How do I implement drone-to-ground distance-measuring for autopilot landings for heights over 10m? I found ultrasonic to be too inacurate, let alone GPS. Maximum height is 1000m, Vmax is 100 km/h, Vaverage is 72 km/h. The drone is plane-like, no *copter or so.
Thank you for any input!
You are probably looking for a radar altimeter, but I think that 1000m height will be a challenge if you want to build it yourself, because of power required to get detectable reflection at such distance. Couple hundred meters may be more realistic target for home made low power radar.
Here is schematics of radar landing altimeter that is usefull to about 1000 feet.
On real aircraft, they will have both a Radar Altimeter and a Barometric Altimeter. The Barometric Altimeter is used at higher altitudes and the Radar Altimeter is used during takeoff and landing to gauge distance to the actual ground (i.e. at altitudes where terrain elevation changes are a significant concern - typically 5000 feet).
In reality, a single sensor will probably not be accurate enough to do what you want. Most of what I know is related to AGVs (Ground Vehicles), but I think that some of the same principles apply.
You probably want to use a combination of sensors to get the accuracy that you need. Some of these can be quite expensive.
GPS: A standard GPS module should be able to get you down to about 1m +/- accuracy. If you step up to a differential setup (one station on the ground, one on the plane), then you should be able to get significantly more accuracy, but at a much higher cost. Something like 10cm or even 1cm should be possible (with velocity data), but with a significantly higher cost.
INS: You can supplement your GPS system with intertial measurements. The boom in MEMS devices has made relatively decent solid-state sensors available at consumer prices. Adding accelerometer, gyrometer, and magnetometer data to the GPS data should make the signal more accurate, and account for possible "glitches" in you GPS readings.
Radio-assisted navigation: I'm not entirely up on this, but many airports use a radio-assist to help land the planes. You may be able to research how these systems actually work and implement your own (legally, of course).
For a more detailed look at some of these considerations, I would check out DIYDrones. They have put together some pretty tightly-integrated systems using GPS, INS, Barometers, and a large array of other sensors. They have also tackled some of the difficult filtering challenges that come with multiple sources of data in an airborne system.
A barometer would do quite well you get something like 10 cm resolution, the only tricky bit is that your drone will need to know the barometric pressure at ground level and that tends to change with the weather.
If you want really high performance location control, then you will probably not get around a vision based system with a high-power computer that can recognize the the landing strip and hit the right zone at the right speed.
If you are going to land on landing sites under your control, I would place several radio emitters around the site and compare signal power. That's the only reliable & easy to implement way.
If you want to land anywhere - only GPS (+-1m possible in USA), ultrasonic or laser measurements are valid options, but none of them are perfect.
A laser rangefinder will give you good precision and accuracy, and is designed for your expected distance, but may be heavy (due to the optics) and will resolve the distance to a point rather than a larger area.
The measured result may change rapidly if you're going over terrain that has a lot of variation (such as a forest or city), and it may be hard to get a reading over reflective surfaces such as water which will not return much of the beam in the direction it came.
However, this should be considered as an option. Consumer handheld rangefinders for hunting or golf run from $50 to over $200; I'm not sure about commercial prices for integration into a system like a UAV.
I always wanted to try this:
Mount a downward-facing camera on the UAV. The quality is mostly irrelevant. Grab frames from it at some fixed interval. Analyze pairs of images to determine how fast the ground appears to be moving. There are a lot of options here for algorithms. Now, given your GPS speed (not airspeed!) you how how fast you are actually going and how fast the ground appears to be moving. At 0 altitude the (properly scaled) apparent movement would be 1:1. As you gain altitude the apparent speed of the ground would slow down.
