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Thread: Build a simple radar altimeter

  1. #16
    Registered User Dan Thomas's Avatar
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    Re: Build a simple radar altimeter

    Ten feet of heavy chain. A bell in the cockpit attached to a string to the chain. When the bell rattles, flare the airplane.

    Dan

  2. #17
    Registered User Empirical's Avatar
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    Re: Build a simple radar altimeter

    The ultrasonic version might be a lot easier to build, but in terms of performance, generally there is no comparison between a radar altimeter and an ultrasonic altimeter:

    Radio frequency (radar): fastest signal speed possible, stable - unaffected.
    Ultrasonic: 873521 times slower signal speed, highly affected mostly by temperature, humidity.

    Requirements for the ultrasonic altimeter:

    - High power ultrasonic transmitter.
    - The transmitter should be set at an optimum angle forward, depending on the cruising speed (the receiver should be set at an angle backwards).

    Requirements for both radar & ultrasonic (to avoid loose the signal or accuracy when climbing, diving or turning):
    - The transmitting antenna or transducer should be always level (the ultrasonic should also be turned slightly forward ).
    passive way: mounted on a spherical bearing with a weight,
    active way: 2-axis servo with a sensor (gravity, gyroscope)

    Issues with ultrasonic:
    - 1 second of lag time for each 343 meters (1127 ft) of altitude: e.g at 1000 meters (3281 ft) altitude and at a speed of 100kmh (60mph) each measurement will refer to where the aircraft was 2.92 seconds or 81 meters (266 ft) back.
    - loss in accuracy when the aircraft is gaining or loosing height.
    - loss in accuracy when the aircraft is turning or following a curved path
    - possible loss of the signal in sharp turns.




    I'm not currently an aviator, but I guess that even at 100 meters (328 feet) at 100kmh (60mph), I'd want something a lot better than the 0.3 sec lag time that ultrasonic offers. So I think ultrasonic would be only competitive if installed on a helicopter flying slowly at low altitude.
    Someone experienced (in flying an aircraft) might have a better opinion on the importance of lag time in various conditions.

    EDIT: removed excessive blah, blah
    Last edited by Empirical; May 8th, 2011 at 11:04 AM.
    A wonderful harmony arises from joining together the seemingly unconnected.
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  3. #18
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    Re: Build a simple radar altimeter

    If you want to try out the ultrasonic approach (which I think will work fine for what you want--height above water from about 100' to zero feet) this would be the simplest way to try it out: Buy one of these ultrasonic tape measures ( Amazon.com: Zircon Ultrasonic Measure with Laser Targeting: Home Improvement ) and see if it works (duct tape, or passenger pointing it out the window during the approach, etc). If it does, you can get fancy and take it apart: mount the display on your panel, put the circuit board somewhere safe, and figure out where to put the transmit/receive horn somewhere where it won't get wet but will still work. The only two modifications you'd need are 1) A small transformer to let it work off of acft power and 2) an "activate" circuit that energizes it and generates a "ping" for a new reading approx every 1/2 second.

    There area also laser versions of these construction tape measures that do the same thing ( Amazon.com: Ultrasonic Distance Meter Measurer Measuring Tool Laser with USB2.16 AGPtek All-in-One Card Reader: Electronics ) though the range is only up to 60 feet. Again, you'd need to make arrangements for the device to quickly "shoot" a repetitive series of measurements as you land. Neither dispaly is optimum--a bar graph leading down to zero would be more useful than numbers. I think the laser light reflectivity off the water might be a little more "iffy" than the ultrasonic approach, but it would be very easy to see if these things work.

    Adapting a commercially-available product is going to save you a lot of time compared to finding an appropriate circuit, buying all the parts, etc.

  4. #19
    Registered User Empirical's Avatar
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    Re: Build a simple radar altimeter

    Quote Originally Posted by Vigilant1 View Post
    [...]which I think will work fine for what you want--height above water from about 100' to zero feet[...]

    Assuming 30.5 meters (100 ft) altitude and say 70kmh (43.5 mph) landing speed, my estimation is that this won't work or it won't be reliable because the transmitter and the receiver are not separate (low power transmitter) and the bell is highly directional. If pointed straight down, the reflection will appear behind the transducer and if turned slightly forward, the bell won't look straight to the reflection and will loose significant gain.

