D-BAUG - Departement Bau, Umwelt und Geomatik - ETH Zürich

Highlights ▪ High-Tech Measuring Systems
GPS as Rotational Sensor
Can GPS be used to estimate the orientation
and rotational velocity of the antenna?
by S. Häberling, A. Geiger / IGP
To this day, the satellite-based navigation systems such as the
Global Postioning System (GPS) are mainly used for positioning
applications. An additional method to retrieve 3D rotation anglesfrommeasurementswithasingleGPSantennainreal-time
was demonstrated by a master thesis.This information about
the angular velocities and the full 3D rotation in addition to the
position would enlarge the applicability of GPS in machine control
and air navigation.
The method is using the properties of the circularly polarized
electromagnetic waves that are transmitted by the GPS satellites.
The rotation of the receiver antenna relative to the satellite
antenna generates a phase shift called “phase wind-up”
(Fig.1).Depending on the type of rotation and on the elevation
angleof theincomingsignal thisphaseshift,measuredfromdifferentsatellites,allows
thedescriptionof thefull3Dantennarotation.
For the detection of small phase shifts in GPS raw data it is necessary
to build a linear combination between signals with different
frequencies to eliminate error sources. After a statistical
signal processing with appropriate filtering the results indicate
that minimalrotationalvelocitiesof3–3.5°/saresignificantlydetected
depending on time of day (e.g.ionosphere) and satellite
constellation.Thealgorithmdevelopedfor thedeterminationof
the full 3D rotation generates stable solutions.This was verified
by simulations and real measurements (Fig.2).
74 ▪ D-BAUG Annual Report 2009
Fig. 1: A simplified schematic illustration of an incoming circularly
polarized electromagnetic wave. Due to the rotation of the
GPS antenna (blue to red) an angle (phase shift Δφ) occurs between
the two dipole vectors (dashed). They can be described as
a projection from the polarization plane (grey) onto the antenna
plane.
Fig. 2: Measurement setup with the rotation simulator and the
mounted GPS antenna.