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Maddog

Radio link for data

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Hi, what do you guys use for streaming the instrument data to the ground?

I am primarily looking into something that will have low-power consumption and a range of 3km for streaming digital data (gathered by a MCU) to an RX on the ground.

Any thoughts?

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Hi Maddog,

I am also working on a long range data link. My current specs are 500mW and I have several candidates. There are a number of constraints but if latency isn't an issue, you have a lot of options. If however, you are looking to control your plane with the link as well, then you want the lowest latency possible (each data packet is about 22mS on regular radios so you don't want much more than that for latency). This narrows down the candidates considerably, to make 2 or 3 possible ones, and some of them are very expensive. Size is also another consideration, as they come in various form-factors, some being more suitable than others. At this point, it looks like 868MHZ/900MHz is the best choice, with 70cm (433MHz thereabouts) a close second mainly because it needs a longer antenna. Lastly, the bitrate is also a limiter. The lower your bitrate, the greater your distance - more or less. So at 4800kbit/s you can probably easily get 5km line of sight. At 9600, 3km should not be a problem at all.

Daniel

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Hi Daniel,

I am not planning to control the plane, at least not in real-time, but rather in command mode, thanks to your DragonOSD, which i was hoping to use more as a pilot option.

I can drop the bob-4 and use the I/O's for the downlink and some sensors. The duplex comm is in a future perspective, so for the beginning I will may start with simple broadcasting.

So at 4800kbit/s you can probably easily get 5km line of sight. At 9600, 3km should not be a problem at all.

That's 4.8Mbps roughly, is it that good or was it a typo?

Not knowing the packet headers i can't tell how much practical data i can get at 4800bps (600bytes/second) will that be suffice for 5hz data update for speed, heading, alt... and 1hz update for batt voltage and current, temperature?

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Maddog,

If it is just a data downlink and you want something simple I'd opt for 433 of 900MHz, 500mW should get you a long way and won't interfere with the 2.4GHz transmitter most of us will have bolted onto their plane.

Either way I'd want to have some error correcting mechanisms in there, at least a checksum for each data packet, can't do much with sensor data that isn't verified to be correct. On second thought it might be easier to just add a raw data stream to the video downlink, possibly use the audio channel? With QPSK modulation you should be able to get up to 56Kbps if the available bandwidth is halfway decent.

However not sure what the latency will be if you factor in all the processing needed. QPSK modulators are readily available in IC format, hence this might be easier than you might think? They're mostly designed for modem use, on crappy low bandwidth copper lines, hence a strong RF link should be better than miles and miles of copper.

Cheers,

Sander.

Edited by ssassen

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