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GPS100 User’s Manual
Page 6 Revision 1.2
Magnetic variation has different values at different locations on the earth with most areas
undergoing change to the magnetic variation over time.
The GPS100 indicates magnetic variation anywhere on Earth using the World Magnetic Model
(WWM2005). It is important to understand that the WMM2005 geomagnetic model only
characterizes that portion of the Earth’s magnetic field that is generated in the Earth’s fluid
outer core (the main magnetic field). The portions of the geomagnetic field generated by the
Earth’s crust and upper mantle, and by the ionosphere and magnetosphere, are not
represented in the WMM2005. Consequently, a magnetic sensor such as a compass or
magnetometer may observe spatial and temporal magnetic anomalies when referenced to the
WMM2005. In particular, certain local, regional, and temporal magnetic declination anomalies
can exceed 10 degrees. Anomalies of this magnitude are not common but they do exist.
Declination anomalies of the order of 3 or 4 degrees are not uncommon but are of small spatial
extent and are relatively isolated.
On land, spatial anomalies are produced by mountain ranges, ore deposits, ground struck by
lightning, geological faults, and cultural features such as trains, planes, tanks, railroad tracks,
power lines, etc. The corresponding deviations are usually smaller at sea, and decrease with
increasing altitude of an aircraft or spacecraft. In ocean areas, these anomalies occur most
frequently along continental margins, near seamounts, and near ocean ridges, trenches, and
fault zones, particularly those of volcanic origin. Ships and submarines are also sources of
magnetic anomalies in the ocean.
However, from a global main field perspective, the declination (D), inclination (I), and grid
variation (GV) RMS errors of WMM2005 are estimated to be less than 1.0° at the Earth’s
surface over the entire 5-year life span of the model. Also, the RMS errors at the Earth’s
surface horizontal intensity (H), the vertical component (Z), and the total intensity (F) of
WMM2005 are estimated to be well below 200 nT over the entire 5-year life of the model.
Thus, the WMM2005 meets and exceeds the accuracy requirements detailed in MIL-W-89500
(Defense Mapping Agency, 1993) for the entire life span of the model. To learn more about the
WWM2005, please visit http://www.ngdc.noaa.gov/seg/WMM/DoDWMM.shtml.
4 Maintenance
Regular maintenance is important to ensure continued proper operation of the Maretron
GPS100. Perform the following tasks periodically:
• Clean the unit with a soft cloth. Do not use chemical cleaners as they may remove
paint or markings or may corrode the GPS100 enclosure or seals.
• Ensure that the unit is mounted securely and cannot be moved relative to the mounting
surface. If the unit is loose, tighten the mounting screws.
• Check the security of the cable connected to the NMEA 2000
®
interface and tighten if
necessary.