# Cheapest solution for Mode C Transponder and ADS-B?

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#### rv7charlie

##### Well-Known Member
I find performance criteria in the regulations, but not a list of “approved position sources”. Can you provide a link to that requirement and list?

Thanks,

BJC
While not a definitive answer, a stroll through the Navworx debacle might give you a feel for meeting the 'performance criteria'. I never bothered to examine each weed (as in 'get down in...'), but it seems that the issues revolved more around what extra data the FAA requires to be embedded in the GPS output, than pure position accuracy. It's also informative to note that there are AMOCs (alternative means of compliance) for the Navworx, but each specifies a particular *IFR certified* GPS for the position source. None allow an open-ended 'meets performance criteria'.

Charlie

#### Toobuilder

##### Well-Known Member
HBA Supporter
Log Member
I use the D180 as the encoder for my Rocket, but used standalone encoders seem to be readily available cheap.

Concerning the position requirements, it seems that WAAS is a hard line, but TSO'd is fuzzy. I can't see how the FAA would ever know where your GPS was built so long as your PAPR was clean. (Although they DO ask the manufacturer of the GPS on your PAPR request).

#### rv7charlie

##### Well-Known Member
Well, back when the Navworx fiasco was unfolding, you could buy dozens of different stand-alone WAAS GPS 'pucks' for well under $100; a lot of them for <$30. A lot of *those* were and are being used with perfect performance, supplying position data to various EFIS products and tablet apps. I've got one driving the GRT in my -6. So if it is easy to fool the FAA, and Navworx had access to those same GPS modules, why didn't they just use one?

Not saying I disagree with the desire, or even the practical reality that a $20 puck would get the job done safely, but if it would work out from the FAA's perspective, no one would be shelling out$500 for the approved position source.

IIRC, the issues get down in the weeds of what extra data (beyond accurate position) the GPS is required to supply to the ADSB towers via the plane's ADSB emitter.

#### Built2Fly

##### Well-Known Member
@FinnFlyer, I'd love to see your code. I am interested in the format of the message and the setting on GTX-327 to receive that. I found it hard to find those information. Maybe we can document the working combination here in case it is needed by someone later.

@GeeZee, yes, it is hardly worth the effort since it can be bought so cheaply. But that is fun of building experimental, right? Doing things in your own way and learn along the process.

#### GeeZee

##### Well-Known Member
HBA Supporter
I find performance criteria in the regulations, but not a list of “approved position sources”. Can you provide a link to that requirement and list?

Thanks,

BJC
Nope on the list. If you look at AC-20-165B there are multiple sections related to“position source” requirements in general. And yes an advisory circular is not a regulation but if you want to use a GPS that’s not already being used/approved by a ADSB OUT equipment provider you’d better be prepared to prove it meets the requirements of AC-20-165B. As to Toolbuilders comment about how would they know? I don’t know the answer to that either. You could try a generic GPS and see if you get “filtered” by the FAA as a non compliant emitter

#### Built2Fly

##### Well-Known Member
@BJC, @GeeZee, @rv7charlie

It is hard to find a GPS not supporting WAAS these days. I have gone through a few datasheets and find almost everyone has WAAS.

However, I just talked to David from Uavionix this morning. And he mentioned to me that not all WAAS are the same. Or in another word, not all WAAS is compatible with echoUAT or compliant with ADS-B requirement. This may have to do with the integrity and anti-spoofing requirement. This is also listed in the position source section that @GeeZee linked above.

