Streaming 1080p/60fps up to 20km wirelessly is mind boggling to me. What kind of tech is used to make that happen? seems like it'd require a lot of power.
The video quality degrades at longer distances. You can't get the full bandwidth at the maximum distance, but you might see enough to navigate.
Communicating with a drone up in the sky is easier than something like your WiFi or your cell phone because you have a nice, clear line of sight to the drone. Fly behind a hill, building, or some trees at distance and the drone will lose connection and go into safety mode.
The free space path loss at 20kM is 126dB at 2.4GHz in perfect conditions, or 134dB at 5.8GHz. If you start with the 1 Watt nominally allowed by regulations, that's +30dBm. Subtract 126dB and you're left with -96dBm. That's a weak signal, but it's actually close to the receive sensitivity of the WiFi card in your laptop, believe it or not. I would guess the DJI gear uses narrower channels than WiFi to achieve a better noise floor than the 20MHz (or wider) channels you get with WiFi.
The 20km figure is really an extreme upper limit. Realistically you'd probably need a high-gain antenna pointed in the direction of the drone to achieve it.
That's super interesting! Would you have resources to recommend to learn about that?
For instance: where do you get that "the free space path loss at 20kM is 126dB at 2.4GHz in perfect conditions, or 134dB at 5.8GHz"? And why does 1 Watt translate to +30dBm?
30 dBm = 30 decibels wrt to 1 milliwatt. 10 decibels = 1 bel = 10-fold increase of the base quantity, so 30 decibels = 10 x 10 x 10 = 1000x increase, so 30 dBm = 1000x 1 mW = 1 W.
I don't know enough to offer any real sources, but I'll share that I've found ChatGPT very useful for learning stuff like this - really solidly documented and established science/engineering.
I asked* "When making a wireless transmission what is the signal loss at 20km for 2.4Ghz?" and it gave an excellent explanation of free space path loss, stepped through the calculations, and gave the correct answer.
I've had it explain various kinds of first-order filters, especially for electric guitar circuits too. And asking follow up questions works well with how my brain works.
> but I'll share that I've found ChatGPT very useful for learning stuff like this
I get your point, but no way I use ChatGPT. Other than all the ideological issues I have with it, I just can't trust it.
> and gave the correct answer.
Are you saying that because you believe it did, or because you already knew the answer? I don't trust ChatGPT, so I don't want to learn anything it says. And I don't need it for stuff I already know.
Wikipedia shows sources, does not hallucinate, and shows the same content to all the users (meaning that if there is a mistake, someone else can actually report it).
Those are accurate details, but if you zoom out they're still contextually very similar - a swathe of text on a subject written by someone (or something) you have no veritable trust in but is usually fairly accurate.
You should independently fact check either source. Wikipedia is slightly easier, but after you have a base subject matter understanding you should have the terminology to validate anyway.
I suspect you just don't like it, which is perfectly fine.
> Those are accurate details, but if you zoom out they're still contextually very similar
I cannot go against that. To me, the points I listed make them fundamentally different.
> I suspect you just don't like it, which is perfectly fine.
Because you don't understand why I see it as a problem that people don't make the difference between Wikipedia and ChatGPT does not necessarily mean that I am just making up arguments because I don't like it...
That's fair. I think we're arguing semantics here. Expanding my "you just don't like it" would be "you have immovable philosophical issues with one, ergo you won't pick it regardless of its accuracy". I didn't mean to imply you were just trying to be contrary - apologies if it came across that way.
I do like the ideals you have there, but I think for most people a tool is just a tool, and if it ms mostly accurate they're happy.
Because there are other ways (simple Google search, asking an EE friend, etc) of validating the answer. Often I'm looking to learn a concept so it isn't like I'm using it to solve a bunch of things. Just validate my learning. It is easily as good as having one of my EE friends explain the concept to me.
The channels can't be too narrow bandwidth and still fit any sort of 1080p video feed in them. If I'm remembering my signals courses right, raw 1080p at 60fps would need something like 6GHz, so even at 100:1 compression they'd still need 60MHz channels.
This is a major advantage DJI and other Chinese drone makers have over US based ones. Technically acquiring an SDR/DSP implemented LTE baseband is not a major difficulty. Getting Qualcomm to let you is quite a challenge.
Looking at FCC listings and product pages you can tell that DJI are also throwing power at the problem. The EIRP on DJI devices is very high. They go up to 33dBm (2W EIRP) on the most modern devices like the Mini 4 Pro.
Keep in mind that this only applies to the FCC regulations. In Europe (CE regulations) the claimed range is 10 km.
The mini 3 pro has a CE regulations claimed range of 8 km, but after 2.5 km I pretty much loose connection. If I turn the drone to face my direction, I might be able to fly it a bit further, but at this point it is so hard to control it, that there is no point.
Btw, according to regulations, you can not fly it without line of sight. So, in practice the "legal" range is a few hundrend meters. I have yet to see an observer with binoculars. :)
Transmitting long distances lives on a kind of spectrum. On the one end you have MOAR PWER and on the other end you have signals analysis and robust codecs (? I'm not sure "codec" is right. Maybe encoding? Can someone answer this?) that allow for lost information. The signals analysis will clean the incoming signal and the codec/encoding will allow for robust recreation of the captured data.
Most wireless transmission technology lives somewhere in-between the extremes of the above spectrum. Signals analysis is quite advanced and codecs/encodings are also quite advanced at this point.
My RF knowledge is hobbyist only but I think the word you're looking for is "modulation", e.g. LoRa is able to work over significant distances at low power because of its clever chirp spread spectrum modulation method.
Is there any system that works for at least 10-20km without line of sight, supporting a video feed usable for guidance, without cell phone network coverage?
Maybe something where there are two drones, and one flies high with line of sight to both the user and the low-flying drone so that it can act as a relay like a satellite would?
Or something that uses radio frequencies that bounce over the atmosphere, and if bandwidth is not enough uses an automated pilot plus the ability to send photos every few seconds so the user can tell it in which general direction it should fly?
Or something that uses Starlink/Iridium/etc. to relay data?
Those things are all possible, but you’re not going to find it in a COTS product.
The key terms in the link budget is directionality. If you use a phased array (like Starlink) or a parabolic dish, you can boost your range tremendously. This is how 4x4 MIMO WiFi can achieve gigabit speeds with the same power limit.
I think it would be very useful for scouting in woodland/backcountry use including search&rescue.
At 50 km/h, it moves 10-20 times faster than a hiker, and at <1000$ it's more than 100x cheaper than a plane or helicopter.
But requiring line of sight is a strong limitation because it means that you need to fly over the tree canopy and might not see under it, and you need to fly really high if target is on the other side of a mountain or hill and again you might not see well.
And of course cell phone coverage is not guaranteed so you can't use the easiest solution of putting a cell modem on it.
