Navigation Magic

"I must go down to the seas again, to the lonely sea and the sky,
And all I ask is a tall ship and a star to steer her by . . ."
John Masefield

The main objective of the navigators is to maintain the spacecraft on the planned trajectory for the duration of the mission. The navigation team provides the project with predictions of the trajectory of the spacecraft, planets, and Saturnian satellites. They also plan and generate the trajectory correction maneuvers (TCMs) required to maintain the spacecraft on the pre-planned trajectory. When playing billiards it is much easier to sink a long shot if, after the ball has been shot, you are allowed to nudge the ball a couple of times with the cue stick as the ball approaches the pocket. Without these small adjustments the spacecraft would miss Saturn by many millions of kilometers. After the fact, the navigators also reconstruct the spacecraft path for, among other things, comparison with science observations.

Clever navigation techniques

In order to plan for TCMs and measure the spacecraft trajectory, the navigators use a number of clever techniques to locate the spacecraft and celestial bodies precisely. Three basic data types are necessary towards this end: doppler, ranging, and optical navigation. Each data type provides a different type of information; when used in concert, the accuracy of spacecraft and celestial body measurements can be very high.

Doppler is a way to measure the speed an object that is approaching or receding from the Earth. In simple terms, a Deep Space Network antenna sends a signal up to the spacecraft which is then directly returned. If the spacecraft is approaching or receding from the tracking station, the signal is returned a tiny bit faster or slower respectively. If you've noticed how a car's beep, or the sound of an airplane engine sounds lower after it passes you by, you understand how doppler works. Measuring this difference in frequency can help pin down the spacecraft's speed in the solar system, and therefore give navigators clues as to precisely where it is headed.

Ranging uses the fact that light has a finite speed to determine the distance from the Earth to the spacecraft. Signals sent to the spacecraft are received and quickly returned, and the delay between the signal sent from the Earth and the same signal received is equivalent to the distance from the spacecraft to the Earth. Ranging is similar (but much more precise) to mailing a letter to yourself to see how long the postal service takes for delivery. When used together with Doppler the spacecraft's position and speed can be determined very accurately.

Optical data consists of pictures of celestial bodies against a star background taken with the spacecraft's cameras. The measurements extracted from these pictures can be used to determine where the spacecraft is with respect to the celestial body in the field of view; however, in many cases optical data is also used to determine where the celestial body is, and not the spacecraft. This is particularly important for the less well understood satellites of Saturn that have uncertain orbits.

Navigation for Cassini

The navigation for Cassini naturally separates into a number of nearly independent phases, each with their own demands. During the early part of the cruise segment, the focus is on successful planetary flybys, in particular that of Earth. The TCM or propulsive maneuver strategy around these planetary flybys typically requires three maneuvers between successive encounters: the first soon after the encounter to clean up error dispersions, and the second and third to provide an accurate delivery to the next encounter. In addition to these maneuvers there is a large deep space maneuver between the two Venus encounters.

After the Earth encounter, navigation is much the same as in the early part of cruise, except an additional propulsive maneuver is added before and after the Jupiter encounters due to the long time intervals of these periods.

During the Saturn approach the navigators complete preparations for tour navigation and calibrate the optical cameras to prepare them for taking optical data. Some optical images of the satellites are taken, and a flyby of Saturn's most distant satellite Phoebe planned for 19 days prior to Saturn arrival.

Saturn Orbit Insertion places the spacecraft on an orbit with a five month period. Near the farthest point in the orbit the spacecraft performs a large propulsive maneuver to raise the orbit's closest approach point to Saturn out of the main ring system and targets the spacecraft to Titan so the probe can be released. Twenty-four days before the Titan flyby, the spacecraft (and probe) are targeted to the exact probe entry point on Titan. Two days later, the probe separates from the spacecraft, and two days after that the spacecraft performs a propulsive maneuver to set up the flyby and probe relay events. Navigation leading to the delivery of the Huygens probe uses both Doppler and Range and images of Titan and the major Saturnian satellites.

During the tour the navigation activities support both the updating of the nominal tour trajectory and the control of it with cleanup and targeting maneuvers. The objective of navigation activities is to maintain the preplanned sequence and timing of activities while accounting for expected variations in the positions of the satellites and the atmospheric model of Titan. Tour navigation uses a mixture of Doppler and Range along with optical images of Titan and the other satellites. An average of six images are collected each day and transmitted to the Earth.

In general, the tour maneuver strategy uses three propulsive maneuvers between each encounter: a cleanup maneuver to correct for errors in the previous flyby, and two targeting maneuvers for the following flyby. The cleanup maneuver occurs two days after the flyby and requires tracking data to determine the errors in the flyby. The first targeting maneuver is generally placed when the spacecraft is furthest from Saturn, and the second occurs three days prior to the encounter.

Prior to Saturn approach , the positions of most satellites can only be known to within 1,000 to 3,000 kilometers (600 to 1900 miles). During the approach phase, optical images will be used to reduce this uncertainty to 100 kilometers (60 miles). Once in the tour phase, the uncertainty is expected to drop to several tens of kilometers. The positions of the satellites will be updated periodically as part of the navigation activities.

Prof. Manuel Grande
Last updated: 1 July 2004