I have recently been pushing the limits of the XBee Pro to see how far it can go.
Unfortunately my current plane (SkyFun) only has a 38 minute duration and a maximum travel distance of 20km, this means the furtherst I have been able to go is 10Km (a 20Km return trip).
I have a 14dB patch antenna which has still shown signal of 85%+ out at this distance.

I wanted to find out how far the XBee could really go.
This meant planning a one way mission.

I spent about a week looking for the perfect start and destination, plotting the course and arranging access to the landing location with a local farmer.

The start point was my normal local park and the landing location was a farm paddock 16Km away.
The flight was over sparsely populated farm land and at a constant incline meaning I could increase altitude during the flight in order to get greater range.

My plan was to launch and monitor the flight from the launch location until I lost contact with the plane via Telemetry. After this I would drive to the landing location where the plane would be circling and manually land.

I calculated that even if I maintained contact with my plane during the flight and it made it all the way that I would still have time to make the 12 minute drive without running out of battery power.
With a tail wind, the plane would average 60Km/h at 45% throttle.

With everything planned and 45 test flights already in the bag with this airframe I was ready.

I launched and the plane took off to the waypoint as planned.
I was hoping for at least 12Km as this was ideal conditions.
The point on the map above shows the point I lost contact which is 10.6Km from the launch location.
The XBee maintained a very strong signal up until the last Km and then it began to drop off very rapidly.
It begins to be unusable below about 40%.

I packed up the ground station and drove to the destination which took just 11 minutes.
The plane was circling perfectly as expected and I put it in FBWA and landed without incident (apart from the slightly startled flock of sheep).

It turns out that the 10Km I had been flying to is around the maximum range of an XBee.

I am working on a much bigger aircraft that will have a 1 hour plus duration at a higher speed so I am going to have to look into more long range options for telemetry. GPRS / cell is preferable as we have excellent coverage here and fully routable and cheap internet plans. However, for experimentation I have been surprised at the distances that can be achieved with an XBee with a cheap patch antenna added on.

Here are a couple of tips for people wanting to achieve maximum range..

 - Minimize antenna cable length. My XBee is mounted on the back of my patch antenna and I run a long USB to my laptop. This maximizes gain and reduces lost signal.
 - I use either the ground or my car to enhance the ground plane effect. Putting the antenna in front of my car door or right beside my car makes a big difference to the distance. Also keeping it as close to the ground as possible.
 - Altitude is king, the higher you can fly the greater the achievable distance. Due to the fact my destination was on an incline, my end waypoint was 400m high relative to my starting point.
- Fly long straight missions so you can line up your antenna properly, I am building an antenna tracker that removes the need for this but Ardustation2 is currently broken since the last Mavlink updates.

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  • Rigel,
    we tried unbricking them but to no avail. Before we were successful in unbricking with the first xbee for like two or three times but after that it went dead. With the second xbee, unbricking didn't worked at all.

  • Marianna,

    What do you mean by your Xbees are broken?  They may just need to be unbricked.  See "Unbricking an Xbee" at the bottom of this page: http://code.google.com/p/ardupilot-mega/wiki/Wireless

  • Thanks for posting! It's very useful information!

  • Is the xbee rpsma 900hz is sensitive? We already have two xbee broken. :(

  • Developer

    APM aleady has a telem failsafe, it was meant for use with joystick control, but can be enabled on any model, and without joystick control.

  • Ok, so I've pretty much answered my own question.  For those interested see:  http://diydrones.com/profiles/blogs/xbee-868-revived regarding the workaround of the 868 duty cycle limitation.  If anyone knows of a better way of doing it please share.

  • @Jeb
    Yes, I definately disable RC failsafe. You only need to do a short search on here to find out how many people have crashed or had accidents due to RC failsafe. I personally think its more dangerous to fly with RC failsafe turned on that what it is with it turned off.

    I do agree, a Telemetry failsafe is a brilliant idea. I have previously posted my thoughts on how it should work:

    If uplink is lost for more than X seconds, then do an action.
    There are a number of potentially valid actions that could be useful options in the event of a lost uplink, it would be nice to be able to pick which option you wanted to use on a per mission basis.

    • Return to Home
    • Return to previous waypoint and backtrack through waypoints (in hope of
      picking up signal again and getting new instructions)
    • Go to a fixed location and autoland
    • Abort and land / crash immediately
    • Circle
    • Circle + Accend (in hope of picking up signal)
    • Circle + Decend (graceful abort mission)
    • Shortest distance, pick the closest waypoint from either home or destination
      and go to that.

    A loss of RC tx link is of minimal concern and shouldn't really need to be a failsafe causing condition for a proper UAS. Telemetry is much more serious.

    In my case I only attempted this test after 40+ succesful fully autonomous flights with minimal or no manual interference and NO crashes or unexpected behaviour. If you know that when you lose contact with your plane that it will be at its destination when you get there, then a failsafe is not required (especially in a telemtry range test scenario where loss of link is almost guaranteed).

  • Hi UT

    I'm not quite sure what you mean by workarounds for getting over the 10% duty cycle on the 868? Isn't the 868 designed to operate at 100% of its rated 24Kbps?  Also, what is the minimum data transfer rate needed for proper communication between APM and the GCS?



  • So you've obviously disabled the RC failsafe RTL function.  As I don't think anyone has set up an Xbee based failsafe, I have to think that flying in such a condition is inherently unsafe.  Believe it or not, most of the UAV's operated in theater right now DO have a geek on an RC transmitter.  Not the big boys, but the smaller ones (shadow, silverfox, raven, etc.).


    But it brings up a really good thought:  Xbee based failsafe.  How hard would it be to implement a lost data-link failsafe?  At least in the US, this is required to even be considered for a COA or experimental certificate.  I'm not even saying that RTL would have to be the mode, just so that if all communication with the aircraft is lost, I KNOW what it is doing.

  • To me there are pros and cons associated with going cellular.  The pros are the obvious lack of having to worry about frequencies and licensing and all of that...but to me a real drawback is that I REALLY like to fly far out away from people when I am testing / playing with this technology, and there may not be good coverage out in the middle of nowhere.


    @Grips - If there IS good GPRS / Cellular coverage where you intend to fly it might be a good solution to your licensing / spectrum concerns.

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