Synchronised crashes -- coincidence or not?

[Kim Taylor]

Chris

I believe that the commands are broadcast simultaneously on 2.4GHz and 900MHz, at least that's what the blurb on the T9 site says, in particular referancing the FrSky X18

Kim 

[John Lee]

On Jeti it transmits on 2.4gHz and it monitors the round trip - Tx>Rx>Tx signal integrity. If that fails it switches to the 900mHz backup. There is no telemetry on 900mHz.

 

The Jeti telemetry gives you the signal strength on both Rx aerials (or all 4 if using 2XRx) and the round trip signal quality as a 'Q' percentage. That measures every packet of information sent to the Rx, which then sends it back to the Tx which compares outgoing & receiving packets giving you a % of the successful commands. In the 7 years I have been using Jeti the 'Q' has only dropped from 100% on a handful of occasions in flight and then only to 70-80% ish. The signal from the Rx to Tx for telemetry is weaker than the Tx's signal so even at 0% Q you could still be in full control, just the telemetry has failed first, which will prompt a voice warning.

 

It has never given even a suggestion of a control problem.

[Martin Harris - Moderator]

I understand that a lot of model flying in the USA takes place in high RF environments, such as parks close to residential and industrial areas and with larger participant turnouts than the UK’s typical dozen or so flyers in a rural setting and 2.4 GHz saturation is a significant problem. I remember a lot of discussion about this at least 10 years ago on a largely American forum and the situation is unlikely to have improved!
 

[Allan Bennett]

Just going back to the comments about wind sheer;  There's a line of trees at the south boundary of our flying area that can give rise to down currents caused by swirling when the wind is from the south, but our two crashes were well away from any trees, the wind was almost dead calm, and the sky was just a sort of uniform grey.

[john stones 1 - Moderator]

 I'd go coincidence.

[Andy48]

On 04/11/2024 at 12:52, Allan Bennett said:

There's no suggestion it was a collision, EvilC57; the models was well separated and heading different directions. 

 

We're in North Hertfordshire, and I can't see any notice of jamming trials there.  The closest seems to be Scotland over a month ago.  As for telemetry, there were no low signal or lost signal warnings on my FrSky system during flight, and failsafe is configured to shut the motors on loss of signal, not throw the model all over the sky as it did just before crashing.  But I don't have logging so can't prove it one way or the other.

Basically you must have logging if its a FrSky system, and cases like yours are exactly why it is useful on every flight.  Its easy to enable for any model.

[Andy48]

On 05/11/2024 at 16:50, Martin Harris - Moderator said:

I understand that a lot of model flying in the USA takes place in high RF environments, such as parks close to residential and industrial areas and with larger participant turnouts than the UK’s typical dozen or so flyers in a rural setting and 2.4 GHz saturation is a significant problem. I remember a lot of discussion about this at least 10 years ago on a largely American forum and the situation is unlikely to have improved!
 

 

Am I right in thinking they also allow much higher radio transmission power in the US so that more devices could be within range.

[Don Fry]

On 05/11/2024 at 14:58, Konrad said:

. But over time we see a very strong indication of a 15 meter wide band of crashes that extend 1Km a way from our field. We have not isolated the cause of this but it is in the data so it is real.

Be very careful there. There is a pitfall in statistics. If a victim expects a given result, that result gets reported, and the amalgamated data reinforces the expectation in future. A self perpetuating cycle. 

[Martin Harris - Moderator]

Joke and associated comments moved to "funnies" thread - I could see no connection with this topic?!?

 

[Konrad]

There are no conclusions being made just an observation that at each end of the runway there are a lot of crashes reported. Also that there is a swath about 15m wide where the density of reported crashes  is significantly higher than the general area other than at the ends of the runway. (This does seem to align with a notch in the adjacent hills).  Now there is no way of knowing the rate of crashes as a function flights or flight hours flown over the area. 

 

Yes, a lot of the crashes at the ends of the runway have a note about the signal, frame or data packet lost. This might just be antenna orientation issues as well as other RF issues. 

