OFJ Field Journal from Lou D'Amario - 11/2/95

CONTROLLING THE ALTITUDE OF THE IO FLYBY

On October 25, the Navigation Team completed the final test and training exercise before the Jupiter encounter. This final test simulated the work the Navigation Team does for the "JOI Tweak". What is the "JOI Tweak"? Between now and the Io flyby (which occurs four hours before Galileo's closest approach to Jupiter), the mission plan allows for three trajectory correction maneuvers (TCMs) to control the flyby conditions at Io. The Io flyby reduces the speed of Galileo (called a "gravity assist"); this reduces the velocity change (known as "delta V") required at the Jupiter Orbit Insertion (JOI) burn by about 175 meters per second (378 mph), which results in a large propellant savings. Since the spacecraft has a limited amount of propellant, it's always good news to hear that we can save some more.

As the spacecraft gets closer to Io, the Nav team's aiming accuracy increases. With the final TCM (five days before the Io flyby), we are able to control the altitude of the Io flyby to within plus or minus 115 kilometers of the desired value of 1000 kilometers. If Galileo passes closer to Io than 1000 kilometers, the spacecraft's speed is reduced more than was planned; in that case, we can reduce the JOI velocity change and save propellant. On the other hand, if Galileo passes further from Io than 1000 kilometers, its speed is reduced less than planned, and the size of the JOI delta V needs to be increased (using more propellant).

So, the "JOI tweak" is simply a late adjustment of the JOI burn, based on our improved knowledge of the altitude of the Io flyby. The final TCM, which takes place 5 days before the Io flyby (or, in Nav shorthand, Io - 5 days), is based on radio tracking data up to Io - 6 days. Three days later, with three additional days of tracking, the uncertainty in the altitude of the Io flyby drops to plus or minus 75 kilometers. Based on this improved estimate of the Io flyby altitude, the Navigation Team computes a new JOI delta V. Then, commands are prepared and sent to Galileo to change (or "tweak") the JOI delta V value in the computer on board the spacecraft.

On November 1, the Relay/JOI Readiness Review was held. Managers and analysts on the flight team presented to a review board the operations plans and spacecraft sequences for receiving and storing on the Orbiter the data sent from the atmospheric Probe and for performing the JOI burn. I was one of many people who gave a presentation. My presentation, which took most of about a week to prepare, discussed the navigation results for the Orbiter Deflection Maneuver (ODM) and TCM-26.

ODM, which was performed two weeks after the atmospheric Probe was released, was the first maneuver to target Galileo for its flyby of the moon Io and the first TCM to use the big 400 Newton engine (although it uses different letters, the Orbit Deflection Maneuver is a Trajectory Change Maneuver, too). Trajectory Change Maneuvers are never perfect, because 1) the navigators do not have perfect knowledge of where Galileo is and where it is headed at the time of the TCM and 2) the actual change in velocity performed by the rocket thrusters on Galileo never turns out to be exactly what was commanded. So we keep performing TCMs until the errors are so small that they no longer matter. TCM-26 was performed about one month after the ODM as part of this trajectory error correction process. The improvements are fairly significant: after ODM, the Io flyby altitude was about 1800 kilometers away from the desired value of 1000 kilometers. After TCM-26, the miss will be no more than a few hundred kilometers (and perhaps a lot less).

The next Trajectory Change Maneuver (TCM-27) is scheduled for November 17 (20 days before the Io flyby, or Io-20). The design of TCM-27 starts next week on November 8 (nine days before the maneuver is performed). My next journal entry will contain the results of the design of TCM-27. Stay tuned!