code (or via ARINC429, if you are lucky – there was a post on how, from an avionics guy in the USA, but I can’t find it) and that is as accurate as the two transponders. The azimuth comes from the four antennae; two resolving L-R and two resolving front-back, and you use the signal levels, and the TAS box gets the aircraft heading (via X/Y 400Hz, or ARINC429 if you are lucky). The distance comes from timing the return packet and that’s really accurate.
A Mode S target is of no additional help, except that the extra data can be used to display the target tail number, in very narrow situations (N-reg only, and only on some Avidyne display devices – not possible over ARINC429).
But with ADS-B IN, you get the target’s GPS position, but you can’t do a warning on it unless the TAS box has the “own ship” 3D position.
Peter wrote:
But with ADS-B IN, you get the target’s GPS position, but you can’t do a warning on it unless the TAS box has the “own ship” 3D position.
I dont think ADS-B out sends GPS altitude. It sends pressure altitude hence comparing that you the own ship only needs pressure sltitude input just like a normal TAS system.
JasonC wrote:
I dont think ADS-B out sends GPS altitude. It sends pressure altitude hence comparing that you the own ship only needs pressure sltitude input just like a normal TAS system
ADS-B Out includes Geometric altitude which is altitude above the WGS-84 ellipsoid. Position is latitude and longitude, so the aircraft position in space must be known. ADS-B Out requires a position source, it is expected to include latitude-longitude from a GPS. Own ship position could be determined from a 1090 Mhz receiver if it sniffed the ownship broadcasts or had an interface from the ADS-B Out device that included own ship position. The Avidyne transponder does not integrate a position source, but some transponders do such as the GTX345/335 (option), the Stratus ESG, the L3 Lynx.