 

I'm not too concerned with the reported data not being accurate to the best of the reporter's abilities. What I am concerned with are the unreported incidences. I'm going to assume that most of these are of known origins. Like mid-airs or my improper energy management and are evenly distributed across the field in accordance with the same  unknown, unknowns

 

But yes, sorting through raw data to find any meaningful patterns or correlation's takes some skill in statistics.  We now suspect some kind of high gain antenna but as of yet haven't found anything to support this.

 

I fly dual band equipment based on my personal experience with the 2.4gHz dropping out, at close 1/6 the range of 900mHz when performing ground RF range checks.* I've also seen (twice) that I've lost the 2.4 gHz link yet still showed and had full flight control in the air and at extreme visual range (thermal gliders). Ok, I was at these extreme ranges flying a dot, because I knew I had the longer range of 900mHz.

 

*Take the opportunity to see this for yourself when you see one of your fellow fliers with this dual band equipment. Ask him to do a range check with both 2.4gHz and 900mHz and note when they drop out. Your confidence in the range of 2.4gHz equipment will be shaken!

Edited November 7 by Konrad

[GrumpyGnome]

I don't know anyone with a dual-band transmitter to demonstrate.  But similarly, I don't know anyone that has become 'out of range' flying VLOS where they can see their model sufficiently well to see what it is actually doing.  

 

I do find it a little odd that if range in 2.4 is such an issue a) there's not more noise about it, and b) AFAIK only FrSky of the major brands, sells dual band.... maybe it's a specific US/North America problem, and other brands have dual band jn the pipeline.

[Chris Walby]

IMHO people are mixing the range of the various frequencies and the interference of those that are currently in use.

I have model that has very marginal power to weight and one of our club instructors flies it very large circuits as in turns it washes its speed off (he can see that far out, but I can't) on DSM2 and its never given us a problem.

 

Others have detected loss of signal inside the perimeter of the flying field which is only a few acres.

 

I have flown when we had 24 models in the air at once and there was no loss of signal crashes.

 

IMHO the problem is intermittent external influences and as the UK has lower TX/RX transmit power levels we have less issues than the US. Its the same discussion regarding mobile phones on the flight line, put transmitters very close to other electronic equipment and you will get problems.

[Konrad]

RF noise has a direct effect on the effective range (the ability of the RX to extract the data packet).

 

The RF environment is very different today from that in which 2.4gHz was first introduced 25 or so years ago.  For example we now have G5 cell phones and their impact on the RF spectrum. The notion that rural area are safe from these changes is false. The floor (base line noise) of the 2.4gHz band has risen significantly. This is why the use of dual band protocols is gaining popularity. We really don't need, for RC flight, the data rates we get with the higher frequency. So moving down the RF spectrum makes since. Like the dual frequency protocols of 2.4gHz the same logic for redundancy applies to dual band. This is why most new purchases are moving to dual band as it addresses today's RF environment and that of the future.  Frequency hopping 2.4gHz works. It is just that frequency hopping across two bands works better.

 

I posted to show that there are real null spots in the RF coverage in most fields, particularly with the very high, line of sight, frequencies we use. I tried to show that at one of my flying fields we are trying to locate (isolate) these by requiring that crashes be documented.  All flying fields have areas for higher probability of a crash. Most of these are at the ends of the runway, but not all.

 

I mention that while I fly line of sight, that 900mHz give better performance at longer ranges. I give anecdotal evidence with the statement that I have flown at extreme range with thermal gliders. This was a 4 meter ship built predominately with carbon. As such RX antenna placement is problematic. Even with the thermal ship 2.4 gHz is more than adequate. It is just that 900mHz offers another level of security with the RF link. I tried to say that I wouldn't be flying like I was without 900mHz. That is different than saying I needed 900mHz to fly the way I want. (Flying dots really is not fun)!!

