Hi,
Till now, it's impossible for me to landing with FPV mode, maybe it's caused by large optical view of camera ?! in most case, aircraft touched down suddenly !!!

So, it's possible to build a simple variometer with accelerometer ?
exmle : http://www.snesometel.com.tn/images/PDF/ADXL330.pdf
We have 300mV/g, but I have no idea about the value of "g" in "normal" landing

The other idea is to use an usltrasonic trasnducer to detect the approching of airstrip ? (like the car parking radar)

What do you think ?

An ultrasonic radar would be much better, the variometer (either accelerometer or pressure sensor based) being usable only on perfectly flat terrain.

I don't have any experience with one though, and I don't know how well the signal is going to reflect on grass. It's an interesting project anyway, thanks for bringing this up!

An accelerometer would give you the change in vertical speed, not the vertical speed. You could be coming in for a landing at 1 m/s vertical speed and the accelerometer would still read 1g. You could integrate but without an external ref it would be prone to error.
The ultrasonic idea seems good though, at least indoors, http://www.sparkfun.com/products/639 this looks like a good, lightweight ultrasonic range finder with six and a half meters range. You'll need to connect an FPV video text generator thingy between your camera and transmitter that can interface with the sensor.

ultrasonic reflexion can perfectly work if the reflector (in this case the ground) is mobile ?

How about flying your FPV aircraft with a safty pilot on a buddy box that can take over during landing, at least until you know what a good one looks like?

I have no friend

Originally Posted by Alphactif

ultrasonic reflexion can perfectly work if the reflector (in this case the ground) is mobile ?

If it works imobile (with ground), it is going to work the same with mobile ground, because the speed of the plane is much lower than the speed of sound.

The Sparkfun sensor is nice, you would use the analog ports and create a variable frequency audio tone for the FPV transmitter. But again, only if it works, I think grass absorbs (ultra)sound..

There s no way to use ultrasonic reflection differently ?
If we sent a constant amplitude (or volume) of ultrasonic sound, the amplitude or reflected sound will be highter than the distance to the ground is low.
No need to calculate time between sent and reception sound.

Other idea is based on two laser diodes, we will observe two red points in ground, more ground clearance decreases more points are close

There are many ways a radio/sonic radar can work, from plain measurement of the interval between send and receive to doppler effect exploatation and sawtooth variable frequency impulse methods.
Not a single one is based on the received amplitude, because it is extremely inaccurate and imprecise. The ratio of reflection depends very much on the material, ground in our case that is not the same everywhere.
Whatever method you choose, there must be enough reflection from the object, which might not be grass's case.

The laser pointer idea is very good for night but I don't think you'll be able to see the dots in full sun.

200mW DVD writer laser diode are very powerfull, I think that visibility of laser depending essentially on the resolution of camera.

Ok the amplitude reflection is depending to the reflector surface, it's necessary to calibrate the sensor to the ground surface before flight

I think it is difficult to land in FPV because there is no point of reference on the screen. May be if you have fixed horizontal line on the screen and adjust camera while on the ground in a way that fixed line matches horizon you have a good reference of plane attitude during descent. Just a thought...

I for one have no specific difficulty in landing FPV. I came into FPV with tons of experience in FSX though

It's possible to do a project like this but without µC ?

Thermin with arduino (1 min 15 sec)

I Only need to generate sound (100Hz to 1000Hz) function of obstacle distance, more lower is the distance to the obstacle more higher is the frequency of sound.

Here is a synoptic schematic of circuit without µC.
The Tx sent ultrasound wave every 1 second if no echoes, but, in presence of obstacle, Rx signal are amplified, and transmitter send immediatly an other ultrasound wave.

At 300m/s (30cm/ms) the tx sent the second wave after 8ms, ( 120 X 2 / 30 =8ms), the spaker sent an audio sound at (1/8) X 1000 = 125Hz

At the lower distance (approx 15cm) this frequency is 1000Hz

what do you think about this idea, and what's the best electronic circuit to do this ?