As the title suggests really. Background to this is on a fly with krikey, we had some really strange crashes where all control was lost and we spiralled into the ground.
With no real concrete evidence, and after reviewing the flight footage, our best guess is that there was another strong 2.4ghz signal in the area which wiped us out
FPV Crash: Were we killed by RF interference ? (http://www.youtube.com/watch?v=YNWPyWCmw_U#ws)
I was left with a few confusing issues to explain this, which are basically gaps in my knowledge. Obivously in the 'olden' days of having a fixed frequency, if something was on the same frequency then expect weirdness and glitches. But we're on slightly more modern stuff now, our 2.4 system have a bind between the tx and rx so any other stuff on the same frequency can be ignored, we also jump all over the place to other frequencies within the band.
So my question is - how does this all work. I understand what a PPM stream looks like, basically a series of pulse widths where the high (or low) signal is the PWM value of a channel with a low (or high) gap in-between each, and a longer 'sync pulse' at the end of every 22ms (or however long) pulse.
What I didn't understand is where any addressing stuff would go - i.e how does the TX say "this is from me".
How do the tx/rx coordinate moving frequencies within the band
Is there something extra in the PPM stream to denote addressing and extra info, or a different sort of 'packet' or something ?
If this is all working, I was trying to work out how my signal could get 'overloaded' by another really strong signal source. Again, I can see how my weak tx signal would get wiped out by a strong one, but if there was nothing addressed to the rx, it would lose the signal (and in the case of my wing - failsafe into RTH on the storm).
It kind of looks like it could 'hear' the fact my radio was addressing it on the frequency, but then the actual stream got wiped out by the strong signal. Is that possible ?
If you want to learn a bit on how the systems use hopping :
FHSS Radio Advantages (http://www.youtube.com/watch?v=j5mWG-vnFtM#ws)
A bit more to help you out :
How spread spectrum RC systems can share the same part of the 2.4GHz band (http://www.youtube.com/watch?v=aZGcyuezMik#ws)
Cheers Ian, that was super helpful. My networking background really does't help me as I tend to put things into the same sort of protocols. But the simple PN code thing that Bruce explained shows very nicely how I can not lose signal, and get completely wiped out at the same time.
Ouch!
That sux Wayne - I can 'see' wifi interference on my 2.4 vid link - shame you havnet got a spare 2.4 rx to have a 'look'
A RFExplorer would also show it up nicely...
http://micro.arocholl.com/index.php?option=com_content&view=article&id=48:introducing-rf-explorer&catid=40:article&Itemid=64 (http://micro.arocholl.com/index.php?option=com_content&view=article&id=48:introducing-rf-explorer&catid=40:article&Itemid=64)
A couple of nasty crashes mate.
What I don't understand is:
1. With the falcon crash, you had no control, but just before impact the throttle cuts, I know it's prob impossible to remember when you actually cut the throttle. But did the throttle stick and then just before impact did you gain control, but to low to recover.
2. From my limited understanding of rc control, if you rx doesn't receive a valid control signal, it goes into failsafe, but both yours didn't.
3. If the camper was putting out mega wifi, it's further away than you were with you valid control Sig. Distance has a big part to play in how strong an RF signal is. Bearing this on mind, when we flew at MD in formation, both badlands and myself were transmitting on or around 2.4g with your plane a lot closer to us than your control Sig. If there was going to be a lock out due to swamping it would of happened then I would of thought.
You are correct in wanting a reason why this has happened, it knocks your confidence in you gear if you don't know why.
You could try and replicate it. Have your plane sat onto of a Wi-Fi router and walk away with your controller until you loose control. Make sure the router is busy doing something, download a large file or stream a video.
you need a wifi detector, can be an app on your smart phone, but a stand alone with a directional antenna would be better.
Bloody Apple have removed any wifi hunting apps, so finding signal strengths of things will need something other than my phone. I'll look into it. Those rf explorers look lovely, but there's quite a hefty price tag.
With the Falcon crash Bruno, I had some influence it would seem - at the start when I was trying to turn out of the roll, it slowed it down a little, but then went over anyway. I would have automatically having taken the throttle off when I felt there was no hope... though I can't remember how near the ground that was. The spin seems to go in pulses, so almost like the elevons went full over, then relaxed a touch, and then went again. I'll have a play with some wifi signals to see it I can make anything happen.
Quote from: CurryKitten on July 04, 2014, 06:02:08 PM
Bloody Apple have removed any wifi hunting apps, so finding signal strengths of things will need something other than my phone. I'll look into it. Those rf explorers look lovely, but there's quite a hefty price tag.
Google "2.4ghz sdr" and follow the rabbit hole. You can build a spectrum analyser for about £20 if you have the time to research it. I'd say the best thing you can do though is get RSSI in to your OSD.
Aren't you talking about coming over to the dark side on UHF anyway?
RTL-SDR is brilliant, but needs some knowledge and extra kit to work at 2.4GHz.
I have a couple of dongles for Amateur Radio work, cheap to buy here:
https://www.cosycave.co.uk/category.php?id_category=61 (https://www.cosycave.co.uk/category.php?id_category=61)
Software:
http://sdrsharp.com/ (http://sdrsharp.com/)
Google around for the Android app of SDR Sharp for a tablet - works pretty good too providing your tab has a USB port.
Mr RC-CAM did a short taster video on using a dongle as a spectrum analyser some time ago
https://www.youtube.com/watch?v=uAXp4Zgecdk (https://www.youtube.com/watch?v=uAXp4Zgecdk)
RTL-SDR is useful at frequencies between about 50MHz and 1.8GHZ right off the bat (great for UHF).
With an HF converter, I can tune down to VLF
http://www.nooelec.com/store/ham-it-up-v1-0-rf-upconverter-for-software-defined-radio.html#.U7cYsrlOXcs (http://www.nooelec.com/store/ham-it-up-v1-0-rf-upconverter-for-software-defined-radio.html#.U7cYsrlOXcs)
Also useful at 35MHz !
Another option might be to use a wireless laptop/netbook and the freeware version of InSSIDer:
http://www.techspot.com/downloads/5936-inssider.html (http://www.techspot.com/downloads/5936-inssider.html)
https://www.youtube.com/watch?v=9z9OZptBpvE (https://www.youtube.com/watch?v=9z9OZptBpvE)
Not tested by me, but might be useful for nowt?
Hope you figure out what brought your models down.
Nigel.