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CLOCK ALIGNMENT, ORBIT DETERMINATION QUALITY ASSURANCE

This document outlines strategies for ensuring clock alignment and orbit determination quality assurance for the GRACE USO satellite. The relative clock accuracy needs to be within 0.16 ns for a 0.5 µm range error and the common clock bias under 100 ns to maintain orbital precision. The use of advanced geometric and K-Ka light time corrections, including GPS sync and incorporating CHAMP data for overlap analysis, is discussed. Additionally, the necessity of parameter tuning for in-flight quality assurance is emphasized, along with latency considerations and expected improvements.

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CLOCK ALIGNMENT, ORBIT DETERMINATION QUALITY ASSURANCE

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  1. CLOCK ALIGNMENT, ORBIT DETERMINATIONQUALITY ASSURANCE od_constellation.pl tdp2clk1b.pl gni1a2gnv1b.pl kbr_prefit.pl

  2. Why • GRACE USO’S Run Free • Relative clock needs to be < 0.16 ns for 0.5 m range error • Common Clock Bias < 100 ns => < 0.7 mm orbit error • Orbit Determination • Geometric Corrections to Phase Center • K-Ka Light Time Correction

  3. How, od_constellation.pl, + • GPS fixed to FLINN Solution • Currently 3 cm comparison to IGS • < 3 cm scatter from high elevation SLR with older less accurate FLINN • Latency ~10 days • Solution Strategy • 30 hour data arcs (6 hour overlaps) • Reduced Dynamics • Accelerometer may be incorporated later • White noise clocks • Epoch State, Drag, Solar Scale, Stochastic Accelerations

  4. Functional Tests • Use CHAMP data in place of GRACE • 3 Days Successfully Processed for GRACE A & B • Some parameter tuning will be necessary for GRACE

  5. In Flight Quality Assurance

  6. Preflight Quality Assurance • Look at CHAMP orbit and clock overlaps • CHAMP has a poor oscillator • Look at Ground Station Clock Synchronization with GPS

  7. CHAMP Clock Overlaps

  8. CHAMP Clock Overlaps

  9. Station Clock Tests Expected Improvements: USN1, Currently 10’s ns reference clock changes

  10. CHAMP Orbit Overlaps

  11. Summary • Clock Calibration/Orbit Determination Process is preflight ready • Some Post launch tuning is necessary

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