Hey all, I'm doing work with testing various radios for telemetry, and I was wondering if anyone had done work with sniffing and collecting the Mavlink packets for 3DR, XBee, microhard, Xtend, etc.  I'm interested in being able to just look at the raw data coming in live (perhaps converted to text) and I have a USRP with GNU Radio and sigmira.  I figure it shouldn't be that hard to code up what I need to detect preambles and such, but my problem is I have no idea how to go about dealing with the FHSS.  If I listen for a couple seconds I can see a repeated pattern in the hopping so I can then predict the next frequency, but I have no idea how to code this.  Has anyone worked on software for sniffing these packets, or does anyone know of some hardware available to listen in, without being the radio that pairs with the drone radio?

A screenshot of the visible hopping pattern:

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It would be earlier to get a third xbee or 3dr radio, then programmatically step through each netid until you "hear" MAVLink. Then hand that off using a modified (silent, no responses) mav proxy .... Then you could feed the data from mav proxy to any gcs of your choice.
If they are broadcasting legally, it may be harder for the 3dr, since you will not know the freq range (defaults are not recommended) but that you could identify with your gear.
I believe you'll find it is DSSS, not FHSS.

GFSK is what is implemented I think.

Sorry, I should have qualified, I was talking about Xbee. I thought that was understood since he mentioned spread spectrum rather than digital modulation. I was typing on my mobile. 

Hello man, I know your ask is here a long time, but I'd like to know if you finished your sniffing. I am needing to do the same. Thank you.

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