MARS EXPRESS INTERPLANETARY NAVIGATION FROM LAUNCH TO MARS ORBIT INSERTION: THE JPL EXPERIENCE “18TH INTERNATIONAL SYMPOSIUM ON SPACE FLIGHT DYNAMICS” Dongsuk Han (1), Dolan Highsmith (2), Moriba Jah (3), Diane Craig (4), James Border (5), Peter Kroger (6) (1 – 6) Member Technical Staff, Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena CA 91109 (USA), [email protected] ABSTRACT December 2003. The following day, MEX performed a The Jet Propulsion Laboratory (JPL), California Institute of Technology and the National Aeronautics maneuver to divert from the impact trajectory necessary for Beagle 2 to a path that allowed the spacecraft to and Space Administration have played a significant role enter orbit around the Red Planet. It arrived at Mars on in supporting the safe arrival of the European Space 25 December 2003, performing a flawless capture Agency (ESA) Mars Express (MEX) orbiter to Mars on maneuver [2]. 25 December 2003. MEX mission is an international collaboration between member nations of the ESA and A critical aspect of such interplanetary navigation is the NASA, where NASA is supporting partner. JPL's orbit determination (OD). The OD evaluates radiometric involvement included providing commanding and and interferometric tracking data to estimate the tracking service with JPL's Deep Space Network (DSN), spacecraft position and velocity, along with other in addition to navigation assurance. The collaborative dynamic modelling quantities like solar radiation navigation effort between European Space Operations pressure. These parameters are then used to predict the Centre (ESOC) and JPL is the first since ESA's last deep trajectory and design maneuvers to keep the spacecraft space mission, Giotto, and began many years before the on its intended path. MEX launch. This paper discusses the navigational experience during the cruise and final approach phase of JPL heritage on deep space missions stretches back for the mission from JPL's perspective. Topics include 40 years, highlighted by flight-tested capabilities for technical challenges such as orbit determination using trajectory modelling and orbit determination. The ESOC non-DSN tracking data and media calibrations, and Flight Dynamics Division (FDD) developed new modelling of spacecraft physical properties for accurate navigation software that required validation. The representation of non-gravitational dynamics. Also primary goal of the relationship between ESOC FDD mentioned in this paper is preparation and usage of and JPL Navigation was to produce comparisons of the DSN Delta Differential One-way Range (ΔDOR) output from their respective navigation tools, both measurements, a key element to the accuracy of the before and after launch, to seek agreement in their basic orbit determination. capabilities. 1. INTRODUCTION 1.2 JPL Navigation Responsibilities Mars Express was the first Mars interplanetary mission The specific responsibilities of NASA regarding the for the ESA, as well as its first deep space mission since MEX mission are set forth in a Memorandum of Giotto, a mission to Comet Halley in the mid-1980s [1]. Understanding (MOU) between ESA and NASA [3]. At that time, ESA established a relationship with the The MOU requires NASA to provide the following NASA JPL to assist with the deep space navigation. The navigation related items: success of the cooperation with Giotto led to the • Delta Differential One-way Ranging ( DOR); establishment of a similar relationship for tracking and Δ Approach navigation support; navigation services for the Earth-to-Mars phase of • NASA-derived tracking data types for ESA MEX. • validation of its navigation performance; • Pre- and post-launch navigation support in 1.1 Mars Express Orbit Determination consultancy, independent cross-verification of navigation, consultancy on interplanetary MEX launched 2 June 2003 from the Baikonur operational issues and necessary tracking data Cosmodrome in Kazakhstan. Trajectory correction produced from the NASA DSN to ESA; maneuvers were made along the way to precisely target • All relevant review information as required the release location of the Beagle 2 lander on 19 supporting the above items. 