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GPS Attitude Determination

This document presents novel approaches to GPS attitude determination, crucial for emergency scenarios like crew return vehicles from the International Space Station (ISS). It outlines a comprehensive algorithm that tackles the challenges posed by integer ambiguity and includes both traditional and improved methods. The study utilizes state-of-the-art GPS equipment, including the Trimble TANS Vector, with multiple antennas forming baselines to enhance accuracy. Applications are detailed, including simulations and hardware data verification, paving the way for future advancements in astronaut safety and navigation solutions. ###

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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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