@rv7charlie, can you provide link or search terms for the Navworx GPS you mentioned? Uavionix is recommending their SkyFIX, and that's a $500 GPS. #### Built2Fly ##### Well-Known Member @rv7charlie, a quick search turned out that FAA suspended approval of Navworx because of non-compliant GPS used. Maybe it is not such a good idea to get those used Navworx GPSs. #### rv7charlie ##### Well-Known Member Please go back and re-read my comments, a little more carefully and in context. The FAA put Navworx out of business over their inability (or unwillingness) to get GPS 'right' in the FAA's eyes. Sorry if my comments caused confusion; the Navworx thing was such a high-profile legal battle that I thought everyone was aware of what went down. Charlie #### Built2Fly ##### Well-Known Member @rv7charlie, sorry I am new to this and am learning as I go. I may not have the necessary background information even it might be well known here. For example, I know nothing about Navworx a few days ago. But I really appreciate your inputs, and I am learning from it. Any idea is good idea, and there is not bad knowledge. Thanks. My interest is to find the cheapest solution that is compliant to FAA regulation in my area. I think we know now that the Navworx GPS might not be a good choice even they can be had for a cheap price. Any other ideas what other (cheap) GPS might work? #### BJC ##### Well-Known Member HBA Supporter BTW, there is an old thread on this topic. I have a Garmin GTX 327 coupled with a GDL 82. BJC #### rv7charlie ##### Well-Known Member Built2fly, If you have a viable Mode C transponder already installed (or find a deal on a Garmin 327 or similar), I haven't seen a cheaper option than the EchoUAT with their GPS, at around$1500. If you already have an IFR approach-certified GPS in the plane, its GPS output can feed the ADSB. Like BJC said, the Garmin GDL82 will work, but it's a few hundred$more, and without 'In'. If starting totally from scratch (no existing XP), the Stratus ESG is about$3K, again without 'In', and it's Mode S, so no anonymous mode, if that matters to you (it does to me, just on general principles).

Not sure what you mean by FAA regulation in your area. If you're in the USA, the requirement for ADSB is virtually identical to the old Mode C requirements; generally speaking, if you don't fly above 10K feet AGL or in Class B/C airspace, you don't need it. Can't help if you're not from around 'here'. ;-) I have heard that there are a lot of other options (but not UAT, like the EchoUAT) available to pilots in Europe/Australia, but that's just hearsay.

Charlie

#### Built2Fly

##### Well-Known Member
@rv7charlie,

Thanks for the ideas. I am in San Diego, California. My pain is that I am under the Mode C veil, so I need to deal with that mandate. And I like to do it as cheaply as possible (but still legal). I just went out and bought a Garmin 327. Of course I don't have to use it if there is other better solutions, but that is the cheapest modern transponder I can find.

It seems that the choices are so far (assuming $350 already sunk in the Garmin 327): • Stratus ESG,$3K: + all-in-one, - no "in", - no anonymous mode, - price
• Garmin 327 + Garmin GDL 82: $1800 additional, - no "in", - more wiring, - price • Garmin 327 + Uavionx echoUAT / SkyFYX EXT:$1400 additional, + "in", + UAT anonymous, + less wiring
The last one is the cheapest. The only place I might be able to shave a bit is the GPS. I am not sure if there is any other option or the extra incompatibility trouble worth the price saved, but it could be a fun experiment. Any thoughts?

#### Toobuilder

##### Well-Known Member
HBA Supporter
Log Member
...Not saying I disagree with the desire, or even the practical reality that a $20 puck would get the job done safely, but if it would work out from the FAA's perspective, no one would be shelling out$500 for the approved position source....
I'd just about bet that the puck sold for $500 is identical to those that can be purchased for$20. Not to say there are not some poor performers out there, but that $500 is not likekely an attempt to recapture any certification efforts. #### rv7charlie ##### Well-Known Member I'd just about bet that the puck sold for$500 is identical to those that can be purchased for $20. Not to say there are not some poor performers out there, but that$500 is not likekely an attempt to recapture any certification efforts.
Then as I asked before, why didn't Navworx take that path? While I agree that none of them are worth the money charged, there must be some 'nerd-level' data or error checking that the FAA expects, and doesn't show up in an off-the-shelf GPS puck.

Why not give it a shot, & let us know how it goes? I've got a $25 puck I can send you if you have the ADSB xmtr. #### rv7charlie ##### Well-Known Member • Garmin 327 + Uavionx echoUAT / SkyFYX EXT:$1400 additional, + "in", + UAT anonymous, + less wiring
The last one is the cheapest. The only place I might be able to shave a bit is the GPS. I am not sure if there is any other option or the extra incompatibility trouble worth the price saved, but it could be a fun experiment. Any thoughts?
I haven't found a less expensive option that is guaranteed to work, unless you've already paid the freight for an IFR WAAS GPS navigator.

