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RE: Reply to: Re: [Paparazzi-devel] distance measurement for landing.(El


From: gisela.noci
Subject: RE: Reply to: Re: [Paparazzi-devel] distance measurement for landing.(Electro-Technik)
Date: Sun, 22 Nov 2009 08:31:17 +0200

 

 

-----Original Message-----
From: address@hidden [mailto:address@hidden On Behalf Of Chris
Sent: Friday, November 20, 2009 9:38 PM
To: address@hidden
Subject: Reply to: Re: [Paparazzi-devel] distance measurement for landing.(Electro-Technik)

 

 > "Even if local atmospheric pressure could be measured perfectly

 > accurately, with no errors, would this be the best parameter to use to

 > facilitate an automatic landing?"

 

 From what  i have found out so far by reading and thinking all possible

scenarios a barometer will not do the job.

What is needed is a definite, accurate and dependable method to measure

the distance from the ground with an accuracy of at least 10cm and a

latency of no more than 40ms.

 

---10cm is a little excessive - between 20 to 30cm is fine. A 10cm resolution is also fine.

 

Something like

1) someone sitting to the landing gear with a hand held

    laser distance meter :-)

2) A laser or optical  range meter but for all terrains.

 

--- to heavy and requires a laser of reasonable peak power to cope with surface specularity variation.

 

3) A UWB range finder

 

---- This is a very good option, and is what I am working on. The problem with this technology is that the process relies on VERY fast pulses being generated for the TX pulse, an the range delayed RX trigger sample pulse. At 300mm per nanosecond, the TX and RX pulse width needs to be least than 250picosecond total duration, to be able to ‘measure’ distances of 150mm.(this gives pulse rise and fall times around 90ps – WHOOSH!)  A 1ns pulse will give a range ambiguity of 300mm. These short pulse are only possible using quite esoteric (read – expensive -) components, Step recovery diodes, Marks Bank generators, Tunnel Diodes, etc, and so such solutions are not yet so readily available, but the growth in this field is rapid, and we will se more of this. The main driver for this technology right now is the automotive market – reverse parking ranging and 360 degree ranging for collision avoidance.

 

The second complex part of this technology are the antenna. The antenna design is quite different from conventional RF and Radar antenna in that the antenna should not be resonant and should be very broad band, from around 2 or so GHz up to perhaps 8 to 10GHz. The pulse rise times of 90 to 100ps gives an antenna response requirement up to 10 or so GHz. The most critical element of the antenna is the requirement of ‘non-resonance’. If the antenna is resonant, it will ring electrically when the pulse is applied. The duration of the ringing will directly influence the resolution of the system; if the ringing persists for 40 to 50ns ( very typical, sometimes even longer

if for example a simple dipole cut to resonance at 5GHz is used) the your resolution is approx half the ring time , say 25ns = 7.5meters!!!

 

Also the antenna should be directional, so that only the area of interest is ranged. Reflection from the aircraft itself need to be eliminated.

 

This system works very well and good results can be achieved for the medium and longer ranges where resolution can be sacrificed, with easily available components. The very short ranges (where we need very fast pulses to get good resolution) is a little more difficult. It is an ideal solution for an airborne seek and avoid sensor, both for other aircraft in the same air space, and for terrain avoidance.

 

There are some ‘commercial’ units starting to appear :

 

http://www.getradar.com/PDFfiles/PER24.PDF

 

 

4) A normal RF range finder

 

--- To heavy and power hungry ( normal RF Doppler rangfinder)

 

 

5) A powerful ultrasonic range finder.

 

--- This is what I am using for autoland. I use 4 Ultrasonic transducers, pulsed with 100v peak to peak, 40KHz, 15 pulses per transmission. The transducers are all wired in phase. A single RX transducer is used for the reflected pulse detection. I can detect up to 22meters of a hard surface, with no TX/RX misalignment. However, in flight the range is easily reduced to 3 to 5 meters due to :

 

      Surface reflectivity variation. I fly in the Namibian desert areas, so, soft sand and gravel plains are the order of the day and such surfaces can absorb more than 80% of the transmitted signal.

      Aircraft roll angle. The Ultrasound transducer is VERY directional, and an aircraft roll angle of 10 degrees will result in a loss of received signal.

 

      The receive transducer amplifier gains are critical, to high results in to many falls rangings, to low reduces range, and the RX tranducer MUST be vibration isolated from the aircraft, esp if an IC engine is the prime mover ( vibration).

 

      This system does work and very well indeed. We have implemented autoland as follows:

 

      Fly upwind to the descent spiral. Enter the spiral at 100m AGL, and spiral down to 40m AGL. Exit the spiral on a tangent, heading in to the wind, down the landing glide slope. We control the descent rate to around 2 to 2.5 m/s, aiming for a pre-touchdown point an estimated 5m AGL. During this descent we control rate of descent and airspeed, looking for a valid ultrasound range return. As soon as a ground range is detected, we aim the aircraft at the touchdown point, trying to keep descent rate less than 1.5 to 2m/s. At 1m above ground we reduce descent rate to 1m/s, and as soon as that is achieved, we cut throttle, and hold pitch and roll level, and at 300mm above ground flare to a pith angle of 5 degrees, and touchdown.

 

Do not try to use the maxbotics or similar Ultrasound units for this type of application; there is simply not enough energy in the TX pulse.

 

6) A vision range finder either on the airplane or at the ground station.

 

---On aircraft is can be a bit bulky and heavy perhaps. System is reliant on lighting conditions, ground specularity, etc. Try, for example, to let an auto-focus pocket camera focus on a flat surface that is strewn with uniform shaped gravel, with little colour or contrast variation. Sometimes it does not manage to focus, and vision ranging systems can suffer the same effects. Lighting contrast also affect the CCD considerable, and a repeatable reliable result is not guaranteed.

 

6) a wire connected to a potentiometer which drags below the

    aircraft  (say 50 cm lower) :-)

 

--Forget the pot; use thicker wire and and make hook in the end. String up a wash line and catch it….

 

Joe

 

Chris

 

 

 

 

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