Posted by Scott James on September 20, 2008 at 12:24am
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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Hello. Yesterday i thinking about this problematics. My friend need tracking UAV because he have on UAV video transmitter on 5.8GHz. For good video signal, receiver antenna needs to be directed to transmitter antenna. My version of the solution of this problem is: On airplane is the GPS and RF GFSK modem module. RF modem module have omni-directional antenna and high output power. Frequency of the RF modem is around 430MHz. RF modem transmit the GPS data. Antenna ground tracking device have RF GFSK modem too, with omni directional antenna also (or one-direction antenna with wider angle then antenna for video receiver). For tracking you need to know only two angles: one is azimut second is latitude angle. You dont need to know distance from you to UAV. I attached the picture of the problem. On picture needed angles is gammaN and gammaH. From GPS data you can know the alfaA, bettaA, alfaL, bettaL angles. Also from GPS data you can know latitude (h). It is enough for rating needed angles. Before start you just to must calibrate ground antenna station. Software of the antenna tracking device need to know coordinates of the ground antenna placement. You just need to place UAV GPS-FSK modem traking module on ground antenna station and let software set this coordinates like ground antenna station coordinates. Then you need to turn ground antennas exactly to north and set this position like default. So now you can fly with UAV. Tracking device will rate azimut and latitude angles from GPS data and turn antenna exactly to UAV. So this is my idea.
P.S. Maybe the math problematic can be more simple. Amateur UAV will not fly on hundreeds kilometers. So i think that is possible transfer spherical tracking problematic onto plane tracking problematic. The rate code will be more simple and faster.
P.S.2 Sorry for my english.
Could the groundstation Xbee which is receiving telemetry from the UAV be used to point a tracking antenna? Maybe an additional arduino board could take in telemetry from the GS xbee and then send out ftdi to laptop for groundstation software and also use outputs to drive servos?
Also, the RTL variable could be used if you launch from where your antenna is. No need for a GPS on the ground? or even program in your starting coords before flying?
I'm glad people with the technical knowledge are pursuing this. Fantastic work.
From my limited code trading skills i take it that you planning to use Xbee modems to transmit the NMEA data back to the ground and also use standard servos for 180 degrees of movement. Is this correct?
yeah similarly not totally i hv a project of tracking antenna of UAVs my first assignment is to choose which type of MOTOR is to choose or best for it to change the coordinates of the antenna while tracking it from the base station plz help me out
I was considering this very same issue. I have seen tracking antennas that actively track an object by signal strength. This sounds like a great idea along with the dual GPS approach. I wonder if they can even be melded together for a whichever works best scenario.
I was planning to have the GPS on the UAV and a GPS on the antenna like you have mentioned.
I wanted to build an arduino based system with an electronic compass, GPS and gyros to calculate how much the motors actually turned the antenna.
Ideally it would be great if I could do it only with the electronic compass but I figured that would be used to align North and the gyros would be needed to make sure the that movement on all 3 axis would be accessible and fed back into the equation for adjustments.
I was planing on trying to calculate the path of the UAV and to account for turns and descents by basically calculating the turn rate and/or decent rates extrapolating the path if signal is lost temporarily.
Besides I have to figure out the tracking code so that I can track moving objects from the UAV. Stationary waypoints are a thing of the past. :^)
I like the dual antenna idea by the way.
If I ever get anything working I will try to post what my results are.
I have a BASIC STAMP book that contains the code to control a Yaesu G-5500(?) Az/El antenna controller. I can dig it out if anyone is still interested in this discussion. I had considered making one a couple of years ago, and am interested in this project again.
My current thought is to use a 3axis Accelerometer to keep track of the antenna azimuth and elevation, with the 3rd axis maybe controlling polarization in the case of a yagi. This 3rd axis may not be necessary, but may be nice if you plant the whole setup on uneven ground. Given that the base station would be stationary, would accelerometers be sufficient? Since there would be no real centripetal forces to deal with, I am inclined to think they WOULD be, unless someone can explain to me otherwise.
Also considering coaxially mounted spotter scope with video capability to allow visual/motion tracking on laptop.
Thoughts?
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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
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 ;-)
Replies
P.S. Maybe the math problematic can be more simple. Amateur UAV will not fly on hundreeds kilometers. So i think that is possible transfer spherical tracking problematic onto plane tracking problematic. The rate code will be more simple and faster.
P.S.2 Sorry for my english.
smerovanie antenn.PNG
Also, the RTL variable could be used if you launch from where your antenna is. No need for a GPS on the ground? or even program in your starting coords before flying?
Anyone think this is doable?
From my limited code trading skills i take it that you planning to use Xbee modems to transmit the NMEA data back to the ground and also use standard servos for 180 degrees of movement. Is this correct?
Thanks,
Nick
I was planning to have the GPS on the UAV and a GPS on the antenna like you have mentioned.
I wanted to build an arduino based system with an electronic compass, GPS and gyros to calculate how much the motors actually turned the antenna.
Ideally it would be great if I could do it only with the electronic compass but I figured that would be used to align North and the gyros would be needed to make sure the that movement on all 3 axis would be accessible and fed back into the equation for adjustments.
I was planing on trying to calculate the path of the UAV and to account for turns and descents by basically calculating the turn rate and/or decent rates extrapolating the path if signal is lost temporarily.
Besides I have to figure out the tracking code so that I can track moving objects from the UAV. Stationary waypoints are a thing of the past. :^)
I like the dual antenna idea by the way.
If I ever get anything working I will try to post what my results are.
My current thought is to use a 3axis Accelerometer to keep track of the antenna azimuth and elevation, with the 3rd axis maybe controlling polarization in the case of a yagi. This 3rd axis may not be necessary, but may be nice if you plant the whole setup on uneven ground. Given that the base station would be stationary, would accelerometers be sufficient? Since there would be no real centripetal forces to deal with, I am inclined to think they WOULD be, unless someone can explain to me otherwise.
Also considering coaxially mounted spotter scope with video capability to allow visual/motion tracking on laptop.
Thoughts?
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