25m Altitude/Distance/Range sensor

I have a RC model airplane capable of carrying max 800grams payload (after all my onboard electronics, batteries, GPS, video camera). I want to try
auto takeoff and landing. Especially for the auto landing, it important to be able to determine the slop of the final landing approach. For this intended application, I need to accuratelly determine the height of the airplane. (The static pressure sensor alone is not accurate for this).
So, the obvious question; 1. Do you know any range/distance/height/altitude sensor with the following features; * LIGHT WEIGHT * Low power (and hopefully low cost) * Measurement range 25m-25cm * Measurement resolution (better than) 10cm * Measurement rate 10 Hz
And also 2. Would you suggest any method that helps to line-up the airplane with the landing field (runway). (Currently I'm planing to use the GPS data but it seems to be not very accurate.)se
Cheers,
Falco
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Falco wrote:

There are a couple of good methods for detecting the ground. Aircraft use a few methods, but a Radar altimeter is the most widely used. See http://www.roke.co.uk/aviation/miniature_radar_altimeter.asp This will most likely be out of your budget and weight range though.
If the weather is clear, you're not too high, and the ground is fairly reflective, a laser range finder with RS232 output will work rather nicely. See:
http://laseroptronix.se/dispu/ilm400el.html
You might also try hacking one of these cheaper consumer products:
http://www.professionalequipment.com/xq/ASP/ProductID.4231/id.15/subID.125/qx/default.htm
As for lining up with the centerline and glide slop of the runway, aircraft use multiple technologies for that including ILS, MLS, and DGPS (see http://www.edu-observatory.org/gps/dgps.html ).
The resolution, price and weight of ILS and MLS will not make sense for a small model though, plus you need to have multi-million dollar ground-based transmission equipment for each, plus very expensive receivers on board. Although the receiver is relatively inexpensive, DGPS has a resolution of 3-6 meters. The last time I flew RC, our runway was 25 feet wide with tall grass on either side. Not wide enough to overcome the DGPS error. I suppose if you had a wide enough runway you could at least line up with it. With laser distance detection, you could over come the error of the DGPS and control your low-level altitude. I would rely on a digital barometric pressure sensor or the DGPS altitude for descending to pattern altitude and follow the DGPS altitude down until the laser picks up the ground.
You might also consider using Ultra Wide Band Radar for ground detection, see http://www.uniroma2.it/fismed/faculty/Stadero/papers/osee.pdf
Have fun,
Shawn
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Don't airplanes have a visual method available for the pilots using lights on or near the runway. I am talking about the system that they use for the glide slope.
Is your video good enough for a computer to process this image?
A different idea is to use two ground based cameras to view the approach area. A computer would calculate the angles and generate the 3-d location of the plane. Superbright LED's on the front of the plane may provide a bright enough light source to help the process.
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snipped-for-privacy@yahoo.com wrote:

They have different types of visual glide slope indicators using directed multi-colored lights, but you would not want to use them as part of an auto land system. Plus, this is a model airplane landing at what I presume is a non airport.
Shawn
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<Falco> wrote:>I have a RC model airplane capable of carrying max 800grams payload (after

The other answers you've had haven't been especially encouraging. I think that's because no off-the-shelf solution exists which meets your specs. Laser rangefinding gets the closest, but isn't very light-weight (in a model aircraft context) or low-cost.
What *is* light-weight and low-cost is sonar. A few tens of dollars and a few grammes gets you a sensor which will read out to 5-10m. I don't know how well sonar fares against moving (relative to the sensor), soft (the vegetation around most flying fields) targets though. It wouldn't be a very difficult or expensive experiment to find out if sonar could do the job, if you already have an instrumented aircraft (which you do).
Tim
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Did I really still have that sig?

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This idea is sort of far-fetched so take with a grain of salt.
Use an optical mouse with modified optics (specifically, focused at infinity and with a better f-ratio) to measure optic flow beneath the aircraft. Particularly you are interested in the forward/backward angular rate. Subtract the pitch rate from your IMU. You know your velocity over the ground pretty well from gps data. Compute altitude from velocity and corrected forward optic flow.
Thoughts?
chris
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