Primarily for general aviation discussion, but other aviation topics are also welcome.
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#1613276
Marvin wrote:...What your describing is Fruit and Garble.

Ground Based Receivers and Extractors as well as this used in TCAS are a little more complex in their front end design to address the issues as well as changes being made in the interrogate frequencies and therefore reduction in replies required to determine a SSR position.

A software tuned receiver is not even close to the same level of firmware in the decoding process.

Can any one guarantee - no.
.


Ahaa, you fell into my carefully laid trap Mr Bond :D

I am in agreement, on your statement regarding Ground based receivers (I am not so familiar with the front end TCAS design), but I believe you are right: in order to make sense of the overlapping data you need incredibly high quality front end receivers and huge compute power in the digital signal processing at the back end, in order to differentiate.
You are not going to get that kind of equipment sat on the coaming of your average GA aircraft.

In fact some of the early tests which were done regarding congestion on 1090Mhz (I have the reports somewhere), the analysis could not even be done realtime, it required post processing of the data in order to analyse effectively, with the caveat given as (I'm paraphrasing here from memory) :-
'at some point in the future computer power will be capable of handling the data in realtime'
its kind of like leaving the Nuclear waste cleanup to future generations!

So ground (or space) based stations will possibly make sense of the ADS-B data, in-aircraft systems, will not be able to differentiate and disambiguate the ADS-B data

I understand that their is a discussion regarding whether the 1090Mhz frequency could be used to handle simultaneous transmissions of OOK and FSK - this could be a solution, but I am sceptical that this would work, I would have thought that the harmonics generated during the switch between the 2 FSK frequencies would interfere with the 1090 OOK receivers

Thx
Lee
G-JWTP liked this
#1613290
leemoore1966 wrote:
gaznav wrote:...“Why aren't we raving about this?”. Because it hasn’t quite hit the nail on the head for many of us - the use of another proprietary signal standard, like FLARM, means that it just adds to another standard to detect by extant receivers like other ADS-B In, FLARM or PowerFLARM receivers. The internationally recognised standard of ADS-B is the way ahead and when, or if, PAW starts pushing out ADS-B we really will have something to “rave about”. :thumleft:


I may now really upset the apple cart :?

Firstly lets not confuse an international standard with CAP1391
Can devices which conform to CAP1391 be used outside of UK airspace - No, so how is this international ?

More importantly regarding ADSB on 1090Mhz using OOK (On/Off Keying), how much capacity is available in this spectrum some simple - back of a napkin analysis

ADSB/DF17 emits 4 packets per second
2 x Airborne Velocity
2 x Airborne Position
Each packet consists of a preamble (8uS) and the Data Block (112uS)
that is a total of (8 + 112) x 4 = 480uS every second
I am not great at Math so lets round that to 500uS for simplicity

In the most perfect of worlds, if all ADSB emitters could be synchronised then 2000 emitters could all run without ever corrupting each others data.
If that was a motorway, it would mean every car driving bumper to bumper without touching one another

The world is not ideal, these are not synchronised and so this will never be the case - which means they will stamp on each others broadcast time slot in a random and undefined manner.

This is further complicated, ADSB is not the only emitter on 1090Mhz, we also havethe following transponder transmissions :-
Mode-A (20uS)
Mode-C (20uS)
Mode-S (64uS)

Each of these transponders are interrogated by multiple ground based radar systems and air based TCAS systems, so these are NOT N-times per second, they are as many times per second that they are interrogated - now you start to see how congested the spectrum is.

Now consider the range of these transmissions, I can receive ADS-B transmissions from 200km away using PilotAware on the coaming. So imagine a geographical area with a radius of 200km all broadcasting simultaneously where any transmission can overlay any other transmission.

