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Need help with understanding Spectrum Analyzer Test.

Discussion in 'Inspire 1 Discussion' started by Joe R, May 1, 2016.

  1. Joe R

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    Hello,

    I have a pretty fair understanding of electronics but I lack experience and knowledge about radio signals, wifi systems, etc.
    I've done some research on the subject but I know that some of you have experience and knowledge that is far superior to mine.
    I'm trying to understand how the radio control system and the video feed works with the Lightbridge systems found on Inspires and Phantoms, I don't own a Phantom but I assume they use the same system.
    I'm trying to figure out what makes it possible for these aircraft to fly so far with a such a high degree of reliability.

    I use a Wi-Spy 2.4x Spectrum Analyzer receiver (USB type) and I'm using the older Chanalyzer 4 software made by MetaGeek. I've been using this setup for about three years and have found it to be very reliable and useful.
    Just a quick example: I used my SA (Spectrum Analyzer) at a location that looked very congested with not only high amplitude signals but most were reddish orange in color indicating that those signals were very dense or strong. I was convinced it wasn't safe to fly my S800 (No Lightbridge). A friend was with me that also had an S800 but he decided that it was safe, he lost control of his bird and crashed it beyond repair. He lost a $2K camera too, it was a total loss all the way round! :-(
    He now has his own spectrum analyzer and takes the warnings seriously. Lesson learned I guess.

    The setup for my test:
    I placed the I1 and the controller 400 ft. apart in an open field with no houses within 600 ft.
    I noticed that the signal was only 10 MHz wide (I should have set the SA for frequency instead of channels).
    I also noticed that the closer I got to the I1, the stronger the signal, the reason I say this is because the signal got higher in amplitude when I got closer to the I1. The amplitude of the signal got much weaker as I approached the controller 400 ft. away from the I1.
    In addition, the farther I walked away from the line between the I1 and the controller, the weaker the signal.
    This makes me believe that the signal between the I1 and the controller is more like a direct beam rather than radiating equally in all directions. It seems that the width of the beam (where the signal is strongest) is about 100 feet in diameter. This was done with no obstructions.
    I'm not sure if this was the right way to conduct the test or not. Is multipathing an issue with the I1 and the controller being only 5 ft. off of the ground? The field was covered by 8 inch tall grass so I'm thinking that the signals wouldn't be too likely to bounce off of the ground. If I have this right, if it was a hard dirt field, the signals may bounce off of that surface and cause multipathing?

    I can't help but to think that a direct beam between the I1 and the controller is what makes long range flights possible (not being high in amplitude but dense and in a direct path).
    I also feel that flying at close range in heavily congested wifi areas is possible because of this beam. Am I wrong about this? Most UAVs without Lightbridge simply can't fly through and over cities without losing control, it's at least very rare that those UAVs have successful flights in those conditions.

    To me, if I can better understand how this system works, it would help me to make better "Go" or "No Go" decisions.
    I realize that the app will (or at least should) warm me of potential signal strength issues in congested areas but I'd rather use my SA to catch what the app may miss.

    I have been a licensed private pilot since 1987 with nearly 2,000 hours in a manned aircraft (SEL) and have been flying 3D RC helicopters and photography UAVs for the last six years. (Photography UAVs for the last four years). When I first started with aerial UAV photography, I didn't have GPS receivers or self leveling gyros. It was all seat of the pants flying back then but I did have an A/V transmitter with a monitor for FPV. No telemetry though.
    I mention this because I'm in this commercially. I still film for fun but I don't want to take risks simply because of ignorance on my part. In other words, I don't want to think everything is Ok when it really isn't.

    Off topic. I've noticed in some threads (don't remember what forum) where people thought that AMA insurance covers commercial operations. FWIW, it doesn't. AMA only covers recreational.
    Hill & Usher is the only broker I'm aware of that will provide commercial liability and accidental coverage for your UAV. They can provide complete cover for your UAV for 7 to 10% of the total cost of your rig as long as you follow the rules and regs. The deductible is 10%, if your rig is worth $5.000, you pay $500 for a total replacement. Usually you wouldn't lose the controllers or the iPads so it would only be for the cost of UAV itself unless someone stole the whole thing. Yep, they cover theft as well. They also cover submersion in water even if you can't recover it, you just have to show the flight recorded in the iPad as proof that it decided to try becoming an illusive submarine.
    Just trying to help those who need the right insurance to cover your *** (UAV).........what do you think I meant by ***? :D Dirty minds.......Mmmm Mmmmm :cool:

    One of my 3D helis has "WTF" on the canopy, most people think it means What the F*** ? , It clearly means "Without The Flybar" since it's FBL. :D Ok, that was WAY off topic. :)

    Please have a look at the video below that was sped up by 600% at some points.
    I'd really appreciate any information, clarification and/or advice I can get!

