Downloading the Webb Scope's data starts with a 6-month scheduling scramble
Missions bid to secure comms time on Deep Space Network
The remarkable images and data captured by the James Webb Space Telescope are being sent to Earth after more than six months of jockeying to secure network resources, NASA has revealed.
The Webb (JWST) phones home over the Deep Space Network (DSN), the world's largest and most sensitive scientific telecommunications system. The DSN's three facilities – in Goldstone, California, USA, Madrid, Spain, and Canberra, Australia – are positioned approximately 120 degrees apart in longitude to allow communications at most hours of the day.
However the DSN is not dedicated to the JWST. It also handles comms with several dozen other missions, ranging from cubesats to the most distant operating human probe: Voyager 1, currently over 14.9 billion miles (24 billion kilometers) from Earth. As its realtime operations page demonstrates, it's connected to several missions at a time.
Those missions must all secure time on the network before mission controllers can plan how to operate and maintain the distant craft – and, of course, to download data.
NASA must therefore plan how best to share the network's coverage for all those missions.
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The team managing JWST therefore has to negotiate with other groups six months in advance, to schedule time to communicate with and download data from the space telescope.
NASA's Mission Operations Center (MOC) draws up a mission scheduler – a rough plan that arranges time with the DSN to keep all the other ongoing space missions ticking along too. "It's not as simple as picking up the phone and calling the telescope," Kari Bosley, the lead Webb mission planner at the Space Telescope Science Institute (STscI), said.
"In order for Earth to connect with Webb there are a few things that happen prior to scheduling a contact. The mission planners at STScI work together with the mission schedulers at JPL to create contacts with Webb. The Flight Dynamics Facility at NASA's Goddard Space Flight Center sends the MOC at STScI the view periods in which the observatory is visible from those three different DSN sites.
"The mission scheduler compares those times to what is available in the scheduling system where other missions are competing for time with their spacecraft. All missions require specific amounts of time to communicate with their spacecraft, and the timing depends on where the spacecraft are in space. There are times when multiple missions request the same resource at the same time. When this happens, our mission scheduler at JPL will negotiate with other missions to come to a compromise that satisfies all of the missions," she explained.
A rough schedule is then sent to the mission planners up to six months in advance. The program is fixed for the first eight weeks unless an emergency arises – like trying to fix a spacecraft veering off track.
The DSN allocates different types of radio waves to communicate with probes. Instructions are sent to the JWST via the S-band, and its position, trajector, and health are received at this wavelength too. Scientific data, on the other hand, is sent using the more powerful Ka-band frequencies.
"We use Ka-band to downlink stored science and engineering data, and some telemetry from the spacecraft. If we used S-band to downlink data, it would take many days to download each day's data. With Ka-band, it takes much less time, and we can usually complete download all of the stored data in a couple of hours," Bosley said.
The team usually contacts the JWST two to three times per day, maintaining communications for two to six hours at a time. The telescope will automatically continue to snap more images and collect measurements of light from distant galaxies, and store the data in its solid-state recorder that will be downlinked during the next point of contact.
NASA is currently developing other types of space communications systems as it explores sending humans to the Moon, Mars, and beyond. The DSN will be too slow to communicate with future astronauts exploring and living in space. ®