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“Attitude on a Chip” Single Antenna GPS Attitude
(SAGA)
E. Glenn Lightsey
Associate Professor, Aerospace Engineering
The University of Texas at Austin
Problem
Autonomous vehicles are undergoing miniaturization• Satellites, unmanned aerial vehicles (UAVs), robotics
Sensors must be available that provide required information• Many sensors are not suited to miniaturization for size, cost,
and complexity reasons
• A device capable of determining platform attitude pointing to within a few degrees
Based on a GPS receiver, it also provides:• position, time, and relative solutions
Highly suited for vehicle automation
Highly suited for miniaturi-zation (could ultimately fit on a single chip and antenna) current working
prototype< 2” x 2” x 4”
Our invention
Principle of operation
GPS signal-to-noise ratio (SNR) measurements, when coupled with an external vector sensor, such as a Three-Axis Magnetometer (TAM), can be used to provide coarse attitude determination accurate to within 5˚ to 10˚.
SNR g * (1+cos())
cos() = Â • L
 is the antenna boresight vector
L is the normalized LOS vector to the GPS satellite
is the incident angle of incoming GPS signals, called the antenna off-boresight angle
ˆ
ˆ
GPS measurement model
A quaternion point rotation can be used to relate the nominal antenna boresight vector with the rotated body-fixed vector:
Ârot = (2q2 – 1) Âk + 2(q • Âk) q + 2q0 (Âk × q)
The SNR measurement model is the cosine of the angle between this rotated antenna boresight vector and the corresponding GPS line-of-sight vector:
G1 = Ârot • L = cos()
Y1 = ƒ(SNR)
ˆ
0
Three-axis magnetometer (TAM)
Simulated investigation• Truth – 10th order IGRF
2000 model
• Measurements – 6th order IGRF 1995 model plus 0.3mG measurement noise
Spacecraft implementation• Honeywell HMC2003 three-axis magnetic
sensor
• Atmel ATmega128 microcontroller interface circuit
Testing results
Formation Autonomy Spacecraft with Thrust, Relnav, Attitude, and Crosslink
Two student-built nanosatellites delivered to U.S. Air Force Research Labs
Expected launch in 2007
SAGA will fly on two satellites