# Build a simple radar altimeter

Discussion in 'Instruments / Avionics / Electrical System' started by XAviator, May 5, 2011.

1. May 9, 2011

### dannicoson

#### Member

Joined:
Feb 27, 2003
Messages:
15
0
Location:
Round Rock, TX
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 2. May 9, 2011 ### Lucrum ### Lucrum #### Well-Known Member Joined: Jun 10, 2008 Messages: 956 Likes Received: 189 Location: Canton, GA 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. 3. May 9, 2011 ### Empirical ### Empirical #### Guest Actually, it would be 15 feet (14.9787 pointed straight down, 15.0209 at optimum angle). 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. 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. +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 a moderator: May 9, 2011 4. May 10, 2011 ### XAviator ### XAviator #### Member Joined: May 1, 2011 Messages: 13 Likes Received: 0 Location: CT 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. 5. May 10, 2011 ### XAviator ### XAviator #### Member Joined: May 1, 2011 Messages: 13 Likes Received: 0 Location: CT 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

6. May 10, 2011

### Lucrum

#### Well-Known Member

Joined:
Jun 10, 2008
Messages:
956
189
Location:
Canton, GA
Understood, like I said I wasn't necesarily trying to put down your interest in it just mentioning my own experience with them.

7. May 10, 2011

### XAviator

#### Member

Joined:
May 1, 2011
Messages:
13
0
Location:
CT
No worries. I thought it was better to let ppl know what my intention are, it wasn't really clear from my previous postings.

8. Dec 9, 2011

### Paul Shankland

#### New Member

Joined:
Dec 9, 2011
Messages:
2
1
Location:
Flagstaff, AZ
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