Usually we think that in RF transmission, the strongest signal wins over, which is great, I will only receive the strongest transmissions, which are closest in proximity right ?
Wrong, On/Off Keying does not work like this, it is a CARRIER-ON / CARRIER-OFF method so data corruption occurs very easily with overlapping transmissions, this is not the same as FSK which is a method that uses a 2 symbol carrier FREQ-1 / FREQ-2, importantly, a FREQ is always present, and never silent

Nobody has done any detailed analysis to know what the system break point is - but the FAA did envisage this was a problem and have encouraged ADS-B/1090(OOK) for aircraft flying above FL180 and UAT/978(FSK) for aircraft flying below FL180. proactively addressing any possible congestion issue.

So to the question
gaznav wrote:...or if, PAW starts pushing out ADS-B we really will have something to “rave about”. :thumleft:

Hmm, well how about, if those encouraging usage of this spectrum, in the manner specified were to indemnify the manufacturers and end users, that should the system fail and need to be retracted - all costs and investment would be compensated.
If the capacity of the system is as robust as those who promote it, then this is perfectly reasonable, as this is a zero cost sum - right ?

Who wants to volunteer first ?

Thx
Lee


I thought that mode s (select) would only respond directly when integrated directly to the individual unit. ie it is selective as to who reply's.

That is why all the NAA's wanted it so as to free up the spectrum and thus release SSR codes.

If that's the case wouldn't that free up the airwaves for the GPS data on the ES?

Get geeky if you like!

I'd like to know.

G-JWTP
#1613323
Regarding Mode-S that is my understanding as well - it is selective. Whereas an older Mode A/C Transponder is not and so every ground-based secondary RADAR, airborne early warning aircraft, military air-defence capable ship and fighter jet that interrogates will run the risk of FRUIT (False Replies Unsynchronised In Time) - simple detail of FRUIT here http://www.radartutorial.eu/13.ssr/sr14.en.html. Put simply if an interrogator asks for a reply then every transponder in the secondary RADAR’s beam will be asked for a reply - for an AEW aircraft that can be a 200nm+ radius circle. Then if every other aircraft/ship/ATC is also interrogating then you may get FRUIT. However, as someone who has many thousands of hours sat behind fighter aircraft RADARs and flying in early warning aircraft then I have yet to see FRUIT for real - so what I am saying it is not that common in my humble opinion.

So IF there is problem with saturation from Mode A/C transponders then the answer is either 100% ADS-B or 100% Mode S (which we know was batted down by smaller aircraft users on the grounds of size, battery life and high cost). So that leaves the one answer.

@leemoore1966
Whilst I agree with some of your points as ever, I do believe that we need to be absolutely clear here. ADS-B IS an international standard, and SkyEcho pushes that out, but what is yet to be allowed outside of the UK is portable ADS-B Out as the forward leaning CAA and CAP1391 has allowed in the UK. So IF another nation in future allows portable ADS-B transmission in the future (which I would guess is a good possibility depending on the UK’s experience and various trials) then the SkyEcho will come straight out of the box and work there, because it is pushing out transmissions to the internationally recognised ADS-B. My comments are not supposed to be a dig at your current products and I think that the work that you, Keith and the rest of the PAW Team have done is excellent - as you know I agonised over which way to go over PAW, FLARM and SkyEcho for many months. As we all go through this journey together, we are all learning and adapting to new thinking on this. I personally think the Rosetta is a great leap forward for you guys and my only reservation is the continuance of the P3i portion - that’s my opinion and it could turn out to be very wrong (although I did talk my parents out of buying Betamax many years ago and chose VHS instead!).

Finally, to all, I see that the SkyEcho2 is now advertised here: http://www.radartutorial.eu/13.ssr/sr14.en.html. £419 all in and it claims to work with FLARM and more importantly PAW. So if it has an ability to transmit and receive PAW data then you would get the OGN capability as well. So adding up either Rosetta or PAW Classic with this £419 then for £600 to £700 you would end up with a device that pretty much detects everything and makes you visible to a lot of aircraft. So we are definitely getting there :thumleft:
Last edited by gaznav on Wed May 23, 2018 8:39 pm, edited 1 time in total.
#1613329
G-JWTP wrote:
I thought that mode s (select) would only respond directly when integrated directly to the individual unit. ie it is selective as to who reply's.

