[xtophe]

BTW - as far as I'm aware nearly all Flarm units do provide barometric information, based on a calibrated pressure sensor - it's needed for the IGC logfile.

Actually, from the reverse engineered radio protocol (as used by OGN) only the GPS altitude is transmitted.
But again it is a case of arguing the altitude might be off by a couple hundred feet vs a MAC

[Rob L]

The ATC advisor went on to say that the CAA is looking at cooperative EWS, ADS-B out, to be mandated for all aircraft from 2024 onwards.

For all aircraft? Including (for example) weather balloons? WWI aircraft in flying museums? I think not

[Buzz53]

Which is here: CAP1391 Appendix D.

Also relevant to this discussion is Appendix A Table 8, which is a comparison of different systems, and -in particular - interoperability between them.

Thanks but I don't think that is the one that Gaznav quoted, which is an earlier and more general report that I can no longer find, and which I think (and hope) is more relevant to this discussion.

The report appended to CAP1391 is quite narrowly focussed and deals primarily with whether a lot of new low power emitters will or will not impact negatively on existing TCAS systems (using high-power transponders) which is a worthy cause but maybe not of prime interest to most of us. In particular it has nothing to say on how the low power devices will fare in a more crowded environment. Rather like the very reduced requirements of CAP1391, it is more about not interfering with existing users than proving the equipment does what's required. For example there seems to be no mandatory receiver performance standard in CAP1391, only the transmitter, so you could come up with a "certified" device which is actually deaf as a post and quite useless.

I think the "Certification" issue is much over-egged. For CAP1391 the product is self-certified by the manufacturer, not by an independent body. So the same as PAW in fact! (OK, in the case of CAP1391 the CAA needs to then accept the self-certification but I imagine, from a position of ignorance, this will be a paper auditing exercise).

Alan

[Dave W]

Thanks but I don't think that is the one that Gaznav quoted, which is an earlier and more general report that I can no longer find, and which I think (and hope) is more relevant to this discussion.

My apologies - you are correct. @gaznav referenced report QINETIQ/14/01558, whereas I linked to the report @patowalker mentioned as an Annex to CAP1391, which is from the following year (QINETIQ/15/02265).

The former doesn't appear to be publicly available- at least, not via Google search.

[gaznav]

Here is an extract from a NATS report:

It is expected that voluntary carriage of ADS-B by GA will increase as GA users see the direct safety benefits it provides, hence the CAA’s and NATS’ ambition to promote the use widespread use of ADS-B on 1090 MHz, including operators of aircraft that already have a transponder.

This section is reproduced from the NERL concept for GA surveillance because it examines the consequences for the radio frequency (RF) spectrum if a large number of ADS-B transmitters are introduced into the operating environment.

One of the two radio frequencies used for secondary surveillance, 1090MHz, is already known to suffer from high levels of occupancy caused by responses to Mode S radar and Traffic Alert and Collision Avoidance System (TCAS) interrogations (on 1030MHz). ADS-B uses the same reply frequency and there has been a concern that a significant increase in the number of devices using 1090MHz may reduce the probability that ground radars will detect:

› A single response to a selective interrogation, which may cause further interrogations. The update of the track may be delayed while an additional interrogation is made and the possibility of False Replies Unsynchronised in Time (FRUIT) is increased. FRUIT is a term used to describe the situation when the transponder response is received by a radar that did not make the initial interrogation, and,

› Responses to a Roll Call, because failure to register responses to the roll call may delay the initiation of a radar track.

QinetiQ were contracted to simulate the impact on Roll Call Round Trip Probability (RTP) and Probability of Detection (PD), which will be experienced by existing radars when the RF environment changes. Their model simulated the effects of introducing 828 Low Power ADS-B Transceivers (LPATs) prototypes into the operating environment at any one time. The full results are published in references 1 and 2.

The conclusions of these reports indicate that, provided the transmitted power of LPAT was limited to a maximum of 20W, there is likely to be:

1) A measurable increase (46.3Hz to 58.0Hz) in the rate of long format Mode S FRUIT. However, this is a small percentage (~1%) of the total FRUIT in the scenario, which remains stable.
2) A slight drop (0.42ppts to 0.52ppts) in RTP for all Interrogators of Interest (IoI). However, reference 2 shows there is likely to be a significant decrease in RTP if the effective transmitted power is increased to 45W for each ADS-B device.
3) An insignificant drop (0.010ppts to 0.019ppts) in PD for all the IoI.

