# Question about Sonic Doppler Shift Speed Measurement

1. Oct 25, 2007

### sll914

BACKGROND:

Myself and a number of others are doing a thing called Dynamic Soaring with our special purpose built radio controlled gliders. We are getting them going very fast; in the range of 300-320mph. We have verified a top speed of 309mph using radar guns but that is the highest that they will read. Hence we are looking for different methods to measure the speeds that will go as high as 350-400mph.

Currently, we are trying to research if we can accurately measure the speed of our radio controlled gliders using the doppler shift of sound waves emitted from a fixed frequency horn mounted on the plane. I know some groups are using this method with success but our scenario has one twist.

We fly in strong winds (60-80mph) and where we measure the speed of the plane, the air masses between the plane and the groundbased microphone will vary from +/- 80mph. For instance, the plane may he in a 80mph head wind and the microphone will be in zero wind.

Heres a video and informative thread if you're interested:

http://www.thud.us/videos/rc/socal-ds-2007/norco-100in-kinetic309mph.avi

HERES THE QUESTION:

As to using doppler shifted sound waves to measure the speed of a moving object --

Since wound waves use the air as a medium to travel, will the perccieved doppler shift be affected by movement of the medium? For instance, if the plane is travelling at 350mph directly towards the groundbased microphone and the air is still, we could deifnitely measure the amount of doppler shift and show that the plane is going 350mph. If the plane, however is travelling with a ground speed of 350mph, and is flying in a 50mph headwind, and that head wind is present all the way to the microphone, will the doppler shift indicate a groundspeed of 350 or 300mph? Now would it make any difference if the plane is in a 50mph headwind but there is no wind at the microphone? How about total turbulence (+80mph/ -80mph) between the two?

I can make a case for either answer but can't conclusively say which is correct.

I could say that since the soundaves are travelling 'upstream', that a grounbased receiver will see fewer peaks per second, so that the perceived doppler shifted frequency is lower. BUT -- I can imagine that as the sound waves are emitted into the 50mph headwind, that the peaks will be closer together than they should be. Therefore, when they hit the microphone, they WILL be travelling slower relative to the ground than they normally would be but the peaks will be spaced closer together than the frequency normally would be resulting in a net of no change in frequency...

Intuitively, I've never noticed a pitch change when people are shouting in the wind, so that supports the case of no effect by movement of the medium.

Thanks for any helpful insights! Not to be picky or anything, but I've heard a few different opinions regarding this question. If you don't mind, please indicate the certainty level of your opinion. Also, it would be helpful to know if you have the background or credentials to be certain or if you're a mildly educated speculator like myself )

Thanks,
Spencer

Last edited: Oct 25, 2007
2. Oct 25, 2007

### Staff: Mentor

I don't think sound is a good match for an accurate measurement of your glider's velocity. As you say, the air is moving due to wind and upcurrents and what-not, so that automatically compromises the measurement.

Just extend the range of the radar, or build your own using ISM (2.4GHz) band components. Stick with RF for the speed measurements -- that would be my recommendaion.

3. Oct 25, 2007

### Staff: Mentor

BTW, many of us have an indicator of our "credentials" in our Pubilc Profiles. Just left-click on a user name, and click on Public Profile.