1.3 JPL Role in Operations • Maneuver summary, which included previously performed TCMs and WOLs reconstructed from Much of the cross-verification of navigation software spacecraft telemetry. and validation of tracking data types occurred prior to • Predicted maneuvers, which included the the MEX launch, making use of tracking data from the preliminary values for future TCMs and WOLs. Ulysses spacecraft [4]. The focus of this paper, • Spacecraft attitude quaternions. however, is the role that JPL Navigation played in MEX • Solar panel orientation (gimbal angles). flight operations during Earth-to-Mars cruise. Each input provided key dynamic modelling Activities performed included a final software cross- information that allowed for more accurate estimation verification test during an intensive two-week tracking of thrusting events, solar radiation pressure, and campaign two months after launch; implementation and trajectory prediction. As an example, without testing of ΔDOR measurements to greatly improve the accounting for the commanded attitude and solar panel accuracy of the orbit determination; and approach orientations, a noticeable dynamic mismodelling navigation support in the form of daily solution signature is introduced in the tracking data residuals. exchanges during the last several weeks before reaching This, in turn, can be aliased into the spacecraft state Mars. Products provided in addition to those required by estimation, producing a less accurate estimate of its true the MOU included estimates of the Beagle 2 orbit. atmospheric entry interface point and landing ellipse. The following sections provide details of each of these 2.2 JPL Models activities to summarize the JPL experience regarding The remainder of the dynamic modelling, including MEX Earth-to-Mars navigation. gravitational perturbations, solar radiation pressure, and spacecraft physical modelling, was performed with the 2. DYNAMIC MODELLING capabilities of the JPL Navigation software. The physical model of MEX has a direct impact on the Of the elements required to deliver a spacecraft accuracy of the SRP computation. In order to properly successfully to Mars, proper dynamic modelling of the model the SRP, JPL implemented a 6-component spacecraft is of prime importance. The gravitational model. This model consisted of three flat plates to influences upon the spacecraft are well understood and represent the spacecraft bus (taking into account the modelled in high fidelity. The forces that become less Beagle 2 lander attached to the spacecraft +Z face), two trivial to model are those due to non-gravitational flat plates to represent the solar arrays, and a self- sources. These forces can be understood as those due to shadowing parabolic dish to represent the High-Gain solar radiation pressure, trajectory correction maneuvers Antenna (HGA). The HGA was modelled as having a 5° (TCM), momentum wheel off-loadings (WOLs), and offset in the spacecraft X-Z plane rotated about the Y- possible outgassing events. Residual acceleration axis by –5°. The solar power arrays were nominally models may also be required due to inexact or pointed at the Sun. The spacecraft bus components were approximate modelling of the dynamics impressed upon oriented along the spacecraft bus-fixed axes. the spacecraft. The spacecraft areas and optical properties were 2.1 ESOC Inputs obtained primarily from the MEX Flight Dynamics Data Base (FDDB). These optical properties were then JPL Navigation relied upon ESOC Flight Dynamics to converted to the appropriate navigation software inputs. provide appropriate information concerning spacecraft Interestingly, a consistent 14% bias in solar radiation dynamics. As part of the JPL Navigation/ESOC FDD pressure acceleration was estimated by both JPL and agreement [5], a set of interface files was defined to ESOC. Because other dynamic mismodelling effects provide this information. The files contained such as outgassing are usually on the same order as the information related to the spacecraft attitude and solar radiation pressure acceleration, it is difficult to planned thrusting events. Other relevant files provided separate or differentiate the source of dynamic as a means of dynamic model verification were an mismodelling. inertial accelerations file and a solar radiation pressure acceleration file. Therefore, in order to refine the solar pressure model, a Various scripts were employed to ingest these interface quiescent time during cruise needed to be found so as to files and produce the appropriate inputs for the JPL allow for the estimation of solar pressure areas and navigation software. On a routine basis, the inputs used reflectivities without the possible aliasing effects from for orbit determination and trajectory modelling other sources (i.e. thrusting events, attitude changes, included the following: solar panel orientation changes, etc.). The quietest arc found (and used) was between August 6 and August 14, 2003. During this arc, there were only two momentum WOLs. The refined areas and reflectivities produced a JPL to produce global ionosphere maps. The solar pressure scale factor estimate near the desired troposphere calibration, however, required local weather value of 1.0, versus the previous value of 1.14, which measurements that are