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Out of line-of-sight communication

Posted by Nate Hill on June 30, 2010 at 11:27am
Hello fellow DIYers. I have finally decided to act on my dream of creating a UAV, and I thought I'd start out by asking what is probably a often-asked question from beginners:

How do you remain in contact with a UAV when it isn't in line-of-sight? (For example, behind a mountain or very far away.)

I'm assuming the regular answer is "well, you don't...", but there must be some ways. One can't use a satellite, but one could potentially use satellite internet with a strong enough transmitter and a bunch of extra electronics, even if it's just for telemetry. What are the possibilities?

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Tags: communication, line, of, satellite, sight, wireless
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Replies

  • David July 1, 2010 at 7:56am
    The concept that it seems like you're asking about is radio horizon; essentially, given an antenna hight, how far could the antenna "see"? It's a fairly simple problem, and you can find a simple calculator (essentially assumes a 0 altitude for the second antenna) here. Also, you can do some reading on the calculation itself. The short answer is that, assuming a lack of topography, the radio horizon for a 2000'-high antenna is 63 miles. (Bolded to answer your question quickly)

    Now for just a little more info than you want: the rest of the calculations that you'll need to do for BVLOS operations can be started here, and the site contains some great explanations of the basic concepts of RF link budgets.
  • Nate Hill July 1, 2010 at 7:50am
    Wow these are great ideas. The relay drone seems to be one of the simplest solutions. Thanks! I'm assuming I could mount some kind of repeater or transmitter in such a drone without too much difficulty.

    So lets say, all implications aside, I have a relay drone circling at 2000 feet. What would be a general line-of-sight range for the UAV I'm controlling? I know this number is calculable and dependent on my equipment, but just generally speaking: 1 mile? 10 miles? Farther?
  • Ryan June 30, 2010 at 11:46pm
    Sat Comm, Relay Station, Hand Off Relay Control Station, or Relay UAV

    Of course you can also fly higher!
  • Dave Buckley June 30, 2010 at 8:51pm
    Hello Nate,

    Notwithstanding what may or may not be "legal" - [it will vary from country to country and state to state], would it be feasible to set up a remote drone as a relay station?

    You could have the relay platform linger at a position where line of sight (LOS) will be maintained for the duration drone 1 will be out of LOS, of the ground unit. When LOS is lost from the ground controller, relay through the second drone which maintains LOS to the first drone.

    When LOS is regained, control switches back direct to the ground unit.

    Telemetry from both units to ground will establish when signal from the main unit is low or lost, and could transfer through the relay unit.
  • Alex June 30, 2010 at 12:16pm
    You might get a different answer from everyone, so for what it's worth here's mine.

    I'm in a university UAV team so we have all kinds of safety regulations when we fly. Long story short we have an external-to-the-autopilot end-all-be-all safety switch, which can (and will) take control of the aircraft in the event of a simultaneous autopilot and RC failure. For test flights we command it to essentially glide down (into the trees).

    For our competition (AUVSI Student UAS competition) that gets turned into a 'flight termination mode' where the aircraft terminates itself. Hard rudder and hard elevator -> spiral of doom.

    Additionally, if *just* RC fails, but the autopilot is still alive, it's allowed to try to fly back to an area with an RC link for a set time (3 minutes IIRC). For us this is just a GPS waypoint set at our groundstation; it climbs to a reasonable altitude and tries to fly home, circling over a spot right near the runway. This also gives us time to repair a broken link on the ground if it's on our end here. After that timeout, even if the autopilot is heathly and it's still got gas/battery, it goes into flight termination mode. (So you don't want to set the waypoint overhead... cause if you cant repair the link it might just terminate *at* you).

    Finally we have redundant radio links to the plane. The autopilot we use (CloudCap's PiccoloLT) we talk to via 900 MHz link, the RC uses a 2.4GHz link, and we talk to our flight PC (onboard payload computer, unrelated to autopilot or RC) over 802.11a (5 GHz).

    In the event of an RC (2.4GHz) link failiure, we can fly RC through the autopilot link.
    In the event of a 900 MHz link failure, we can route the autopilot link through the payload's wireless.
    A payload (5GHz) link failure doesn't affect the flying of the aircraft.
    With this combo we can live (and control) through any two R/F links failing.

    At least here (USA) its not legal to fly out of line-of-sight. There are complicated exceptions that take lots of fun paperwork, and if you had those you'd not be asking this question :-) So if you have to ask then no, you cant fly out of line-of-sight (in the USA).

    Buy your friends lunch, get them GPSes and lawn chairs and TX units and place them along the flight path. (This is assuming you have authorization to fly an unmanned autonomous craft at all). This is how our military has to do it. They're not happy about it either.

    PS - ground testing for wireless systems will save you lots of headache. rangetest, EMI-test, find the limits of your system then operate completely within a factor-of-safety of those tested limits.
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