Replies to This Discussion

Permalink Reply by David on July 2, 2010 at 7:58am

I would respectfully disagree that mobile phone technology is a legal radio system. From CFR Title 47, Part 22, Subpart H:

§ 22.925 Prohibition on airborne operation of cellular telephones.
Cellular telephones installed in or carried aboard airplanes, balloons or any other type of aircraft must not be operated while such aircraft are airborne (not touching the ground). When any aircraft leaves the ground, all cellular telephones on board that aircraft must be turned off. The following notice must be posted on or near each cellular telephone installed in any aircraft:

“The use of cellular telephones while this aircraft is airborne is prohibited by FCC rules, and the violation of this rule could result in suspension of service and/or a fine. The use of cellular telephones while this aircraft is on the ground is subject to FAA regulations.”

(Emphasis added)

There are radios that can be legally operated in the air, but cell phones aren't in that category.

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Permalink Reply by Alex on 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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Permalink Reply by Dave Buckley on 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.

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Permalink Reply by Ryan on 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!

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Permalink Reply by Nate Hill on 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?

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Permalink Reply by vau on July 1, 2010 at 10:49am

I thought about a drone relay but perhaps better is a relay baloon/blimp with highly directional antenna from ground location to baloon.

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Permalink Reply by David on 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.

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Permalink Reply by Dr Mike Black on July 1, 2010 at 4:55pm

Hi

In tests, with a 40' raised, tracking yaggi & patcn ant with diversity on 2w, we get results from 32-41 miles, (as long as there isnt a mountain in the way) and the mythical god of all things radio is blowing the luck wind in the right direction.

pretty reliable (and brave if you want 30miles out) :~)

Mike

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Permalink Reply by Dave Buckley on July 1, 2010 at 5:50pm

Great references to the radio horizon explanation, David.

Here's a simple block diagram of a system that might automatically select the signal from the strongest transmitted signal.

It uses the relay drone to retransmit the control signal from the base site for the master drone on a second transmitter.

A comparator selects the receiver with the strongest signal/best signal to noise ratio, and sends that signal to the servos.

It keeps the whole system relatively simple (save for the fact you're operating a second UAV as a relay station)

The same platform could be used to boost your video signal for greater range.

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Permalink Reply by Ryan on July 2, 2010 at 6:06am

Of course if you expect to fly the UAV fully manual during all this relaying business, do expect great delay. Relaying will work best if the UAV is in semi-auto or fully autonomous mode where all you do is sending flight plan commands and receiving telemetry/video

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Permalink Reply by Nate Hill on July 2, 2010 at 7:21am

How long of a delay do you think there were be? Assuming that there was minimal processing and that rather efficient software and repeaters were being used, I wouldn't think there were would be too much of a lag. I'm guessing a tenth of seconds gain every few minutes?

Out of curiosity, how serious would the delay be if the signal was being relayed, say, three or four times before it got to the plane? (ignoring the fact that no one could realistically keep track of that many UAVs)

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Permalink Reply by David on July 2, 2010 at 8:27am

The amount of latency introduced by a repeater depends entirely on how it's set up, signal quality at each node, traffic on the network, etc. The newer generation of the xBee radios already has highly-efficient mesh networking capabilities; I would suggest that you look into leveraging them, or something similar, unless you really like the minutia of data communications. The amount of latency for those configurations is small, and fairly constant over time assuming constant operating conditions. If I have some time over the weekend, I may look through my documentation to see if I have numbers for you, though it will vary based on the packetization settings, retries, etc.

In answer to your second question, under ideal circumstances, the added latency should be a multiple of the number of repeaters.

You'll notice that there's a lot of assuming going on here... which is why I agree with Ryan that you'll probably want to avoid attempting to fly "on the stick" over a network. Not that I think that it's impossible...

By the way, espescially with xBees and radios like them, you should be able to unit test any network scheme before you "need" it. Turn the xmit power way down, and use a directional antenna on your ground station to transmit to a repeater some distance away on the ground to get to your aircraft still in LOS. Some creative RF shielding may be needed, but it shouldn't be that hard, and you'll have a full network system without really risking your aircraft.

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