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 –Hardware data verification

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

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 Crew return vehicle escape transmitters

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

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

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

Linear Least Square (Cohen’s method)

Nonlinear Least Square Result

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