(Toolbuilder, Approach Certified GPS another 'no different' area; you can get current approach plates on a GRT EFIS, and use a WAAS GPS puck for position. It'll work perfectly. Want to test the FAA on legality?)

Charlie

#### Built2Fly

##### Well-Known Member
In summary, here is the least expensive ready-made solution we can get for Mode C Transponder + ADS-B at around middle of 2020:
• Garmin GTX 327, ~$350 used • Serial altitude encoder, ~$50 used
• Uavionix echoUAT + SkyFYX EXT, ~$1400 new (no used available) • Transponder antenna, ADS-B antenna I'd say that's roughly a$2000 solution. If there is anything that is reasonably modern and lower cost, I'd love to hear.

@rv7charlie, @Toobuilder, how do you test the FAA legality on a GPS receiver? I have some ideas on building a cheaper experimental GPS source that might work. Do you just go flying with it and wait for the FAA citation if it fails?

#### Toobuilder

##### Well-Known Member
HBA Supporter
Log Member
Then as I asked before, why didn't Navworx take that path? While I agree that none of them are worth the money charged, there must be some 'nerd-level' data or error checking that the FAA expects, and doesn't show up in an off-the-shelf GPS puck...
It could very well be that Navworks picked a junk component and had to try live with that decision. Maybe Uavonics' puck is a $23 dollar unit that happens to meet the perf requirements consistently - But it could very well be BECAUSE of the beating that Navworks took that Uavonics gets 5 bills for their puck! Not accusing here - just being cynical. #### rv7charlie ##### Well-Known Member IIRC, Navworx went through at least two different receivers, and never did get it right. Reports in the aviation media at the time left the impression that it might have been their defiant attitude toward FAA, more than the (real) failures of their GPS, is what really killed them. Remember, this info is sourced from aviation media, who are only marginally better at reporting legalese than the regular media is at reporting on aviation. The FAA has published specs for performance and how to test (which I haven't bothered to try to read; reading the regular FARs for just flying around can cause a psychotic break from reality).$500, while excessive, is a bargain compared to a $12,000 WAAS navigator that is only legal because it uses a database that our tax dollars paid for and that they get for free. I don't doubt that a GPS could be built (or sourced) that didn't cost any more than the sub-$100 price point, but they are getting paid for proving to the FAA that it works. Any of us could do the same thing; download the requirements and go for it.

Same cost vs price thing is true even in the pure experimental world; I don't know about all EFIS units, but the GRT stuff is a small form factor computer running linux (with a really bright screen and killer software).

#### Built2Fly

##### Well-Known Member
@rv7charlie, @Toobuilder

I think we all agree on a same thing. The "certified" or the lesser "FAA accepted for experimental" class of devices are over-priced. Part of the price we pay there is for the effort of jumping over the FAA hoops. So in some way, we are paying a lot more than the physical stuff that is shipped in a box.

We don't have to pay for that if there is a way to experiment. It is well known that a GPS source can be built for less than $100, probably less than$50. If there is a way to test "pass or fail" for such homebuilt device, I can easily swap and try a few different chips and find the one that works. Then we can share that knowledge here so people can build their own and not having to pay the excessive fee. That's the value of this forum and community.

Do you know if there is any way we can test the "pass/fail" of a GPS source?

#### rv7charlie

##### Well-Known Member
The FAA will send an ADSB performance report upon request. They'll also send them without you even asking, if you're failing any of the performance parameters.
The legality of intentionally flying a non-approved GPS source attached to ADSB is a different question, best posed to your FSDO or MIDO, etc. All the mfgrs had to fly their new devices at some point to test and demonstrate operation, so it's obviously possible to test stuff with the FAA's blessing, in the right circumstances. But the FAA will tell you the conditions; we can't (or at least, I can't).

I haven't heard or read of anyone getting violated for failing a performance check yet, but we all know it's coming at some point.

Charlie