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ADS-B Made Simple

Chaz,
Are you actually using the Trig transponder with it's control head, or the Dynon/AFS transponder?

If it's the Dynon/AFS transponder, then you'll need a firmware update for the transponder to be fully 2020 compliant, and that's not out yet, but it is in testing.

If it's the Trig with the control head, then that's all up to Trig and their STC program for the TT22.
 
Chaz,
Are you actually using the Trig transponder with it's control head, or the Dynon/AFS transponder?

If it's the Dynon/AFS transponder, then you'll need a firmware update for the transponder to be fully 2020 compliant, and that's not out yet, but it is in testing.

If it's the Trig with the control head, then that's all up to Trig and their STC program for the TT22.

So I see that the Trig TT31 is approved for use with the Garmin 400W series, but the TT 22 is not. How hard is the FAA making this?
 
If it's the Dynon/AFS transponder, then you'll need a firmware update for the transponder to be fully 2020 compliant, and that's not out yet, but it is in testing.

It is good to know the Dynon/AFS transponder is already in testing for full 2020 compliance. This kind of "after sale" support is another reason why I am SO GLAD I went with a full SkyView system for my airplane.

:cool:
 
Clarification

I am using the SV-261 with the AFS 5500/5600 combo.. & the AF-ADSB ..

The reports I'm receiving indicate the config passes the ADS-B tests; that's great.
 
So I see that the Trig TT31 is approved for use with the Garmin 400W series, but the TT 22 is not. How hard is the FAA making this?

The TT31 and TT22 have different firmware and hardware. So this isn't really the FAA being onerous about ADS-B, its the normal STC process where you can't assume that because one device worked fine, a different one will as well. Trig is working on the STC for the TT22, it just isn't 100% complete yet.
 
I am using the SV-261 with the AFS 5500/5600 combo.. & the AF-ADSB ..

The reports I'm receiving indicate the config passes the ADS-B tests; that's great.

So here's the $64 million dollar question that those who worry about paperwork are asking: How is it that the reports say you are okay, but your configuration (GTN w/ TT22) is in fact not approved (yet). This is what keeps some government types awake at night, causes Mac to write strange articles, etc.
 
So here's the $64 million dollar question that those who worry about paperwork are asking: How is it that the reports say you are okay, but your configuration (GTN w/ TT22) is in fact not approved (yet). This is what keeps some government types awake at night, causes Mac to write strange articles, etc.

There are a couple flags set in the configuration that tells the FAA you are compliant.

I had a generation one version of an unit (not looking to bash any vendors) that improperly sent the FAA the flag indicating I had a compliant GPS. When I upgraded to a third generation hardware/firmware from the same vendor, they had fixed that issue and correctly transmitted that I had a non-compliant GPS.

Things like this are clearly going to cause more confusion with folks. I suspect the FAA's willingness for the one off confirmation of things working and providing a detailed report may wane as we get closer to the deadline.
 
The compliance report tells you a system that was installed with all the data showing it COULD be compliant is operating properly and is compliant. It cannot detect devices which are not compliant but are sending out the bits saying they are. Remember, a large part of compliance is about a system that does the right things in the 1 in a million failure case, not the 99.9999% of the time when everything is working right. If you have no data about the pedigree of the design of the ADS-B out device, you can't even be sure it's not lying about the compliance bits (like rleffler experienced)!

If you want to be 100% sure you are compliant, you better have a document from your ADS-B OUT vendor stating that their equipment meets the requirements of FAR 91.227 when connected to your specific GPS position source and specific firmware versions. Generically, this means a STC has been issued for your EXACT ADS-B OUT device and your EXACT GPS. Other documentation is acceptable too though. It's up to you as the pilot to verify that you are compliant with 91.225 and 91.227. If you don't have a document saying you are, then you better have some other data to back it up.

Of course, none of this matters until Jan 1, 2020. 91.227 and 225 don't even apply until that date.
 
The compliance report tells you a system that was installed with all the data showing it COULD be compliant is operating properly and is compliant. It cannot detect devices which are not compliant but are sending out the bits saying they are. Remember, a large part of compliance is about a system that does the right things in the 1 in a million failure case, not the 99.9999% of the time when everything is working right. If you have no data about the pedigree of the design of the ADS-B out device, you can't even be sure it's not lying about the compliance bits (like rleffler experienced)!

