I have been thinking of ways to increase the range of the data modem or video feed from a UAV. And it always comes back to having better directional antenna on the ground that has to be pointing at the UAV. My problem is I don’t know too many people that would love to come with me to some remote area and hold a stick up to the sky with an antenna on it and point it at a UAV for hours at a time! I can’t see why?? :)

Then I thought it can’t be that hard to make a tracking antenna, can it??

Putting the mechanics of it aside for a moment….
If you had a GPS receiver on the ground (base station), and the feed from the UAV’s GPS it could be done. If you edited the Ardupilot code (another version running on the ground just for the antenna control) so that the GPS data from the UAV was constantly updated into the waypoint 1 position, and changed the rudder part of the code to give you a 360 degree value. This would give you the direction to point the antenna. Then all you would need is the difference in altitude between the two GPS units (Ground and UAV to get the height of the UAV), and then the distance to waypoint data to calculate the angle that the antenna would need to be pointing up from the ground. (Tan- x = altitude/distance I think?) Then all you would need is a reference to north (Compass module) to make sure it all syncs up. Somehow…
The only problem is if it lost track of the UAV it wouldn’t be able to find it again. Hmmmmmm..

This could even be used in a car following a UAV as long as the car was level and not going up or down a steep hill. I guess you could compensate for that as well if you had enough time on your hands.

I found this but there is not much info.

The hard part would be physically building the antenna mount.
I not planning on actually doing this anytime in the near future as I am still trying to build a working UAV but, can anyone think of a better way to do this?

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The only better method I can think of it is to do what I do, which to ask a small child (preferably one of your own) to keep the antenna pointed at the plane. Doesn't work under battle conditions ;-)
The other option is, I could get my wife to wear that super stylish helmet mounted antenna.
But of course I would have to fully test it to make sure that it worked in Hell at sub arctic temperatures! ;-)
You are correct, with two GPS units, (or a GPS in the air, and just knowing the coordinates of a spot on the ground) it is relatively easy math to figure out the Azimuth and Elevation from the antenna to the aircraft.

I think this is a really cool idea with lots of potential. I'd love to hear if you make any progress on it. I've done some similar stuff with pointing antennas at high-altitude balloons, and in a fixed mode with long-range wi-fi. I've learned some valueable lessons, so I wanted to share wiht you a couple potential gotchas to think about....

1) How do you calibrate the antenna's pointing direction? If you know you need to point 15deg east of north (based on the above calculation) how far do you command the servo to turn? If you don't need really good accuracy pointing the thing the direction you think is north and saying that is zero might work, but sooner or later (depending on how far the craft gets away from you and your antenna radiation pattern) that rough guess pointing won't be enough.

2) Can you repeatedly give your servos the same amount of power and be sure they turn the same amount? This is a problem that people have solved on the aircraft, but it has partially been solved by the feedback of the aircraft state. If you command a servo to point 10, then 40, then 80, then 230 degrees, will it be able to accurately point in all those directions, without accumulating any error?

Both #1&2 can be addressed by adding some feedback mechanism. For starters pointing direction and elevation angle sensors would be nice. Depending on accuracy required an electronic compass may be enough to give you direction. It would probably be pretty easy to rig some gravity sensor (an object that hangs strait down), and is compared to the angle of a known fixture of the antenna as its elevation changes. Another option (which could be used in combination with the above) would be to have some power output meter from the antenna, and feed back that in to a PID, and constantly re-point the antenna so that it keeps pointing at the spot that gives the strongest signal. This could be challanging to do in two dimensions, especially with the signal from the aircraft changing dramatically when it changes its aspect to the antenna (unfortunately this will always happen at the time when the PID will want the MOST confidence of its pointing).

Another option may to add some type of optical tracking system. Since an airplane usually flys against a sky that doesn't have many features, it should be possible to program a camera to look for the "black dot" flying out there, and then command the antenna to move to keep the black dot in the center of the screen. This would ovbiously get substantially more complicated when the aircraft is low to the ground, with all the buildings, trees, antennas, etc in the background. However, then it should be possible to detect the aircraft based on its motion relative to the background...you'd just need more resolution than the open sky scenario (which might be a good assumption because usually when it it close to the ground, it is relatively close to the antenna...otherwise you risk getting the antenna eclipsed by an object on the ground). So basically all you have left to worry about is other airborne objects (birds, other airplanes, etc) or fast ground objects (cars, trains) getting into the same frame as the aircraft. I'd imagine that it would be a really tough problem to solve if one passed in front of the other going in approximately the same direction (just different ranges).

3) What's the required speed of pointing? I'm not sure how big (or heavy) of an antenna you have, but it might require some large servos to move at all! Then there's the question of how quickly it will have to re-point. If you're going to be flying miles away from the antenna (then #1&2 will be more important!) there probably won't be a huge requirement to get fast pointing...however if you ever want to do a low level flyby of the location where the antenna is located, it may have to do a 180 degree slew in a matter of a couple seconds. Imagine an aircraft flying directly over an antenna at 100m altitude, at 100km/hr (27.8m/s -> round to 25m/s). So...4 seconds before the flyover the antenna is pointed at a 45 degree angle (100m over 100m up), and 4 seconds after the flyover (8 seconds from start) the antenna will have rotated 90 degrees to 40 degrees above the ground on the other side. This is far from a worst case scenario...often times small planes come much lower that 100m and faster than 25m/s.

