RC3000 Antenna Controller Appendix HGPS Hemisphere GPS Compass
HGPS-2
The following simple diagram illustrates the situation of the two antennas measuring the signal phase
from one GPS satellite.
The vector between the two antennas is often referred to as the baseline. One antenna will be
designated as the primary antenna and the other as the secondary antenna. The true heading derived
will represent the heading from the primary antenna to the secondary antenna.
The GPS compass engine can measure the instantaneous phase of the carrier at both antennas but it
can't directly determine the total number of carrier waves between the antennas. The compass engine
must be told the actual distance between the two antennas. Knowing the baseline length and phase
information from multiple GPS satellites, the compass engine sorts through the many (possibly thousands
of) combinations of orientations between the two antennas. The compass engine's algorithm eventually
determines what heading must exist between the two antennas in order to fit all the data collected.
This discussion of the GPS compass algorithm is extremely rudimentary but is presented in order to
highlight some critical items associated with the installation of the unit:
1) the most critical item is that the two antennas be positioned where they have unobstructed views of the
sky. If the antennas don't receive signals on a direct path from the GPS satellites, the phase measured
won't be representative of the true geometry of the antenna array. If the direct path to a GPS satellite is
blocked, the antennas may still receive a signal reflected off of some surface (multipath) in the area of the
antennas. This reflecting surface may be buildings in the area or other structures associated with the
satellite antenna system. The GPS compass algorithm can't distinguish between signals received directly
or from a reflected path. It is possible that the algorithm can determine an incorrect heading solution
based on multipath signals. Note that the high performance antennas supplied have a large ground plane
area in order to minimize the chance of multipath reception.
The Hemisphere manual states that the antenna array must be positioned with no structures
above its horizon.
2) the distance between the antennas must be carefully measured and programmed into the compass
engine via the RC3000 antenna controller.
Procedures to address these items are discussed in the installation section.
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