On Sunday, April 1, 2018 at 2:16:01 PM UTC-7, Richard Pfiffner wrote:
Installing the antenna under the instrument panel cover is not a good idea.

Richard
www.craggyaero.com

I think it depends, what other options there are and what is in the way of a *broad* sky view from the antenna. It may well be the best install option because others might be worse.... like under the turtledeck or an external antenna somewhere on the fuselage (yuk), how good or bad those other options are going to vary a lot by glider.

Possible issues include metal bracing struts, other wires and coax, Flarm antennas, metal instrument panel parts, instruments themselves, and yes maybe even that old chestnut of interference from other GPS antennas. I notice lots of loose wires and cables flopping around under some panels... not a good idea. In some cases a useful test might be to temporarily stick the transponder antenna on top of the instrument glareshield with nothing else near it,... and see if that works OK with a test flight and FAA ADS-B report.. If that does not work then you have zero chance of working with the antenna below the panel.

Lots of behind the panel installs will require having stuff moved around to make the ADS-B GPS antenna the central antenna placed high under the glareshield, with nothing metalic in the way of a *broad* sky view. I hope this is not brain surgery, but it does require going into it with an expectation that a great antenna location/install is required, and you must pull FAA reports to know things are working OK.

2020 Compliance has stringent requirements. NIC=7 (0.2 NM) containment radius may not sound hard, but SIL=3 means 10^-7 probability of a position report being outside that radius, which makes this a challenging requirement, and requires the GPS antenna has a great broad sky view.

If folks want a quick install and can't get this to work easily with their setup then doing SIL=1 is an option to consider, you lose ATC visibility and ADS-B Out required airspace flight privileges.... and still pull a FAA report to check TABS installs. TABS has a requirement of NIC=6 (0.5 NM containment radius) and SIL=1 (10^-3 probability of being outside that radius). (FAA ADS-B reports will flag NIC=6 and SIL=1 in red, but they are fine for TABS).

And a reminder all the above is for experimental gliders with a TN72 GPS Source, anybody with a certified glider who wants 2020 Compliant ADS-B Out needs their A&P to be installing a TSO-C145c TN70 GPS. Using an STC and sending a "notice only" 337 to FAA HQ.... but hopefully *not* the normal process for a major change field approval. If your A&P is not familiar with doing an ADS-B Out install in a certified aircraft have them contact me, or an A&P or others who have been involved in doing these in gliders.