Time-varying gravity and magnetic applications
Key research question: What is the contribution of climate scale changes in the geopotential field to navigation applications?
In addition to positioning services, NOAA supports marine and aviation navigation products. NOAA’s National Geodetic Survey works closely with NOAA’s Office of Coast Survey to support their marine navigation products and services. According to the 1974 International Convention for the Safety of Life at Sea (SOLAS), the use of a magnetic compass is mandatory on commercial shipping. As such, NOAA’s marine charts include the deviation value (in degrees) of the compass’ magnetic north from the true north. This deviation angle is called the magnetic declination and has been a challenge for mariners for many years. This is because the magnetic deviation angle varies with both geographic location and time.
Geological evidence and observations collected from magnetic observatories indicate that the Earth’s magnetic field has been constantly changing and the location of Earth’s magnetic north and south poles gradually shift. The location of Earth’s magnetic north pole has been monitored since 1831. The location of the north pole has been observed to be gradually drifting north-northwest by more than 1,100 kilometers over the past century.
At NOAA, the World Magnetic Model is considered the standard model for navigation, attitude, and heading referencing systems using the geomagnetic field. The National Geodetic Survey works closely with the National Geospatial-Intelligence Agency’s Office of Geomatics that produces the World Magnetic Model. NOAA’s Marine Charting Division at the Office of Coast Survey uses this magnetic model (current epoch is from 1/1/2019 to 12/31/2024) and monitors the total magnetic variation (Geomagnetic Declination) and its change on an annual basis (Secular Variation). They create geospatial features that are added to charts, such as Isogonic Lines ( lines connecting points of equal magnetic variation), Isoporic Lines (lines connecting points of equal annual rate of change of any magnetic element), and Agonic Line (lines connecting points where there is no magnetic declination and where a freely suspended magnetic needle indicates true north).
Another geopotential field used for navigation is the Earth’s gravity field. A high-resolution map of the gravity field can be used by autonomous vessels for navigation using principles similar to the ones used for navigating with a digital elevation model. The spatial variability of the gravity field creates features that provide horizontal location information, and the gravity field gradient (slope) can indicate if the platform is too close or too far from the ground. The type of systems that can navigate using high-resolution gravity field datasets are gravity gradiometers, and they can be used for navigation as stand alone systems or integrated with GNSS or attitude sensors. Gravity gradiometer measures specific force due to gravitational attraction, where spatial changes in the gravitational field are a result of the nonhomogeneity of the Earth, oblateness, the Earth’s rotation, and variations in the Earth’s terrain. The magnitudes of these differences are very small, and their measurement requires sensitive instruments. A gravity gradiometer is an instrument that measures the difference in the Earth’s gravitational acceleration across a known fixed baseline.
In 2019, the National Geodetic Survey established the Geoid Monitoring Service (GeMS) to account for geoid changes at climate-scale times. This product is actively investigating all potential physical processes that could modify the geoid over time and how to properly incorporate these changes into the national geospatial infrastructure and other civilian and military applications. The main requirement for GeMS is to monitor geoid change in North America to maintain a centimeter-accurate vertical datum. Input sources for GeMS include several geodetic space missions that collect gravity observations over the whole globe and provide an accurate height relationship between the gravity field with respect to a given ellipsoidal height that is used as a reference datum: the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow On (GRACE-FO), GFZ CHAllenging Minisatellite Payload (CHAMP), and Gravity Field and Steady-state Ocean Circulation Explorer (GOCE). The typical ground resolution of GeMS is on the order of 100s up to 1000s of km (medium- to long- wavelength).
Peer Review Publications and Conference Presentations
Ahlgren, K. 2022. “GeMS Validation Survey,” NGS Webinar series, May 22, 2022, Silver Spring, MD, USA. https://www.ngs.noaa.gov/web/science_edu/webinar_series/gems-validation-survey.shtml
Ahlgren, K., V. Childers, T. Damiani, R. Hardy, J., Kanney, N. Kinsman, J. Krcmaric, D. Roman, D. Smith, D. van Westrum, D. Winester, and M. Youngman. 2019. A Preliminary Investigation of the NGS's Geoid Monitoring Service (GeMS). NOAA Technical Report NOS NGS 69, Silver Spring, MD, USA, pp. 120. https://geodesy.noaa.gov/library/pdfs/NOAA_TR_NOS_NGS_0069.pdf
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