I have been following the news about the launch of the PAZ satellite (a SAR imaging sensor). The orbit will be sun-synchronous. I know this is important for optical sensors to make sure they have the same illumination conditions. My doubt is why is it relevant for an active sensor? I have heard the same explanation (illumination conditions) but since it is SAR satellite, shouldn't it be independent of that?
While I don't actually design / choose orbits for SAR satellites, my understanding is as follows.
Fundamentally, it is a question of meeting a set of mission parameters. Parameters such as global coverage with predictable revisit times, sufficient GPS coverage, and data downlink opportunities. By piggy bagging on existing infrastructure, the cost is reduced and much of the existing infrastructure is designed to support optical satellites that do require sunlight.
As such, the existing physical systems point towards a sun synchroneous orbit.
Furthermore, I'll challenge your assumption that the received signal wouldn't be influenced by being acquired at different points during the day. A central element in remote sensing is to look at changes over time, and for that you generally want to minimize the number of variables that change between two given acquisitions, in order to isolate the signal from changes from the signal from variance. One way of doing that is to acquire the image with similar illumination and view angles. The illumination will matter, even for SAR images, as illumination will impact other physical parameters, such as moisture (both in the air and on the surface) and plant "behavior" (think sunflowers and those plants that furl up their leafs at night). These indirect physical changes that respond to illumination will be reflected in the SAR signal, and a sun-synchronous orbit will help minimize impact from such things.
Also, a SAR instrument needs a lot of electricity since it is an active sensor and therefore a sun-synchronous orbit keeps the instrument's solar panels well illuminated, therefore allow plenty of electricity for the instrument.