On Tuesday, November 13, 2012 3:22:40 PM UTC-8, Andy wrote:
On Nov 12, 7:18*pm, T wrote:

I've been told that the ADS-B out data includes GPS quality/

reliability information that comes from the WAAS receiver. Basically a

"trust me" code. Hopefully the cost of WAAS GPS recievers will come

down in costs as we approach 2020.



The Trig transponders have provision for setting the GPS data

integrity level in the configuration menus. If I had one I'd connect

any available GPS and set the integrity to LOW. I'm not asserting

that is either approved or legal.



From the installation manual:



"GPS System Integrity Level

An important metric for ADS-B ground system behaviour is the SIL or

System Integrity Level. It is intended to reflect the probability that

the GPS position source is providing erroneous information. A detailed

analysis of the contribution to system integrity is outside the scope

of this manual, and the installer may need to carry out a system

safety analysis to determine the best value to set. However, a

reasonable guideline might be:

Equipment

Transmitted Integrity Level

VFR only GPS or uncertified installation

Low

GPS installation certified for en-route and terminal IFR navigation

Medium

GPS installation certified with augmentation, such as WAAS or LAAS

High"



Andy

It sounds like you are talking about an older DO260a (the RTCA spec that defines 1090ES data-out) version of a Trig transponder or at least the documentation that came with one. If you are playing with ADS-B data-out with a Trig transponder, even if you are using a non-compliant NMEA GPS source, you probalby want to get the current DO260b firmware and setup instructions from the factory (no longer included in the manual).

DO260b significantly changes some of the over the air transmission of SIL, NIC, NACp, and NACv data that describes the GPS systems specs and performance and how that data is provided.

Its unclear how the FAA ADS-B infrastructure will deal with DO260a transmitters in future, I've heard some opinion they will just be ignored past a certain date.


Darryl