This series of delay-doppler radar images of the asteroid 4769 Castalia (also known as 1989 PB) were taken at approximately 2 minute intervals on 1989 August 22 at the Arecibo Observatory. The horizontal axis is Doppler frequency, and the vertical axis is the time the echo was received ("delay"). These two scales map differently onto distance, which has been approximately compensated by replicating the pixels in the vertical dimension, yielding a resolution of approximately 150 m (horizontal) by 300 m (vertical).
The images are fainter at the beginning and the end of the
sequence because the antenna gain was a fairly strong function
of zenith angle when these images were taken. The published
results (Ostro et al., 1990, Science 248:1523)
have corrected for this factor. These images have been
stretched to look decent on a browser, rather than for
Each image above is the sum of 27 separately reduced (Fourier transformed) data sets, or "looks". With some limitations, these looks can be traded off between resolution and signal-to-noise ratio. The image here shows this effect for four of the images above. The left image in each pair is as above. The sampling frequency was 16 ms, and there are 64 complex samples in each look, giving a resolution of 0.95 Hz (~150 m), and allowing 27 looks to be summed (the total time of each observation is about 30 s). On the right, the same data were re-transformed using 256 samples in each look, followed by 256 zeroes. Thus these images have a frequency resolution of 0.24 Hz (~40 m), and a frequency sampling of 0.12 Hz, but the signal to noise ratio should be 2 times worse, as only 6 looks can be summed. Only the frequency resolution can be adjusted in this way, so the delay resolution is unchanged. Just for grins, you can see this matrix as an MPEG animation (a measly 72 kB).
Steve Ostro and Keith Rosema have further information on 4769 Castalia and other asteroids including 4179 Toutatis and 1620 Geographos in their Asteroid Radar Research pages.
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Last modified 1996 May 15