World Magnetic Model (WMM) (original) (raw)

Background

The WMM consists of a degree and order 12 spherical-harmonic main (i.e., core-generated) field model of 168 spherical-harmonic Gauss coefficients and degree and order 12 spherical-harmonic Secular Variation (SV) (core-generated, slow temporal variation) field model. WMM2025 supersedes WMM2020 and should replace this model in navigation systems. Also included with the model is computer software (available in C) for computing the magnetic field components X, Y, Z, F, D, I, and H in geodetic coordinates and their temporal variations. The spherical-harmonic expansions used to compute the magnetic field components are the same as the ones described in The US/UK World Magnetic Model for 2025–2030: Technical Report.

Earth's magnetic field, as measured by a magnetic sensor on or above Earth's surface, is actually the sum of magnetic fields generated by a variety of sources. These fields are superimposed on each other and through inductive processes interact with each other. The most important are these geomagnetic fields:

The observed magnetic field is a sum of contributions of the main field (varying in both time and space), the crustal field (varies spatially, but considered constant in time for the time-scales of the WMM), and the disturbance fields (varying in space and rapidly in time). Earth's main magnetic field dominates, accounting for over 95% of the field strength at Earth’s surface. Secular variation is the slow change in time of the main magnetic field. The WMM represents only the main geomagnetic field.

To create an accurate magnetic field model, it is necessary to have vector component measurements with good global coverage and low noise levels. The European Space Agency's SWARM satellite mission is presently the most suitable magnetic observing system. Also available are ground observatory hourly mean data, although with poorer spatial coverage. The observatory data can provide valuable constraints on the time variations of the geomagnetic field.

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Compasses have been used for several thousand years to determine direction. They point in the direction of magnetic force at the user's location, and the direction it points is, more often than not, in a different direction than geographic north (toward the North Pole). A more precise direction is achieved by knowing the angle between them (magnetic declination). However, declination changes with location and time, and a geomagnetic model is often used to correct for it. Since the changes in geomagnetic fields are difficult to predict, timely model updates (every five years for the WMM) are required for navigational accuracy. The WMM satisfies all these criteria and is therefore widely used in navigation. Examples include, but are not limited to, ships, aircraft, and submarines. Aircraft and satellites commonly use magnetometer based attitude (roll and pitch) control.

GPS

Why do we need magnetic navigation when the Global Position System (GPS) is readily available? GPS provides precise point location but only measures travel direction when in constant motion. A GPS receiver must collect several sets of latitude and longitude pairs to obtain direction. In addition, GPS signals may become blocked due to obstructions, adverse terrestrial and space weather, ionospheric conditions, or being underwater or underground. Hence, compasses complement GPS receivers to attain precise and immediate navigational headings for air, ground, and water-based systems. Electronic compasses and the WMM commonly co-exist in GPS receivers.

Antennas and Solar Panels

Antennas (e.g. satellite dish television) and solar panels often need to be precisely oriented for maximum performance. The WMM's declination information for specific locations is often employed by companies to orient their products correctly.

Consumer Electronics

While the WMM is traditionally used for navigation, it is now acquiring new utilities in consumer electronic devices with built-in digital compasses. Many of the new generations of smartphones and digital cameras take advantage of the WMM to estimate bearing. The availability of low-cost, small, and energy efficient electronic compasses allow for magnetic direction in portable electronics to be commonplace. NOAA is providing support to application development engineers to port WMM to their devices. For example, WMM comes pre-installed in Android and iOS devices, thereby bringing its use to more than a billion devices around the world. NOAA/NCEI has developed an application called CrowdMag that allows users to collect their own magnetic field data using the magnetometers in their phone. This app sends data anonymously back to NOAA so it can be used to help validate and expand future magnetic models.

Mineral Exploration

Oil and mineral exploration companies use airborne and marine magnetic fields to detect magnetic signals from Earth's crust. These small amplitude signals (typically 100s of nT), must be separated from the large main magnetic field (typically 20,000 to 60,000 nT). Companies use geomagnetic models to extract these small magnetic signals from the survey records. A new application uses geomagnetic models for directional drilling. Oil wells are often drilled horizontally from a conveniently located platform. An electronic compass located behind the drill head (bit) provides the engineers with accurate orientation of the bit.