“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 Â 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:  rot = (2q 2 – 1)  k + 2(q  k ) q + 2q 0 ( k × 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: G 1 =  rot L = cos(  ) Y 1 = 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