Garmin sued because they could see that Uavionix Tailbeacon is exactly what the market wanted, was about to take the entire low end of the FAA mandated ADS-B OUT market, and indeed that is what happened. The benefit for those of us who bought a Garmin product anyway is the Garmin response was to sell the GTX 335 for A&P mechanic installation, not just dealer installation.
My off the cuff, not particularly thought through comment on static pressure and barometric altitude measurement for portables minus a transponder is that I have noticed opening the static system to cockpit static pressure on my planes makes only a small difference to indicated barometric altitude.
The SkyEcho 2 has an onboard barometer
How does that work in a cockpit, where the pressure is typically a few mb less than outside (depending on speed, aircraft shape, etc)?
was to sell the GTX 335 for A&P mechanic installation
Surely that would have been an illegal restrictive practice anyway? It’s like the old debate about some STCs being limited to dealers only.
My experience (only) is that for slower aircraft, moving the static source inside the cockpit is not hugely important. Some CAR3 certified aircraft don’t even have a static port – the factory setup was vented into the cockpit. One could imagine that most of the aircraft flying without a transponder might fall into that category (slow) and that the altitude error induced by a portable, barometer equipped ADS-B OUT unit would not be huge.
Garmin’s practice with almost everything they sell is to restrict sales to dealers, for dealer installation, unless its for an Experimental aircraft. An exception was forced by competitive pressure for the GTX335, but that practice remains for the GTX345 that is one notch upmarket but differs only a little technically – its purely a business-based practice
Silvaire wrote:
Garmin sued because they could see that Uavionix Tailbeacon is exactly what the market wanted, was about to take the entire low end of the FAA mandated ADS-B OUT market, and indeed that is what happened.
Garmin’s patent was broad. It was used in the GDL88/84 installations as an option when the transponder did not have a serial output that provided the data. Navworx patented a different method, but they are out of business because they challenged FAA on compliance of GPS source and the FAA won that battle.
Peter wrote:
I wonder how this works in Europe, and particularly where there is no transponder.
It has a built in encoder and can be installed in an aircraft in the US in aircraft that don’t have a transponder installed because they don’t require one because they lack an engine driven electrical system. A hangar neighbor is considering installing one on his aerobatic home built that has a battery, but no alternator so other aircraft will know where he is located.
Sure; what I was getting at is how the lack of an accurate barometric reference is handled.
On the UK social media scene, and with the current £250 push, and with the CAA and the various “industry interests” pushing the “mid-air protection” aspect rather than (obviously) the surveillance or the drone-avoidance aspect, a lot of people are running around like headless chicken trying to get “something”, but if the baro altitude is going to be say 100ft off, you have to maneuver rather more than if you could rely on it.
The tolerance in a grey-code altitude encoder is 125 feet, so a 100 foot error due to static source position on one of the portable devices is of little importance.
It is quite relevant if you are using one of the boxes which are claimed to pick up transponders (but actually show only relative altitude, no azimuth or distance) and you are flying under the 2500ft LTMA where the base of CAS is 2500ft, tops of ATZs are say 2200ft, ATC have the CAIT software watching your transponder if you have one, ATZ owners have the recently authorised ADS-B IN monitoring (from which they can generate MORs) and you get a traffic warning on which you can act only vertically 
For absolute altitude you use your aircraft instruments. For not smashing into another airplane you should be interested in relative altitude.
The relative altitude is not being determined using the same reference – unless both “portable” boxes are using GPS altitude and then, assuming geoid correction is being done right…
