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We're working on a model rocket project. And we need two transmitters and one receiver. One of the transmitters is for the rocket body and the other is for the scientific payload in the rocket; the receiver is for the ground station.

As far as we've researched, RFM95/96 is really good at long range telemetry to use for point-to-point. We've tested it also.

However, we plan to use two transmitters simultaneously so we need two different channels but there aren't any options about channels on RFM9X devices and when we try to transmit data simultaneously, the receiver can not receive the packets in order.

How can we design the telemetry system that works with the rate higher than 1Hz using 2 TX and 1 RX nodes? Ground Station must be collecting the sampled data at least 1 Hz rate.

RFM96/95: https://www.hoperf.com/modules/lora/RFM96.html

Faruk UNAL
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    You forgot to ask a question. You forgot to link to the product datasheets. There's an edit link under your question. – Transistor Mar 31 '19 at 14:15
  • Re-read the data sheet, these things have plenty of frequencies. But you also need to consider what is *legal* - continuous transmission is not. Typically these are used in a frequency hopping configuration with a multichannel SX1301 based receiver. Actually workable systems are quite a bit more complex than your present thinking. – Chris Stratton Mar 31 '19 at 16:21
  • @Transistor So sorry, I've corrected my faults. – Faruk UNAL Mar 31 '19 at 16:48
  • @ChrisStratton Can you explain the method which use SX1301 and frequency hopping configuration? As far as I've understood, there will be two different modules for transcievers(RFM96) and the receiver(Special receiver based SX1301)? – Faruk UNAL Mar 31 '19 at 16:54
  • Your goal most likely will required an operating license, not an ISM band. – Chris Stratton Mar 31 '19 at 17:04
  • @ChrisStratton It's a competition for university students and we will be allowed to use other frequency bands however we also want to use ISM bands (433.05 - 434.79 MHz or 902 - 928 MHz). – Faruk UNAL Mar 31 '19 at 17:09
  • *Record* your data on the device rather than transmitting it - consider whatever fraction you transmit to be advisory only. Take time to understand legal usage in your jurisdiction. Any (scarcely credible) claim of being "allowed to use other frequency bands" would need approval of the license holder who will be operating your equipment under their license, and will need to stay within the restrictions of that license, so you had best start talking to them *right now* before committing to a course of action. – Chris Stratton Mar 31 '19 at 17:21

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Read the datasheet for the RFM95/96, particulalry section 4.2.7.3. Automatic Restart when Packet Collision is Detected. The configuration you have is valid, BUT if you try to send data too fast you will end up with many collision (wireless is a Collison domain for a single Rx).

Ensure the Tx packet rate from both your Tx's is low to minimize collisions, LORA is not for high rate transmissions.

Jack Creasey
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  • Our receiver shall collect the data from each transmitter at least 1Hz rate. The size of data is about 50 byte. Most of the time there is a collision. We should check the availability state of the receiver before sending data. Is there any method for this? – Faruk UNAL Mar 31 '19 at 17:07
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All three modules are transceivers, so it should be possible to get the two on the rocket to cooperate with each other. For example, the rocket body module could listen for the payload packets, and transmit its packet immediately after the payload packet ends. The payload module could leave enough time between packets for this to occur.

Dave Tweed
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  • Smart! We will try it however communicating with them by different channels will be better for us because we have to send commands them separately to verify the sensor of them after the rocket is placed to the launcher. – Faruk UNAL Mar 31 '19 at 16:58