NASA News item Ground Segment Testing a Success for NASA’s James Webb Space Telescope says:

Testing teams have successfully completed a critical milestone focused on demonstrating that NASA’s James Webb Space Telescope will respond to commands once in space.

Known as a “Ground Segment Test,” this is the first time commands to power on and test Webb’s scientific instruments have been sent to the fully-assembled observatory from its Mission Operations Center at the Space Telescope Science Institute (STScI) in Baltimore, Maryland.

Since reliably communicating with Webb when in space is a mission-critical priority for NASA, tests like these are part of a comprehensive regimen designed to validate and ensure all components of the observatory will function in space with the complex communications networks involved in both sending commands, and downlinking scientific data. This test successfully demonstrated the complete end-to-end flow from planning the science Webb will perform to posting the scientific data to the community archive.

I don't know where JWST is at the moment, but I'm pretty sure it's on Earth and not very close to any of the Deep Space Network dishes. Unlike lower frequencies, the bands used for deep space communications won't reflect off of the ionosphere, so rather than linking to the DSN and letting that system broadcast to JWST, the Mission Operations Center at the Space Telescope Science Institute (STScI) in Baltimore, Maryland must have used the internet and either a mock-up of the DSN converting to S or X-band locally, or they bypassed the radio link entirely.

Is it known how this was done?

This question reminds me of What are these white cylindrical objects pointed in different directions in this photo of the Saturn V instrument unit?

  • $\begingroup$ Interest / Related: The British Jodrell Bank Radio Telescope has been used for interplanetary probe communications. The near field of the antenna extends to beyond the atmosphere, making RF comms testing with target systems pre-launch 'a bit problematic'. $\endgroup$ – Russell McMahon Sep 7 '20 at 10:40
  • $\begingroup$ @RussellMcMahon I can imagine that if the Jodrell Bank Radio Telescope was transmitting and there was a system under testing on the ground in the vicinity, then some leaked power could certainly reach it. I'm not sure though that what "extends to beyond the atmosphere" can be called the "near field" of the dish. Usually that term refers to distances comparable to either the size of the antenna or the size of the wavelength. If you can find a citable source for that info about testing, please consider posting an additional answer. Thanks! $\endgroup$ – uhoh Sep 7 '20 at 10:53
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    $\begingroup$ I was looking at such a reference a few hours ago - I'll see if I marked it. That was for a two feed interferometric feed from the same dish - they said the near field was outside the atmosphere at GHz frequencies and could be much more. || My Jodrell bank mention was based on a remembered statement to that effect long ago by I think JB people itself (possibly during a visit there?) but that source is not locatable. More maybe ... || I'm aware that the USUAL stated NF distances are in the wavelengths and or of dish size order. I've recently seen NF statements of >10km and 100s of km. $\endgroup$ – Russell McMahon Sep 7 '20 at 12:29
  • $\begingroup$ @RussellMcMahon Oh! In the case of a multi-dish interferometry with a baseline of tens or hundred of kilometers the interferometer's near field could be outside the atmosphere, but usually that's done for receiving rather than transmitting. A sparse array isn't much help over a single dish when transmitting. $\endgroup$ – uhoh Sep 7 '20 at 12:34
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    $\begingroup$ No - this is single dish with feed horns overlapping physically (hard on the mind). And from what I read in other summaries (which do not give any details) they are geeting single dish NFs or 10's to 100+ km. | And yes - I know this is quite contrary to anything in the usual NFC and similar literature. More maybe ... $\endgroup$ – Russell McMahon Sep 7 '20 at 12:40

This is done in a anechoic chamber. Such chambers are padded on the inside with material that absorbs electromagnetic waves. On the outside, the rooms are shielded like a Faraday cage, so that no external radiation enters the room.

They put the whole satellite in the room and by putting a transmitter and receiver in the room in the right locations and using scaled power levels, you can simulate end-to-end communication without having to launch the satellite.

This article on the ESA test facilities explains how this works:

[These rooms] are shielded anechoic chambers with internal foam coatings that absorb reflected radio waves to simulate the boundless nature of space. The metal walls form Faraday cages, blocking all external signals such as TV broadcasts, airplane and ship radars and even mobile call phones. [...] HERTZ is a hybrid antenna testing laboratory which combines in the same anechoic chamber a Compact Payload Test Range (CPTR) and a Near Field Range (NFR). Their purpose is to verify in-orbit performance of antennas, payloads and complete satellites with complex antenna farms.

Here's a photo of what such a room looks like with a satellite in it:

Satellite in anechoic chamber at Airbus facilities (Satellite in anechoic chamber at Airbus facilities - source Credit: ESA – S. Corvaja)

  • $\begingroup$ I don't think this is how a complete "ground segment" is checked. I think this is how an antennas and radio bits are checked. This is an excellent post and I wish I'd asked the question to which it is an answer. But the ground segment as described in my block quote is different than what this test would address. I'll be happy to post an appropriate question that this will nicely address tomorrow. $\endgroup$ – uhoh Sep 5 '20 at 11:25
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    $\begingroup$ @uhoh The purpose of the end-to-end test, as stated in the quote you provided, is to do a full loop from mission controls to spacecraft back to mission control, to see if commands to the spacecraft illicit the correct response. The "ground segment" is an antenna, transmitter/receiver, and a bunch of computers. The only way to do an end-to-end test is by putting and appropriate transmitter and receiver (maybe not the DSN one in this case) in the room with the satellite and hook it up to the computers (quite possibly via a dedicated link between the test facility and the mission control center). $\endgroup$ – Ludo Sep 5 '20 at 12:34
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    $\begingroup$ Oh, so there was a transceiver in the chamber along with JWST that was linked to by Mission Operations Center at the Space Telescope Science Institute (STScI) in Baltimore, Maryland? Okay cool! $\endgroup$ – uhoh Sep 5 '20 at 13:31
  • $\begingroup$ Have a look at Figures 6.1 and 6.2 here webthesis.biblio.polito.it/12578/1/tesi.pdf (pdf page 64). This is a totally different procedure and uses separate Faraday cages for the two elements with an RF link between them. So I disagree that the test that I've described in my question was necessarily done "in a anechoic chamber." Maybe it was and maybe it was not, but I think for the test I've asked about I'll need some specific source or citation about it, rather than just "I'm sure they did it this way". $\endgroup$ – uhoh Nov 26 '20 at 0:44
  • $\begingroup$ See also DSN Telecommunications Link Design Handbook, 305, Rev. B Test Support $\endgroup$ – uhoh Nov 26 '20 at 0:59

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