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OFJ Field Journal from Ed Hirst - 10/13/95


These past few weeks I have been working on figuring out the best telecom (telecommunications) configuration for the first time we try sending down the atmospheric probe data. This event is scheduled just after arrival at Jupiter in December of this year. Usually, telecom configurations are quite standard but, in this case, special analysis is required due to the position of the spacecraft, the Earth and the Sun. The spacecraft is very close to being on the opposite side of the Sun as seen from Earth and the spacecraft's radio signal is passing very close to the Sun (within 7 to 4 degrees). At this angular separation, the solar activity could disrupt the radio signal and, if intense enough, could cause the loss of Probe data. The correct telecom configuration can reduce the effect of the Sun on the radio link.

What is this telecom configuration stuff all about? Let me give you an example. We use deep space antennas (70 meters in diameter- about the length of a 747 jet!) to listen to Galileo's radio signal. These antennas can be configured in what is called two-way mode or one-way mode. In one-way mode, the antennas are simply listening to the radio signal. Two-way mode, however, allows us to listen to the radio signal as well as send commands to the spacecraft and collect "two-way Doppler" data for navigation. From that description, you might think - why wouldn't you be in two-way mode always? Well, like all good things, there is a cost to being in two-way mode. The additional equipment that is used while in two- way mode reduces the end-to-end strength (from Galileo to the radio signal processors) of Galileo's radio signal. A weaker signal means that the rate at which Galileo can send data to the ground goes down, which means that we get less data back. Part of my job is to schedule two-way and one-way time so that we maximize data return while ensuring that commanding and navigation needs are being met.

So, why is this important? Why don't we wait until the spacecraft comes out from behind the Sun to return the data? Well, the atmospheric probe's mission is the first of its kind. It is the first time an outer planet's (those beyond the asteroid belt) atmosphere will be studied directly. So, it's tremendously important, and we want to get the data back safely to Earth as soon as possible.

What we plan to send back in this first attempt is the Probe "symbol data set" which comprises approximately half of all the Probe data. It includes all of the Probe data down to where Jupiter's atmospheric pressure is 10 times that of the Earth (the 10 bar level. The full data set is expected to go down to the 20 bar level or greater).

We have scheduled other opportunities to return this same data set, but these opportunities are not until January 1996 (about 3 weeks later). This first return is important because it will provide the Probe principal investigators with a quick look at the Probe data set and they will be able to determine whether the Probe mission was successful or not.

Away from work, I've been playing a lot a sports lately. This past Wednesday our volleyball team played its first (and last) playoff game. The playoffs were single elimination, best of 3 games. We lost our first game by about 10 points, but played much better in the second game. At game point, we were losing by, again, about 10 points, but rallied behind killer serving. We managed to get within a few points before losing the game and the match. Oh well, we will get them next season.

I also had a basketball game this past Thursday. Lost that one also, but this one was more disheartening than the volleyball loss. We were up by 10 points with 1:47 on the clock. The opposing team started fouling us to stop the clock and we started making lots of mistakes. They managed to catch up and tie and we went into overtime. n overtime, it was back and forth and we kept making mistakes. With 10 seconds left in overtime, we were down by three and I took a three point shot. My chance to be the hero or the goat (I was 2 for 4 from behind the 3 point line)..... GOAT! We will get them next time.

I am looking forward to this weekend. I plan to hit the beach. The forecast is for shoulder to head-high waves AND Santa Ana winds (off-shore flow). It should be pretty good!



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