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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?

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  • $\begingroup$ 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'. $\endgroup$ Sep 7, 2020 at 10:40
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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. $\endgroup$ Sep 7, 2020 at 12:29
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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 ... $\endgroup$ Sep 7, 2020 at 12:40
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    $\begingroup$ 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! $\endgroup$
    – DrSheldon
    Jun 23, 2021 at 17:18
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    $\begingroup$ @uhoh: i.stack.imgur.com/33fHU.png $\endgroup$
    – DrSheldon
    Jun 24, 2021 at 0:04

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Partial answer..?

In lay mans terms I think there is confusion between a 'complete end-to-end flow from planning the science Webb will perform to posting the scientific data to the community archive' test and an 'environment' test, which is where everything as assembled is tested in specific places such as an anechoic chamber.

In this case they are only talking about getting everything talking to each other as a wholly assembled spacecraft and also talking to the ground support network via simulated DSN and then getting the data out to appropriate networks on Earth. Including getting the GSN to talk to the assembled spacecraft via the same network.

The key point was that the spacecraft had not yet been communicated with as a wholly assembled spacecraft. Prior to this the test (including environmental tests) had only been done with individual systems independent of each other and therefore not as a singular whole spacecraft (for example, subsystem or individual system design tests, performed as early as practical in the spacecraft development program).

This is the first time they have all been put together in one place, been linked up, and then expected to talk nicely to each other and to Earth.

Kind of like individual teams have been tinkering in their own basements for months uptil now.

Now is when they all come together in one room, plug everything in to each other and flip a switch, hoping it all works...

https://www.universetoday.com/147597/james-webb-is-working-perfectly-on-the-ground-next-trick-doing-it-from-space/#more-147597

Recently, the testing teams conducted the critical “Ground Segment Test,” where the fully-assembled observatory was powered up and to see how it would respond to commands in space.

To summarize, the ground segment test involves crews conducting the full end-to-end process that begins with mission planning and ends with the scientific data it obtains being posted to the community archive.

As Amanda Arvai, the Deputy Division Head of Mission Operations at STScI, said in a recent NASA press release:

“This was the first time we have done this with both the actual Webb flight hardware and ground system. We’ve performed pieces of this test as the observatory was being assembled, but this is the first ever, and fully successful, end-to-end operation of the observatory and ground segment. This is a big milestone for the project, and very rewarding to see Webb working as expected."

To simulate the distances involved once the JWST is in space – 374,000 km (232,000 mi) at perigee and 1.5 million km (930,000 mi) at apogee – the Flight Operations Team relied on a special emulator that simulated a radio link between the observatory and the DSN.

Commands were then relayed through the DSN emulator to the JWST, which is currently located in the cleanroom at the Northrop Grumman facility in Redondo Beach, California. (answering OP's query on location at the time the question was asked)

https://www.digitaltrends.com/news/nasa-james-webb-ground-segment-test/

https://www.sciencetimes.com/articles/29953/20210302/james-webb-space-telescope-reaches-completion-final-functional-performance-test.htm

testing teams were successful in completing two important milestones that verified the internal electronics of the observatory are all functioning, and that the spacecraft, including its four scientific instruments, can both deliver and receive data appropriately through the same network they will utilize in space.

The said trials are known as the all-inclusive systems test, which occurred in the Northrop Grumman, and the ground segment trial.

Following the individual tests and it being assembled altogether, the Final Ground Segment Test begins:

  • creating a simulated plan that each of its scientific equipment would follow

  • Commands to chronologically turn on, move and operate each scientific instrument were then transmitted

  • Flight Operations team linked the spacecraft to the Deep Space Network (using the test hardware related here) - via special equipment used to imitate the actual radio connection between the Deep Space Network and Webb

  • For Webb, one example test was during a simulated flight environment when the team successfully practiced seamlessly switching over control from its main Mission Operations Center at Space Telescope Science Institute to the backup Mission Operations Center at the Greenbelt Maryland-based Goddard Space Flight Center of NASA.

  • send Software Patches to the Observatory successfully (See other SE posts about OTA reprogramming)

  • establish a backup plan that is not expected to be needed, although it is necessary to practice and perfect

  • successfully send numerous software patches to the observatory while it was doing its commanded operations.

  • Analyse the data that is sent back by the observatory whilst it carries out its (albeit simulated) mission.

