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OFJ97 Field Journal from Leo Cheng - 2/5/97

Today was one of those days. Yesterday, I had released what was supposed to be the final version of the sequence of computer commands for the E6 (E for Europa, 6 for the sixth orbit) Encounter . It's been over two months since we started the development of the command sequence (the set of instructions that operate Galileo). And now at the end of the finish line, we've found a problem...yikes! Well, better catching the problem late than never.

I have a very challenging job on Galileo. I am the Sequence Integration Engineer or SIE on the E6 Encounter command sequence (or just "sequence" for short). I lead a group of 20-30 people to create a sequence which will collect the best data on Europa so far.

I also have the "birds eye" view of everything that happens on Galileo. I need that view, because I am responsible for piecing together hundreds of commands. Instead of a jigsaw puzzle, my end product is a command sequence that will maximize our mission objectives...ummm...without breaking anything. :-)

To do this, I first plan activities on the spacecraft that keep Galileo healthy. Then, I "integrate" or put together my commands with the commands that collect the science data. What we get is a listing, in time order, of all the commands that will be loaded into Galileo. This listing is then distributed to all of the members of the flight team. This includes the subsystem engineers (who are concerned with just one small part of the spacecraft, like power), the science representatives, and the navigators. All of us review the sequence, looking for mistakes.

The mistakes could be an isolated one, like commanding the camera to use a green filter instead of a red filter. Those are usually easy to fix. The mistake that we found involves more than one system. These are more complex. In our case, it involved two science instruments: the Extreme Ultraviolet Spectrometer (EUV) and the Heavy Ion Counter (HIC).

The EUV and HIC share a common data buffer. A data buffer is computer memory used to temporarily store data collected from an instrument. It's like using one mailbox, and one address, to store mail from two families. But in the case of EUV and HIC, only one "family" can use the mailbox at a time. You would have to tell the post office to deliver mail for the EUV family at say 10 am, and the HIC family to deliver at 2 pm.

In our mistake, HIC had told the central computer (our "post office") to deliver data (our "mail") before EUV was done (at "10 am" instead of "2 pm"). We wouldn't have broken anything on the spacecraft, but HIC would have lost some data.

Did we fix it? Yes, we did. It delayed our schedule a little, but it was still before we "uplink" or send the commands up to Galileo. The final step before we uplink the command sequence is to generate the the actual computer code that Galileo's central computer understands. Just like loading a program on your desktop computer, we load this into Galileo's central computer. Unlike your desktop computer, we do this with radio signals, hundreds of millions of miles away.

No one said flying a spacecraft at Jupiter is easy.


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