Sniffing Mavlink packets

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:

Tags: FHSS, USRP, mavlink, sniffing

Views: 218

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Permalink Reply by Michael Pursifull on June 25, 2012 at 6:09am
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.
Permalink Reply by Michael Pursifull on June 25, 2012 at 6:11am
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.
Permalink Reply by Michael Pursifull on June 25, 2012 at 6:13am
I believe you'll find it is DSSS, not FHSS.
Permalink Reply by Jake Stew on June 25, 2012 at 8:29am

GFSK is what is implemented I think.

Permalink Reply by Michael Pursifull on June 25, 2012 at 8:55am

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. 

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