If you want to be 100% sure you are compliant, you better have a document from your ADS-B OUT vendor stating that their equipment meets the requirements of FAR 91.227 when connected to your specific GPS position source and specific firmware versions. Generically, this means a STC has been issued for your EXACT ADS-B OUT device and your EXACT GPS. Other documentation is acceptable too though. It's up to you as the pilot to verify that you are compliant with 91.225 and 91.227. If you don't have a document saying you are, then you better have some other data to back it up.

Of course, none of this matters until Jan 1, 2020. 91.227 and 225 don't even apply until that date.

One wonders why such levels of exacting performance in the transponder and position source are/will be required of VFR aircraft, especially when not operating in Class B airspace. It makes sense for airliners, bizjets, King Airs, etc. and probably even for all IFR operations, but it really seems like overkill for us little guys who may not ever talk to any kind of ATC service.
 
One wonders why such levels of exacting performance in the transponder and position source are/will be required of VFR aircraft, especially when not operating in Class B airspace. It makes sense for airliners, bizjets, King Airs, etc. and probably even for all IFR operations, but it really seems like overkill for us little guys who may not ever talk to any kind of ATC service.

If you never talk to atc then maybe you never go near class B or C; if you stay below 10,000' too then you can just ignore the whole thing. Quite possible for easterners. Out west, it can be tough to safely fly over the mountains and stay out of the mode C required airspace.
 
If you never talk to atc then maybe you never go near class B or C; if you stay below 10,000' too then you can just ignore the whole thing. Quite possible for easterners. Out west, it can be tough to safely fly over the mountains and stay out of the mode C required airspace.

My point is that the full 2020 mandate is excessive, because it's written to satisfy the needs of large, fast aircraft operating in crowded airspace in IMC. The technical requirements are overkill for VFR operations in light airplanes, and almost certainly a result of some desk-driver who imagined that nothing smaller than a King Air ever operates in the applicable airspace.

Full disclosure: 90% of my flying has taken place within Atlanta's Mode C veil, underneath the Class B. Yet since my private checkride, I have spoken to ATC on only two occasions--both during BFR's here in Savannah.
 
Separation requirements have nothing to do with whether you're VFR or not, or the type of aircraft you fly ... If you're in controlled airspace, especially class B, you're in busy airspace with lots of targets, and for dependable, accurate positioning, everyone needs to be reporting their position to the same standards ... If there's a 737 reporting really accurately, and an RV-7 reporting far less accurately, you loose the ability to keep separation with close margins ... the aircraft with the least reliable signal needs a bigger buffer around it.

I suppose the technology could allow for this, each aircraft gets a given "buffer" around it based on the quality of the position reporting ... but that could be a big headache for ATC, especially where there are fixed routes, corridors, etc. that depend on a given level of accuracy (i.e. approaches and such).
 
The tough requirements to meet for an ADS-B position source have to do with the deep analysis of failure cases and proving you are 10^-X reliable. It isn't hard for a cheap GPS to produce very accurate position 99.99% of the time, and to prove that.

Thus, in VFR the argument is that legally, separation IS different in VFR because separation is via eyeball, not via controller vector. It's your fault if you hit that fully visible airliner, not the GPS, no matter what the GPS is saying.

So, in VFR, where it's already up to the pilots to stay apart from one another, why do we require a position source that is just as reliable as it is in IMC, instead of allowing some reduction in the assurance of that device given it is only a backup to the Mark 1 Eyeball, not the primary separation device?

Here's an interesting point: Today's radars only have about 1/4 mile position accuracy when you're 30 miles away from the radar, and that's only with 95% confidence. Additionally, the latency is as high as 12 seconds, which at 250 knots is a full mile of flight.

ADS-B requires that the 95% position accuracy be 0.05 NM (5X better than radar) and that the 99.9999% accuracy be 0.2NM or better (tighter than radar's 95% accuracy). So ADS-B is spec'd at being WAY better than the radar we use today. On top of that, ADS-B must have a latency below 1.6 seconds.

So in the end, even for VFR applications, the ADS-B position source is required to be about an order of magnitude more reliable and accurate than current radar is. Allowing VFR aircraft to give up some of that over-the top reliability seems reasonable.
 
Today's radars only have about 1/4 mile position accuracy when you're 30 miles away from the radar, and that's only with 95% confidence. Additionally, the latency is as high as 12 seconds, which at 250 knots is a full mile of flight.