That is why all the NAA's wanted it so as to free up the spectrum and thus release SSR codes.

If that's the case wouldn't that free up the airwaves for the GPS data on the ES?

Get geeky if you like!

I'd like to know.

G-JWTP


Correct and that is one of the techniques for reducing the amount of returns.

Mandatory Mode -S anyone?
gaznav liked this
#1613341
Except that autonomous devices, such as SkyEcho, will blat out their stuff at regular and frequent intervals independently of any radar. Once the gazillions of drones take to the skies, and all flying machines are mandated to carry something, won’t that mean things getting rather congested?
#1613355
Re: “Gazillions of Drones”
I believe there have been plenty of studies in this area and it involves having low power outputs for such a problem. One such paper on the subject is here: https://www.mitre.org/publications/technical-papers/ads-b-surveillance-system-performance-with-small-uas-at-low-altitudes

The conclusion to this report that looked at high densities of drones using ADS-B with lower numbers of manned aircraft was:

Presented results showed how key parameters affect current system performance if fleets of UAT equipped SUAS are added to the national airspace. It has been shown that there are ranges of transmit power and traffic density that are compatible with the current system. This analysis was modeled, and as with all models, assuptions and constraints affect the results. A feasibility evaluation for a particular future fleet should include specific details on the fleet size, distribution, transmit power, number of aircraft simultaneously transmiting, manned aircraft traffic, distance from SUAS to ground stations, terrain details and other parameters.

I’m expecting this is all a part of the ongoing trials by NATS/CAA on the use of ADS-B with various other agencies/Govt depts/companies plus the work of the Electronic Conspicuity Working Group. I’m also assuming their collective minds are examing this and many other issues as we go forward to a solution - not forgetting that our own regulator, the CAA, has said that ADS-B is it’s prefered solution announced nearly a year ago - https://www.caa.co.uk/News/ADS-B-can-help-reduce-airspace-infringements-and-mid-air-collisions,-says-CAA/
uAvionix-Ramsey, Marvin liked this
#1613366
Hi Lee

Ah, now I see your point. So that issue is why the US have opted to reduce the data used on ADS-B on 1090 saving it for just position and traffic reporting and then pushing all the fancy stuff like weather, TIS-B and airspace info onto 978 on the UAT. Plus also reduce the individual transmissions on 1090 by getting a lot of GA onto 978 ADS-B but then sharing data via Ground Based Transmitters (GBTs) across the 2 frequencies. But the US are wasteful of the spectrum in my opinion, they still sell and allow the fitment of brand new Mode A transponders and the use of Mode S is not as commonplace for GA as it is in Europe as I understand it. So that means 1090 is congested in busy airspace and why they elected to go into the ADS-B/UAT solution over 10 years ago to resolve their particular and to some extent unique problem.

I guess that is why NATS/CAA are trying this radical solution compared to the full USA ADS-B/UAT/GBT piece? That is expensive and possibly not necessary; especially as Mode A starts to dwindle more and more in Europe.

I see Mode A transponders’ effects on our spectrum like others view old dirty diesel cars on our environment. If we clean up our act as we are doing then introducing more cleaner solutions (like ADS-B and/or Mode S) should see this not being an issue for some time - maybe never if another technology comes along? Or if all else fails then we dig deep and go down the US route.

