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

[SteveHutt]

Hi Richard,
Thanks for that info. I am in contact with another Trig v1.4 firmware user who has been keen to know whether he needed to upgrade. Your post was helpful. I am still awaiting authoritative response from Trig.

Hi Welsh Pilot,
Thanks for posting the Becker data. Do please post further updates when you have new info.

Regards,
Steve

[scsirob]

This ADSB lark, and the way the manufacturers have implemented it all seems to be a bit of a pig's breakfast, considering they happily sell their transponder's to the GA user.

When the GA user connects a certified GPS then the SIL/SDA values should correspond to their respective class of reliability. It's this trial connecting non-certified GPS sources that bring about some issues. We can't blame transponder manufacturers for not anticipating 'illegal' use of their equipment.

I wonder which NMEA parameter the transponders are supposed to look at for determining the right values in a certified installation. PAW should then easily be able to send the right string.

[SteveHutt]

I wonder which NMEA parameter the transponders are supposed to look at for determining the right values in a certified installation. PAW should then easily be able to send the right string.

Rob,

In general it does not work like that. Most transponders define a list of supported GPS devices (or a generic NMEA option) in their setup controls. By selecting the GPS device you are effectively telling the transponder which SIL/SDA/etc values to use in the ADS-B Out messages (and which format to expect to receive GPS data in). Unfortunately, often which values are used for a given selection is not published in the transponder manuals - something I have raised with the CAA through the CAA Conspicuity Working Group. Also, many of the certified GPS do not by default use NMEA as the data output format. In the Trig, etc scenarios, if a certified GPS was able to/configured to output NMEA and was connected to the Trig transponder with ADS-B Out switched on then the Trig transponder would automatically broadcast ADS-B Out messages which declared the setup as "Unknown" (sort of equivalent to uncertified).

This is not to say that some sophisticated highend airliner-type kit may be designed such that GPS devices do pass SIL/SDA-type values to the transponder to use in the ADS-B Out messages. But I have yet to encounter this degree of integration in GA-type avionics.

The ADS-B specifications need to cater for all possible situations, including the most complicated fault-tolerant avionics systems designs that airliners likely have. That is why it often appears overkill for GA. But in the radio communications arena GA & CAT are all playing in the same sandpit so have to live within a common specification. This push for uncertified GPS ADS-B Out is simply UK GA saying that we think if regulators want GA to participate in ADS-B then the technology bar had been set too high for GA.

Steve

[Moffrestorer]

I had a definitive reply from Becker Avionics today ( also relayed by HM dealership).

"Our ADSB solution in BXP6401;

The Surveillance Integrity Level is set to 0,
The System Design Assurance is set to 2 (it corresponds to Software and Hardware Design Assurance Level C).

Sorry, but those parameters are hard-coded and there is no possibility and not allowed to change them."

So there we have it, the Becker transponder is unsuitable for the NATS trial. Obviously, a certified GPS could be connected, but at what cost and weight penalty? In a Eurostar, even though ours is registered as Category A, there is not a lot of payload to play with.

Rob, I agree with your comments following my previous posting in this thread, but it seems different manufacturers applied different "solutions " to ADSB, witness the approach taken by Trig and Garrecht. When we were in the market for a Mode S, enhancing conspicuity was uppermost in our minds and Extended Squitter/ADSB seemed to offer great promise. It appeared to me from sales literature at the time that the Becker unit was capable of both ADSB out and in, unlike Trig where a separate receiver had to be purchased for "in". I was disappointed to find out later on that our Becker only offers ADSB out, also. I have since learned that for GA, some Mode S offerings did not even include ES, and none of them seem to have ADSB In, as standard. Basically, we appear to have been provided only half of what is required, in a rush to get units to market to meet the Mode S take-up deadline that was being pushed for by EASA and CAA, at the time.

[Welsh Pilot]

Yes I was also led to believe that the Becker had ES back in 2008 when I purchased it. I took this up with Becker and their response was they did not advertise ES. I'm sure they did but I can't prove it after all this time. That's £2000 wasted!

[The Westmorland Flyer]

I think, in fairness, that the NATS trial is demanding unusual and, one might argue, counter-productive parameters that the transponder manufacturers probably never imagined would be called for. Insisting that we set parameters that make our ADS-B out invisible to TCAS, for example, is plain bonkers.

Hopefully firmware upgrades will start appearing soon.

[SteveHutt]

I had a definitive reply from Becker Avionics today ( also relayed by HM dealership).

"Our ADSB solution in BXP6401;

The Surveillance Integrity Level is set to 0,
The System Design Assurance is set to 2 (it corresponds to Software and Hardware Design Assurance Level C).

Sorry, but those parameters are hard-coded and there is no possibility and not allowed to change them."

Thanks for that info. I must admit that most of my past focus has been on the SIL=0 setting. I need to go deeper into the SDA setting.
I just looked up some details on SDA and found this (this is American FAA documentation from a draft of DO-260B)

2.2.3.2.7.2.4.6  “System  Design  Assurance”  OM  Code  Subfield  in  Aircraft  Operational Status Messages
The System Design Assurance (SDA) subfield is a 2-bit (―ME‖ bits 31 – 32, Message bits 63 – 64) field that shall define the failure condition that the ADS-B system is designed to support as defined in Table 2.2.3.2.7.2.4.6.
The supported failure condition will indicate the probability of an ADS-B system fault causing false or misleading information to be transmitted. The definitions  and probabilities associated with the supported failure effect are defined in AC 25.1309-1A, AC 23-1309-1C, and AC 29-2C.  All relevant systems attributes should be considered including software and complex hardware in accordance  with RTCA  DO-178B (EUROCAE ED-12B) or RTCA DO-254 (EUROCAE ED-80).
The ADS-B system includes the ADS-B transmission equipment, ADS-B processing equipment,  position source, and any other equipment that processes the position data transmitted by the ADS-B system.