ADS-B requires that the 95% position accuracy be 0.05 NM (5X better than radar) and that the 99.9999% accuracy be 0.2NM or better (tighter than radar's 95% accuracy). So ADS-B is spec'd at being WAY better than the radar we use today. On top of that, ADS-B must have a latency below 1.6 seconds.

So in the end, even for VFR applications, the ADS-B position source is required to be about an order of magnitude more reliable and accurate than current radar is.

I sure hope so. Given the capability of today's technology, it would be foolish to go for less, I'd think, and then be in the same situation again in a decade or so.

The whole NextGen program is extremely comprehensive and multi-faceted. ADS-B is just one part of it.
 
ADS-B with uncertified GPS

I mentioned this on the other ADS-B thread but as this thread has got onto the subject of uncertified GPS I thought I would re-post.
The CAA in the UK are well aware of the issues for GA of insisting on certified GPS and are now investigating the consequences of authorising uncertified GPS data as the position input to ADS-B.


General Aviation ADS-B Trial in Southern England


NATS had envisaged issuing an AIC back in the autumn initiating a nationwide trial of GPS connectivity to Mode S transponders as an enabler to ADSB in/out for the GA fleet. At the request of the CAA we have not issued that open invitation to participate but have written to approximately 180 clubs and flying organisations offering the opportunity to participate in a formal trial. A sample letter is copied below;

Dear Sir/Madam,

Re: General Aviation ADS-B Trial in Southern England

As part of the development of tools to help improve flight safety by enhancing situational awareness, NATS is conducting a trial with general aviation pilots to assess the accuracy of position data from a non-certified GPS. NATS is inviting owners of suitably equipped aircraft to participate in the trial to gather sufficient data to make the study meaningful. The data can be broadcast via a Mode-S transponder with ?Extended Squitter? (ES) functionality otherwise known as Automatic Dependent Surveillance- Broadcast (ADS-B).

The purpose of this trial is to assess;

a) The typical accuracy of ADS-B reported positions from non-certified GPS sources,
b) The impact of low integrity GPS data on ATC surveillance systems, and
c) The interest of GA users in ?situational awareness? applications that assist a pilot to visually acquire nearby traffic.

The trial will be conducted for an 8 month period from 1st January 2015 to 31st August 2015 in airspace in southern England. Further information is included in attachment 1.
Aircraft owners will need to submit a minor modification for approval to the appropriate airworthiness authority to allow the GPS to be physically connected to the aircraft?s transponder. The CAA has agreed to waive their fees for the minor modification to Annex II aircraft during the period of this trial, provided the owner registers for the trial prior to submitting the modification.
Once the modification is carried out, the participant will need to do nothing more other than fly their aircraft with the transponder and GPS turned on. Participants may be required to disconnect the GPS source at the end of the trial.

NATS will not use this data for any purpose other than the trial, such as for the provision of an ATC service. If the non-certified GPS source is used with on-board equipment that detects aircraft via ADS-B, the equipment will not be certified for collision avoidance. Pilots will continue to be responsible for maintaining an effective look out and avoiding other aircraft.

If you are willing to volunteer to modify your aircraft to connect a GPS source to your aircrafts? transponders, please would you provide details of your:

1. Aircraft type,
2. Transponder type,
3. GPS equipment, and,
4. Aircraft engineer who will be conducting the minor modification (if known).

Participants are requested to respond to this invitation either in writing or by email to me at the address below.

Thanks in advance for your interest, and I look forward to working with you.




Attachment1: General Aviation ADS-B Trial in Southern England ? Trial Description

The purpose of this trial is to assess;

a) The typical accuracy of ADS-B reported positions from non-certified GPS sources,
b) The impact of low integrity GPS data on ATC surveillance systems,
c) The interest of GA users in ?situational awareness? applications that assist a pilot to visually acquire nearby traffic.

The trial will be conducted for an 8 month period from 1st January 2015 to 31st August 2015 in airspace in southern England.
NATS low-level ADS-B coverage is centered on southern England. The diagram below is indicative of current coverage at 3500ft.An additional ADS-B receiver will supplement coverage to the north-west of this region in Spring 2015.Higher level coverage extends to most of England.

