OFJ97 Field Journal from Jim FH Taylor - 2/7/97
THE SPACECRAFT IS OK - - IT WAS JUST A CORONAL MASS EJECTIONI am just back from my 15-minute lope that takes me on the horse trails around two of JPL's boundaries. I use this time alone at the end of the day to reflect on the events of the day. I usually also remember two or three more things to finish up before I can go home.
On the trail I looked up at the glow where the sun had been. It was the sun that changed almost everything I worked on today. It started with a voice on the network from the station in the California desert that was tracking the Galileo spacecraft. The voice was calling our mission controller whose area is three floors below my eighth floor office. The station operator at Goldstone said her equipment had "lost lock" on the signal coming down from Galileo and she had been unable to reacquire it after several attempts. My attention perked up.
In my role as the telecommunications systems engineer on the Galileo flight team, I make it my responsibility to know how the spacecraft is communicating with the earth. Besides my personal computer, I have an engineering workstation on which I can display various measurements about the two radio links with Galileo. These are the "uplink" (the signal that we send from earth up to Galileo that is received by the spacecraft) and the "downlink" (the signal that Galileo sends us that is received by the tracking station). Besides my telephone, I also have this voice communications assembly to hear the station operators at the three Deep Space Network sites in Australia, Spain, and California.
Before long, the mission controller called me and asked if I was following the situation. At that point, we didn't know why the station couldn't decode the downlink. Perhaps something had happened at the spacecraft to change the signal. Or there could be some problem with the station receiver software. But, most likely, the problem was with the sun. Since Galileo's position in the sky right now is still quite near the sun, the sun's radio emissions sometimes interfere with the spacecraft's radio signal. Several days this week, the stations in California and Australia had trouble receiving Galileo's signal. Most of the data matched what we expected if the sun was interfering. Some didn't. And there was the fact that a new version of the receiver software was installed at the Australian station just before the "track" on Tuesday (February 4th). That day, we lost 47 frames (over three hours) of digital data. If all was well, we shouldn't have lost any.
When the mission controller asked for my help to diagnose the problem today, I called up a graph showing what's known as receiver phase error data. Radio waves, when they are undisturbed and easy to receive, can be likened to the waves in a calm ocean, having regular peaks and troughs. If you are in a boat when the ocean is disturbed by a storm, the surface builds up into higher peaks and deeper troughs. Not only that, the spacing between the wave peaks, and even the direction the peaks travel in, become irregular. In the same way as the boat and its passengers sense a stormy ocean, the phase error measurement gives an indication of how irregular the radio waves have become. It thus tells me how disturbed or difficult it s going to be to receive the downlink.
On my workstation screen, I could see large phase errors beginning about the time the station could no longer decode the signal. It surely looked like solar interference. I had e-mailed Richard Woo, a solar radio scientist at JPL, last night. Richard was just one of the people I worked with during the week, each with his or her own area of expertise. One member of the receiver development team, Robert Kahn, faxed me plots of the difference in phase of the signals received at California and Australia at the same time. This was a clue. Another member, Sue Finley, brought over plots of data of the signal processed through an algorithm; she said it looked similar to other times the downlink experienced solar scintillation.
Wade Mayo and Ray Piereson, members of my Telecom Unit, queried the project data base to produce statistics about the decoding process. Each set of statistics is a block of words (such as "bit rate", "signal level", and "number of corrections") along with numbers (such as "20", for 20 bits per second, and "-168 decibels" for a signal level.) Each block of words and numbers applies to one frame of data. Remember, we lost 47 frames (and successfully decoded 3 frames) on February 4.
The statistics baffled us. The information didn't match previous solar effects. Because not all of our information was consistent, we still didn't have a clear cut idea what was causing the problem. Even if the equipment on board the spacecraft was OK, we were worried that this problem could continue or occur again, causing us to lose still more data. Each scientist whose experiment depends on receiving particular data would be upset if the lost frames happened to contain THAT data.
The possibility of an equipment problem on the spacecraft always worries me the most, puts that feeling of dread in the pit of my stomach. All of us on the flight team tend our spacecraft with great care. We feel bad enough when anything causes science data to be lost. It can be devastating when an "anomaly" occurs, when some part of the spacecraft breaks down, threatening the mission. I would feel quite uneasy until the station could get the data in "lock" again. And I wouldn't really feel released until we were more certain of the problem's cause.
I am an engineer. But I need to describe a little more science here. Radio waves and light waves are of the same electromagnetic nature. It's just that radio waves are much longer than light in wavelength. Have you ever looked at another person through the clear but shimmering air above a hot fire? The person's figure you see is distorted by the shimmering. Even without smoke you can't make out details as well as through clear air. Now imagine that instead of looking at another person, you had a large mirror on the other side of the fire and were looking at yourself. Your image, passing through the disturbed air region twice, would look that much more distorted. It's the same with our radio waves when they pass close by the sun on their way from the spacecraft near by Jupiter. Charged particles, the "solar wind" flowing outward from the sun, create an effect on radio waves much like the heat from a fire disturbs the air above it. The effect is intensified if we send an uplink to the spacecraft, and the spacecraft "reflects" the distorted signal back as the downlink.
It so happens we were sending an uplink this morning. The mission controller and I agreed that we should turn the uplink off. That way, we would receive a signal that had passed near the sun just once. It takes a radio wave over 50 minutes to travel one way from the earth to Jupiter now. We would have to wait twice that long to see if our idea worked. I began to feel some relief when my phase plot showed a large decrease at the expected time. And the relief was still greater when the voice from Australia announced they had "Reed-Solomon decoder successful." We knew then for certain the spacecraft was OK.
My phone rang this afternoon just as the sun was setting. It was solar radio scientist Richard Woo, apologizing for not getting back to me sooner, but he too had had a very busy day. He said that there had been a very large mass ejection from the outer portions of the sun (the corona) late on Monday (February 3). Traveling at 1000 kilometers per second, it would take this huge blob of hot plasma 9 hours to intercept the path between the earth and the spacecraft, where it would create havoc with our radio signal. The time of the coronal mass ejection (CME, as he called it) matched when we lost our data. Bingo!
Richard Woo told me there is a site on the World Wide Web [the URL of the home page is http://lasco-www.nrl.navy.mil/lasco.html, and click on "movies"] where you can watch a movie of the "Feb 3 CME". Oh, dear! I have to get some viewing software into my computer before I can do that.
My phone rang again just as I started this journal page. Richard Woo again. Though he had people waiting for him, he wanted me to know he had confirmed that there had been a "great" CME today. "Maybe great for solar scientists," I mutter, "but not so great for Galileo telemetry or the people who depend on it." Richard is excited by these events. He is coming in this weekend to correlate its timing with Galileo radio signal data type called doppler. I gave him the time the California station lost the Galileo data signal. We'll tie the loose ends up early next week. I stopped by the office of the Galileo mission director, the second-in-charge, to tell him of the CME. He said he will sleep better this weekend knowing we understand the data loss.
Now I'm going ride my motorbike the nine miles home to Sierra Madre. I expect to sleep well tonight also. Sunday, my wife Barbara and I plan to go to a movie, "Hamlet", that's playing in Pasadena. She said it's four hours long. I have to decide if I'm going to take my beeper, the one I have as a member of Galileo's anomaly resolution team. Yes, I decide, I will wear the beeper, but I'll put it on "vibrate" mode.
I wonder what new telecom problems I will work on in the new week.