ADSB antenna woes
 

I have my avionics shop making some major mods to my RV8 panel. One mod was to add a Navworx 600b ADSB. My luck, consistent as it is, allowed me to pay the certified price a few weeks before the feds allowed non-tso'd units which sell for a grand or so less. We discovered that the unit had a bad gps and had to return it. That done I was anticipating a Florida early spring flight but got this message today:


"Hi Don, good news and bad news. Good news is I got the ADSB back, re-installed, and after fussing around with it for a while got it to work. Bad news is that it will not work with the cheap little GPS antenna they supplied with the unit. It does work with a Garmin GA-35 WAAS GPS just like the one currently installed for your GTN-650 (I think its under the cowling at the top edge of the firewall). So we need to install another GA-35 somewhere..."

As you can imagine I am eagerly looking forward to spending $300 or so on an antenna.
Anyone have an issue like this?

VR,
Don

The 600 is, as you say, a TSO'd unit, and as such the TSO should say which antennas are needed to keep the TSO valid. If the 'cheap little unit' is listed then it should work, and if it does not you need to take it up with Navworx. But if the only antenna listed is the $300 one, you are stuck - although Navworx should have made that clear. So, what does the paperwork say?

GPS antenna for 600B
 

Funny thing. I just downloaded the install manual for the TSO'd 600B, and went to table 4-4 which states which antenna's can work with the 600B. I was surprised that the GA-35 WAAS GPS antenna was not listed, but the GA-56 non WAAS was. Also approved was the AT575-43 which is the only antenna listed for the 600-EXP unit.

Go figure. Is this correct, or is my old mind playing tricks on me.

B

That is correct.

I think the avionics shop listed the GA 35 simply because that's what they had available to try to get the system to work. If they can find a cheaper and equally effective one and that's what they'll use. I know they had been in contact with naveorx tech-support. As I understand it (and I don't) it has to do with signal strength. A weak signal and the unit won't function.

There's a guy on the Beech forums selling Garmin GA 56 antennas for $45 each. You have to sign up to read the messages, so here's the details:

Selling GA56 antennas , 45.00 each includes domestic shipping , I have 51 of these time yo clear em out....Alan 856 419 5209....

You'll want to confirm, but it looks like the GA56 is approved with the 600B.

David

I have a GA56 connected to my 600B and it works just fine. Larry

The GA 65 is the older non-WAAS antenna IIRC.

Will it give the accuracy to be a certified ADSB GPS source when the 2020 rules come into effect?

According to the Navworx manual it should. The antenna doesn't make the 430W WAAS. The receiver does. The antenna just lets the receiver see the satellites. It just so happens that the new receiver was certified with a different antenna.

Jesse - not to be argumentative, but the antenna DOES make the 430W units approved. If you look at the GNS480 (all of which were WAAS from day one), there are multiple antennas listed, some of which fall into the category where a pre-departure satellite geometry predictor must be consulted to know whether the approaches at destination will be able to meet the minimum performance standards based on RAIM availability, and some of which, like the GA35, which eliminate the need to consult that satellite predictor.

What it comes down to is the GA56 antenna has very different performance characteristics, particularly in a parameter called discrimination, at low elevation angles. The GA35 is much better able to use signals from low-elevation satellites than the GA56. That's why it's the antenna Garmin used in the 430 "WAAS" upgrade, even though both the GA56 and GA35 are both very capable of receiving the WAAS signal.

For those who don't know, the WAAS signal is a separate signal from the GPS signal. To make the 430 meet the TSO requirements for more critical approach certification (C-146a), the WAAS signal is used as a mandatory part of TSO compliance. There are quite a few other differences between a 430 and 430W, including the antenna, which make the combined package meet the more stringent approach requirements called up in TSO C-146a versus the less-stringent TSO C-129a.

I find it funny that Garmin called the upgrade from 430 to 430W the WAAS upgrade when my 10 year old Lowrance 2000c handheld GPS had WAAS capability right out of the box. It would have been much more appropriate had Garmin called it the 430/146a.