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RC4600 ACU Board Set
3.2.6 Magnetic Variation
In order to calculate satellite pointing solutions, the antenna's orientation with respect to True North must
be known. The RC4600 uses a compass to measure the local horizontal component of the earth's
magnetic field. The earth's magnetic field is very irregular as shown in the following map.
The magnetic field also changes slowly over time. The following table shows how the magnetic variation
for Washington D.C. has changed over the last 250 years.
Year
1750
1800
1850
1900
1950
2000
The RC4600 uses the International Geomagnetic Reference Field (IGRF) model to determine the local
magnetic variation (difference between Magnetic North and True North). This model demonstrates how
the earth's magnetic field and how it is changing. The IGRF is based on world-wide observations and is
updated every five years. The IGRF model cannot account for short term effects such as magnetic
storms, etc.
Local magnetic variation is calculated given the antenna's latitude, longitude and the current date. The
magnetic variation calculation cannot account for isolated local anomalies or external effects (power lines,
train tracks, etc). The azimuth scanning functions (2.1.1.2.2.3.3 and 2.1.1.2.2.3.6) are used to
compensate for these heading calculation errors.
Chapter 2
Magnetic Variation
-3.3
-1.0
-2.5
-5.5
-7.5
-10.6
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