Re: [Paparazzi-devel] Long Range Flying Wing

Cheers,
Reto

Am 10.05.2012 um 02:15 schrieb Stephen Dwyer <address@hidden>:

Hello Reto,

We use 9XTend modems as well and used to have exactly the issues you

describe regarding the downlink working (mostly) fine but not being

able to change settings properly. I had discussed this with Chris

previously, and he recommended polling as he did here.

After playing with the various settings randomly, we found it

difficult to find a good setup. So, we looked into the issue further.

After closely reading the manual and studying the state flowcharts, we

hooked up a logic analyzer to the tx and rx lines on the ground and

airborne sides as well as to the rx led and tx power pins on the

modems.

We discovered that the default settings for the modems do not work at

all. While in operation it appears as though the settings don't

change. Looking at the logic output, one observes that more often than

not, the groundside modem receives a command packet at about the same

time the airborne side receives a telemetry packet. Each modem goes

into transmit mode and attempts to send the packet, resulting in a

collision. Under streaming or basic reliable mode (no

acknowledgements), both packets are sent but not received because each

modem is transmitting at the same time, and thus cannot receive. In

acknowledge mode, the default settings have the same retries with no

delay, so both sides continuously attempt to transmit but never

receive an acknowledgement. Each modem receiving a packet at the same

time seems to happen very often (almost always).

Instead, each modem must be configured a little bit differently. We

found the both acknowledged mode and polling acknowledged mode work

quite well. Polling mode essentially eliminates all packet collisions

but induces a bit more latency in one direction (from remote to base).

Normal acknowledged mode has some collisions, but these collisions

seem always to be resolved.

I have posted some sample configurations here:

http://paparazzi.enac.fr/wiki/Modems#Configuration

I am sure other configurations work, and we haven't flown with the

above configs, but it seems to work rather nicely in lab testing with

a TWOG + Aspirin. Thanks to Chris for pointing us in the polling

direction initially.

Hope that helps anyone having issues.

Thanks,

-Stephen Dwyer

On Tue, May 1, 2012 at 12:59 AM, Reto Büttner <address@hidden> wrote:

Hi Chris

We have problems over 25 km with 9600 baud. Something must be wrong in

our setup. We use a dipol antenna on the plane as well, very similar

to yours.

Thanks for the tip to keep the modem body and the antenna in a

straight line. In our current configuration we have a 90 degree bend.

We will check on mounting it in a straight line.

Cheers, Reto

2012/5/1 Chris <address@hidden>:

I get over 25km with 115200 (haven't tried further) and a 8dbi panel antenna

without any problems.

At 9600 i have gone as much as 40km out and the signal was rock solid both

ways so i bet it could

probably go further than that.

One thing to watch for is the airborne antenna, don't use the normal 900Mhz

l/4 whip antenna,

use a dipole antenna (l/2) or a J pole.

If you use an antenna that can be rotated (swivel) keep the modem body and

the antenna

in a straight line, it makes a big difference.

I use this antenna

http://www.dpcav.com/xcart/900MHz-Antenna-Compact-Dipole.html

You may also try a V dipole but not made from pcb (pcb antennas have a lot

of losses).

I haven't tried 9600bps and joystick control though only at 115200.

As i said i had up link problems at normal mode but when i switched to the

polling mode

all problems went away, instant response to GCS and joystick commands with

no visible latency.

Nice plane you got i am sure that it must have almost no drag as it looks

really slick.

Chris

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From:	Reto Büttner
Subject:	Re: [Paparazzi-devel] Long Range Flying Wing
Date:	Thu, 10 May 2012 13:44:24 +0200