In summary, the widespread use of LPAT type systems by the GA fleet does not significantly compromise the performance of ground radar interrogators, provided the maximum transmitted power is limited to less than 40W when measured at the antenna. This finding is reflected maximum transmitted power requirement in the CAP1391 (reference 2) standard published by the CAA in 2016 which permits the maximum as 40W at the antenna port.

List of documents referenced in this document:
1 Low Power ADS-B Transceiver (LPAT) RF Environment Modelling Study – Phase 2. QINETIQ 2012.
2 Low Power ADS-B Transceiver (LPAT) RF Environment Modelling Study. QINETIQ 2014
3 ED-73E, Minimum operational performance specification for secondary surveillance radar Mode S transponders Vol 1, EUROCAE, May 2011
4 Simultaneous Transmission by Electronic Conspicuity Devices and Mode S Transponders, CAA, August 2017
5 CAP1391 Electronic conspicuity devices, CAA, April 2018

Source: https://www.nats.aero/wp-content/upload ... _issue.pdf

I seem to recall reading the 2014 study once, but I can’t find it either. But I have only ever read about the 2012 study.

@Straight Level - you asked who funded the study. I don’t know 100% for sure, but I believe it was the CAA and NATS. Here is QineitiQ’s history:

QinetiQ was formed in July 2001, when the Ministry of Defence (MOD) split its Defence Evaluation and Research Agency (DERA) in two. The smaller portion of DERA, was rebranded Dstl (Defence Science & Technology Laboratory).

The larger part of DERA, including most of the non-nuclear testing and evaluation establishments, was renamed QinetiQ and prepared for privatisation. QinetiQ became a public private partnership in 2002 with the purchase of a stake by US-based private equity company the Carlyle Group.

In 2003, QinetiQ signed a 25 year long term partnering agreement (LTPA) under which we provide UK MOD with innovative and realistic test and evaluation of military and civil platforms, systems, weapons and components on land, at sea and in the air. In February 2006, QinetiQ was successfully floated on the London Stock Exchange and the Carlyle Group sold its stake in the company.

So, as a tech company that provides scientific advice, it lives or dies on the accuracy of the scientific reports and recommendations it makes. Just because the CAA and NATS may have paid for the report, I would suggest that a £1.3Bn pound venture like QinetiQ are unlikely to falsify it’s report’s finding into this somewhat small matter and risk it’s global reputation as a scientific consultancy? If that is what you were getting at?

[rjc101]

I can see the return to a directional radar antenna not being a problem.

Does the report cover air to air transmissions or just air to ground (radar). Having worked on reports such as this, they thoroughly cover the specific question asked. Also having commissioned reports from similar companies, the actual question has to be carefully format too.

Depending on how many transmitters are visible to one another in the air, there is the potential for the transmissions to overlap causing the receivers to miss messages (which can be made up of multiple packets) due to corruption. To some extent this is could be mitigated by transmitters closer to the receiver having the power to “blast through” an overlapping transmission from further away, depending on how well the receiver can reject the “noise” from the other lower power transmission in the background.

The same is true for all EC transmissions of course, but at 20W the impact radius of this is higher than the likes of FLARM or PAW which operate at much lower power ~500mW.

It looks like the report asked a specific question about the impact on the radar network. Anyone have a study of the impact for airborne ADSB receivers?

[Straight Level]

So, as a tech company that provides scientific advice, it lives or dies on the accuracy of the scientific reports and recommendations it makes. Just because the CAA and NATS may have paid for the report, I would suggest that a £1.3Bn pound venture like QinetiQ are unlikely to falsify it’s report’s finding into this somewhat small matter and risk it’s global reputation as a scientific consultancy? If that is what you were getting at?

