A routine mission to photograph NASA from our day job almost ended in tragedy as our XbeePRO 900's lost contact & the vehicle headed for a roof.

The XbeePRO 900's sucked from day 1. The problem seems to be they only do 50mW compared to the 90mW of the original XBee PRO 2.4Ghz. They also spread the signal more omnidirectionally, making for all around weaker reception.

900Mhz normally had 10% packet loss while occasionally getting nothing while the 2.4Ghz normally got all packets & occasionally nothing. You could get better reception by rotating the 2.4Ghz wire antenna in azimuth & keeping it parallel. Rotating the 900Mhz wire antenna didn't really do anything. Finally the 900Mhz interfered more with all the other avionics.

Upgrading to the 900Mhz equivalent of an XBee PRO for $42 was too good to be true. You need to unload more money in high gain antennas & haul around a tracking mount in addition to your laptop, batteries, transmitter, & video downlink.

Best to stick with 2.4Ghz unless you're loaded with government loans or know someone at AIG.

So ignored the warning signs during test flights, insisting the 900Mhz hype was true. Sure enough, while scrutinizing video telemetry, saw the vehicle was suddenly pointing the wrong way & was over a roof. Took manual control, but the radio was gone.

We don't do RTL because of the chances of an autopilot malfunction sending it into a human or an engine spinning up on the bench. Best to let it drop if the radio dies.


Fortunately it was 1 of the few non government buildings still inhabited & with tenants who could access the roof.

It somehow crashed right side up, mostly undamaged & took 4GB of roof photos. Both main & preview cameras captured the action. If it was a pure autonomous flight, leaving us enough bandwidth to monitor the radio, this wouldn't have happened, but had to play with our $50 UART cam.



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Comment by Thomas J Coyle III on October 30, 2009 at 4:52pm
Jack,

You gotta to adhere to the 6 Ps of success: Perfect Planning Prevents P**s Poor Performance. Nice work otherwise.

Regards,
TCIII
Comment by Greg Fletcher on October 30, 2009 at 5:55pm
Jack ,
I have that X-bee, but haven't flown it yet. I have one question. How did the loss of telemetry cause the crash, are you doing some ground based control computing ?
Comment by Jack Crossfire on October 30, 2009 at 8:27pm
Information on the aircraft is on vicacopter.com. It wouldn't have hit the roof if it was an onboard autopilot. Switched to ground based autopilot 1 year ago when 100ft was the mission & there was a lot of hype about new 900Mhz radios. Now that the mission is 400ft & the performance of new radios is not as rosy, it's not as clear if ground based autopilot has a future. 2 B sure, you should buy Xbees & support the sponsors, but don't expect crazy full duplex, dynamically unstable controllers to work as well as traditional downlinks.
Comment by Angus Peart on October 31, 2009 at 12:04am
I have got 25km+ out of the XBee 900MHz XSC modules with small 3dbi whip antennas, and at least 10km using the same arrangement with non-XSC Xbee 900's... I suspect something else could be the problem here? I tend to steer clear of those wire antenna XBees.

Comparing power outputs doesn't really work when the radios are different frequencies, but 50mW @ 900MHz should go further than 90mW @ 2.4GHz.
Comment by Jack Crossfire on October 31, 2009 at 3:21pm
There is a high gain antenna plan some time in the future & the XBee PRO 900's are fine products, in the right applications, with the right antennas, but our taxes went up too much this year to go any farther with 900Mhz.
Comment by Garry Qualls on October 31, 2009 at 3:30pm
What application are people using to orient and place their photos to make these movies?
Comment by Jack Crossfire on November 1, 2009 at 3:08pm
Video stabilization is done in Cinelerra. Since that is a professional program, don't have a recommendation for diydroners to use for their own video stabilization.

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