The Minimum Operational Performance Specifications for ADS-B allow the provision of position information from a non-certified GPS source, provided the Surveillance Integrity Level (SIL) parameter is set to ?unknown? (zero). Therefore, you may participate in this trial with a non-certified GPS source, so long as the SIL parameter in the ADS-B message is set to ?unknown?. The positions reported via ADS-B will be compared with radar track data to establish the impact of low integrity data on surveillance tracking systems.

Not all transponders are suitable for this trial and the method for connecting the devices depends on the transponder and the GPS equipment. NATS and the CAA will work with you, your engineer and the transponder manufacturer to determine whether your aircraft and equipment are suitable for this trial and how to implement the modification. The CAA will also provide guidance for how to submit the minor modification request to EASA, see also http://www.caa.co.uk/default.aspx?catid ... ageid=9259.

Owners of EASA type aircraft will need to consult with a licensed engineer to submit an EASA form 32 to request a minor modification. Under EASA rules, it is permissible to submit one modification for multiple aircraft if they are the same aircraft type and the same equipment is used. Owners of Annex II aircraft will need to request approval of a minor modification from the CAA. The CAA has agreed to waive their fees for Annex II owners during the period of this trial.

In both cases the modification will need to demonstrate:

? The installation of the GPS equipment does not affect the safe operation of the aircraft,
? The SIL is set to zero, and,
? The GPS is not be used as the primary navigational aid
? The connection between the aircraft?s transponder and the GPS unit is non-permanent.

If the non-certified GPS source is used with equipment that detects proximate aircraft via ADS-B, the equipment will not be certified for collision avoidance. Pilots will continue to be responsible for maintaining an effective look out and avoiding other aircraft.

It is clear, from the months of debate within this thread, that a number of individual FLYER Forum members would be interested in participating in such a trial and I would urge those people interested to contact Adrian Price via the details above, and help us progress this exciting development.
 
"Thus, in VFR the argument is that legally, separation IS different in VFR because separation is via eyeball, not via controller vector. It's your fault if you hit that fully visible airliner, not the GPS, no matter what the GPS is saying."

Yes, but it's ATC's fault if the 737 was vectored too close to you because your position reporting is off by x many miles. Even ff you're in class B and doing everything right (other than reporting an inaccurate position), ATC could still vector a 747 on top of your head because oops, he thought you were a couple of miles to the left.

Could be because the GPS itself has issues, but it could also be installation issues for example.

Sounds like the ADS-B messaging can provide the necessary information regarding accuracy for the system and ATC to use it and make decisions accordingly (NAC, NIC, SIL, etc.) ... So non-certified systems could just declare the lowest levels of these things, and ATC could just give you a bit them a bit more room!
 
Accuracy

Separation requirements have nothing to do with whether you're VFR or not, or the type of aircraft you fly ... If you're in controlled airspace, especially class B, you're in busy airspace with lots of targets, and for dependable, accurate positioning, everyone needs to be reporting their position to the same standards ... If there's a 737 reporting really accurately, and an RV-7 reporting far less accurately, you loose the ability to keep separation with close margins ... the aircraft with the least reliable signal needs a bigger buffer around it.

Is that really the case?

Doesn't the ADBS standard output signal include the error margin?

Low accuracy (a relative term in this case) targets could simply be given more spacing. If they are slow this would be easy to do.
 
Gil,

You are correct, and I did mention that same idea later on.

you could simply but a "ring" around your target on the screen, and have the systems understand that "nothing should enter this area". Actually I suppose this works in 3D so it should be a "bubble".
 
I still don't buy it that experimentals have no path to compliance.

I would bet that a large percentage of those 8,800 GA aircraft that are already equipped are experimentals thanks to the amazing experimental avionics vendors we have available to us these days.
 
No 'ASD-B Out' == No Flight Following - Really?

Hi Pahan,

Nice read. I am surprised about this on page 6 of Rev D:

After January 1, 2020 an aircraft without an approved ADS‐B ?Out? solution cannot enter airspace that today requires a transponder and will not be eligible
for other radar services, such as VFR Flight Following.

After thinking about it a bit, it makes sense. This is the first read that made that connection.

Thank you,
 
I still don't buy it that experimentals have no path to compliance.

I am not convinced either.

Can anyone reference a FAR or other regulation that would indicate this is the case?

Follow along with my reasoning and please point out where I might be missing something.

I see nothing in 91.225 or 91.227 that prevents the installation and use in an E-AB aircraft.

