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
"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.
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.The Doppler effect is used to measure the speed at which a spacecraft is approaching or receding from the Earth. A Deep Space Network antenna sends a signal up to the spacecraft, which then returns it. If the spacecraft is approaching or receding from the tracking station (or if the Earth is moving toward or away from the spacecraft), the frequency of the signal returned appears to be a little higher or lower, respectively. If you've noticed how the sound of an airplane engine sounds lower after it passes, 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.
One might wonder why a signal is normally sent up from the Earth to be returned by the spacecraft, to measure speed via Doppler effect. Why not just listen to the spacecraft without sending it a signal? It's a matter of obtaining a highly precise measurement. The frequency sent from Earth is extremely stable (unvarying), because it is generated by high-precision equipment (a hydrogen maser), maintained and operated in a carefully controlled environment. The transmitter on the spacecraft, however, is small, lightweight, and subject to temperature variations. Thus it cannot by itself achieve the needed stability. Instead, the spacecraft's transmitter takes advantage of the stable uplink signal from Earth, to generate an equally stable downlink signal to send back to Earth. (To avoid interference problems, the spacecraft multiplies the received signal by a known fraction before returning it.) The resulting Doppler tracking system is able to measure a spacecraft's speed to within a fraction of a millimeter per second.
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.
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.
