Verifying Mode S ES ADS-B Out transponder setup - Can PilotAware help?

[bnmont]

 I managed to get this today from my TRT800 transponder output.

[Moffrestorer]

Hi Lee,

I have installed latest software 20151114 and see the "traffic" screen. Can you explain how the validation test should be done? I see CAT on the traffic screen but without the data analysis (SIL, SDA etc), as included in your screen-shot

[Admin]

Hi Lee,

I have installed latest software 20151114 and see the "traffic" screen. Can you explain how the validation test should be done? I see CAT on the traffic screen but without the data analysis (SIL, SDA etc), as included in your screen-shot

Hi Chris,
I was also surprised by this, the information sent by the CAT is infrequent.
I spoke to a few people about this, and I have not yet got a good set of answers.
My theory at the moment, is that the intention is, unless told otherwise, assume that the GPS is certified, and therefore no need to send these parameters.
If someone could confirm or dispute this, that would be a great help.

I asked TRIG how often they will periodically transmit these parameters, and they said :-
5 Seconds on the ground
2.5 Seconds in the air

I asked the same question of Funke, but did not get an answer, although if you look earlier in the thread, there is a posting by Brian Montilla with his TRT-800 settings captured

I hope others will chime in but I think it seems a little vague at the moment

Regarding 'Can you explain how the validation test should be done', I will leave that to others, hopefully Steve Hutt can provide some guidance here, I can only capture the information, I am not sure I understand what the parameters are yet for PASS / FAIL

Thx
Lee

[Moffrestorer]

Thanks Lee,

I think Brian's TRT-800 screen shot may have been the result of a test via the NATS verification process, judging from the text.

Regards,

Chris

[Admin]

Thanks Lee,

I think Brian's TRT-800 screen shot may have been the result of a test via the NATS verification process, judging from the text.

Regards,

Chris

No, it was just an early engineering build I made available to Brian, and have been trying to polish up since
Thx
Lee

[SteveHutt]

Hi Chris,

I believe Brian's screenshot was from a special test version of the PAW Traffic screen and not from the 20151114 release.

Regarding LAA MOD 14 approval for a Mode S ES transponder with uncertified GPS position feed, the process is unchanged at this moment in time and is as documented in the LAA MOD 14 form. Right now the PAW 20151114 Traffic display is a means to ground check a transponder setup prior to doing the currently required test flight in the NATS ADS-B ground receiver coverage area ( http://fasvig.org/nats-uk-ads-b-3000ft-coverage-area ).

I have suggested to the LAA that they give consideration to the use of the PAW Traffic display as a replacement for the MOD 14 process test flight. This would be especially beneficial for those who live well outside the area. The LAA are still considering this idea. Obviously, the PAW Traffic display is still very new so needs to be proven.

As to how it could be used, I have just been speaking to Lee about this. If a PAW is run in range of a test candidate a/c for the Mode S ES w/ uncert GPS, PAW will detect the message transmissions that contain the required fields and present them in the PAW Traffic Display. This screenshot could be captured and returned to the LAA as part of a new modified MOD 14 process. The key fields required by the MOD 14 process are SIL and SDA, both of which are required to be set to zero to pass the verification. And achieving SIL=0, SDA=0 is dependent on support by the given transponder manufacturer, by model, by firmware version and by how the transponder has been configured.

The way the PAW Traffic Display is populated is driven by the way data for inflight use is managed by PAW. Data can be 'aged' (dropped from the display if no new updates are received from a given target. This behaviour can be especially prevalent I think when people are experimenting with a PAW at home in their livingroom with poor antenna receiption (in fact what I am doing myself and I have seen this). This data 'aging' would not likely occur when using PAW in the open in the vicinity of a candidate ADS-B test a/c.

A little understanding of how ADS-B messages work does help. There is not just one single ADS-B message format that contains all of the data fields that is repeatedly broadcast by a transponder. There are many different message types/formats, each containing different data but keyed to the a/c hex code, and these different message types/formats are not all sent at the same time interval. Lee has had to write code to capture the different message types from a single a/c as and when they arrive and bring all the data together into the table in the PAW Traffic Display. That is why a row in the table can be partially completed and the gaps becoming filled in later.

Hope that helps.

Steve

[Moffrestorer]

Hi Steve,

As a follow up to my previous posts concerning use of Becker BXP 6401-2 for ADSB Out, because of the SDA=2 issue, I have requested quotes for the two certified GPS receivers "sources" mentioned in the Becker installation manual.

Received one this evening from the USA for only $3995. Add to that, VAT?  and installation, it would actually be cheaper to replace the Becker with a transponder which the LAA would accept with a non-certified GPS.

