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On December 7, 1995, the Big Three events (Io Closest Approach, Probe Relay, and Jupiter Orbit Insertion) captivated the thoughts and energy of most of the people on the Galileo Project. These were monumental engineering feats that would determine our paths for the next 2 years. But for those of us on the Radio Science Support Team, a fourth event---the Earth Occultation by Jupiter---caused us to focus our attention on the workings of the Deep Space Network (DSN).
Earth occultation occurs when the spacecraft travels behind a planet---in this case, Jupiter---as seen from the Earth. We record the radio signal at the ground station as it passes through the ionosphere and atmosphere of the planet both on the incoming (called ingress) and outgoing (called egress) legs. From this data, the Radio Science investigators cull electron density profiles of the ionosphere and temperature-pressure profiles of the atmosphere along the raypath. The electron density profiles provide scientists with one more piece of information on the structure of the magnetospheric puzzle that is Jupiter. The temperature-pressure profiles help describe the dynamics of the atmosphere along the raypath at the time of the experiment.
Although only four people were officially on duty during the observation, everyone on the Support Team wanted to be on-hand for not only the Big Three but also for the Occultation Experiment. Those who were to be on duty were instructed to stay away until 4 pm. Well, everyone was here by 2 pm.
We had been preparing for this experiment for several months. In the 2 months preceding the experiment, we had conducted three operational readiness tests (ORTs). We also had countless meetings and conversations with, and sent and received dozens of messages to, the Network Operations Project Engineers (NOPEs) and other DSN support personnel. We needed the best possible frequency predicts for the experiment; so, during the week of the experiment, our operations plan called for two trajectory update deliveries by the Navigation Team (NAV) and an update to the time-of-ingress. The Radio Science receiver doesn't "lock" onto a signal---it is tuned using frequency predicts---no feedback loop! If the frequency of the actual signal is too far off from the expected value, we might miss the signal entirely.
So, there we were on Thursday night, waiting for the NAV Team to call us with an update to the time-of-ingress. The scheduled time for the update was midnight (PST) on Thursday night.
We heard from NAV around 12:15 and the difference between the predicts delivered on Wednesday and the update was about 25 seconds. This meant that we could be off by as much as 37 Hz, but we felt that we could absorb this in the bandwidth we were using (the bandwidth was about 2500 Hz).
As the pass began, we listened intently as station personnel read back information on the status of the Radio Science equipment.
At 2:02 am (PST), we saw the carrier pop up. Just as it should! Finally, we were receiving the FIRST SCIENCE DATA IN THE ORBITAL TOUR!!!
We continued to monitor all of our displays and listen to the voice traffic between the station and the Network Operations Control Center (a.k.a. the Dark Room). Right around 3:22 am (PST), we all gathered around the spectrum display---seven heads all vying for the best view. Suddenly, the signal seemed to dim like a bulb in a brownout. Then, in the next refresh, it popped back up---at a level higher than it was during the pass. Then it was gone. The spacecraft had passed behind the planet and all communication was cut.
Everything was nearly perfect. The occultation happened right on time. The signal disappeared just as predicted. The station personnel did all the right things at the right time. The DSN Operations Engineer soon faxed us a copy of the spectrum and it looked great!
There was nothing more to do until close to 7 am (PST) when the spacecraft would come out from behind Jupiter. Several of us took a nap. (Do you know how hard it is to get comfortable and sleep in an office chair??)
Before the appointed time, 6:53 am (PST), we again gathered around the spectrum display waiting for something to happen. And...the signal popped right back up at the appointed minute! The Radio Science equipment recorded the signal through the end of the observation (at 8:13 am (PST)) without incident.
At 8:14 am in the Radio Science real time area, the scenes from the previous day showing ecstatic flight team members celebrating the successful completion of the JOI maneuver were replayed. We gave each other high fives and shook each others' hands. We were all quite happy and relieved that our efforts had paid off. Then, we all went home and got some sleep.