Here is the link to AC20-165 "Airworthiness Approval of Automatic Dependent Surveillance - Broadcast (ADS-B) Out Systems":

http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC 20-165A.pdf

You will note in section 1-1b that it states:

"b. This AC is not mandatory and does not constitute a regulation. This AC describes an acceptable means, but not the only means, to install ADS-B OUT equipment. "

Since this is not mandatory and does not constitute a regulation, clearly it does not have to apply to E-AB aircraft, so we can ignore all the talk of STCs and such in this document.

I have been unable to find any mandatory regulations that apply to E-AB aircraft that says an installation of ADS-B Out equipment by the owner or builder in an E-AB aircraft will not be in compliance as of 2020 as long as it passes the required tests, much the same as with a Mode C transponder.

Thoughts?

-Dj
 
I still don't buy it that experimentals have no path to compliance.

I would bet that a large percentage of those 8,800 GA aircraft that are already equipped are experimentals thanks to the amazing experimental avionics vendors we have available to us these days.

This same topic was brought up recently on the Dynon Forum. They answered in a very clear matter and can be summarized as follows:

"As has been discussed, the only thing an experimental needs to meet is the FARs, specifically 91.225 and 91.227. We've discussed this with the FAA and they agree.

91.227 is the real issue, and it's the thing the AC-20-165 was written for. 91.227 requires you to meet specific performance requirements. These are not simple requirements- they fall into the integrity of the ADS-B system in the aircraft as a whole, including the position sensor (GPS), the ADS-B out device, and all the software and wiring between them.

The use of a TSO'd GPS and the use of TSO'd ADS-B out does not guarantee compliance. A TSO'd GPS can send data out in a proprietary, undocumented way (ahem, Garmin, with their ADS-B data format) that is initially reported to meet the requirements of ADS-B and then turns out not to (ADS-B format to ADS-B+ format). When a ADS-B OUT device wants to use this data from this GPS, it must be proven that it interprets it correctly, which has also been gotten wrong before in a TSO'd ADS-B OUT device.

So, the FAA very reasonably wants to know that BOTH devices have been tested TOGETHER, and that TOGETHER they really do meet the requirements of 91.227 in the real world, not just the bits you transmit to say you are compliant. If you want to use two untested devices, that's on your shoulders to prove that it works, and that it meets the 1-in-100,000 flight hour failure requirements."


[My emphasis added]

So in order to be 2020 compliant, and this applies to ALL aircraft including EAB, you must use an approved combination position source/transponder not just two individually approved devices. You can see an approved combination list (as of June 2014) in the article ADS-B Installations are Beginning to take off on page #6. This list keeps growing and should soon include a lot more equipment combinations commonly used in EAB aircraft. For example, in August 2014 the Trig TT22/31 and G430W/530W combination was added to the approved combination list.

Based on the speed of change avionics are experiencing, especially in the EAB realm, 2020 is still quite far away. Besides, who knows what will happen between then and now. IMHO this is a situation where the phrase "hurry up and wait" is very applicable.

:cool:
 
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And I agree with Dynon on this one. This is another thing to consider for folks waiting on a less expensive position source solution.... The combo will have to make that list to be worth anything.

Still nothing I see excludes experimental.


This same topic was brought up recently on the Dynon Forum. They answered in a very clear matter and can be summarized as follows:

"As has been discussed, the only thing an experimental needs to meet is the FARs, specifically 91.225 and 91.227. We've discussed this with the FAA and they agree.

91.227 is the real issue, and it's the thing the AC-20-165 was written for. 91.227 requires you to meet specific performance requirements. These are not simple requirements- they fall into the integrity of the ADS-B system in the aircraft as a whole, including the position sensor (GPS), the ADS-B out device, and all the software and wiring between them.

The use of a TSO'd GPS and the use of TSO'd ADS-B out does not guarantee compliance. A TSO'd GPS can send data out in a proprietary, undocumented way (ahem, Garmin, with their ADS-B data format) that is initially reported to meet the requirements of ADS-B and then turns out not to (ADS-B format to ADS-B+ format). When a ADS-B OUT device wants to use this data from this GPS, it must be proven that it interprets it correctly, which has also been gotten wrong before in a TSO'd ADS-B OUT device.

