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OFJ Field Journal from Stephen Licata - 10/26/95 (2nd)


The Galileo Flight Team spends many months planning complicated science and engineering activities, but sometimes the most nervous time is spent waiting for the results to appear on the computer screens at their desks. As part of my job, every once in a while I call a meeting of the Galileo Data Flow Working Group to make sure this process runs smoothly at critical points in the mission. I'd like to share with you one of our recent assignments - planning the release the Probe from the main spacecraft so that it could begin its descent into Jupiter's atmosphere.

First, let's look at how the spacecraft data gets to Earth. On Galileo, the Command and Data Subsystem gathers information on all parts of the spacecraft (cameras, science instruments, even the heaters and gyroscopes) and creates a "status report". This report is sent to earth through the low-gain antenna at a radio frequency around 2300 on your FM dial (Don't bother trying! The signal is put into a secret code to protect it from static and interference in space and the Earth's atmosphere). On the ground, these radio signals are captured at the Deep Space Network by huge tracking antennas the size of a football field and converted into the computer language of 1's and 0's. The tracking station computers tell us at what time the signal was received and then send it by a special network of wires and radio relay antennas to JPL.

Here at JPL the Data Systems Operations Team converts the data into a structure that can be understood by JPL's computers and passes the data on to a set of computers and computer programs called the Multi-mission Ground Data System (MGDS). The MGDS separates the data into categories of "spacecraft science data", "spacecraft engineering data", and information about the tracking station (for example, how strong the radio signal was when it was received at Earth).

Immediately some of these data go to the Mission Support Area where the mission controller, a person called the "Ace", looks at the data and promptly calls the tracking station if there is a problem with the data stream.

At the same time all the data related to the spacecraft goes to another part of the MGDS called the Telemetry Delivery Subsystem which is like a library where people with special permission (like the scientists) can go to grab the data that they want. If the data they want are parts of a camera picture, these data first have to go to yet another area called the multi-mission Image Processing System before they can be looked at by the scientists and others.

Currently the time required for the data to go from the Deep Space network to a scientist's computer screen varies from 10 minutes to an hour (for the pictures). Early next year, we will be changing the computer software on the Galileo spacecraft such that this time delay may stretch to two hours but we will be able to get many more pictures each day from the spacecraft, so scientists are willing to be a little more patient!

In planning the release of the Galileo Probe last summer, we had a few special problems. First of all, since the Probe, once released, cannot talk to the Galileo Orbiter until it begins to enter Jupiter's atmosphere, we wanted to be very sure that everything was working fine before we let it go. Therefore, the Probe Engineering Team asked that we first turn on the Probe using an electrical cable from the Orbiter to make sure everything was working correctly. Then we had to switch the Probe to its own internal power before cutting this power cable (just like a baby being born gets separated from its mother!).

A second challenge was that the computer equipment used by the Probe Engineering Team is not the same as used by the rest of JPL. Therefore, we needed to record the Probe information on special data tapes. For each of these two key events - turning on the Probe and switching it to its own internal power, we commanded the Galileo spacecraft to issue two status reports each time. This allowed us to be VERY sure that there were no problems with the spacecraft reports.

As it turned out, this was a lengthy process, with data coming in from the spacecraft as late as 10:30 p.m. and these tapes being delivered sometimes at 2:30 am.

My job in all this was to identify where the data was going when it arrived at the Deep Space Network and to negotiate between the Probe Engineering Team and the Data Management team (the data tape folks) this rather unusual set of delivery times. In the end, everything ran very smoothly and the Probe Release on Wednesday July 12 went off perfectly!

We will be going through a similar process in December when the Probe enters the atmosphere and starts sending back "real data". Even now I am updating that same tape delivery schedule. Keep your fingers crossed!



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