The visually opaque clouds of Venus still pass radio and some microwaves, and so powerful radar signals can be used to map the surface. This is probably done using some kind of delay-Doppler technique, the Arecibo is big but it's not going to resolve features this small, and even if it could (which it can't), building a map pixel-by-pixel would not be a reasonable use of time.
So I would assume the gap is somehow related to the delay-Doppler geometry, but I can't figure out how.
The caption for the color map shown below from the National Air and Space Museum says:
A radar map of Venus collected by CEPS scientists. This shows most of one hemisphere of the planet, with a thin band near the center for which the mapping technique cannot produce an image. Dark areas are smooth or covered by fine material from impact craters, while bright areas are very rough. These maps are being used to study geologic processes beneath the dense atmosphere, and to search for changes in the surface over time. (emphasis added)
Why can the mapping technique not form an image for this slice?
The monochrome image is from the Lights in the Dark item Ground-Based Radar Reveals the Surface of Venus and carries the following caption:
Radar map of Venus’ surface made from signals sent from Puerto Rico and received in West Virginia (Credits: B. Campbell, Smithsonian, et al., NRAO/AUI/NSF, Arecibo)
The last monochrome image comparing two nearly-identical images from 1988 and 2012 show the band in the same place for both observations. I can't figure out why this is so.
I'm looking for a technical explanation, not just some guesses, or "it's related to..." type answers. Thanks!
above: from here
above: from here
above: from here
above: Some geographical reference points on Venus, found here.