Why do radar maps of the surface of Venus have missing slices?

Question

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.

Answer 1

These maps were produced with doppler-delay imaging, which uses a combination of range and motion to identify the radar return from each specific point on the surface and build a map.

Longitude is determined from doppler shift - as Venus rotates, one side of the planet is moving towards Earth while the other is moving away.

Latitude is calculated from the time taken for the signal to return - the equator is closer to Earth than the poles.

This method has two limitations that prevent imaging the doppler equator.

• Equivalent North and South latitudes are the same distance from Earth, and cannot be distinguished if the beam is aimed at the equator. This can be mitigated by aiming the beam at the northern hemisphere so that the return from the south is much weaker, but near the equator the ambiguous points are too close together to distinguish that way.

• Latitude resolution is much lower near the equator. Near the poles, the view is edge on and two close points will have a significantly different distance from Earth. The equator appears flat and two points must be a lot further apart to show a measurably different range.