# How many communication antennas does a satellite need if it belongs to a constellation?

I assume (but I may be wrong) that in a communication satellite constellation such as the one spaceX plan to put into service:

• each full duplex link requires a dedicated antenna
• each satellite communicates with a subset of the other satellites in the constellation (some if not all of the sats that are in line of sight)
• a subset of satellites directly communicate with the ground segment
• an active antenna requires a substantial amount of energy to work
• there may be some spare antennas on a satellite

Given all those assumptions, I wonder how many antennas per satellites are required for the constellation to be able to work as expected?

• I'm not sure your assumptions are all correct. You will not need a dedicated antenna per link, and your view of "substantial" may not be correct - they can run very low power antennae. Feb 28 '17 at 7:43
• @RoryAlsop now I wander how the power needed by an antenna affect its usabale bandwidth (the spaceX constellation is made of communication satellites). Feb 28 '17 at 8:50
• You may want to read google.co.uk/… Feb 28 '17 at 9:00

We are living in the 21st century now. One phased array can support a large number of beams, each pointing in a different direction, each moving at a different rate. All the steering can be done electronically.

So no, you don't need one antenna for every link. Just a few spacecraft-fixed, phased arrays is all that's necessary.

Here is a drawing of an Iridium Next satellite. The SpaceX satellites will look very different, but for the purposes of your question, the Iridium Next is a good enough example.

The large (orange) hexagonal array on its belly is the L-band (1-2GHz, 30-15cm) is a 48-beam transmit/receive phased array antenna to communicate with multiple ground "customers". Each beam is "generated" in signal processing electronics, and a different phase and possibly (probably) amplitude would be sent to each of the sub-units with the array.

The higher the frequency, the smaller the array can be and still produce tight, steerable beams.

This excellent, detailed answer about GAIA's phased array describes the situation where the spacecraft is spinning around its axis but needs to keep a continuous data lock with Earth. The phased array is designed to handle this and keep a lock over 360 degrees azimuth and 30 to 60 degrees elevation with respect to the spacecraft's mid-plane. It's all done using math and fast processing to generate the correct signal amplitude and phase in each sub-element so that the beam is always optimized.

So the answer is "a few" if you count a single fixed array as an antenna. They are made of many small sub-units, but these are not independent antennas, they are all sharing the same signal.

above: Conceptual drawing of Iridium-NEXT from Harris.

above: Testing the GAIA phased array, from here (paywalled here).

• As for the Iridium NEXT sats, apart from the main mission antenna, they have 4 cross-link antennas for the "neighboring" satellites - one for the sat in front of it and one for the one behind it one the same plane; then one for the next satellite in the eastward and one for the westward plane. Further, they have 2 (?) antennae linking to the ground stations (that's for callers to an Iridium number who do not use an Iridium phone themselves, but e.g. a landline). Apr 21 '17 at 19:53
• @eerie Thanks! I'd originally tried to include some information like that to the answer but I found the drawing so confusing I could not figure out what is what. Are the four cross-link antennas also phased arrays? It seems they'd have to be steerable somehow. If you'd like, you're welcome to edit the answer and add some information on the cross-link beams, or suggest some place where the cross-link antennas are discussed or at least have an arrow pointing to them so I know what is what. Thanks!
– uhoh
Apr 21 '17 at 23:55
• I unfortunately do not know nor really understand about phased-array. But I read that the in-plane links are fixed, the other ones are steerable. Maybe that helps. I add an image to the answer with the descriptions inserted. A lot of technical information can be found here Apr 22 '17 at 19:06