 

As to the American comments I usually fly by myself or with less than 5 guys. I fly coastal cliffs as such half the of the directions that RF comes from is ocean. So from that direction I'm likely only seeing ship to shore communication, as I can't see Hawaii, and weather radar.

 

That comment about transmitters causing equipment issue is real.  But remember that radiation (RF frequency) power dissipates with distance at a rate of 1/R^2. 

See this thread:  https://forums.modelflying.co.uk/index.php?/topic/60066-kingtech-vs-frsky-tandum-radios/&tab=comments#comment-1024644

 

 

 

So the take away with my posts, are that RF nulls are real and that dual band protocol mitigate these very well. This is why most new equipment at my fields are now dual band.

 

While real, I place little confidence in coincidences.

[Andy48]

My last flying site had a microwave link running across the field. I would get a brief signal lost warning when flying through it at the exact height, but no control issues. I changed to Frsky after a number of years with Spektrum when I had 3 near identical crashes in 2 weeks. Eventually I found out why the Spektrum would fail and not the Frsky. Putting the receiver on on a well secured plane in the pits, then switching the transmitter on with the throttle half open, one could observe how quickly the reciever responded once the throttle was closed. The Frsky system was virtually instantaneous, but the Spectrum took at least a couple of seconds before it responded. - Plenty of time for control to be lost. This was a good few years ago now, no doubt Spektrum have resolved this brownout issue on newer systems.

 

A thought did occur to me. During the BMFA world record for the greatest number of model planes in the air at once I didn't hear of any crashes. We had 25 planes up, with no problems at all.

[Konrad]

Off topic warning!!!

 

"this brown out issue" How is this a brown out issue?

 

My understanding of brown out is a voltage (Vss) issue, where the voltage drops to the point where the microprocessor reboots. I don't understand how the closing of the throttle effects this, unless there was problem with the throttle servo (stalled or very high current draw).

 

Most brown out issues are an indication of a poor power supply (battery or BEC). Modern firmware (and hardware) give priority to the microprocessor to speed up the recovery from a power interruption.

 

When one hears about problem with a brand I like to learn what was the problem and the time frame of the problem.

 

I'd like to mention that Spektrum was one of the early providers of 2.4gHz equipment to the RC community. As such they suffered from the learning curve the most. But Spektrum and Horizon Hobbies stood by their equipment and honored all my warranty claims. We all have benefitted from Spektrum!

 

To my mind all that 25 planes in the air means is that the RX's had the ability to frequency hop to another frequency. Or that the probability that both frequencies being saturated for any length of time is very low. I agree 25 planes in the air is a nice stunt but it tells me little. 

Edited November 7 by Konrad

[Peter Beeney]

I think there is one point of interest here, or maybe more one of concern as well, and that relates to the model’s behaviour before the crash. If both models behaved in exactly the same way and it seems very likely they may well have done, then I would've thought that the jamming signal, if that's what was happening, was a fairly sophisticated (powerful) piece of kit. Without knowing very much about the technical aspects of it all I've always thought that a jamming signal would be made sufficiently powerful enough to completely swamp the airways across a wide bandwidth and prevent all the legitimate signals from getting through. If that were the case then I think the receiver would simply go to it’s ‘lost signal - apply failsafe’ condition.

If this happened to a model flying on its own the reason for the crash may have been construed quite differently; a ‘major malfunction’ within the tx perhaps; but as this happened to two models simultaneously it surely points to some sort of outside influence, namely an uninvited radio signal. A very strong jamming signal is one thing and I'm sure it’s perfectly feasible, but taking note of Allan's comment in his second post …… not throw the model all over the sky as it did just before crashing…. this one seems to be able to do something that I thought was very highly unlikely if not impossible these days, namely move the control surfaces to and fro in a random and uncontrollable fashion.

I've always understood that the receiver will only ever send genuine and authentic control signals to it’s servos and if that is the case then maybe we would have to consider the fact that this section of the receiver was being bypassed in some way which to my limited knowledge just leaves  the decoder. I’m pretty sure that when the receiver is in failsafe mode due to no signal it continues to send the control pulses to the servos as the last known good signal, or a preset position as per the throttle.