Gaz,


I've has a quick skim, this is the section that concerns me:-

"standard civil commercial aircraft modelling component, which had been derived from NATS UK radar recordings from 12th June 2006"

also
"Transponding General Aviation Component
This component contains a typical distribution of GA aircraft and is based upon
NATS radar data. The component has been used for a number of years in SIEM
modelling tasks for the NISC. All the aircraft are transponder equipped.
The total number of GA aircraft in the component is 170. "

What is special about 12th June 2006 and why did NATS supply such historical data?
I would want to determine the busiest day in the past 5 years, not only that but the busiest hour in that busiest day, and then extrapolate the ADSB take-up over say the next 10 years, to include all possible new applications such as delivery drones, air taxi etc, not just a typical day.
After all, the system needs to work 100% of the time. Imagine if for just 1 hr of 1 day the system falls over !
Also we don't want to be replacing equipment that becomes redundant in a few years because as it is no longer fit for purpose.
I have Flarm, PAW and ES transponder (I think it outputs SIL 3 ) therefore I'm not wanting the system to fail !)

ADSB is CAA and NATS preferred solution. They asked and paid QinetiQ for the report.
Any consultancy producing a report don't have to falsify anything, just don't report on issues the customer might not want to see.
The report is based on the data the customer supplied
Fox guarding the hen house or what

[PaulisHome]

I've has a quick skim, this is the section that concerns me:-

"standard civil commercial aircraft modelling component, which had been derived from NATS UK radar recordings from 12th June 2006"

also
"Transponding General Aviation Component
This component contains a typical distribution of GA aircraft and is based upon
NATS radar data. The component has been used for a number of years in SIEM
modelling tasks for the NISC. All the aircraft are transponder equipped.
The total number of GA aircraft in the component is 170. "

I'd question this too.

It's not a difficult thought experiment. Just consider mandating all gliders have ADSB out. Now think about Lasham launching a couple of competitions (which they do) - you would have maybe 80 gliders in a small area under the London TMA. Ask whether there is going to be ADSB congestion in that area, and in the CAS directly above it.

That really doesn't look like a 'typical distribution of GA aircraft'. And any conclusion based on that assumption seems to me to be worthy of further examination. (I write as someone who used write that sort of technical report for a living).

Paul

[Cessna571]

SkyEcho and SkyDemon



This wartime cub is now conspicuous.

Couldn’t spot it in the air when I was 600’ above it, within 2 miles, near Royston though, no matter how much I tried.

Turns out that drab green IS excellent camouflage

[Cub]

[Straight Level]

I'd question this too.

It's not a difficult thought experiment. Just consider mandating all gliders have ADSB out. Now think about Lasham launching a couple of competitions (which they do) - you would have maybe 80 gliders in a small area under the London TMA. Ask whether there is going to be ADSB congestion in that area, and in the CAS directly above it.

That really doesn't look like a 'typical distribution of GA aircraft'. And any conclusion based on that assumption seems to me to be worthy of further examination. (I write as someone who used write that sort of technical report for a living).
Paul

Add to that thought experiment:-
There has been poor weather for one or two weekends and it's a super weekend to follow, me and the rest of us take advantage and go flying.
Add to that it's the start of the summer holidays and just about every CAT is airborne .
Add to that Amazon launch their drone delivery service
etc
etc.

@gaznav where in the report does it discuss the effect on ADSB in a 'perfect storm' scenario?
The report assumes a "typical day". Isn't this the critical failure of their analysis ?
We all know in aviation if it can happen, sooner or later it will happen.

I've been in industry long enough to realise if you ask a consultancy to determine if something will work they will give you a good reason as to why it will work especially if you give them the data to make that analysis in the first place.

Interesting !

[rjc101]

This, no doubt thorough, report was to answer a question...

The purpose of this document is to describe the analysis undertaken to support the CAA trial for simultaneous operation of transponders and separate ADS-B equipment on-board general aviation aircraft operating in uncontrolled airspace.

Which is the issue if you carry a portable ADS-B Out box with you into your ModeS equipped aircraft, the combined messages from both devices was unknown. Hence the report was commissioned to determine what impact this may have.

The report was written to asses this specific scope...

The analysis consisted of two methodologies:
1) To monitor engineering and ATC reports for any indication of unusual track behaviour by uncontrolled aircraft outside of controlled airspace.
2) To examine surveillance recordings for the aircraft involved in the CAA trial and attempt to identify instances of single and/or multiple data corruption, which could be attributed to the simultaneous operation of an ADS-B transmitter and a transponder by the same aircraft.

Which is correct, to answer the original question of what happens when you have two transmitters, on 1090MHz, identifying the same aircraft. What happens to the ground radar infrastructure performance.

The report was not commissioned to determine anything to do with congestion of the 1090MHz frequency as a whole.