    With separate and properly angled transmitter and receiver it could work though.
    A wonderful harmony arises from joining together the seemingly unconnected.
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  5. #20
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    Re: Build a simple radar altimeter

    Quote Originally Posted by Empirical View Post
    Assuming 30.5 meters (100 ft) altitude and say 70kmh (43.5 mph) landing speed, my estimation is that this won't work or it won't be reliable because the transmitter and the receiver are not separate (low power transmitter) and the bell is highly directional. If pointed straight down, the reflection will appear behind the transducer and if turned slightly forward, the bell won't look straight to the reflection and will loose significant gain.

    With separate and properly angled transmitter and receiver it could work though.
    I don't know either, but it would cost less than $50 to find out for sure (if it doesn't work, take it back or add it to the tool collection).
    Anyway, at an altitude of 100 feet and a speed of 50 Kts, the plane will have moved about 18 feet between the time of sending and receiving the pulse. I'm not sure how well the signal is focused (approx 5" long horn--probably not too well), but it'd be worth experimenting with. The problems would decrease as one descended. Yes, separate "aimed" transmit and receive horns would help at greater distances, but could prove problematic when closer to the water.

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    Re: Build a simple radar altimeter

    Back in the late 1980's I enjoyed the fact that we had a real radar altimeter in our A6E's. Of course that was full military funding to ensure it really worked. I'm not sure what a general aviation equivalent would cost, I think in the 1990's I priced them in the sub-$3000 range.

    One concern I would have is how well the device works over water, especially glass smooth water. We used our rad-alt flying on off the carriers and I don't remember any limitation there but I remember there were some times we didn't trust our other radar for ranging when pointed at the water. I think this is another case of being able to test and fully understand the technology you are using before you bet your life in it.

    My vote for you would be to use an existing general aviation rad-alt. Then at least you would have the confidence of good testing by the manufacturer so you would know what to expect over land or water. I do think it would be VERY useful for a float plane going in and out of various lakes and ponds to know their relative height above ground.

    GPS would be a very good second given you always know the actual altitude of the lake you were landing on. Are they all charted for surface altitude above sea level?

    How do the old pro's do it in bush-flying country? My bet is purely visual flying?

    As far as learning about electronics, how they work and how to assemble: Yahoo Group electronics_101 Electronics_101 : Electronics Learning Group

    They are a good bunch, all the way from vacuum tube guys to fully digital, they've helped me with a bunch of projects over the years.

    Later, Dan Nicoson

  7. #22
    Registered User Lucrum's Avatar
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    Re: Build a simple radar altimeter

    Quote Originally Posted by Vigilant1 View Post
    I'm not sure of your application, but it's hard to see the utility of a project like this...
    Not trying to be a party pooper but I was wondering the same thing. I've done literally hundreds of approaches in IMC and to be honest I rarely if ever include the radar altimeter in my scan.

  8. #23
    Registered User Empirical's Avatar
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    Re: Build a simple radar altimeter

    Quote Originally Posted by Vigilant1 View Post
    Anyway, at an altitude of 100 feet and a speed of 50 Kts, the plane will have moved about 18 feet between the time of sending and receiving the pulse.
    Actually, it would be 15 feet (14.9787 pointed straight down, 15.0209 at optimum angle).

    Quote Originally Posted by Vigilant1 View Post
    The problems would decrease as one descended.
    Only the straight-down approach will have such problems, since the higher the altitude, the higher the distance that the reflection will appear behind the receiver.

    Quote Originally Posted by Vigilant1 View Post
    Yes, separate "aimed" transmit and receive horns would help at greater distances, but could prove problematic when closer to the water.
    This is not true, since the required optimum angle is only depended on the cruising speed NOT the altitude, because the reflection time decreases proportionally to the altitude. So if you optimize it for a given speed, both the transmitter and the receiver will always point at the reflection point on the ground at any altitude:

    altitude:
    h = 100ft * 0.3048 = 30.48 meters

    aircraft speed:
    a = 50 knots * 1.85200 = 92.6kmh / 3.6 = 25.7222 meters/sec

    the speed of sound at 20 degrees Celsius
    b = 331.45 + (0.6*20) = 343.45 meters/sec

