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Using data from Echo in lieu of or the Baro Encoder for Transponder?

charosenz

charosenz

Well Known Member
I see the ADSB Echo has 2 ports for data in and out. Could the Echo be used to supply altitude data to the transponder in lieu of a baro encoder (while also providing data out for the ADSB? While I have both, it just seems the Echo would be more accurate and of course having one less device simplifies things...Not withstanding the baro works when the ADSB does not have GPS signal....
 
Thinking out loud:
I presume the ADS-B Echo is GPS-based? So it will report actual altitude AMSL
The barometric encoder is reporting altitude above or below the 29.92 level, which depending on the QNH can be vastly different from AMSL.

Mix the two and traffic avoidance gets quite complicated....
 
Lufthans said:
Thinking out loud:
I presume the ADS-B Echo is GPS-based? So it will report actual altitude AMSL
The barometric encoder is reporting altitude above or below the 29.92 level, which depending on the QNH can be vastly different from AMSL.

Mix the two and traffic avoidance gets quite complicated....
Click to expand...
I have done a lot of studying on this since the first post. I see that U-Avionix is going to be providing a free pressure encoder for Echo customers - so the Echo would have pressure altitude data when GPS signal is not available. In the mean time I am just going to follow the schematic in the GTX327 manual and use RS242 pressure data out to provide that pressure alt data to the Echo(ADSB) . (since I have a pressure encoder that supplies RS232 data out (ACK30.9).
 
To Walt’s point above….GPS altitude and altitude encoder provided pressure altitude are two distinctly different measurements. They are generally “close” to the same, but are often different by more than 100 feet. The ENTIRE air traffic control system in the US is based on everyone using pressure altitude…..so throwing a GPS altitude reading into that doesn’t meet the FAR requirements for mode C (see Walt’s comment), and sort of screws up the data in the system for all of us.

Also, GPS altitude does show more significant digits (the nearest 10 feet, if I remember correctly). An altitude encoder reports altitude rounded to the nearest 100 feet. But again, because the ATC system can only see altitude to the nearest 100 feet, sending altitude to the nearest 10 feet doesn’t do any good. Besides that, your transponder likely rounds to the nearest 100 ft before sending the data out anyway.
 
Stevea said:
To Walt’s point above….GPS altitude and altitude encoder provided pressure altitude are two distinctly different measurements. They are generally “close” to the same, but are often different by more than 100 feet. The ENTIRE air traffic control system in the US is based on everyone using pressure altitude…..so throwing a GPS altitude reading into that doesn’t meet the FAR requirements for mode C (see Walt’s comment), and sort of screws up the data in the system for all of us.

Also, GPS altitude does show more significant digits (the nearest 10 feet, if I remember correctly). An altitude encoder reports altitude rounded to the nearest 100 feet. But again, because the ATC system can only see altitude to the nearest 100 feet, sending altitude to the nearest 10 feet doesn’t do any good. Besides that, your transponder likely rounds to the nearest 100 ft before sending the data out anyway.
Click to expand...
Steve, AH....that helps quite a bit. I was not really sure what Walt was getting out but your info helps. I think it also is why the Echo is getting a pressure encoder added by UAvionix for free. Since I have a baro encoder with RS232 out for my transponder I will use that. The Transponder also has RS232 out for the ADSB if I want. I may do this too until the new baro encoder for the Echo shows up.

And kudos to your for a slow build. I ordered one of the last slow build 6A (24483). Which I would not have done had I known the 7 was months away from the market. Oh well. Had a blast with the 6a while I had it.
 
charosenz said:
Steve, AH....that helps quite a bit. I was not really sure what Walt was getting out but your info helps. I think it also is why the Echo is getting a pressure encoder added by UAvionix for free. Since I have a baro encoder with RS232 out for my transponder I will use that. The Transponder also has RS232 out for the ADSB if I want. I may do this too until the new baro encoder for the Echo shows up.

And kudos to your for a slow build. I ordered one of the last slow build 6A (24483). Which I would not have done had I known the 7 was months away from the market. Oh well. Had a blast with the 6a while I had it.
Click to expand...
Glad it helped.

FYI, I have an echoUAT/skyFix setup for my ADSB. My SL-70 transponder “snifs” the transponder code and altitude being sent out. Works fine. I am also on the list for the uAvionix echoUAT encoder add on, and will install it when it arrives. Let me know if you have any questions WRT my setup or yours.

As for the slow build…..yea, I was one of those slow builders with a slow build kit. Pre-punched wing skins and firewall was all I got. But, I was able to use the -7 firewall forward plumbing routing….helped a lot.
 
I am pretty sure that the adsb-out specs require that the transponder and adsb-out device get their pressure altitude from the same (single) source.
 
BobTurner said:
I am pretty sure that the adsb-out specs require that the transponder and adsb-out device get their pressure altitude from the same (single) source.
Click to expand...
Generally you are correct. For some reasons (unknown to me at least) uAvionix got approval to do what they did WRT adding a secondary blind encoder to the echoUAT setup. The second encoder is a backup to the xponder encoder, in case the xponder is not pinged frequently enough by ATC radar.
 
Stevea said:
Generally you are correct. For some reasons (unknown to me at least) uAvionix got approval to do what they did WRT adding a secondary blind encoder to the echoUAT setup. The second encoder is a backup to the xponder encoder, in case the xponder is not pinged frequently enough by ATC radar.
Click to expand...
The idea is that both transponder and adsb should send out exactly the same pressure altitude. So yes, a backup is allowed but only when there is no PA coming out of the transponder.
 
Stevea said:
Also, GPS altitude does show more significant digits (the nearest 10 feet, if I remember correctly). An altitude encoder reports altitude rounded to the nearest 100 feet. But again, because the ATC system can only see altitude to the nearest 100 feet, sending altitude to the nearest 10 feet doesn’t do any good. Besides that, your transponder likely rounds to the nearest 100 ft before sending the data out anyway.
Click to expand...
Modern encoders that send data via 232 do have 10ft resolution.
 
Walt said:
Modern encoders that send data via 232 do have 10ft resolution.
Click to expand...
Yep. My modern digital encoder does allow selecting 10 ft vs 100 ft resolution. I'm just not sure my slightly less than modern Apollo SL-70 (using the serial input vs grey code input) sends out 10 ft resolution......at least the number it displays on the front pressure altitude reading is only to the nearest 100 ft.....reads "xx00" all the time, even with the encoder set to 10 ft resolution.
 
Stevea said:
Glad it helped.

FYI, I have an echoUAT/skyFix setup for my ADSB. My SL-70 transponder “snifs” the transponder code and altitude being sent out. Works fine. I am also on the list for the uAvionix echoUAT encoder add on, and will install it when it arrives. Let me know if you have any questions WRT my setup or yours.

As for the slow build…..yea, I was one of those slow builders with a slow build kit. Pre-punched wing skins and firewall was all I got. But, I was able to use the -7 firewall forward plumbing routing….helped a lot.
Click to expand...
Gosh lucky you. My 6a had no pre-punched holes at all. I used the sniff set up on the 6a when I had the baro encoder with a grey code configuration (no rs232 set up at that time). Since then I bought the encoder with the serial data out that is going in my next build (since I sold the -6a).
 
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