Latest Update 20160708

[Ian Melville]

If a transponder with ADSB transmits two packets then the mode S and ADSB  would have the same hex code? I didn't think it did transmit twice? Hence a bearing less Mode S that also transmits ADSB, would not be bearing-less?

What we are discussing above is mode S only transmissions.

[tnowak]

So there aren't any flags in a Mode S (non ADS-B) transmission that could possibly help differentiate between CAT and GA?

I seem to recall setting various parameters in my Trig TT21which relates to my aircraft size and cruise speed etc. I don't think they were related to my ADS-B connection but don't have my manual to hand just at the moment.

Tony Nowak

[exfirepro]

Hi Doc/ Tony / Ian,

If it was easy, we would have done it, believe me.

Yes, as Ian says, with an aircraft fitted with ADSB and Mode S, we get two sets of data, but Lee then filters out the Mode S using the 'hex' address, as the ADSB gives us exact positional data which are then presented to your nav system.

The problem arises with straight Mode S, which is used by a significant proportion of GA, as well as by a significant proportion of CAT instead of ADSB. Having no positional information (except altitude) we have to assess and present risk information based solely on the strength of the received signal. The problem is that signal is generated from a transponder the output of which can vary from about 70watts minimum at the antenna (e.g. Trig TT21 or similar) up to 500watts at the antenna for CAT. That's before we take into account significant variation between the fit of even the same transponder in individual aircraft, which can be considerable. The only reliable filter to differentiate between a close GA signal or a further out CAT one is the human eyeball I'm afraid.

Regards

Peter R
Mode S Development Tester

[EricC]

ADSB usage. From the table I guess that only three aircraft
are ADSB equipped.

Am I correct?     

[Keithvinning]

Yes

[exfirepro]

ADSB usage. From the table I guess that only three aircraft
are ADSB equipped.

Am I correct?   

Eric,

At first sight, Keith is correct - the rest of your traffic screen entries 'appear' to be Mode S. The significant factor is normally the fact that containing no positional information PAW cannot display any DISTance from the contact aircraft to your PAW.

Whilst I have experienced lots of 'non-ADSB' CAT aircraft, including Dash-8s and other FlyBe aircraft in particular, I am very suspicious that so many 'CAT' aircraft - including Boeing 737s, 747s and 787s  at such high altitudes would not be fitted with ADSB out equipment.

If they were on the ground, knowing your proximity to Blackpool Airport, I would suspect that some of these - for example the bottom two - were Ground Transmissions from otherwise ADSB equipped aircraft (which appear on PAW in the same manner as if they were Mode S), but most of these are, as I say at high altitude.

I wonder if there is something else going on here. Perhaps Lee might be able to shed some light on this.

Regards

Peter

[tnowak]

Hi Lee,

Then what about trying to use the "aircraft category" and "speed" flags to differentiate between CAT and GA mode S bearingless targets?

I have asked Trig about these and it seems all Mode S transponders will have this data.

From Trig:

"The aircraft category is reported whenever the flight ID is changed, when it’s interrogated, and is squittered every 5 seconds (if the aircraft is ADS-B equipped).
The airspeed is reported when replying to a Mode S interrogation (usually from a TCAS).
The aircraft category is reported in the “Aircraft Identification and Category” extended squitter message in the “Emitter Category” field.
The airspeed is reported in the “Long Air-Air Surveillance” message in the “RI” field."

I remember setting these values when I got my Trig TT21

Tony

[Moffrestorer]

Hi Tony,

Don't wish to rain on your parade, but the parameters you mention are all part, I believe, of the ADS-B message, and will be absent if the aircraft is only Mode S i.e. Is not transmitting ADS-B Extended  Squitter and is therefore only " bearingless" in this particular case.

Regards,

Chris

[tnowak]

Hi Chris,

The info from Trig tech support seemed to suggest the data wasn't only for when ADS-B was configured/enabled.
I had to make those settings when I first installed my transponder (3 years ago) well before I decided to add the ADS-B capability.

Tony

[Moffrestorer]

Hi Tony,

It funny but the darned Becker BXP 6401 Mode S transponder our flying group purchased in 2010/2011 didn't require us to make any of those settings. It's meant to be capable of outputting ADS-B ES if a GPS source is connected. All we had to do was connect the pre programmed address module which carries the 24bit unique code. It just seems to show how much further Trig are ahead of the game, I suppose. However I still think the parameters that you had to program were probably a precursor for you to add ADS-B Out functionality. This is evidenced but the term "squittering" that appears several times in the quote you posted from Trig and only applies to ADS-B Extended Squitter which is overlaid on top of basic Mode S, if so enabled.

Regards,

Chris

[Deker]

The signal strength is very low on those A/C that show no distance.
Maybe some of the position data is corrupted hence not showing the distance?

[Paul_Sengupta]

Yes, I've found this, if the signal strength is low, it won't be able to get a position.

Usually lower than this though, in the single digits.