So, the FAA very reasonably wants to know that BOTH devices have been tested TOGETHER, and that TOGETHER they really do meet the requirements of 91.227 in the real world, not just the bits you transmit to say you are compliant. If you want to use two untested devices, that's on your shoulders to prove that it works, and that it meets the 1-in-100,000 flight hour failure requirements."


[My emphasis added]

So in order to be 2020 compliant, and this applies to ALL aircraft including EAB, you must use an approved combination position source/transponder not just two individually approved devices. You can see an approved combination list (as of June 2014) in the article ADS-B Installations are Beginning to take off on page #6. This list keeps growing and should soon include a lot more equipment combinations commonly used in EAB aircraft. For example, in August 2014 the Trig TT22/31 and G430W/530W combination was added to the approved combination list.

Based on the speed of change avionics are experiencing, especially in the EAB realm, 2020 is still quite far away. Besides, who knows what will happen between then and now. IMHO this is a situation where the phrase "hurry up and wait" is very applicable.

:cool:
 
Just to show how confusing this can be, the above quote is from AOPA, quoting a press conference at Osh. But in fact only the TT31, not the TT22, is currently approved to work with the 400W series.

Do you have a link to the latest FAA approved ADS-B equipment combination list? I have been looking for it but haven't been able to find it. It would sure help being able to check the list once in a while for updates.

Still nothing I see excludes experimental.

Yup. I agree with you.

:cool:
 
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ADS-B for dummies?

Okay guys, this is a great discussion; very informative, but also causing my brain to hurt!
For us non-avionics guys; this would be SOOOOOOo helpful!

I realize some of the complexity, but don’t have a clue.

Would this be a fair summary? Please correct as needed.

To fly in the U.S. after 2020, and use UAT/978 - we need;

1. ADS-B out transmitter....... .... . . . .. . $3000
2. GPS source- approved with #1.. . .. ...$2000
3. Antennae-top............... . . . .. .... . . . ..$500
4 Antennae – bottom....... . . . .. . . . . . .....$500
5. DIY mount, wire, beer, bandaids etc. .$500
6. Avionics shop test, proof of compliance .$500
( similar to current transponder re-cert?)

Delete cost and equipage of GPS source if you have one already
Add cost of receiver if you want ADS-B IN of course
Add cost of 1090 ES if you want to fly internationally
( maybe Canada, Caribbean, Mexico?)

if I have only steam gauges and an old KT76, rip 'er all out and put in a SkyView 'n stuff?
 
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That is pretty much my understanding, given
3. top antenna is for gps
4. bottom antenna is for UAT
6. So far as I know, the FAA has said nothing about testing

Also, IIRC, Walt recently offered a package (#1, 2, and ADSB-in) for about $3.3K US.
Bottom antenna is a standard transponder antenna, less than $500.
 
Perry,
You don't need a top and bottom antenna on a GA plane (that's "diversity" which is needed on airliners). You also don't need Mode-S to fly other places in the Americas- they don't have Mode-S mandates. Your KT76 will be all you need.

Generically an ADS-B OUT device (978 or 1090) on an experimental can use a $17 non-TSO'd transponder antenna, so that's not a big cost. The bigger one will be the GPS antenna.

You have a few options:

1) Replace your KT76A with a Trig TT31 which is a drop in replacement. The TT31 is a fully TSO'd Mode-S with ES transponder. No new antenna needed. A TT31 is $2,400.

You would need a GPS for this by 2020. The TN70 is sold today but is $3,000+. At least it includes the GPS antenna.

Hopefully someday soon there will be a cheaper option.

2) Add a UAT to your system. There are companies selling a UAT OUT device with a approved GPS for about $4,000. Beware most of these need a certified GPS antenna for another $500+. Some of these have UAT IN (weather and traffic) so that's a nice bonus.

3) Go SkyView. Which of course is the best option, but I do admit you will spend a few extra pennies here. And the GPS eventually too.
 
Perry,

1) Replace your KT76A with a Trig TT31 which is a drop in replacement. The TT31 is a fully TSO'd Mode-S with ES transponder. No new antenna needed. A TT31 is $2,400.

You would need a GPS for this by 2020. The TN70 is sold today but is $3,000+. o.

However, IF you already have a Garmin 400W/420W/430W, it is an approved position source for the TT31 (may need a software upgrade, "free" except for labor). For those with the 400W series receivers, this may be the cheapest upgrade path (antennas already in place; sell your old transponder, net cost will end up under $2K. Add $500 if you want ADSB-in).
 
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