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OFJ Field Journal from Randy Herrera - 11/20/95

A RADIO EXPERIMENT BEGINS AS GALILEO DISAPPEARS FROM EARTH

Well, the Radio Science crisis I described in my last journal has subsided. We discovered early last week that there were software changes in the July upgrade which only the programmers knew about. We (the users) didn't. AAARGHHH! So, now at least we know what the problem is (one computer is too slow compared to another computer and so it is dropping information in the process). Luckily, we do have some ways to work around this.

It's almost 7 pm on Tuesday night before Thanksgiving and The Smiths are serenading me as I type this. I just finished editing our Radio Science Handbook. This is a resource document that our team puts together right before every major experiment. We list useful things like phone numbers and beeper numbers. We also explain the different parts of our instrument (both the Deep Space Network (DSN) part and the spacecraft part). We list the individual responsibilities of the members of the Support Team. There's a section on references that we use and monitoring systems that are useful to us. There's a section on procedures that are common to all of our experiments. And, finally, we explain each of the upcoming experiments including the appropriate setup for the Radio Science System at the DSN ground stations. Well, I'm finally done and tomorrow I will take it into the documentation section to be photocopied. Whew!! (See--language skills are very IMPORTANT!)

Have I spoken about our BIG experiment on December 8? Well, I'll just tell you about it anyway (hee hee). On December 8, the day after we reach Jupiter orbit, the spacecraft will go "behind" Jupiter as seen from Earth. This is called an "Earth Occultation by Jupiter." As the spacecraft begins to move behind the planet, the radio signal will begin to pass through the Jovian atmosphere. The atmosphere acts like a lens and it will bend (or "refract") the radio signal. We will record the signal at the ground station (for this experiment, it will be the one in Madrid) using special equipment. For radio science, we really are only interested in the center of the signal called the carrier. The equipment at the ground station will sample the carrier at the rate of 5000 times per second and we will record the signal for almost three hours.

Now, the navigation team knows the position of the spacecraft really well. So, by combining the navigation information with the information from the recorded radio signal, our Radio Science investigators can develop what's known as a refractivity profile (that is, they can show refraction as a function of height above the planet).

The neat part is that we can actually use this information to examine what's in Jupiter's atmosphere! The investigators have a pretty good--but not perfect--idea of the composition of Jupiter's atmosphere. Different mixtures of gases will have different refractive effects on the radio signal. So, they'll fine tune their composition "model," changing around the mixture of gases until the model gives them a refractivity profile like what they see in the data. Then, based on gas physics, they can determine temperature and pressure profiles along the path of the radio signal--all from a distance far above Jupiter's atmosphere.

Pretty Cool, huh!!

The next two weeks will be spent making final preparations for the Experiment. It actually begins at 2:02 AM on Friday, December 8 (yes, that's 2 in the morning!). But, we have lots of things to do before then. In fact, by the time it starts, we shouldn't have anything to do (if all goes well)! We'll just sit back and watch as the station records our data.

Well, that's all for now.

 

 
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