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Mode S Transponder/ADS-B Sanity Check

J

JoeLofton

Well Known Member
I'm ready to replace my inoperative Mode C xponder with a new Mode S, so I've been wading through info on transponders and ADS-B capabilities and limitations. This is what I think I know - please give me a sanity check before I place an order for a Trig TT21 or TT22 (both transmit/receive on 1090 MHz). I fly day VFR.

...The Mode S transponder signal satisfies FAA requirements for transponder use regardless of the status of the ADS-B network deployment.
...Connecting my Garmin 496 to the Trig will allow a more accurate fix on my position (but the 496 doesn't provide the position integrity required for IFR).
...Sending the GPS-enhanced signal MAY allow me to receive TIS-B traffic info and display it on the 496 (not sure about this one).
...Traffic info is transmitted by the ground system on both 1090MHz and 978MHz.
...Weather info is transmitted only on 978MHz, so I won't receive any weather with the Trig.
...If I decide later (after the ADS-B network is built out) to utilize the weather info (FIS-B), I will need a universal access (978MHz) receiver, and will somehow have to signal that I can receive on 978MHz.
...I read somewhere that the FAA may require the higher power transponder (i.e. TT22) for all planes by the 2020 deadline. Any truth to this?

Thanks for your input ... Joe
 
Here is what I know about Mode S and ADS-B and hope it is accurate:

Mode S is entirely different than ADS-B. It allows you to receive traffic info from a ground base (TIS) and only in selected metropolitan area. I have it and the coverage is very limited. The cost for certifiying (transponder check every two years) is almost double the cost of a regular transponder with mode C.

Garmin makes a squitter for its mode S transponder that satisfies the ADS-B requirement. I believe it gets installed inside of the transponder and it needs to be done by Garmin.

If I was going to do it again, I would not get a Mode S transponder as I end up adding a Zaon so I can receive traffic info at all times and not only in busy air space.

Hope this helps.
 
496.

The 496 can only handle FIS-A. This not only doesn't have weather, but the traffic information is more restricted than the FIS-B.
I can't remember the details, but fewer targets and targets need to be closer.
You can get more details from http://www.navworx.com/

My plane is setup with Navworx, AFS EFIS, 496 and mode C transponder.

Kent
 
JoeLofton said:
...If I decide later (after the ADS-B network is built out) to utilize the weather info (FIS-B), I will need a universal access (978MHz) receiver, and will somehow have to signal that I can receive on 978MHz.
Click to expand...

No signal output necessary to receive ADS-B weather.
 
I have the Dynon transponder which is really a Trig. It's mode S and extended squitter. I have a 978 UAT receiver but get very limited TIS B traffic. Main issue is awaiting Trig to do a software update that will allow for 1090 traffic to be received on a 978 receiver. Certified stuff so it's been taking them quite a while.
 
It is my understanding that a Mode S (ES) txp, including the Trig, in order to transmit to ADS-B, must get it's position data from a "Certified" WAAS GPS system to be legal. Dan
 
Back to the Drawing Board

Dan, you're right. The Mode S transponder with extended squitter (gotta love the terminology) may not even recognize a GPS interface unless the GPS is certified for IFR.

So a Mode S transponder won't do anything more than a Mode C for me at this point. I may go with a Trig anyway because of very limited depth behind the panel. Would like to get my transponder control head onto the panel instead of down on the floor behind the stick.

Thanks for the input. I'll worry about ADS-B sometime later.

...Joe
RV-3
 
According to Dynon, their Mode S transponder (which is made by Trig) will trigger ADSB-IN through it's own non-WAAS gps location data, but will likely not qualify for the ADSB-OUT mandate unless there is either a certified WAAS signal given from another source or the rules are changed. Currently they send a "reduced reliability" bit (if I understand it correctly - I may not) telling the ADSB receivers on the ground that the GPS location is from a non-certified source, but it's enough to trigger the ADSB-IN traffic data to be transmitted back up to the aircraft.

I don't pretend to know this stuff for certain, if you have better knowledge please enlighten all of us....
 
SDA

You pretty much hit the nail on the head, Greg.

One of the bits of data in the messages broadcast by an ADS-B transceiver is a parameter called System Design Assurance (SDA) level. SDA is preset by the installer based on the certification status of your ADS-B transceiver and attached GPS. It basically tells ATC and the rest of the world how reliable your ADS-B transceiver and GPS are.

Right now the FAA's ADS-B ground infrastructure apparently lets some users get away with using non-certified ADS-B transceivers and GPSs to access TIS-B by setting the SDA "reliability flag" to show that those boxes and installation haven't been certified. This won't always be the case; if the FAA doesn't close this loophole sooner, anybody equipping with ADS-B for the 2020 mandate will be required to have TSOd/certified equipment.

Dave
 
Thermos said:
You pretty much hit the nail on the head, Greg.

One of the bits of data in the messages broadcast by an ADS-B transceiver is a parameter called System Design Assurance (SDA) level. SDA is preset by the installer based on the certification status of your ADS-B transceiver and attached GPS.
Dave
Click to expand...

Installer - meaning builder of the experimental aircraft? So we would need to set that bit to indicate non-certified equipment unless we are feeding a certified WAAS signal to it, correct? It's not factory-set and locked? For the record, I'm installing the Dynon transponder and will be feeding it GPS location data from my certified 430W.

I'm just seeing the potential for abuse here, in that some users could set that bit (either intentionally or through ignorance) to show a WAAS source when there is not in fact a certified WAAS GPS location.
 
Last edited:
airguy said:
Installer - meaning builder of the experimental aircraft? So we would need to set that bit to indicate non-certified equipment unless we are feeding a certified WAAS signal to it, correct? It's not factory-set and locked? For the record, I'm installing the Dynon transponder and will be feeding it GPS location data from my certified 430W.
Click to expand...
Yes, it could be the builder, or an avionics shop, or whomever does the installation and initial setup of the ADS-B transponder. I'm not familiar with the Dynon unit but if they intend it to be used for ADS-B Out in compliance with FAR 91.227 after January 2020, it'll have to be TSOd.

airguy said:
I'm just seeing the potential for abuse here, in that some users could set that bit (either intentionally or through ignorance) to show a WAAS source when there is not in fact a certified WAAS GPS location.
Click to expand...
Yup. ADS-B shifts the burden of accurate and reliable position to the user; whatever your transponder puts out is what ATC and other aircraft will see. Tricking the system, even if it's unintentional, is kinda scary.

Dave
 
User/Installer Determines SIL

From the Trig T21/22 installation manual, as part of system configuration (performed through the menu):

6.1.10 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
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

So it is up to the user/installer to select the appropriate SIL. I would personally set the SIL correctly to reflect my installation because I have no desire to be run down in the sky.

Good to know that a VFR-only GPS can be used to augment position reporting - that should improve the position info that ATC is working with.

Maybe someone from the ATC world can chime in on how the SIL affects traffic separation/advisories.

...Joe
 
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