So to keep from losing communication (unless a several second blackout is OK) you'd have to mathematically calculate a bubble around your antenna, and keep the aircraft from performing certain maneuvers inside that bubble. I haven't thought to hard on the best way to go about it...the scope of the problem depends entirely on how fast you can turn your antenna.


Let me know how this project goes...I'm really interested in hearing about it!
Tom
I worked out a more detailed example of the slewing speed needed in the fly-over example. (Low level aircraft flying directly over the antenna)

Here's a drawing of the setup:


And the various speed/distance/elapsed time that could be used with the drawing (to be clear, this is the length in meters of a Distance Unit (DU) and length in seconds of a Time Unit (TU)):


The elevation angles at points along the flight path.

The helmet was also my thought but what if its not in sight? Aim in the general direction?

The helmet wouldn't look that bad, kind of cool if you wear the proper attire. ;) LOL

A friend of mine in the UK that does Aerial Photography has a backpack with an additional monitor for the client to view, he has a high gain antenna out the top of the backpack and just makes sure his body is pointed toward the aircraft. You can view it in this link.
http://www.horizonimaging.net/equipment.htm

Troy
There is defiantly a lot to consider.

I wonder if you could use the distance measurement to control a relay that could switch over to an omni directional antenna if the UAV was within say 500m of the ground station? More that 500m and it switches back to the directional antenna. This would eliminate the need to worry about close flyovers if the servos were to slow?
As long as the switching didn’t upset the data flow too much I guess.

Another thing I have been thinking about is does the mount need to travel more than 360 degrees. What if the UAV fly’s a circle around the antenna? Or if you were following the UAV in a car, what happens when it fly’s behind you then to the left then in front of you then to the right of you etc… Ideally it would need to be able to just keep rotating it without cords being tangled up somehow…

Maybe it’s easier just to stick with say a 320 degree range and never let it fly behind you.
The bow tie would be a must have!
We did this with a Yaesu antenna pointer. It had an analog output (1 to 5v I think). Anyway, We built a little basicstamp board with a A/D on it. This gave is a reading from 0 to 255 (8 bit... a little lack of forthought there). However it wasnt that bad because even if I used a dish it was well within the 5 degree beamwidth of the antenna. We were using a Yagi so we were well within the tolerances. It was a fixed location antenna so we knew where we were. The "UAV" was actually a cessna tu-162 with a 10 watt 2.4 ghz analog video transmitter and a 900mhz datamodem. Even in a basic stamp with a co-processor we were able to capture the telemetry stream with GPS coordinates and have the antenna follow the aircraft flawlessly. I even wrote in an prediction algorithm to keep the antenna pointed right at the aircraft instead of slightly behind it. The tracker had both pan and tilt and the math isn't that hard.

Also a little side point..Forget using long antenna lines too. Use a 6 inch peice of coax and put the receiver right at the antenna. run your line level stuff off the tower or tripod. This will really,really make a difference!
I was planning to give it a go on the Arduino. (I'm still quite new to it though. The local supplier is having a full day workshop on it here in Melbourne next month. I probably should go and learn all the in's and out's of it.)
That way it could all be incorporated into the antenna mount so all it would have is a connection for power and a Rs232 port. Nice and simple!

Do you still have a copy of the Basic stamp code? I would like to take a look at it is that is ok? Thanks
we talked about this in another post...ummm...i will try to find it and send you the link. I am going to attempt to do this.

I think, wtih all the things that you are putting in, you are forgetting that a directional antenna has a cone for the transmission and recieving (Tx/Rx), not just point to point. I picked up an antenna with 110*y and 30-40deg*x. This means that as long as the base of the cone is on the horizon, i don't have to worry about an entire axis of rotation.

Another method is that you have two antennas, an omni for near-by transmissions, and a directional for when it gets far. that way, when you have to do the derivaties for the the airspeed relationship, you don't have wild movements and the (TxRx) cone covers a wide area.

In fact, that is the method i will be using lol. It just makes life easier.
I don't work at that company anymore. The code is now lost to history... They don't have any programmers anymore and have shifted away from doing live transmissions.
Yes, I mentioned the 2 antenna thing a fewposts up. I just had another Idea.
I was thinking that the main problem is that if the tracking antenna lost the UAV it would not be able to find it again. (Let’s just pretend it’s 15 Km’s away). Most of the high gain antenna's have a small angle of reception like 8 degrees.

But what if the antenna had it's own list of the waypoints as well as the plane. If it lost the signal for more than (x) seconds the antenna could go to the next waypoint and wait for the UAV to come back into contact.
The easy way would be to load the waypoints into the Arduino controlling the antenna and also into the UAV. But if you were clever you could set it up so that when the switch was flicked to engage the autopilot it downloaded to waypoints to the antenna.

I’ve just started playing with it on my Arduino; so far I’ve only got it to display events to a Parallax Serial LCD using the Soft Serial Library which displays info from a Compass Module (HM55B). Next I will bring in 1 GPS with a fixed antenna location and then add in a second GPS for the antenna.
I am going to need some long cables (or just use a sim on my PC) for ground testing as I don’t have a wireless data modem yet!

I thought it could be cool to add the LCD screen to show things like, distance to the UAV, it’s ground speed, which waypoint it was up to and maybe even its altitude. You could then also display messages when things went terribly wrong!! (I can't find your UAV!) (Your an idiot for trying this!) :)

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