Peter wasn’t referring to unapproved GPS sources but certified non-SBAS IFR GPS sources such as the GNS430.
As I understand AMC 20-24, ETSO C-129A or TSO C-129(A) GPS sources can be used for “configuration 1”. The GNS430 conforms to TSO C-129A.
Apologies – you’re quite correct. I was thinking along the lines of a non-certified VFR GPS system such as a GPS150, Skymap etc which have been connected to transponders such as Becker and Filser but which can’t be used as a fully-certified ADS-B OUT solution.
One slight issue for the person certifying the GNS430 as a suitable position souce might be the additional requirements in the CS:
The compatibility of the combination of the transponder and the GNSS receiver for conformity to AMC 20-24, including for latency, is explicitly stated by the manufacturer of the transponder.
Garmin issued an updated version of software for their GTX transponders last year to allow interface to a non-compliant (not meeting DO-260B) position source but I haven’t checked in the install manual to see if they actually list specific GPS sources.
The TSO is not the issue. Even the US mandate allows for TSO C129 non WAAS position sources. The issue is that the legacy GA TSO C129 GPS systems do not provide the instantaneous integrity values and other data necessary for the position to be trusted. So the ADS-B position that is emitted goes along with a containment radius and accuracy that are each set to unknown or untrustworthy. One could design a converter and add fraudulent integrity and accuracy data to indicated the containment was a value other than untrustworthy. These values are continuously calculated by the position source and are expressed in terms of codes for various ranges. For example these are the NIC (Navigational Integrity Code) values broadcast by ADS-B. A non compliant source will broadcast 0 or >20 NM to unknown
Also NACv and NACp (vertical and lateral accuracy 95% probability values) are also coded with NACp values between 3 meters and 10 NM in 11 codes, with 0 being Unknown accuracy and what would be transmitted by a position source that did not provide the data.
Any changes in NACp, NACv, SDA, and SIL must be broadcast within 10 seconds and NIC within 12 seconds. They are dynamically determined values but SDA and SIL don’t change in a simple system with only one position source. None of this data is provided by a GNS430 non WAAS or by a KLN94, but there are some stand alone position sources that are TSO C129.
Doesn’t any “portable” ADS-B OUT device which coexists with a Mode S transponder need you to configure the 24 bit Mode S aircraft ID in it?
I guess it could pick it out of the Mode S transmitted data, taking some care to not pick up Mode S emissions from the plane sitting next to yours 
Peter wrote:
I guess it could pick it out of the Mode S transmitted dataI understand it is what Garmin AutoSquawk and aAvionix SkyBeacons do.
It might do interesting things if you power up this box, with your Mode S not yet on, and another Mode S carrying plane is not far away Also it is quite likely to pick up a stronger signal from a plane parked next to yours, than from your own, due to airframe shielding.
Yep, a friend of mine has two planes, one of which was about to be rebuilt when the ADS-B OUT mandate came to our area. In order to stay legally airborne he quickly installed a Uavionix skyBeacon and signed it off (he’s an IA). The existing transponder in that plane was very old and apparently very weak so he had to turn up the sensitivity on the skyBeacon to get it to couple with his transponder. Then he had occasional problems with the unit thinking nearby traffic was him…somehow this messed up his (separate) ADS-B IN but I’ve forgotten the details.
The plane is now all over the hangar in pieces with the panels also removed, so none of the above will matter in the future.
Does ADS-B radiate baro altitude and/or GPS altitude, and how does this vary between certified and uncertified emitters?
A portable device could receive the pressure altitude from the aircraft’s Mode C (or Mode S) transponder’s radiation, otherwise it would need its own sensor and that will be easily 200ft+ off due to the cockpit getting a very slight vacuum effect (readily visible by selecting alternate static). The point is that if there is no Mode C/S txp running, it can’t possibly be radiating a usably accurate baro value.
How is this resolved, with uncertified emitters like most of this lot?
The traffic display on a portable IN device in the cockpit provides relative altitude for other traffic, so it wouldn’t seem to matter what altitude baseline is used for OUT as long as it’s the same between aircraft. I think it’s GPS data, all ADS-B OUT transmitters have a GPS receiver, no? It works pretty well in my experience, better than relying on ATC. When I make visual contact it always seems to be at the expected relative altitude.
ADS-B doesn’t contain any altitude as far as I know, the altitude you get if from the Mode-S component of the signal, which is relative to QNE.
EDIT: I was wrong:
ADS-B provides altitude, aircraft flight ID and vertical air speed. Will it also furnish horizontal air speed?
In fact, ADS-B reports two kinds of altitudes: barometric and geometric. Barometric or pressure altitude is the one pilots know best – this is the altitude that is displayed on the altimeter in the aircraft. Geometric altitude is calculated by GPS (Global Positioning Satellites) as the height of the aircraft above the earth ellipsoid. These two altitudes are not the same, but having both allows for applications that require one or the other as an altitude source and provides a means of verifying correct pressure altitude reporting from aircraft.