    Sorry for the long writeup!!

    I tried to find information already covered on this subject but couldn't find anything specific. Maybe I didn't look hard enough? Sorry if this is a repeat.

     
  2. The Editor

    The Editor Moderator
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    Hi,
    Although a spectrum analyser can be a useful tool for examining noise floor and general band chatter it wont do you much good in trying to understand the FHSS technology used in the Inspire and most RC systems.
    First of all the signal isn't transmitted as a narrow beam or in fact any kind of directional signal. Both the RC and the Inspire use omnidirectional fixed dipoles that radiate a pattern roughly resembling a doughnut in shape.

    The reason you are seeing a difference in power or amplitude is because the RC/Inspire will ramp up or down the output of its transmitter depending on what is asked of it.
    Additionally, the control signals are sent as encoded 'packets' of data which are frame locked and have parity checking for both integrity and security for each packet.
    Although you will see a carrier on the Spectrum analyser differing in amplitude, it will not give you the entire picture as to the integrity of data etc.
    The reason the Inspire (and other RC) aircraft are able to fly long distances on such small power link is due to the robustness of the FHSS system. This is able to 'ignore' everything except the correctly encoded data stream coming from the RC. If too much interference is detected the system can change or 'hop' frequencies without dropping any data packets.
    It's a very clever and technologically mature system. :)
     
  3. Joe R

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    Thank you for having a look at this for me!

    I do understand packets and the dipoles resembling the shape of a doughnut. I've done some research on encoded and decoded data but not so much on FHSS which is Frequency Hopping Spread Spectrum, I did know what it stood for and that it hops from one channel to another, some at random like the DSMX transmitters. I will be researching that one a lot more, specifically on Lightbridge systems.
    I did another test where the I1 was stationary in the house with the SA at the I1 as I walked up the street with the controller. My test was to see how often it would switch channels. It changed channels a few times as I was walking. I was using camera controls as I was walking to make sure it was sending and receiving at least some data. The fact that it will switch channels depending on which channel has the most activity on it at the time is great!
    I always fly in "Auto" channel select mode for that reason. It must switch channels based on what the I1 is around as well (since the I1 is typically the part on the move).
    I assume that Lightbridge systems only hops to another channel when it needs to rather than hopping at random like the DSMX transmitters? That was rhetorical. I'll research it, of course, if you want to elaborate so others can understand it better, that would be great!

    I think that the advanced, secure encoding and decoding that's going on as well as constantly analyzing the spectrum activity is awesome!
    I assume that the I1 is looking for packets sent only from the controller it's linked to and ignoring everything else.

    What you said makes sense.

    Now I have a good direction to go in for more info.

    I'll still have to figure out what a go or no go situation would look like on the SA (for this platform).

    Thanks again!!
     
  4. The Editor

    The Editor Moderator
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    Don't forget, the Inspire and Lightbridge system is a little more complicated than a standard FHSS system from the likes of Futaba etc.
    The reason being is that the video stream is also transmitted on the 2.4ghz down link as well. This is encoded and shot down the data stream interleaved with two way communication protocols.
     
  5. Joe R

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    A "little" more complicated? Ah, that may be understated. :D
    I can't help but feel that the word "Link" carries a lot of weight in terms of secure data transfer.
    The idea of rejecting signals that pass through the link between the I1 and the controller is very much unlike most standard FHSS transmitters and receivers.

    My son is career Air Force and trains people on multi-million $ wireless robots for EOD (Explosive Ordinance Disposal). I had the privilege of touring their facility in Tx, he says that their wireless systems are very similar to the lightbridge. He could definitely explain exactly how it works but isn't allowed to explain it to non military people.

    They have a gun that can takeover a drone in mid flight and land it safely by just pointing the gun at at it and lowering the gun's aim till it reaches the ground. They can control the signal strength to the point where they can bring it closer to them if they want. The drone will move wherever the gun is pointed.
    It was demonstrated there and was an amazing thing to see. It's no secret though, they posted a video of it on YouTube. They plan to sell them to law enforcement and other officials but are holding off till they can insure who has access to them.
    There's serious talk about allowing airports use them to down drones near manned aircraft.
    I think it's a great idea considering the way things are these days. Nothing to worry about if the drone operators aren't breaking the rules and regs.

