FFT - how to increment in time?

I'm working on some research with a professor, and we're looking at data collected by an x-band radar array looking at ocean waves as they approach the coast (the radar is on land, and we can see about 3 miles out).

What we're trying to do is perform an fft on the signal using Matlab, and then manually increment the fft in time to see if we can inverse it and get back an image similar to one taken by the radar at a different time.

We gave it a shot but the inverse transform quickly went to garbage as our delta_t's got bigger. Small delta_t's gave back an image that looked more like the original image but everything was sortof blurred.

I can provide some of the code, but does anyone have any general advice on how to do this or know of any resources/examples to check out?

Thanks!

[sixwwwwww]

Can you explain what do you mean by increase/decrease in delta_t? Do you mean scaling in time?

One comment beside this is that the expansion in one domain(e.g. time domain) results in compression in other domain(frequency) domain and vice versa.

[Image Analyst]

Some screenshots would help. Use the image icon to attach one. Use snag.gy website to help you paste in your screenshot after you've hit alt-printscreen to save the current window into the clipboard.

[Walter Roberson]

I suspect they mean to use the fft to project what the image would have looked like at a different time.

When one speaks of "image" one is normally referring to space rather than time; for example an audio signal is not represented by an image (except maybe a spectrograph). I do not know what steps are taken to convert the signal of an radar to an image. Known angle for the signal? Buffer each location for a period, perhaps brightness proportional to recentness ?

Answer 1

[Youssef Khmou]

Bruce,

You have to clarify "increment FFT in time", you mean in frequency? you mean you want to predict the spectrum at t+dt? then you need to know the velocity of propagation of that wave means the amplitude and phase of that collected signal,

did you try the translation property f(t+dt)-->F(f)*exp(j*f*dt) ?