# What is a "re-radiation system"?

In SpaceX's Echostar XXIII Hosted Webcast after T+ 00:03:50 they mentioned the "re-radiation system" as a part of the payload fairing that increased it's value and possibly added to the benefit of recovery of the fairing post-launch, rather than just "ditching it".

What is it, why is it so valuable, and why wouldn't just a low tech, low cost, not-worth-mentioning-in-the-video solution like a little piece of coax with a dipole on each end be enough? Any radios on the satellite would be optimized for strong outgoing and weak incoming signals as they would be in orbit tens of thousands of kilometers away from a ground station, so just some weak coupling would be enough for communication over a distance of few tens of meters.

edit: An example of an RF window (another alternative solution) is described in Section 4.1.3 of Proton Launch System Mission Planner’s Guide; SECTION 4; Spacecraft Interfaces.

note: The video is set to start at 21:29 (T+ 00:03:30) so you can see the fairing separate. The re-radiation system of the fairing is mentioned after another 20 seconds or so.

• Your post seems to assume this is a passive system. The hosted webcast doesn't say enough about it to understand whether this is a passive or active system. Do you have other evidence that this actually as simple as you describe instead of more complex like the talking heads here hint at? Mar 20, 2017 at 9:42
• @Caleb holy granola - you are right! I just scrolled up and looked at the text, and that does not look like how I remember writing the question at all! Thank you for point it it out - I'll fix it right now. I must have closed the page without saving somehow.
– uhoh
Mar 20, 2017 at 10:00
• @Caleb I've rejected a few edits that keep trying to force the video time into the link. I'll take care of that when I'm ready, but I don't have to do it right away as the link jockeys keep trying to force me to. There is no fire, no emergency here. Please consider the amount of work I've put into this question and this answer, and assume I'm a big boy and can manage my own links now. Thanks!
– uhoh
Mar 20, 2017 at 10:22
• I you don't want to link to it at least mention that the timestamp is 21:45. The timestamp you give in the question is launch time but you're specifically talking about comments in a hosted webcast and (esp from mobile) this is frustrating to seek for. Deliberately making it harder than it needs to be to find what this question refers to is counter-productive. Mar 20, 2017 at 10:25
• No the explanation didn't come through. I still disagree with your reasoning and rejection, but if I'd seen the rejection message I wouldn't have just submitted it again! At the very least I would have taken that potential issue into consideration made a less obtrusive edit that didn't change the primary link but did note the current run time time stamp (maybe as a secondary link) (which is also something you could have done as an improvement on the edit suggestions rather than rejecting it three times). Mar 20, 2017 at 10:52

I was intrigued by the question and in my search, I found a discussion in another forum. In this discussion, they attach a page from another launcher (Sea Launch) but I guess the concept is similar.

I guess these systems are valuable because the satellite can be mounted on top of a launch vehicle for a long time. During this time you might get limited or none physical access to it. Creating an option to communicate with the satellite during this time period can help with testing and debugging the system.

By the way, unlike big satellites, CubeSat or any secondary payload will usually be launched in a non-operating mode. From the moment of CubeSat delivery to the launch coordinator, you can not gain any access to it. A system that creates an ability to conduct these final test will be highly appreciated but to the best of my knowledge is currently not available for secondary payload.

http://www.spacex.com/sites/spacex/files/falcon_9_users_guide_rev_2.0.pdf

See page 37, right at the top. It describes a system for post-integration payload antenna testing. This would not be a passive device, as a passive device would not be able to pass on sufficient information about the waveform produced by the payload's antenna.

I'm extrapolating here, but it is likely a complex system of antenna elements fed into some very expensive interferometers. This data would be used to reconstruct the waveform to verify antenna gain as well as possibly beam steering and side lobe leakage. The antenna's gain is very difficult to test in the fairing because the available distance is nowhere near the focal length of the antenna.

In the case of EchoStar it may be looking for interference between different elements of the payload's array to make sure it will not produce interference which could result in uneven service.

The other big possibility I can think of is that the entire fairing is covered in tuned passive elements that absorb the outgoing signal on the inside and re-emit on the outside. The trouble with this is that the re-emited radiation isn't going in the same direction. You get some phasing but it would still create a lot of noise in the waveform. I suppose that does sound more like what they call it.

• I only see the following phrase "Through-fairing RF antenna (re-radiation) systems are available as a nonstandard service;..." That's seven words if you count RF as two words. A piece of coax with a dipole at each end would match that description as well as a million dollar box of optoelectronics would. The rest of your answer is unsupported personal speculation. Sorry to sound harsh, but I didn't ask what it might be, or what one thinks it could be.
– uhoh
Mar 20, 2017 at 11:25
• The rest of that sentence, "they are intended for use during payload antenna testing while on the launch pad, not for use during flight.", is important. I can surmise that it's not a piece of coax with a dipole at each end because it's expensive enough to make recovering the fairings attractive. I can surmise that it is intended to characterize the waveform of the payload's antenna system because anything less would be cheap, and I can surmise that interferometry will be used because that's necessary when you can't put your sensors far from the transmitter. Mar 20, 2017 at 11:35
• OK you might be right! Surmization alone does not a good stackexchange answer make. It's probably not even a word :) I am not sure how much antenna testing can be done in a confined space. I think they meant to say transmitter testing - the whole system, not just antennas. For GEO applications, antennas should have highly engineered patterns for specific footprints on the Earth, and I think it might be hard to test that inside the fairing. However, I could be very wrong, and if one did make a phase-mapping near field detector array that would definitely be something you wouldn't wanna ditch
– uhoh
Mar 20, 2017 at 11:42
• Normally yes, but since you're not likely to get a better answer I thought I would chime in. Mar 20, 2017 at 12:36
• I've just added a bounty, any more thoughts on the subject?
– uhoh
Nov 7, 2018 at 1:26