    Since both the aircraft and the sound wave will cover the same distance horizontally until the moment (t) the sound is reflected on the ground (while the sound wave will travel faster at an angle), a right angled triangle is formed with the hypotenuse (b) being the optimum path for the sound signal and the opposite (a) side being the horizontal distance. Their speed analogy will be equal to their distance analogy. So to find the (hypothetical) speed analogy for the adjacent side of the triangle (the altitude):
    c = sqrt ( b^2 - a^2) = 342.485 meters/sec

    Therefore the distance covered by the aircraft until the signal is reflected will be:
    d = (a * h) / c = 2.28919 meters * 3.2808399 = 7.51046 feet

    the lag time:
    t = d / a = 0.0889965 seconds

    the optimum angle (from the vertical axis) will be:
    angle = arctan(a / c) * (180 / pi) = 4.29511 degrees

    the total delay between transmit and receive = 2 * t = 0.177993 seconds

    and the total distance the aircraft will cover = 2 * d = 4.578 meters * 3.2808399 = 15.0209 feet.

    Quote Originally Posted by dannicoson View Post
    My vote for you would be to use an existing general aviation rad-alt. Then at least you would have the confidence of good testing by the manufacturer so you would know what to expect over land or water.
    +1. There are many other safer areas to try and learn electronics than flight instruments -except if you just use it as a secondary experimental tool.

    @Lucrum
    Personally I found this thread interesting because it dealt with the choice and differences between radar and ultrasonic as possible flight instruments.




    EDIT: corrected the angle calculation -sorry!
    Last edited by Empirical; May 9th, 2011 at 01:34 PM.
    A wonderful harmony arises from joining together the seemingly unconnected.
    - Heraclitus (Greek Philosopher 535-475 B.C )

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    Re: Build a simple radar altimeter

    Quote Originally Posted by Lucrum View Post
    Not trying to be a party pooper but I was wondering the same thing. I've done literally hundreds of approaches in IMC and to be honest I rarely if ever include the radar altimeter in my scan.
    It's for future use on glassy water landings (my plane is not on floats yet). I'm currently very inexperienced (no float rating yet) and even though there seems to be a very good technique of landing in glassy, I wanted to use this as a aid to get myself confident. I haven't done much research about it more than finding that build-it-yourself radar altimeter on the Internet and posting a question about it here. For the utility: If I can reduce my chances of destroying my 100k airplane I think it's worth it, especially until I get more experience on the water. It's just an aid, nothing more.

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    Re: Build a simple radar altimeter

    By the way, the price for the "low cost" Roke MRA Roke | Miniature Radar Altimeters is 10,551.00 (about $17,200.00). I guess when they stated "Low Cost", it was the cost for them to manufacture it in China, not the cost (price) for the customer to purchase it

  11. #26
    Registered User Lucrum's Avatar
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    Re: Build a simple radar altimeter

    Quote Originally Posted by XAviator View Post
    It's for future use on glassy water landings (my plane is not on floats yet). I'm currently very inexperienced (no float rating yet) and even though there seems to be a very good technique of landing in glassy, I wanted to use this as a aid to get myself confident. I haven't done much research about it more than finding that build-it-yourself radar altimeter on the Internet and posting a question about it here. For the utility: If I can reduce my chances of destroying my 100k airplane I think it's worth it, especially until I get more experience on the water. It's just an aid, nothing more.
    Understood, like I said I wasn't necesarily trying to put down your interest in it just mentioning my own experience with them.

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    Re: Build a simple radar altimeter

    Quote Originally Posted by Lucrum View Post
    Understood, like I said I wasn't necesarily trying to put down your interest in it just mentioning my own experience with them.
    No worries. I thought it was better to let ppl know what my intention are, it wasn't really clear from my previous postings.

  13. #28
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    Re: Build a simple radar altimeter

    Did you try the accoustic measuring tape out? Also, this is interesting... I am an Amateur Extra and built alotta stuff like this. Sure, it's not easy, but not impossible at all. Landing Radio AltimeterLanding Radio Altimeter or you could kludge together a system for cheap from here LOL: radar altimeter | eBay

    Might also learn more her:
    IEEE Xplore - A Landing Radio Altimeter for Small Aircraft

    And at least if you buy a fancy radalt, don't speand almost 20K, spend just over 10K, here <wink>... FreeFlight Systems Radar Altimeters and Indicators

    There are folks also making good radalt type systems with bistatic GPS - google it.

    Best of Luck!
    Paul, Flagstaff AZ, Waco ZKS-7 N50662, ARS NT3L, PhD, 1310, CDR, USN.Ret

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