Or maybe my glass is just too full ;-)
Marvin liked this
#1613372
PS. Overnight afterthought. Just to add that OFCOM and the CAA have signed up to allowing low power portable ADS-B, so this would have been one of the first things the Spectrum Manager will have looked at - ie. would the potential growth of ADS-B on 1090 have a significant effect on safety through frequency congestion; it’s what they are paid to do! I was also thinking about extant traffic density and ADS-B Out users - if there was such a significant problem them ADS-B would be turned off on the ground when all of the aircraft are really close together in high numbers. But look at a busy international airport, especially if close to other business airports and overflown by significant transitting traffic - like Heathrow, then if this was such a problem then it would surely manifest itself during peak times already? There must be literally hundreds of extant ADS-B, Mode S and Mode A within a 30nm radius of LHR and we aren’t seeing or hearing of problems? Adding some low power 1090 ADS-B into that mix around the Uk would barely raise the noise threshold due to the distances involved and FSPL.

So either there has been a massive **** up or we are worrying about something that is very unlikely to affect on current predictions. Sorry, but my glass is still half full.
Marvin, mikeblyth liked this
#1613384
Hi Gaz,
gaznav wrote:PS. Overnight afterthought. Just to add that OFCOM and the CAA have signed up to allowing low power portable ADS-B

Can you post a link to this ?

then if this was such a problem then it would surely manifest itself during peak times already? There must be literally hundreds of extant ADS-B, Mode S and Mode A within a 30nm radius of LHR and we aren’t seeing or hearing of problems?

This is the status today, but we are postulating about the future, not the current - but if you are of the opinion that there is no looming congestion problem - which is perfectly valid given the amount of investigation done thus far, then does this sound reasonable ?

leemoore1966 wrote:...if those encouraging usage of this spectrum, in the manner specified were to indemnify the manufacturers and end users, that should the system fail and need to be retracted - all costs and investment would be compensated.
If the capacity of the system is as robust as those who promote it, then this is perfectly reasonable, as this is a zero cost sum - right ?


Thx
Lee
#1613385
leemoore1966 wrote:...if those encouraging usage of this spectrum, in the manner specified were to indemnify the manufacturers and end users, that should the system fail and need to be retracted - all costs and investment would be compensated.
If the capacity of the system is as robust as those who promote it, then this is perfectly reasonable, as this is a zero cost sum - right ?
Thx
Lee


Lee, can you point to references where business has had such a guarantee and also received compensation as you suggest would be required?

Why do you consider it necessary for this guarantee to be provided?

Are you seeking assurance a from OFCOM so it can protect the open frequency you use for PAW transmission to sustain you’re service model and protect you investment in the development?

Have the OGN network received assurances about the FLARM protocols they have decoded?

Have you received ongoing assurance from the OGN volunteers , I believe they are, that they will continue to support your initiative to rebroadcast data?

Do you consider that your request is reasonable in a business environment?

I’ll stop editing this post now as you’re question opens up so many worm cans IMHO :)
gaznav liked this
#1613393
Hi Marvin
You see the dilemma here ?

For ETSI, we have to meet the requirements and we are thus guaranteed access to the spectrum for the usage in which it is allowed.

For the 1090/ADS-B, it is a large investment on the part of a manufacturer with no such guarantee, because I think we would all agree, if there is an issue of spectrum congestion, then CAT must have priority - and rightly so.

So who should bear the burden of that risk of being denied access, having made a substantial investment ?
Lets not fool ourselves that there is no risk here, if there were not, the FAA would not be using UAT/FSK, and the CAA/OFCOM would not be investigating Low Power ADS-B as highlighted in Gaznav's previous posting

I think what you are saying, is it should be the sole risk of the manufacturer, even though they have no control, this is similar to asking private companies to invest in providing infrastructure in the full knowledge that there is a risk of losing the infrastructure to nationalisation at a later date for zero recompense - would you invest your time and effort in that ?

When more research and data is available on 1090/ADS-B/OOK, this could change, but for the moment I think we (PilotAware) are best placed to carefully concentrate our precious resources on where we can make a real world difference in safety for GA, using a technology which is sustainable, future proof and not lumbered with historical difficulties.

Thx
Lee
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