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GPS Attitude Determination

GPS Attitude Determination. by Jinsuck Kim AERO 681 Department of Aerospace Engineering Texas A&M University. March 9th, 1999. Outline. Motivation Algorithm GPS carrier phase Integer ambiguity problem Traditional and improved methods Application Simulation

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GPS Attitude Determination

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  1. GPS Attitude Determination by Jinsuck Kim AERO 681 Department of Aerospace Engineering Texas A&M University March 9th, 1999

  2. Outline • Motivation • Algorithm • GPS carrier phase • Integer ambiguity problem • Traditional and improved methods • Application • Simulation • Hardware data verification

  3. GPS equipment : Trimble TANS Vector • Four antenna receiver to form three baselines • Provide attitude and navigation solutions

  4. Motivation • International Space Station (ISS) • Crew return vehicle in case of emergency • “Lost-in-Space” : unknown attitude and position • Place independent pseudolites on the ISS (GPS-like transmitters) • Need the relative attitude at the first stage of escape ISS transmitters Crew return vehicle escape

  5. Difference of carrier phase To GPS satellite Frequency = 1575.42 MHz, Wave length = 19cm

  6. Integer Ambiguity Problem • Static search • Finds a solution that minimizes the error residual • Provides a solution even when no motion has occurred • May converge to incorrect solutions (no unique solution) • Motion based method • Collect data for a given period of time and perform batch process • Inherently highly reliable • Takes longer time and requires sufficient relative movement

  7. Improved Algorithm • Quasi-static resolution (Cohen’s method) • Method has been successfully implemented • A prior attitude estimate must be given • Large-order matrix inversion may be required • New algorithm • Does not require any initial estimate • Requires less computational effort • Converge in significantly less time • Need at least three non-coplanar baselines • Minimize

  8. Linear Least Square (Cohen’s method)

  9. Nonlinear Least Square Result

  10. Application • Program development (current work) • Use Matlab or C compiler with fictitious S/C, GPS signal • Compare traditional methods with the improved method • Hardware simulation (next semester) • Import the data from JSC lab using actual GPS receiver (by Trimble) • Simulate LEO space crafts (ISS and crew return vehicle) • Compare linearized and nonlinear least square solutions • Future work : optimal pseudolites locations

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