Doppler Radar Accuracy

I'm not sure what you're asking. Are you asking at what speeds the low speed approximation is no longer valid? Or are you asking something about the doppler radar. Considering your background, I assume it's the later. So what is it about the doppler radar that you want to know?

Hey Roger, my apologies if I stated the question poorly. I would like to know how accurate modern Doppler radars are, i.e., accuracy in terms of the % of measured target radial velocity.
(whatever that may mean :-)

I've looked at some websites and read up on the subject but can't seem to find and definitive equation for the error in the detected velocity. All I can find is qualitative statements like "distance from the station and ground obstructions limit the accuracy of the radar".

I'm curious, does doppler radar detect velocity by frequency shift of the light? Or is it some sort of phase shift effect?

I'm pretty sure radar Doppler implies frequency shift for radial velocity. Radar distance measurement is, in way, a phase shift effect, because it is a time delay measurement.

I'm an idiot. I've been hearing the term Doppler Radar for so long It didn't even occur to me that it uses doppler shift, which it must. I've just thought Doppler Radar=Weather. I see what your asking now. Doppler radar uses doppler shift of em-waves to measure the velocity of rain. I would think it can only measure the horizontal velocity of the rain, but I suppose if different stations were at different altitudes, you could find the vertical component as well. Just thinking out loud here.

So what kind of error is involved. I'm sure the fact that rain is liquid must cause a little havoc. I'll look around when I have some time and see what I can find.

Yep, I think measuring rain with radar will perhaps have a statistical averaging effect. In the case of a aircraft or spacecraft, the scattering of return pulses will probably be much less.

I'm not a radar guy, so I have no feeling for radar accuracy. Hope you, me or someone else find something!

Jorrie, Can you recommend any good physics forums? I have a question regarding quantum mechanics, specifically why Heisenberg assumed gaussian profiles for position and momentum error to determine the heisenberg uncertainty principle.

Hi Roger, my favorite forum in physics is: PhysicsForums (what else?)

They are quite on the ball, active and have categories for lots of things, amongst others, also quantum physics. I am not frequenting any other science forum at the moment.

Thanks, Nice Forum, I'm joined up.

I found some information regarding your question (but not an answer).

I think what you're looking for will have something to do with the "Nyquist Velocity". If you search the phrase you should find some stuff.

Also, I've found some specs on radar systems, see link below:

http://www.gim.bnl.gov/armclouds/mmcr/MMCR_Observa tional_Parameters.html

Jorrie, I tried the forum. Absolutely terrible. Not only did I not get an anwer to my question, I was lectured on things I learned as an undergrad. I'm gonna look for something else. Thanks anyway.

Roger, yep, this can sometimes happen. I found that if you're new there, you have to state questions very clearly and indicate what you know. That way, you may get some very valuable responses.

Anyway, thanks for the radar links - will look over the weekend - very busy with launch of my eBook!

There is well known uncertainty Principle that gives you idea of time and space resolution. It is directly related to Planks constant. Velocity has two parameters and hence will always have greater uncertainty than any one of them.

You also have to look at the frame of reference. Remember you can never increase the velocity of light beyong a limit so Doppler principle does not hold good for very high velocities. It is due to frame of reference changes and limits of measurement or understanding of reality or whatever you call it.

Shyam

Quote: Shyam (1892) wrote: "There is well known uncertainty Principle that gives you idea of time and space resolution. It is directly related to Planks constant. Velocity has two parameters and hence will always have greater uncertainty than any one of them."

Thanks for the comments, but this is not my problem. The question is, is there a simple velocity accuracy spec for Doppler radars? At (say) 10 km range and a radial target speed of (say) 10 km/s, what is the velocity measurement accuracy, %-wise?

I have used SODAR Doppler RADAR.

These can measure wind speed or air velocity in 3D with +/-1km/hour speed for 1km/h to 200km/h range using 2kHz sound wave propogation up to 10kM distance. More distance you require more power in the microphone that generates mono-frequency sound using piezoelectric speakers. Cleaner is the sound better is the accuracy of measurement. Fourier Transforms used to get doppler shifted frequency components. You also do some averaging on several measurements.

These SODAR radars are 3 receivers placed at about 50m in a triangle to get all three X-Y-Z components of the velocity. Radars are often seen at airports.

Shyam

Have a look at

http://www.remtechinc.com/sodar.htm

www.sodar.com/about_sodar.htm

This field is still developing. Most aeroplaces use similar devices to watch out for storms ahead of them. I don't think they get time to change direction, but knowledge can let you put seat belts in order.

Shyam

Thanks Shyam: very usefull! So the SODARs manage about +-0.5% accuracy at maximum velocity of 200 km/h.

I understand one cannot just scale the velocity range up due to 'velocity bins', velocity ambiguity, etc. But I guess if a Doppler RADAR (RF, nor sound) is specifically designed for very high relative target velocities, similar sort of accuracies may be achieved. I'll make a preliminary calculation for the speed at which the measurement error equals to the low speed approximation error and post it on this forum.

Perhaps you can try LIDAR technology, which we use for monitoring chemical contents of clouds from ground using laser beam absorption by chemicals in the cloud and subsequent fluorescence emission spectroscopy. You can actually monitor Doppler shift in laser wavelength due to velocity changes. Remember that there will be other shifts due to particle scattering as you see in rainbow.

RF scattering from clouds is very unlikely and need very powerful sea cost radar for tracking cyclones etc. You can measure velocity of the entire cloud or cyclonic eye. This is a very powerful tool.

Sensing from sky is done using several wavelengths in visible and IR spectra. Remote sensing work is in such areas.

Shyam

Interesting! I could not dig up any speed accuracy figures for LIDAR. It is apparently pretty expensive to extract the LIDAR's red- and blue shifts in real time - one needs optical interferometry.

Of interest to me is the fact that distance can be measured in real time and with millimeter accuracies. So one can get accurate real time speed by simply differentiating distance. This is apparently how Police LIDARs for speed infringements work.

You can easily measure 0.1nm change in wavelength with holographic grating and CCD detector. This can give remarkable accuracy. However, temperature dependence of molecular velocities can cause error. Rayleigh scattering often gives lots of problem.

Simple method of injecting visible trace of gas through rocket engine has been used to monitor air velocity pattern by looking at the drift of the trace of the rocket exhaust gases. Balloon drift is also used in similar way.

Shyam

Best of luck with your ebook. I found it interesting and informative so I hope it catches on. I think your advice on the forum is probably right.