Software:

The DSN Simulator is an updated version of the software described in “Deep Space Network Array Simulator

This software is used for computational modelling of proposed DSN facilities comprising arrays of antennas and transmitting and receiving equipment for microwave communication with spacecraft on interplanetary missions. Such modelling is performed to estimate facility performance, evaluate requirements that govern facility design, and evaluate proposed improvements in hardware and/or software.

The present updated version is capable of modelling the entire DSN and provides for configurable metrics, making it possible to perform loading analyses for alternative future DSN architectures and mission-set scenarios. The present version also features an improved user interface and interfaces for exchange of data with other DSN software and with a DSN mission model database.

Other Software:

The Network Simulator

  • The Network Simulator (ns-2) is a popular open source network simulation package.

Space Based Internet Emulator

  • An emulator for space networking applications was developed by the University of Kansas, called the Space Based Internet (SBI) emulator. SBI software uses an XML document to describe the scenario, which SBI creates in the Satellite Tool Kit (STK) through STK/Connect at the beginning of the emulation.

  • The communication links modelled in the SBI emulator use the quality of service mechanisms provided by the Linux kernel.

This twitter post seems to confirm the above happened:

https://twitter.com/NASAWebb/status/1382376679667073029?ref_src=twsrc%5Etfw

Using a Deep Space Network emulator, Webb completed an important test simulating the process of sending and receiving data from its scientific instruments in early 2021.

"During its final full systems test, technicians powered on all of the James Webb Space Telescope's various electrical components installed on the observatory, and cycled through their planned operations to ensure each was functioning, and communicating with each other."

https://www.viralstuff.ca/nasas-james-webb-space-telescope-completes-final-functional-tests-to-prepare-for-launch/

The electrical boxes inside the telescope have an “A” and “B” side, which allows redundancy in flight and added flexibility. During the test, all commands were correctly entered, all telemetry was received was correct, and all electrical boxes and backup sides worked as planned.

“It’s a happy moment because we were able to demonstrate Webb’s electrical readiness. We are now ready to move forward towards launch and on-orbit operations now that this evaluation has been completed successfully."

Hardware:

https://deepspace.jpl.nasa.gov/dsndocs/810-005/305/305B.pdf

  • Test support may involve elements of the Microwave, Uplink and Downlink equipment, the Telemetry Simulation Assembly (TSA - RF, generate test data for client), the System Performance Test (SPT - analysis of data) Assembly, and Service Management (scheduling and non-real-time interfaces between the DSN and clients).

  • The links to and from the spacecraft are accomplished by direct cabling although Ka-band signals are normally down converted near the spacecraft and transferred to the downlink equipment at the DSN intermediate frequency (100 – 600 MHz).

RF Signal Generation

The RF output of the exciter is translated to the downlink frequency and can be attenuated to whatever level is appropriate to simulate the expected signal-to-noise ratio for the spacecraft signal during its various mission phases.

DSN Compatibility Test Facilities

There are three DSN facilities that are provided primarily to support compatibility testing:

DTF-21, located near JPL in Monrovia, CA., is equipped with simulated front ends for the DSN 70-m and 34-m stations, uplink and downlink equipment, and at least one set of all data processing equipment found in the Signal Processing Centers (SPCs).

  • Simulators for the antennas, transmitters, and microwave control equipment are provided to mimic their responses.

  • Communications are provided by standard JPL/NASA Integrated Service Network (NISN) ground communications interfaces and a Cesium Beam Frequency Standard provides station timing.

  • DTF-21 may be configured to simulate any station at a DSCC.

  • This capability provides a convenient environment for Project/DSN interface testing.

  • DTF-21 includes an RF shielded room in order to isolate devices under test.

  • Has S-band Microwave and X-band Microwave but no Ka-band Reception.

CTT-22 is a 48-foot towable trailer designed and implemented specifically to perform compatibility and telemetry data flow testing at spacecraft manufacturing facilities and to provide launch support of spacecraft from locations other than Cape Canaveral.

  • It provides capabilities representative of those found at a DSCC. It can be relocated to any convenient location around the world.

  • Has S-band Microwave and X-band Microwave but no Ka-band Reception.

MIL-71 is located in the Mission Operations Support Building (MOSB) at the Kennedy Space Center in Florida, USA.

  • The facility is normally maintained in a caretaker status between launches and is implemented as needed, usually to simulate a 34-m Beam Waveguide (BWG) station, for pre-launch project and DSN compatibility.

  • Has S-band Microwave and X-band Microwave and Ka-band Reception.

In summary:

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.

I think it is a Yes to all of those...

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