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OFJ Field Journal from Stephen Licata - 10/26/95


As an engineer on the Mission Control Team, people often ask me "Do you use computers a lot on Galileo?" The answer, of course, is "Yes" but I would like to share with you an aspect of computer programs that most folks don't think about when they are at home playing "Math Blaster" or "Sim City" on their PC or Macintosh. This is the concept of computer program reliability.

An enormous amount of work goes into writing a computer program used by a project like Galileo. We all know that the computer and computer programs can make our lives and jobs easier, but if we have a computer program that makes mistakes, the more we depend on it, the more trouble we get into. For example, there is a "pocket calculator" program in a recent version of Microsoft Windows that makes an arithmetic error when you "subtract" 3.10 from 3.11. On the calculator you get an answer of 0.00, instead of the correct answer of 0.01! This is known as a software "bug."

The term "bug" comes from the old days when computers were the size of a room. Sometimes an insect would actually crawl into the machinery and cause electrical problems!

At JPL we realize that as computer programs get more and more complex (sometimes the list of computer instructions can be 100,000 lines long!), it is very difficult to makes sure there are no bugs. Therefore, we decide to assign a reliability category to each program and this determines how much time we will spend eliminating "bugs". Software programs called Category "A" are so important to the Galileo mission that we do a huge amount of testing to eliminate bugs and we call a special review meeting to report the test results. The people who are on this special committee ask a lot of tough questions and if they come up with some idea that the tester did not think of then it's back to the testing laboratory! It's sort of like doing the wrong essay questions at the end of a chapter in your history book - BUMMER! Go back and do your homework right!

A coworker named Kevin and I recently made some major changes to a Category "A" software program used by the Mission Control Team. This program, called SEG (for Sequence of Events Generation) collects information about spacecraft activities and our Deep Space Network system of radio tracking stations from around the world and combines them all into the daily schedule of key events. This "daily journal" is used by our people at Galileo Mission Control at JPL, called the "Aces", who monitor the spacecraft 24 hr. a day. Although the SEG program has been used successfully for six years, we had to make some special changes to it because of the new computer software being installed on the Galileo spacecraft in the Spring of 1996 (Yes, that software is ALSO Category A!).

For this testing program, the Project wanted me as a "user" of the software to design a really tough series of tests to meet two objectives: First, make sure that in this new version no "bugs" were introduced that caused us to not be able to do things we could do before (this is called a "regression test"). Secondly, we had to make sure that the new capabilities of the software would work, for example recognize and report in the schedule the new kinds of spacecraft computer commands.

My colleague Kevin is actually the engineer who wrote the original version of the SEG program many years ago. I think that you can understand that it is very important that the person looking for mistakes in the software should not be the same person who wrote it. I know that whenever I write a long essay, I always ask a friend to read it over for spelling and grammatical errors. We also take this same approach at JPL and it is part of what it means to be a professional - nobody's perfect but we all work together as a team to make the mission a success.

Anyway, our testing program was apparently the toughest one ever done on the SEG software and when we reported our results to the review committee they were very pleased and felt comfortable that this new version was free of bugs. We put in many hours of overtime, and didn't get much sleep, but we got the job done and even met our deadline!



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