Re: [Paparazzi-devel] Long Range Flying Wing

[Stephen Dwyer]

Hello,

Regarding range issues:

First thing, is the power setting appropriate on both sides?

As Chris mentioned, definitely possibilities of interference from
other equipment. This can be very hard to debug. As an example, in the
past we had horrible problems with GPS reception. After days of
changing things around, we used a spectrum analyzer to debug, which
really helped solve the problem. I realize it can be difficult to get
access to a spectrum analyzer (turned out to be the 500MHz SBC spewing
junk RFI at 1.5GHz ... a nice harmonic, some careful shielding solved
the problem).

We had serious asymmetrical link issues for a while as well. After
some debugging, we discovered that the connector on one of the
antennas was in poor condition. Switching to a new antenna solved all
of our problems. However, this asymmetry also showed significantly
decreased range, even in the better telemetry direction. Again, Chris
mentioned this: antenna selection, antenna/cable setup and orientation
are rather important.

Our modems also run warm, but I think this is to be expected with the
high power. We make sure to allow some air flow around the modems
(i.e. we don't wrap them in foam).

A while ago I wrote a small module to measure the RSSI on the airborne
side to help debug range problems. It was quite a while back and I am
not sure if it is still alive (a quick compile on master failed). I
will try to fix it in the next couple days if I get a chance, and do
some proper documentation on the wiki for it. Unfortunately, it is
currently for LPC21xx only (we use it on a TWOG). Check
conf/modules/xtend_rssi.xml for more info.

Hope that helps.

Thanks,
-Stephen Dwyer




On Fri, Jul 6, 2012 at 11:09 AM, Chris <address@hidden> wrote:
> Well i have flown to a point where the ground station with its 8db patch
> could not receive well
> the planes data but i was still able to give a command to the plane and
> return it home which indicates
> good airborne reception.
> I use as stated the polling mode with no confirmation and retries are set to
> 0.
> I have set the addresses somewhere in the top range, FFF0-FFF1 i think and
> changed the channel
> to 9 (different hopping frequencies).
> One day i had a problem communicating with the plane (the downlink was fine)
> and it proved to be the video tx.
> Since then i have fitted filters to the video tx (high pass) and the modem
> (low pass)
> and all problems were solved.
> The antenna and the modem have to be in a straight line otherwise SWR will
> rise,
> RF will flow through the wires all over the airplane causing havoc and the
> antenna
> radiation pattern will be greatly distorted.
> The same goes for the video tx.
> Chris
>
>
> Hi Stephen
>
> We configured our 9XTends according to your recommendation
> (Acknowledged Polling Mode):
>
> http://paparazzi.enac.fr/wiki/Modems#Configuration
>
> As we are sending lots of telemetry and not using datalink joystick
> control we chose the air module as the base. It gets pretty warm in
> operation. In the workshop everything works fine.
>
> On the field we experience a very asymmetrical link: The telemetry
> data from the aircraft gets well to the ground station, also at range.
> But the commands from the ground station only get to the aircraft at
> very close range. At range we don't get any commands from the ground
> station to the aircraft anymore.
>
> Can you explain this behaviour? How do you achieve a symmetrical data link?
>
> We still have a 90 degree bend between the modem body and the antenna
> on the airplane. On the ground station we have them in a straight
> line. Could this explain the asymmetrical link?
>
> Regards, Reto
>
> 2
>
>
>
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