T - Sapienza PDF
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Sapienza University of Rome The largest European university, with a variety of curricula in many fields. Department of Mechanical and Aerospace Engineering >65 professors and staff researchers, >60 PhD students and research assistants. Research acCviCes in space systems, aerospace structures, propulsion, flight mechanics and fluidodynamics. EducaNon and Degrees -‐ F A E IRST LEVEL DEGREE IN EROSPACE NGINEERING -‐ SECOND LEVEL DEGREE IN SPACE ENGINEERING -‐ SECOND LEVEL DEGREE IN AERONAUTICAL ENGINEERING -‐ M S O P ASTER COURSE IN ATELLITES AND RBITING LATFORMS -‐ M S T S ASTER COURSE IN PACE RANSPORTATION YSTEMS -‐ P D S A T H PROGRAM IN PACE AND ERONAUTICAL ECHNOLOGIES -‐ > 1500 STUDENTS -‐ M E ANY SECOND LEVEL COURSES ARE TAUGHT IN NGLISH -‐ M P D E ASTER AND H COURSES TAUGHT IN NGLISH hFp://www.ingaero.uniroma1.it/ Outline of seminars Tuesday morning 1) Deep Space Navigation Systems: Where do we stand? 2) The European Delta-DOR correlator Tuesday afternoon 3) Bepicolombo: The ESA mission to Mercury; Mercury Orbiter Radio science Experiment (MORE) Wednesday morning: 4) The scientific use of deep space tracking systems Wednesday afternoon: 5) Radio science in deep space missions D S T S EEP PACE RACKING YSTEMS ? WHERE DO WE STAND L I UCIANO ESS D . EPT OF MECHANICAL AND AEROSPACE ENGINEERING U L S NIVERSITÀ A APIENZA BITTT -‐ 29/01/2013 Outline – Seminar 1 -‐ O S VERVIEW OF DEEP SPACE TRACKING SYSTEM -‐ O : , , ΔDOR BSERVABLES RANGE RANGE RATE -‐ C : URRENT ACCURACIES EXAMPLES FROM PAST MISSIONS -‐ E ΔDOR RROR BUDGET FOR RADIO METRIC OBSERVABLES AND -‐ R : 10 OADMAP FOR IMPROVEMENTS HOW TO DO TIMES BETTER The History of Deep Space NavigaNon Accuracy 6 Basic concepts State of a dynamical system: the set of parameters required to predict the future motion of the system. For OD, the minimal set are (r,v) at a given time. Dynamical model: the dynamical laws governing the evolution of a system in time The dynamical state is almost never directly measurable. One uses measurements linked in some way to the dynamical state, such as range, range-rate, angles, etc. When an estimate of the trajectory is obtained, the orbit can be predicted using the dynamical model. The predicted values differ from the true ones because of: Basic concepts • Errors or inadequacy of the dynamical model • Measurement noise, limiting the accuracies of the observations • Errors in the numerical procedure (including truncation and roundoff errors) As a consequence the state determination must be repeated. It is virtually impossible to propagate the state of a spacecraft for very long times without incurring in serious discrepancies. The time interval between updates depends on the accuracy of the dynamical model, the quality of the observations, and the needs of the user. In this seminar I will focus on item 2 and 3 (measurement noise and numerical noise). DifferenNal correcNons Accelerometer Radio Tracking readings Dynamical Computed Observed model observables observables - Observation model Residuals NO Differential corrections Cost function at a of parameters and state local minimum? YES Updated state vector and dynamical model, estimation errors Measurements Good results start from good data • For interplanetary missions, the following measurements are used • Doppler -‐frequency shiR in the carrier -‐ measures the range rate from the ground antenna to the spacecraR antenna • Cassini used X and Ka band with accuracies up to 0.5 mm/sec (1s integraCon Cme); best results: 0.001 mm/s (Cassini radio science, 1000 s) • Since the Earthʼs moCon affects the range rate, informaCon on the right ascension and declinaCon can be obtained. • Accuracy depends up the stability of the ground reference signal (clock), spacecraR down link is coherently locked to the uplink signal. • Range -‐ Measurement of the round trip light Cme • Range typical accuracy of 1 meters (1s) -‐ subject to long term, systemaCc effects
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