2.2. Channel loading
The portable ADS-B transceivers operate on 1090 MHz, which is already a heavily used frequency. Any increase in the transmission rate on this frequency will have some degree of impact on the overall channel loading, which is discussed in Appendix B.

Appendix B, the final two paragraphs.

It should be noted that the modelled GA aircraft in the simulations could alternatively all equip with low power Mode S transponders (Class 2) which have a minimum peak power output of 18.5 dBW (70W) at the antenna port, see reference 3. Furthermore, these Class 2 Mode S transponders would also be ADS-B OUT capable, likely providing the same message rates as modelled above, and additionally producing replies to ground and airborne TCAS interrogations as well as contributing to the occupancy of 1030MHz.

It can therefore be reasoned that although not explicitly modelled, equipping GA aircraft that currently have no secondary surveillance capability with Mode S transponders, instead of ADS-B only transmitters, would detrimentally impact the 1090MHz spectrum occupancy. Hence the portable ADS-B transceivers would seem to be the preferred option if the majority of GA aircraft were to adopt cooperative surveillance.

I would consider the outcome of that report being used to dispel any concern over 1090MHz loading/congestion to be disingenuous.

[gaznav]

ADSB is CAA and NATS preferred solution. They asked and paid QinetiQ for the report.
Any consultancy producing a report don't have to falsify anything, just don't report on issues the customer might not want to see.
The report is based on the data the customer supplied
Fox guarding the hen house or what

That’s a bit like saying that a house survey, that you pay for, that values a prospective property that you are buying, is written so that is doesn’t “report on issues the customer might not want to see“. We know that is bunkum or there would be a serious problem in the housing market - I’ve walked away from properties that I have invested in due to a survey that I have paid for. I have also walked away from the purchase of an AA-5, having paid £300 for an engineer to survey it for me.

Sorry, I’ll say again, QinetiQ is a world leading company that lives and dies on it’s reputation on research and scientific consultancy. This report probably cost a few thousand quid and QinetiQ is worth around £1.3Bn. It would be CRAZY for it to risk it’s reputation and worth for a few thousand quid.

Finally, the choice of 12 Jun 06. So the CAA and NATS chose a day in the summer, towards the start, when it is more likely for people to be flying after the slow improvements in weather from the winter. Why on earth would they choose a day when it is quiet in the middle of summer? Why would they risk challenge by stating the date of the data and also for something that will affect the safety of others that could later be challenged in Court? Again, that doesn’t make sense. You wouldn’t spend all this money and effort to do this by picking a ‘phoney day’ - especially when it involved the safety of people (pilots and passengers and those that live underneath on the ground)?

I’m sorry, this is another mad conspiracy theory developing here. I’ve said it before, I’m a fan of “Occam’s Razor” (look it up), and the most likely thing here is that a scientific report has proven that low power ADS-B is ok for the use that they intend.

[Cub]

I've been in industry long enough to realise if you ask a consultancy to determine if something will work they will give you a good reason as to why it will work especially if you give them the data to make that analysis in the first place.

Interesting !

If you have been in the industry that long you would appreciate that people do not look to falsify or manipulate the data to fit the picture when that would have a fundamental and potentially detrimental impact on your core business?

ANSPs certainly do not take likely any study or suggestion that may indicate an impact on their ability to deliver safe and robust surveillance services. It is what their whole business is based upon.

Suggesting that a highly respected research organisation’s study is somehow bias or weighted towards the answer the customer may want is disingenuous at the least. Regardless of independent studies, do you honestly not believe that the major ANSPs are not continuous evaluations and monitoring the present and future scenarios with an extremely critical eye?

You are correct to point out that the situation is changing constantly with the rapid uptake of ADS-B amongst the GA fleet and the allocation of a second ADS-B frequency to be exploited by growing sectors of the market and data services.

[Cessna571]

What I don’t understand about all the “congestion naysayers”

If Flarm transmits more often than ADSB and a large proportion of those 80 gliders use Flarm, how does it work at all?

How will ADSB become congested but p3i not when P3i transmits more frequently.

it’s just another clutching at straws thing to try and stop people fitting ADSB when it’s already been properly looked in to.

I’m actually concerned about not bumping into people rather than a daft SE2 vs PAW war, but I don’t have a commercial interest in either.