Kevin Criddle

THE GRAVITY FIELD OF THE SATURNIAN SYSTEM FROM SATELLITE OBSERVATIONS AND SPACECRAFT TRACKING DATA

We present values for the masses of Saturn and its major satellites, the zonal harmonics in the spherical harmonic expansion of Saturns gravitational potential, and the orientation of the pole of Saturn. We determined these values using an extensive data set: satellite astrometry from Earth-based observatories and the Hubble Space Telescope; Earth-based, Voyager 1, and Voyager 2 ring occultation measurements; Doppler tracking data from Pioneer 11; and Doppler tracking, radiometric range, and imaging data from Voyager 1, Voyager 2, and Cassini.

Integration of Spacecraft Telemetry into Navigation Operations for the Cassini-Huygens Mission

The Cassini orbiter is the largest and most complex interplanetary spacecraft ever built. Since attaining orbit around Saturn in the summer of 2004, Cassini, along with its Huygens probe, have been continually improving our understanding of Saturn, its satellites, its enigmatic rings system, and of the solar system. One of the hallmarks of the CassiniHuygens Project is the close working relationship between the many teams required to operate such a sophisticated spacecraft. Their ingenuity has enabled them to find new and different ways to improve their processes during Cassini’s prime 4-year orbital tour. This paper will discuss the relationship between Cassini’s Navigation and Spacecraft Teams and the work required to properly configure Cassini’s telemetry system for Navigation. A detailed explanation of how the Navigation Team utilizes spacecraft telemetry and analysis demonstrating the benefits will also be provided. Finally, telemetry requirements for Navigation for future missions will be addressed. I. Introduction Since the summer of 2004, the Cassini spacecraft orbiting Saturn has been working around the clock collecting historic science data and returning it to Earth. The science results have been well documented in papers, journals, mass media, and websites. What is not as extensively documented is the creativity and adaptability of the Cassini Project flight team members to continually improve the manner in which they operate the spacecraft. There are many teams within a project such as Cassini, each with specific responsibilities vital to the success of the mission, and each who must forge close working relationships. This paper will discuss in particular the relationship between the navigation team & the spacecraft operations team, especially the Attitude & Articulation Control Subsystem (AACS), and how continual collaboration between the two teams dramatically improved the spacecraft’s performance and the process by which the spacecraft is flown. Much of the joint effort between the two teams has been spent addressing AACS’s growing concern for the health and safety of the Reaction Wheel Assembly (RWA). A major source of the improvement for Navigation operations was extensive reconfiguration of the AACS telemetry. Examples of how the Navigation team makes the most of the AACS telemetry and its advantages are highlighted. Finally, based on the experience gained from Cassini, telemetry configuration requirements to better serve Navigation’s needs for future missions are offered.

Navigational Use of Cassini Delta V Telemetry

Telemetry data are used to improve navigation of the Saturn orbiting Cassini spacecraft. Thrust induced delta Vs are computed on-board the spacecraft, recorded in telemetry, and downlinked to Earth. This paper discusses how and why the Cassini Navigation team utilizes spacecraft delta V telemetry. Operational changes making this information attractive to the Navigation Team will be briefly discussed, as will spacecraft hardware and software algorithms responsible for the on-board computation. An analysis of past delta V telemetry, providing calibrations and accuracies that can be applied to the estimation of future delta V activity, is described.

Preliminary Maneuver Analysis for the Europa Clipper Multiple-Flyby Mission

A multiple-flyby mission to the Jovian moon Europa has been proposed. Currently known as the Europa Clipper, the primary objective of this mission would be to observe the science-rich environment of Europa. After a launch in 2021 and a 6.5-year cruise, the Europa Clipper spacecraft would orbit Jupiter’s system for a 3.5-year tour. During the Europa Clipper tour, propulsive maneuvers would be necessary to correct the spacecraft’s trajectory due to flyby dispersions. Maneuvers would be accomplished through the use of two independent propulsion systems. The bi-propellant main engine assembly performs large maneuvers, while the reaction control system thrusters handle small trajectory corrections. This paper presents the feasibility of the proposed tour by producing statistical ∆V results given by the reference trajectory and orbit determination covariance analysis. Preliminary results show that the tour’s statistical ∆V average would be approximately 4 m/s per flyby. This result is comparable to the Cassini Mission at Saturn statistical predictions prior to Saturn Orbit Insertion. However, the number of maneuvers within the typical petal orbit petal duration (i.e. approximately 14 days between Europa flybys) could present challenges to the operational schedule, including the placement of contingency maneuver opportunities. This paper describes the navigation-sensitive portions of the trajectory and offers recommendations to improve robustness.

Cassini Orbit Determination Performance (July 2008 - December 2011)

This paper reports on the orbit determination performance for the Cassini spacecraft from July 2008 to December 2011. During this period, Cassini made 85 revolutions around Saturn and had 52 close satellite encounters. 35 of those were with the massive Titan, 13 with the small, yet interesting, Enceladus as well as 2 with Rhea and 2 with Dione. The period also includes 4 double encounters, where engineers had to plan the trajectory for two close satellite encounters within days of each other at once. Navigation performance is characterized by ephemeris errors relative to in-flight predictions. Most Titan encounters 3-dimensional results are within a 1.5 formal sigma, with a few exceptions, mostly attributable to larger maneuver execution errors. Results for almost all other satellite encounter reconstructions are less than 3 sigma from their predictions. The errors are attributable to satellite ephemerides errors and in some cases to maneuver execution errors.