“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