Does anyone know of a sensor (not too expensive) that could let a UAV know it’s altitude relative to the ground height not sea level? I guess what I am asking is…..
Is it possible to hold a UAV’s altitude at say 400 feet above the ground regardless of the changing height of the ground during a flight path?

Thanks

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Laser ranging can work at that range, but it's neither cheap nor small and light. More here.
What about this.

Use Googleearth to get the height over sea level for each of your waypoints, and then do a a small calculation from waypoint to waypoint to correct the flightlevel. It's not exacty what you want, but it's cheap/free!
The BYU Magicc lab has shown some pretty interesting results using an "optical flow" sensor. Imagine an optical mouse with a longer range lens. If you know your ground speed and your optical flow, you can correlate that to an altitude. (pitch and roll changes will bias your results, but perhaps you could filter those errors out or account for them if you can accurately sense your pitch and roll angles.) Also, if you get into any kind of variable terrain, a 400' cushion with a straight look down sensor doesn't give you much margin for error. It would be easy to see how a combination of finding yourself a few feet below your target altitude + a rapid rise in terrain + an ill placed tree or radio tower or water tower or power line could quickly put an end to your flight.

I'm reminded of a previous boss who took out his brand new boat for the first time. It had this cool depth finder and he was happily cruising along watching the depth finder, not where he was going. He said it went something like 50' ... 50' ... 50' ... 49' ... 2' ... !

However, despite the limitations and potential failure modes of any look down sensor, with a little creativity and care, you might be able to get a lot of mileage out of some simple techniques.

Curt.
Thanks Guys!
I just found the Centeye website and I realized that I'd been there before. Then I remembered an earlier post here on DIY drones that linked to it. (I think it was Chris).
Once I blew the dust off their website I could see they had some cool stuff. I sent them an email to see if they were still around? (No response yet).

Has anybody used these sensors or know of anything similar that could be used for altitude hold in a uav?
Hi Scott,

"Geoff" from Centeye here. Lots of people have hacked the optical mouse chips and done some neat things with them. Google the name Jean-Christophe Zufferey and you should get a few hits. As Curt below said, some people from BYU have also had luck with these sensors. Lots of roboticists are using them for other purposes as well- the last time I was at Carnegie Mellon these optical mouse chips were being used everywhere.

You can get them from Avago and Pixart. Their main weaknesses are that they only output one optical flow measurement per chip and they don't work well in lower light levels.

At Centeye we have actually gone through several generations of sensors- we just didn't put it up on the website (for various reasons). We've a new generation coming out, probably Jan or Feb or so of '09.
Hello!

can this sensors measure distance to water (no waves, absolutely calm)?

distance is 0...20-30 m and accuracy is 0.1...0.3 m, speed is about 50-100 km/h.

regards!
oooh thats a tough one and quite interesting.

im not sure just wanted to post so if anyone gets back with a real answer id know....sorry to burst your excitement
Sounds good, I'd like to see what you guys release.

Thanks,

Scott
In general I'm going to say "No" but I think with some more advanced image processing techniques this can be solved. This would require more than a single sensor, and algorithms sophisticated enough to identify, say, debris or stuff floating on the water and the reflection.

I am told that when honey bees fly over perfectly still water they sometimes get confused and end up landing in the water.

In our own work we have yet to try flying over any body of water.
To answer the original posters answer, could you do what pilots do, and use a barometer? If you consistently take the barometer reading, but zero it in when it takes off, I believe it should be always be correct and updated.

Maybe I'm wrong, I'm just thinking of just mimicking exactly what someone uses in private aviation.
Well, the original question of this thread was "altitude relative to the ground height not sea level", so a baronetric sensor would not meet that requirment.
At the Swiss Federal Institute of Technology in Lausanne (EPFL), Laboratory of Intelligent Systems (http://lis.epfl.ch), we have successfully demonstrated autonomous flight in the vicinity of obstacles such as ground, water, trees, etc., using a series of 5 optical mouse sensors and a 3-axes rate gyro on a 400-gram EPP flying wing. The so called "optiPilot" weighs approximately 20 grams and uses a single dsPIC as processing unit. It ensures attitude and altitude stabilisation over flat surfaces, self-recovery in case of diving trajectory, tree avoidance, etc. As shown in the video below, the optiPilot is working fine over water as long as some ripples are visible.

A video and more info are available here. And here is the lower-res youtube version of the video.

A scientific paper describing the control strategy and the technical details is planned to be submitted towards the end of January 2009.

Cheers,

jcz

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