GAIA is an amazing pair of space telescopes that share a single Gigapixel CCD array, with a complex optical system capable of repeated, extremely precise position, brightness, parallax, proper motion, and other measurement of millions of objects. It operates by scanning continuously via a very smooth, steady rotation of the entire spacecraft.
GAIA is located near the Sun-Earth L2 position, about 1.5 million kilometers from Earth. It generates a huge amount of data even after substantial processing by the spacecraft. This results in a data stream of between several and 8 MB/s for 8 hours every day - a period chosen for best uninterrupted reception by a single DSN station I assume.
To produce a strong enough signal on Earth, GAIA uses a "medium gain" antenna, which means it has significant directionality (directivity). The antenna must dynamically point towards Earth while the spacecraft continues to rotate, in order to preserve the data collection quality and quantity. However, since the level of precision necessary and vibration and microphonics minimization precludes a mechanically rotating mechanism for the antenna, GAIA uses a complex phased array antenna to electronically "point" the beam in Earth's general direction continuously.
So far I haven't been able to find any details of how the phased array is configured mechanically, or electronically. Thus:
Question: "How is GAIA's phased array configured, mechanically and electrically?"
In the ESA's GAIA update #04: Gaia Phased Array Antenna Delivered and Integrated it says:
The Radiating Cone is a 14-sided truncated pyramid; its primary structure is machined from solid aluminium alloy. Each facet has two sub-arrays, each comprised of six radiating elements; each sub-array has a series of RF couplers that split the incoming signal to provide the amplitude weighting that determines the radiation pattern of the sub-array. The overall antenna radiation pattern is obtained by combining the radiation patterns from the 14 sub-arrays on the Earth-pointing side of the radiating cone.
The 28 antenna sub-arrays are fed by seven 'Quadri-Modules', which provide low-level gain, phase and amplitude control, power amplification and routing of signals to the appropriate sub-arrays. The Quadri-Modules are, in turn, fed by a 2:7-way splitter that allows the spacecraft's nominal or cold-redundant transponder to feed the PAA.
I think that this is referring to the truncated pyramid in the center here, which is smaller than I expected. Also, although it is hard to tell for sure, it looks like the pyramid is quite steep.
above: GAIA Antenna Support Panel on Ground Handling Trolley. "The Gaia Antenna Support Panel on its ground handling trolley, showing the Sun-pointing face; the Radiating Cone has two sub-arrays of six radiating elements on each of its 14 facets."
above: Rear Face of GAIA Antenna Support Pannel. "The Gaia Antenna Support Panel on its ground handling trolley, showing the rear face and electronics modules. Of the boxes arranged in a circle around the rear of the Radiating Cone (centre of panel), the Electronic Power and Interface Controller (EPIC) is positioned at about seven-o-clock. The remaining boxes are the seven 'Quadri-Modules', which provide low-level gain, phase and amplitude control, power amplification and routing of signals to the appropriate sub-arrays. The 2:7-way splitter that allows the spacecraft's nominal or cold-redundant transponder to feed the Quadri-Modules is at the upper right"