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Sonar over water

nilsen

Senior Member
#1
Hi everyone,

I added and Maxtronic I2C sonar module to the Pixhawk on the Discovery pro so that I can do terrain following in Altitude hold mode and it works very well.

I was wondering if anyone has tried sonar and then have tried it over water?

I would imagine that flowing water would be challenging but I wonder how it would handle over stillish water such as a lake.

thanks!

Nils
 

jipp

Senior Member
#2
i would think it would have 50.50 results, depending if the water was calm or choppy.. as id imagine the beem could bounce back anywhere in choppy water? i too bought a sonar module for my spring project.. glad to hear it works good over land, as i have no water here in the desert.. :)

chris.
 

cranialrectosis

Faster than a speeding faceplant!
Mentor
#3
Hi everyone,

I added and Maxtronic I2C sonar module to the Pixhawk on the Discovery pro so that I can do terrain following in Altitude hold mode and it works very well.

I was wondering if anyone has tried sonar and then have tried it over water?

I would imagine that flowing water would be challenging but I wonder how it would handle over stillish water such as a lake.

thanks!

Nils
Sounds like a good experiment and maybe a Flitetest article!

Man I wish we had a popcorn emoticon here. :)
 

makattack

Winter is coming
Moderator
Mentor
#4
I haven't played with sonar, but in theory, it should work ok and shouldn't be affected by water. It's measuring the echo return of a audio ping right? Sub-human-detectable audio? As long as it's just designed to generate a range value, it seems like flowing, still, or choppy water should produce similar results so long as the detector can detect the return signal without excess noise. Again I have no practical experience with them.

On that note, it seems it would be easy enough to test this out without risking equipment. Just fill a bath tub up with water, and hold the sonar unit above it to read values from either your ground station or even just hooking the sonar to an arduino and dumping the raw values to a console display.
 

makattack

Winter is coming
Moderator
Mentor
#6
Hmmm... technically, that sonar is sort of like a fish finder... only that model's designed to work in air... not water...
 

jipp

Senior Member
#7
im not sure why i was thinking it was a lazer beam when its sound.. looking at my sonar module.. doh, im a idiot.
i would think it would be ok over water.
chris.
 

makattack

Winter is coming
Moderator
Mentor
#8
im not sure why i was thinking it was a lazer beam when its sound.. looking at my sonar module.. doh, im a idiot.
i would think it would be ok over water.
chris.
Ah, you might be thinking of a LIDAR device, which is like a low power laser... or basically, an optical mouse...
 

jhitesma

Some guy in the desert
Mentor
#10
Doesn't sonar penetrate water? Isn't that why it's used for fish finders, in submarines and other aquatic applications? Or is the density change from air to water enough for it to register.

Doesn't that sensor only have a 7m range? That seems kind of low for terrain following...but I admit I haven't looked into sonar too much.
 

cranialrectosis

Faster than a speeding faceplant!
Mentor
#11
SONAR works for bats so I assume it would work over water or bats would splash down.

SONAR works better in water than in air specifically due to the density of water. Sound travels better the denser the medium.

RADAR uses radio waves (often microwaves).

The key I think is to match the length of the wave to the density of the medium; longer waves for denser medium.

I think SONAR would reflect off the surface of water and present as a solid until you are in the water at which point the 7m range may increase tremendously. But that's just me speculatin'. :)

I think the reason we discuss SONAR instead of RADAR is the FCC would pitch a fit if we started blasting microwaves around. :)
 
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makattack

Winter is coming
Moderator
Mentor
#12
Doesn't sonar penetrate water? Isn't that why it's used for fish finders, in submarines and other aquatic applications? Or is the density change from air to water enough for it to register.

Doesn't that sensor only have a 7m range? That seems kind of low for terrain following...but I admit I haven't looked into sonar too much.
Yeah, most applications use lidar for precision positioning, and sonar for obstacle avoidance, but I can see sonar helping with altitude precision when coupled with a barometer.
 

Tench745

Well-known member
#13
I would imagine that the water has enough difference in sonic density that it would reflect just as well as land.
I know on my boat if you get over tall weeds or really soft mud the sonar gets confused. Mud reads deeper because it's full of water, plants read shallower because they're floating closer to the surface. I have a hard time believing you'd get these same sorts of returns from an air-based sonar.
 

nilsen

Senior Member
#14
I haven't played with sonar, but in theory, it should work ok and shouldn't be affected by water. It's measuring the echo return of a audio ping right? Sub-human-detectable audio? As long as it's just designed to generate a range value, it seems like flowing, still, or choppy water should produce similar results so long as the detector can detect the return signal without excess noise. Again I have no practical experience with them.

On that note, it seems it would be easy enough to test this out without risking equipment. Just fill a bath tub up with water, and hold the sonar unit above it to read values from either your ground station or even just hooking the sonar to an arduino and dumping the raw values to a console display.
Over solid objects like tables and floors, and even grass it is spot on, it emits a barely audible click at about 10 Hz and has a low range of 19cm.

I'm wondering if the sound won't be absorbed by the water, I also think that flowing water would cause the sound waves to not return "up" but to be scattered thus affecting the accuracy, maybe it will think it's higer up.

Luckily in Mission Planner you can see the current sonar reading.

Guess it's time to expreiment.
 

nilsen

Senior Member
#15
Doesn't sonar penetrate water? Isn't that why it's used for fish finders, in submarines and other aquatic applications? Or is the density change from air to water enough for it to register.

Doesn't that sensor only have a 7m range? That seems kind of low for terrain following...but I admit I haven't looked into sonar too much.
I read the technical specs on the maxtronic website, the sonar is affected by the prop wash and noise from the props/motors as noise which causes fluctuations so I'm not entirely sure how it would handle going from land to water, that transition would be interesting. I'll wrap my Disco in foam and try it :)

It does only have a 7m range which is configurable in APM, thereafter it switches to using the barometer.
As for terrain following, I meant close proximity to the ground, following motorbikes on the road, low to the ground. 30cm low to the ground :)
 

nilsen

Senior Member
#16
I would imagine that the water has enough difference in sonic density that it would reflect just as well as land.
I know on my boat if you get over tall weeds or really soft mud the sonar gets confused. Mud reads deeper because it's full of water, plants read shallower because they're floating closer to the surface. I have a hard time believing you'd get these same sorts of returns from an air-based sonar.
I'm so surprised by the accuracy while flying, I do wonder how well it will do when flying forward as the sound waves wouldn't return stright up but at an angle, maybe I should actually mound the module at an angle... hmmm.
 

jhitesma

Some guy in the desert
Mentor
#18
It does only have a 7m range which is configurable in APM, thereafter it switches to using the barometer.
As for terrain following, I meant close proximity to the ground, following motorbikes on the road, low to the ground. 30cm low to the ground :)
Yeow! That is low! Braver than me to trust a sensor to fly that low :D

Can't you just enable the sensor for logging data and not control and fly some test flights over water transitions to see how it does?
 

jipp

Senior Member
#20
very cool. :) i look forward to chasing rabbits with a AP machine. fear you wrascialy wrabbits the tarot err i mean carrot will chase you for the best shot.

so many rabbi