Clearly our syndicate couldn't and wouldn't contemplate any of this. We'd keep the Becker, for what it's worth, as a Mode C transponder that broadcasts our unique aircraft identifier, but regarding ADSB conspicuity, we'd have to forget it, and rely, solely on receiving ADSB from other aircraft,  together with the P3i facility afforded by PAW. Good on you Lee!

[ianfallon]

A sad and crazy state of affairs undermining potential safety benefits 

[IainM]

What exactly are the implications for the whole ADSB system of an aircraft transmitting SDA=2 instead of 0?

Will TCAS systems react differently? ATC? UK only?  What are the actual differences in how elements of the system react.

It seems the CAA, NATS, or whoever have chosen a pretty obscure workaround to enable use of uncertified gps, that was never envisioned by a lot of the hardware designers.

One wonders if the side effects this is causing are a lot more likely to cause problems than just transmitting the SIL=0, SDA=2, with an uncertified gps as position source.

[SteveHutt]

Hi Chris,

Thank you for that post. I have sent an email this morning to the CAA Conspicuity Working Group reiterating the issues I have raised previously regarding transponder manufacturer support for the requirements defined for use with uncertified GPS. I used your post (anonymously) to highlight the consequences that can arise.

Hi Iain,
Yes, I absolutely agree with your post. I do not have a handle on the precise reasoning for both SIL=0 and SDA=0 being requirements for uncert GPS ADS-B Out. I am trying to find out. I understand that CAT TCAS will ignore traffic broadcasting SIL=0/SDA=0. I don't know whether CAT TCAS will respond differently to SIL=0/SDA=2 traffic broadcasts.

Lee is, and I am, trying to better understand the circumstances were SIL and SDA are broadcast. Information from Trig says that their transponders when set to 'Uncertified GPS, will broadcast SIL and SDA every 2.5 or 5 seconds. In a GA scenario SIL and SDA are fixed values. However, the ADS-B spec allows for SIL and SDA to be dynamic to enable the switching of inputs between different devices that might require broadcast of a different SIL or SDA. In researching this I came across an old U.S. DoT/FAA document ( http://www.gpo.gov/fdsys/pkg/FR-2010-05-28/pdf/2010-12645.pdf ) that said SIL and SDA had to be broadcast within 10 seconds of a CHANGE in value. This would appear to be supported by what the new PAW Traffic Display shows. I am yet to see a CAT flight in the PAW Traffic Display showing the SIL and SDA values.

Investigations continue. Lack of access to the latest official ADS-B documentation is a significant hindrance.

Steve

[IainM]

I understand that CAT TCAS will ignore traffic broadcasting SIL=0/SDA=0. I don't know whether CAT TCAS will respond differently to SIL=0/SDA=2 traffic broadcasts.

Right now if i understand correctly CAT TCAS will interrogate and react to my Mode S transponder which has no ADSB output.  Presumably this will continue to work and it would only be the gps position data from my ADSB transmission that would be ignored by TCAS? 

Otherwise adding ADSB would be a backward step... 

The more I read about this, the less I see the benefit of my adding ADSB out.  What's the point if TCAS systems will ignore it.

[The Westmorland Flyer]

Yes, TCAS will still interrogate your transponder (regardless of its mode). However, ADS-B, if visible to TCAs, would provide much better positional accuracy.

An ADS-B equipped transponder is actually two systems in one: a transponder that responds to interrogations from ground/airborne radars, TCAS, etc. and a broadcasting system which uses the time between interrogations to broadcast stuff like position data. To all intents and purposes they are completely independent.

I think the decision to force SIL=SDA=0 on us was necessary to get the trial under way. It will have to be reviewed in due course, as it will severely limit the usefulness of GA ADS-B. That's probably a step too far right now for the risk adverse CAA. The next steps should be to evaluate the trial results, discover (to no-one's particular surprise) that non-certified GPSs give just as accurate positional information as the certified units and therefore conclude that more useful SIL/SDA settings can be applied. It might take a while!

[SteveHutt]

Yep, as Westmoreland says....
CAT TCAS will ignore SIL=0 ADSB broadcasts but will respect Mode A/C/S broadcasts/responses from the very same Mode S ES transponder.
However, PAW equipped a/c, and more importantly, GA a/c with other ADS-B In traffic awareness systems WILL see the SIL=0 ADS-B broadcasts.
Steve

[Admin]

Steve mad a comment earlier about not being able t ocatch any commercial traffic sending the operational status messages, so I thought I would post this here.

The A/C is a BA Airbus 380
https://www.caa.co.uk/application.aspx?catid=60&pagetype=65&appid=1&mode=detailnosummary&fullregmark=XLEI

[bnmont]

Just got this screen shot with the latest software.
Somhow did same attachment twice, cant work out how to remove!

(Admin, removed for you, btw, lots of traffic here!)