So is it possible that a powerful enough signal could cause the servos to operate in this random way by seriously swamping the output end of the receiver to the point where the pulses are being severely ‘modified’? I suspect that there is a chance that both models may have had analog servos on board, digitals maybe a bit more difficult to compromise in this way - if in fact this is what was really happening…. Difficult to believe really, it seems to me that the source would need phenomenal levels of power and also be very close to the flying area… and maybe have a modicum of directionality as well..

 

I've never seen or heard anyone having any suspected interference at the patch so I think that if there was an incident anything closely resembling the synchronised pair such as Allan describes then I think we would soon be trying to make some investigations with some knowledgeable people that might be able to explain what was going on. Just an idle thought, is this beginning to look like the sort of device that some official at a sensitive site, i.e. airport, might be tempted to use to shoot down an enemy drone?

 

And finally, just a second and last idle thought, if this pair of receivers were disabled in what appears to be such an extreme manner would the telemetry section still work as normal?

 

PB

 

[Phil Green]

A modern 2.4g receiver will do one of only two things Peter. It will either send the exact commanded positions to the servos, or it will revert to its preset failsafe state. Thats it, one or the other 🙂

However failsafe itself can be one of many states:

No pulses - the servos will 'relax' from their current position with neither drive nor holding power. 

Hold position - servos stay at the last commanded position, which could be midway through a roll or loop.

Preset - servos move to predetermined positions, usually neutral but often chosen to suit the type of model.

Hold & delayed preset - a sequenced combination.

Many receivers offer a choice of failsafe state and its the users legal responsibility to set it appropriately.

That a section of the receiver is being bypassed as you suggest just isnt feasible 🙂

Direct RF Interference is quite possible, from say a high power video or broadband data link (2.4 being an ISM band) and such interference doesnt have to be continuous, its effect can rapidly come and go.

I was talking to Andy Kunz the Spektrum tech, he confirmed my suspicion that just one single packet getting through every 2 seconds would be enough to hold off failsafe, yet obviously would seriously impact your control of the model.

Bear in mind that the purpose of 'failsafe' is to ground the model before it can leave its flying area, which I understand is what happened here 🙂

 

 

 

 

Edited November 11 by Phil Green

[Martin Harris - Moderator]

This is the full description I was given of an odd occurrence on the same day and not too far away from the OP - although much earlier in the day. 

[Allan Bennett]

I'm certainly leaning towards the 'coincidence' explanation.  Flew again yesterday with same transmitter, with no hint of anything amiss.  My synchronised crasher was also flying with his Spektrum trannie without any problem.

 

Just to answer a couple of points raised in the previous few posts, the servos in my model are analogue, and my receiver's failsafe is programmed for throttle cut and all surfaces neutral.

[Peter Beeney]

Many thanks for your answer Phil, much appreciated. When I was writing that post ‘bypassing’ was the only way I could think of to describe what I thought I was looking at. Your description of just one packet getting through every two seconds though could well be the answer here and I’m quite sure in my case I wouldn't have any control at all in these circumstances!  

 

Like Allan and probably just about everybody else my MO would be to set the throttle to close but I also usually ask the control surfaces go to a gentle turn position as well with the expectation that it then won’t stray too far away…

 

PB

[J D 8 - Moderator]

Our club strip is on a military training site, Mostly cadets on exercise and and driver training.

In the past vehicles with extending masts and other gadgets poking from them have turned up with a " Sorry chaps but we think you should not fly today" They do not say what they are doing.   Today with what is going on with drone warfare I think there is a lot more experimentation going on.

[John Stainforth]

Whilst radio interference does occasionally happen, one of the much bigger causes of crashes is pilot error. Near the club field l belonged to in the US there was hot-spot where a disproportionate amount of crashes occurred. This was around the point of turning from downwind to base leg. The general consensus was that the density of crashes on the ground merely reflected where pilots were most likely to screw up.