# How does one test Ground Segment communications with, and “downlink” data from a spacecraft that's still on the ground?

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?

• 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'. – Russell McMahon Sep 7 '20 at 10:40
• 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. – Russell McMahon Sep 7 '20 at 12:29
• 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 ... – Russell McMahon Sep 7 '20 at 12:40
• Related: Right now DSN Now is showing "Human Space Flight: Artemis I" call sign EM1 being tested with dish 26. It hasn't even launched yet! – DrSheldon Jun 23 at 17:18
• – DrSheldon Jun 24 at 0:04