Arecibo Radar Images of Asteroid
Radar image of 4769 Castalia
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
accuracy.
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.
Return to NAIC Home
Mike Nolan
Last modified 1996 May 15
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).