    Thank you!!

     
  6. Mad_angler1

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    As the Editor says it's not a straight forward FHSS link, link from controller to craft is fully FHSS compliant but the video stream back to the craft is not and only hops when the congestion is detected.

    LB is a term DJI have come up with for its communication system, it's multiple technology's all together

    More info on the original lb system that the P3 and Inspire is a derivative of.

    Features:

    2.4G Full HD Digital Video Downlink
    -1920x1080@60fps input and 1920x1080@30fps output


    Extreme Long Distance Transmission
    -Less than 100mW ground unit transmission in full compliance with CE and FCC certification
    -5dBi omni-directional antenna offers an enormous distance of up to 1.7 km (1.05 miles)
    -Optional 14dBi patch antenna boosts transmission distance tremendously to 5km (3.1 miles)


    Multiple Input And Output Connections
    -AV input
    -HDMI input
    -DJI Zenmuse gimbal input (Digital Video/Power Supply/OSD Data)
    -HDMI HD image output
    -USB 2.0 image data output for iOS and Android phones and tablet synchronization


    Built-in Remote Control Link
    -Uplink remote control transmission and downlink video work simultaneously on 2.4GHz.
    -Remote control data travels to the uplink via the TX-Thru Port at the same time that video is streamed back down all without interference.
    -Support for two remote controls: one for flight control (10 Channels) and the other for gimbal control (6 Channels). Control signals are received in the air and then transmitted to the flight controller through D-BUS.
    -When used with a non-2.4G radio transmitter, video only transmission operation is supported. The DJI 5.8G radio transmitter is recommended.

    Strong Anti-Jamming Technology
    -Frequency Hopping Spread Spectrum(FHSS) + Direct Sequence Spread System(DSSS) technology is used for uplink transmission to enhance stability and offer anti-jamming capability.
    -MIMO multi-antenna technology and OFDM modulation methods along with advanced algorithms are utilized for downlink transmission to enhance data bandwidth, improving dynamic performance and increasing stability even in harsh environments.
    -Channel interference is constantly monitored during downlink transmission with automatic switching between 8 channels for the best possible signal.
    -Up to 8 downlinks can be connected for parallel streaming and work independently without interference.

    Low Latency Transmission
    Maximum latency of up to 80ms
     
    The Editor likes this.
  7. Joe R

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    Thank you for the detailed response!

    The range offered "As Is" is more than enough for me simply because I cannot legally fly farther than I can see it and.......it's just not safe to go farther than that.
    For me, that distance is a maximum of 800 to 900 ft. depending on weather visibility and as long as I wear the contacts I requested from my eye doctor, those contacts make it hard to see anything up close but are great for seeing a small bird at very long distances. I have to wear prescription progressive reading glasses when I wear my contacts (tip of my nose grandpa type) so I can see my iPad and the I1 when taking off and landing. I can see very well with this combination. I did contacts and grandpa glasses especially for UAV filming operations. Even though I have a visual observer, I still need to be able to clearly see my UAV in case of an emergency.
    I'm not sure why people want to fly their UAVs BLOS. It's clearly not legal. Not saying that you do but let's face it, flying BLOS makes it impossible to "See and Avoid" because you can't see what's around the UAV when all you have is the media camera and maybe an add on FPV camera.

    Example: I shot a video of a coal fired power plant for a documentary group. I was at 390' AGL doing a 360 pan shot. I could hear a helicopter in the distance so I asked my visual observer to find that chopper and quick because I couldn't see it on my iPad, it's a 9.7 inch iPad and has a matte screen cover to kill the glare. The helicopter was in view but I just couldn't see it on my iPad.
    I had my VO make an announcement on the unicom channel with my handheld STS aviation transceiver that we were there flying a UAV, he made the announced with our location but no response. I already had GSO's approach channel programmed in so I told my VO to switch to the approach channel and make the announcement again but still no response. I have no clue what frequency the manned helicopter was monitoring but we tried to our best to make contact.
    My observer spotted it and it appeared to be headed near our location. MY VO tried to contact it again but still no response.
    With no response from the manned helicopter, I quickly stopped the pan and reduced altitude quick by traveling backward and down and the same time, it's the best way to lose altitude fast. Straight down causes it to get caught up in it's own prop wash. That manned helicopter had turned and was headed straight towards us. Luckily I had it down to about 50' AGL when the helicopter passed over top of us and didn't cause any issues with my I1. Had it been still at 390' AGL, there would have been a real problem because the helicopter was descending for some reason. In the video it appears to be at around 500 to 600' AGL but it appeared to be at the same altitude as I was at when it passed over us. It even passed at the same point where my I1 had just been hovering. Manned helicopters are notorious for flying below 500 AGL.
    If I had still been flying at 390' it's possible that it would have hit my I1, who cares about the I1, just imagine what it might have done to the manned helicopter and the people onboard!!
    Here's the video of the 360 pan. Skip to 1 minute and 15 seconds into the video and hit play to see the manned helicopter I just mentioned, it's in the upper left hand corner of the video. You may have to view it full screen to see it.



