Data sources in Geoid modeling calculations
Research question: What data sources are we using today to measure the physical shape of the Earth’s surface, the geoid, and what potential new methods can we use in the future?
In order to define a regional (over North America and the Pacific Ocean) geopotential surface that will be used for the U.S. national geospatial infrastructure, the NOAA’s National Geodetic Survey (NGS) geoid modeling team incorporates a large suite of gravity observations that are acquired on land, on the water surface, in air, and out in space. The devices that measure gravity with respect to a uniform density gravity surface (i.e., the anomaly in the magnitude of gravity) are called gravity meters or gravimeters. Gravimeters typically measure in milliGals (1 Gal (Galileo) = 1 cm/s2 and g is about 106 mGal). Gravity observations have been conducted at NOAA since the 1890’s when NOAA was part of the U.S. government agency named the U.S. Coast and Geodetic Survey. Thomas Corwin Mendenhall, the superintendent of the U.S. Coast and Geodetic Survey at the time, developed a gravimeter that was used at over 340 survey stations across the US and around the world. In modern times, the specific gravity datasets that were used to develop GEOID2022 include:
Space Gravity Missions
Data collected from space gravity missions are used to reference airborne, marine, and land gravity measurements into a single reference system to describe features on the Earth’s surface (location and height). Although these missions provide global coverage and accurate height relationships between the static and time variable gravity fields with respect to the ellipsoidal height, the typical ground resolution from space missions is on the order of 100’s up to 1000’s of km (medium- to long- wavelength). Common space missions that collect gravity observations include: 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).
In addition, NOAA has incorporated model data from the Gravity Observation Combination (GOCO) project, a satellite-only global gravity field model computed from gravity observations acquired by 19 satellites over a 15-year period. NOAA is also investigating new space missions measuring the distance from the water surface to the satellite (satellite altimetry), such as the Ice, Cloud and land Elevation Satellite-2 (ICESat-2) and Surface Water and Ocean Topography (SWOT). These missions are used to evaluate the geoid products with average water level values.
Airborne Survey Campaigns
NOAA’s NGS conducts its own airborne gravity surveys. NOAA’s most significant airborne gravity survey campaign in recent years was a nationwide airborne gravity project called “Gravity for the Redefinition of the American Vertical Datum (GRAV-D)”. The project started in 2007 and was completed in 2023. In the NOAA geoid modeling effort, the GRAV-D data with a spatial ground resolution of up to 20 km is used to resolve differences and data gaps from gravity land survey. Although airborne gravity cannot produce high resolution gravity products that can be created from land and marine gravity surveys, airborne gravity observations provide uniform results of an area that typically spans 200-300 km in width and length. Calibration and maintenance of the airborne gravimeter is important for accurate survey data. An aircraft is subject to air drag, turbulence, and changes in the survey path — all which can impact the observation during the survey. In addition, flight time is only a few hours a day and the gravimeter needs to be calibrated on a regular basis due to the aircraft’s departure, flying, and landing. Airborne gravity data is collected and processed in sections, where most survey lines are at the same directions with some cross-lines for quality control.
Land gravity surveys
Land surveys are the most accurate and provide the highest resolution of gravity observations. They also require the most resources in personnel and time for a dense accurate survey. NOAA and the National Geospatial-Intelligence Agency have conducted many land gravity surveys in North America using two types of systems:
- Absolute gravimeters that can measure the total magnitude of the gravity acceleration by timing a falling object (nowadays) or a swinging pendulum (historically). The accuracy of absolute gravimeters is on the order of 10-9 of the gravity field. The two biggest challenges using absolute gravimeters are the survey time needed for collecting a measurement and the environmental conditions, i.e., a very stable and quiet environment (vibration free environment).
- Relative gravimeters that provide more flexibility for surveying. These mechanical contraptions are made with thermally stable and robust parts that use a mass on a spring principle to measure the gravity field to an order of 10-8, where the only changes in the gravity field are recorded and not the total value of the gravity field. For geoid modeling, it is possible to collect good data with 15 minute occupation, with a position determination resulting with at least 10 cm in accuracy.
Marine gravity surveys
Most of the marine gravity datasets used to develop the U.S. geoid model were collected by the National Geospatial-Intelligence Agency and academic partners. NOAA receives these high-resolution datasets that provide accurate coverage up to 200 km offshore. By definition, marine gravity surveys measure gravity at sea level, i.e., marine surveys are conducted very close to the geopotential surface. As such, data collected by marine gravity surveys do not require terrain corrections that are needed for land gravity surveying in order to correct for elevation. One of the main corrections to the data is to address the ship's motion at sea that may impact the observation. As such, marine gravity observations are averaged over a distance (low-pass filter) to damp the ship’s motion “noise” but this compromises the ground resolution. Gimbals and a gyro-stabilized platform are typically used to mechanically reduce the ship’s motion on the gravimeter.
Gravity Reductions
Prior to geoid modeling processing within NOAA, all the available gravity datasets (space, airborne, land and marine) collected over North America and the Pacific Ocean are cleaned for outliers and errors related to the platform or the instrument, and are reduced based on the error characteristics of each data set. The calibration and corrections applied to different datasets prior to the geoid modeling are typically handled by other teams at NOAA’s NGS or contractors before the geoid team compiles the data to a geoid model. The corrections prior to geoid modeling are known as “gravity reductions”, and include, for example:
Tidal corrections address periodic gravitational variations in a fixed location due to the orbital motions of the Sun and Moon. In spite of the Moon’s mass being much smaller than the Sun, its gravitational attraction is larger than the sun because of its proximity to the Earth. It is also important to note that Solid Earth tides are considerably smaller than oceanic tides, and they cause the elevation of a point to change by a few cm.
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Emerging technologies
In recent years, scientists investigated alternative methods to observe gravity shifts that do not require a pendulum or a free-falling object. One such method is the use of optical atomic clocks to measure gravity potential differences between specific sites. Atomic clocks at different elevations in a gravitational field tick at different rates. A clock in a stronger gravity potential (e.g., Closer to the Earth’s surface) will run slower than a clock in a weaker gravity potential (e.g., at higher elevation). As a result, the frequency of a clock in a stronger gravity potential is reduced and shifted toward the red end of the electromagnetic spectrum, providing an indication to a change in the gravity field. Another approach is the use of small magnets to measure changes in the gravity field. By generating a magnetic field, it is possible to levitate objects and counteract Earth’s gravity. Tiny changes in the magnetic field of the magnet created by the gravitational influence of nearby objects can then be converted into a measure of the gravitational force.
Peer Review Publications and Conference Presentations
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