Table 2.2.3.2.7.2.4.6: “System Design Assurance” OM Subfield in Aircraft Operational Status Messages

SDA Value		Supported Failure Condition	Probability of Undetected Fault causing transmission of False or Misleading Information	Software & Hardware Design Assurance Level
decimal	binary
0	00	Unknown/ No safety effect	> 1X10-3 per flight hour or Unknown	N/A
1	01	Minor	≤ 1X10-3 per flight hour	D
2	10	Major	≤ 1X10-5 per flight hour	C
3	11	Hazardous	≤ 1X10-7 per flight hour	B

So......
SDA is actually an indicator of System Design Assurance of the whole system, including the position source.
And Becker is hardcoding their SDA to 2 even though they have no idea what the position source device will be.
It would appear that Becker may be interpreting the SDA differently to others.
I wonder if they are setting the value based on the assumed undetected failure probability of just their transponder device?

Does that make sense?

Steve

[The Westmorland Flyer]

Interesting Steve, thanks for that. It sort of ties on with stuff that I am reading in the TT31 installation manual, which indicates that certified GPS such as the Garmin GTN are at certification level C, equal to an SDA of 2. So it looks like Becker has assumed that only a certified GPS will ever be attached, as indeed was the case until the NATS trial came about.

I'm trying to get to the bottom of all this as well, as I am looking into replacing my non ES transponder with a TT31/KT74 (same innards, different panel). Confusingly the TT31 manual talks about certification levels A to D, which is at variance with the nomenclature in your reference. So I am still not certain that I can set an SDA of zero in the Trig kit.

It ain't easy this!

[Pete]

Just to add a little more to the melting pot.
Trig TT31 when configured to ADS-B the TT31 will ask for the aircraft length and width (Wingspan) in metres, and will calculate the appropriate size code for transmission.
SIL Values:
VFR only GPS or uncertified  TIL ( Transmitted Intergrity Level)-------- Low
GPS installation certified for en-route and terminal IFR  -------   -------Medium
GPS installation certified with augmentation, such as WAAS or LAAS---High

Hopefully this week I will get to connect up PA to my TT31.

All interesting stufff.

[Moffrestorer]

Hi Steve,

Your contention made be true, though in your previous  quote from the Becker Manual, they do define two certified GPS units that are designed to work with their transponder, so presumably the reliability of these units could have been taken into account in determining their SDA setting.

However, they apply the same SDA to two uncertified GPS that are also stated as compatible with the transponder, namely Garmin 430 and 530. Although "technically" uncertified (presumably because Garmin haven't submitted to the expense of certification) the reliability data for these units may be sufficiently good to lump them into the same category.

Does an SDA of 2 imply that a TCAS system would take note and act on such transmissions?

[SteveHutt]

Thanks for the various inputs.

I'm trying to get to the bottom of all this as well, as I am looking into replacing my non ES transponder with a TT31/KT74 (same innards, different panel). Confusingly the TT31 manual talks about certification levels A to D, which is at variance with the nomenclature in your reference. So I am still not certain that I can set an SDA of zero in the Trig kit.

For Trig devices, the latest firmware does allow the setting of 'good' values for pairing with uncertified GPS devices. Buying new would get you the latest firmware at time of purchase so ought to be fine.
I have no information about the KT74.

Some older Trig firmware levels may not be 'good'. I have a question in with Trig awaiting a definitive answer to explain exactly what the minimum firmware levels are for their devices to support the settings for an uncertified GPS. Fortunately for Trig owners, Trig will currently upgrade firmware for free in less than a week but the kit does need to be sent back to Trig for the upgrade.

Pete,
Which firmware version is your TT31 running?

Your contention made be true, though in your previous  quote from the Becker Manual, they do define two certified GPS units that are designed to work with their transponder, so presumably the reliability of these units could have been taken into account in determining their SDA setting.

Yes, it could be that Becker have taken the deliberate decision to hardcode their transponder firmware such that it only outputs the correct SDA value when it is paired with a position source appropriate to an SDA of 2. Not helpful for the currently required settings for use with an uncertified GPS.

Does an SDA of 2 imply that a TCAS system would take note and act on such transmissions?

A very good question and one I decided yesterday I needed to chase up.
If a/c "A" broadcasts 'SIL=0,SDA=0' and a/c "B" broadcasts 'SIL=0,SDA=2' how differently will "A" & "B" be treated by systems (airborne and ground-based) that receive their broadcasts
(bearing in mind that the common SIL=0 is already indicating a position source integrity level of 'Unknown')?

Steve

[Moffrestorer]

Hi Steve,

Did you get anywhere with the question of how aircraft A and B, with differing SIL and SDA settings, would be treated by systems that received their broadcasts?

[SteveHutt]

No, not yet. I will report back if I get any info.

[SteveHutt]

See http://forum.pilotaware.com/index.php/topic,242.0.html for first release of PAW ADS-B broadcast verification capability that provides visibility of the various control fields in the ADS-B message that are of interest when seeking to ensure correct configuration of a Mode S ES transponder with an uncertified GPS position source.

Thanks Lee for turning this request around so very fast.

Steve
FASVIG Programme Coordinator

[Admin]

Just to follow up on Steve's posting, you can go to the 'traffic' web page, and you will see something like this