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.

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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."

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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"


There are some papers about antenna design on Gaia, and this one by EADS CASA Espacio has many details on GAIA Antenna:

http://ieeexplore.ieee.org/document/5505027/ Allica, J. C., Alonso, E., Amado, M., Bazán, A., Casares, F., de Witte, E., ... & Serrano, J. L. (2010, April). "Architecture of GAIA satellite phased array antenna" - 2010 Proceedings of the Fourth European Conference on Antennas and Propagation (EuCAP), 12-16 April 2010, pages 4056-4059 (https://scholar.google.com/scholar?cluster=4948320994472611099)

It is behind IEEE paywall, and I have only first page preview. But there is very similar paper "GAIA SATELLITE PHASE ARRAY ANTENNA" from same authors presented probably at some ESA conference later in 2010.

Main requirements for Antenna design: 8465 MHz center frequency, 10 MHz bandwidth, EIRP (Equivalent Isotropically Radiated Power) of +34 dBW, power on antenna input: +33 dBm. Required coverage: 360 degrees of azimuth and 45 +- 15 degrees of elevation with antenna scanning speed of "1deg/min in azimuth and 1.5deg/day in elevation". PAA subsystem has two RF inputs (one per transponder) and 7 outputs ("2:7 Input Splitter" made with microstrips). 7 signals are connected with coaxial cables to 7 amplifiers/beamformers: GAIA PAA subsystem block scheme by EADS CASA Espacio, 2010

Each amplifier/beamformers has 4 units of "QUAM SSPA" (The Quadrimodule) with total 4 outputs (28 signals from 7 amplifier/beamformers). Their outputs are connected to subarrays of antenna grouped by two into facets, each facet has two "arrays of six circular disk patch antennas". With 14 facets the PAA Radiating core is built (4 real facets are visible, other are mass dummies): figure 2: EM PAA radiating cone by EADS CASA Espacio, 2010

With requirement of 45 +- 15 degrees elevation and this photo of test cap installation https://i.stack.imgur.com/Ek1zw.png from the paper we may assume that facets are fixed at angle of 45 degrees to the honeycomb panel visible in your photos.

Sizes of antenna are listed in http://emits.sso.esa.int/emits-doc/ASTRIUMLIM/GAIA_Phased_Array_Antenna/GAIA.ASU.SP.ESM.00002.01.00.PDF "X Band Phased Array Antenna - PAA- Requirements Specification GAIA.ASU.SP.ESM.00002", EADS Astrium 2006 ..for external units of PAA assembly:

shall be included inside a cylinder of 0.40 m diameter (TBC) and 0.45 m height [TBC], of which the LGA shall be assumed to be 0.20m (TBC), centred on the PAA panel axis. ... PAA external units shall be compatible with installation of a low gain antenna (mass = 0.5 kg) and associated RF harness and/or waveguides feeding through the PAA panel, with more than 2*Pi steradian field of view

There is also the drawings - "Figure 1.4-1: Side View of Gaia Service Module (SVM) with PAA Panel" and "Figure 1.4-2: Possible Configuration of PAA Assembly on Antenna Panel (side view)" at page 8 and 9 of http://emits.sso.esa.int/emits-doc/ASTRIUMLIM/GAIA_Phased_Array_Antenna/GAIA.ASU.SP.ESM.00002.01.00.PDF

You may also check some info about "Semi-active Conformal Arrays" ("Concept is adopted for ESA science satellite GAIA"; ESA patent?) from http://data.cedupoint.cz/oppa_Minerva/Roederer_A0M17NKA_prezentace.pdf "The Design and Construction of Antennas" by Dr. Antoine Roederer, Deft TU (which has great overview of beam antennas for satellites) page 33 and below; and GAIA antenna on page 38:

  • 28 linear sub-arrays of 6 radiating elements
  • 14 hybrid couplers
  • Phase-shifters at low level steer the power around the array

Several other papers mention main parameters of antenna array too:

http://emits.sso.esa.int/emits-doc/ASTRIUMLIM/GAIA_Phased_Array_Antenna/GAIA.ASU.SP.ESM.00002.01.00.PDF "X Band Phased Array Antenna - PAA- Requirements Specification" 2006

The PAA allows for a Beam Pointing capability of 360°in Azimuth and 45°±15°in Elevation

The antenna shall provide a coverage of at least

  • • 45°± 15° in Elevation
  • • 360° in Azimuth.

All angles shall be referenced to the antenna coordinate system (Xa,Ya,Za)

https://arxiv.org/pdf/1609.04153.pdf "The Gaia mission" 2016

these numbers guarantee that the Earth is maximally 15◦ away from the boresight of the phased-array antenna, which is used for science data transmission (Sect. 3.2.2)

Two overviews of GAIA: https://directory.eoportal.org/web/eoportal/satellite-missions/content/-/article/gaia-satellite-mission http://spaceflight101.com/gaia/gaia-spacecraft-overview/

  • $\begingroup$ Thank you for the very thorough answer! GAIA's phased array is an amazing example of instrumentation and design, it's great you've taken the time to hunt down and organize so much documentation. I've adjusted the question a bit to match - let me know if it looks OK. $\endgroup$ – uhoh Jan 21 '17 at 6:41

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