    If this had been way out of my sight at like 3 miles, I would never have been able to take evasive maneuvers. I wouldn't have known where the helicopter was or which way I should go. At that distance, I may not have even heard it, especially since a train with only a few coal cars had just passed, I could still hear the train. Checkout the train tracks in the video at 45 seconds into the video.

    Thank you for the addition of DSSS in the mix! That is good info!
    Also, thank you for mentioning MIMO and OFDM modulation! I'm familiar with OFDM modulation and that may be where the ERP-OFDM signal came from in the Spectrum Analyzer recording that I posted. It didn't occur to me that it could have come from my I1 until you mentioned it. Thanks again for that!

    I also noticed there was another active channel at a low amplitude but still high in density at 10 MHz wide that could have been part of the simultaneous back and forth transmissions. I know that both the I1 and the controller are transceivers.

    I've been researching and studying and I'm sure it will become more clear to me soon.
    Again, the info I mentioned above is especially helpful to me.

    I wasn't meaning for my spill about flying BLOS to be taken as anything but constructive.
    I seriously don't mean you personally by the following words, I don't know you and have no reason to judge you. The following words are for those who do fly at altitudes above 400 ft. AGL and do fly BLOS (Beyond line of sight).
    If you look at the video close, then put yourself at that scene, it can make your scalp crawl.
    The loss of my measly unmanned I1 is nothing compared to the lives on that manned helicopter.
    When I got back to my studio and viewed that helicopter in my video, it made me cringe again (first time was at the field where I shot the video).
    Having had a small bird crack the passenger side of the windshield on a rented Cessna 172 on final approach was probably one of the scariest moments of my flying career in manned aircraft. I had a passenger with me and it literally scared the crap out of him. Don't laugh! Now imagine if that had of been a 6.5 lb drone, it would have killed my passenger and maybe even me because of the debris from the windshield.
    Having lost power on another flight in 1992 was the scariest experience since I had to set it down in a field I knew nothing about. I didn't know if it was rough, soft, mud, rocks or ????? I just aimed the nose between two trees so it would rip the wings off where the fuel tanks were in case it caught on fire. At least I would be away from those fuel tanks. Luckily, it stopped about 40 ft. shy of the trees and the field was fairly smooth. It didn't tip but it was a very bumpy ride after slipped it over the trees and stalled it over the field. The lost power was due to a clogged fuel line, it was determined that the mechanic and the AI didn't do their job and were both fired as a result. I rented it from an FBO at GSO which is an international airport.
    After that incident, I found a plaque that read "Flying is hours and hours of boredom punctuated only by moments of stark raving terror"!!! It hangs on my wall to this day.

    It still crawls all over me to watch a video of some guy flying his drone to 4 or 5 thousand ft. altitude and 5 miles away!!! Try putting yourself in the seat of a manned aircraft and see a drone at your legal altitude. It'll scare the crap out of you too!!
    You're safe on the ground when you fly a drone, for the most part the worst thing that can happen to you personally is to damage or destroy your drone and I guess it's worth the risk, it's just money, right?
    Well.....what about the real person (a real person just like you) in the manned aircraft that's in his airspace and sees something like a drone that could potentially kill him or her!!!
    Are you willing to take that kind of risk if your LIFE is at stake?

    Sorry but it's not about how far or how high you can fly, it's about how safe you can fly!!!

    There is a VERY good reason for the rule to keep in your sight!!! It's not just a way to control what you do like the FAA is on a power trip.
    The FAA's main concern is to keep our NAS as safe as they can for everyone!!

    My post wasn't to see how far or how high I could fly my I1 reliably, it was about understanding the system so I could be better informed on how safe I could fly with it still in my sight.

    Thanks again!

    Joe