Draft:Satellite gravity

Satellite gravity is the study of Earth's gravity field using satellites. It focuses on how changes in the distribution of snow, ice, and the Earth's crust affect Earth's gravitational field.^[1] It takes advantage of how gravity always keeps satellites in orbit.^[2] According to Newton’s law of gravitation, the gravitational field between two objects has a linear relationship with the product of their masses and an inverse relationship with the square of the distance between them.^[3] Satellite gravity uses terrestrial, airborne, shipborne, and satellite sensors.^[4]

History

Since the beginning of the space age in 1957, satellite geodesy, a new area of geodesy, was created.^[5] At that time, the goal of satellite gravity was to obtain detailed insights about Earth's gravity field.^[6] At that time, Sputnik-1 was put into orbit by the Soviet Union on October 4, 1957.^[7] Later that same year, on November 3, Sputnik-2 was also put into orbit.

Earth's gravitational field

Earth's gravitational field reflects its surface mass redistribution and its inner structure.^[8]

Satellite orbit

If a satellite passes above a mass inhomogeneity or anomaly, its orbit will be perturbated, such as increasing or decreasing its distance from the Earth.^[9] The closer a satellite is to the Earth, the more sensitive it is to the gravity of the object below it.

The satellite motion and orbit statistics are observed to generate insights about the forces acting on the satellite.^[10]

Research

Researchers typically use data from satellites that:^[11]

orbit as low as possible (usually 200 &endash; 500 km from the ground)
uninterrupted orbit
separation of gravitational and non-gravitational forces

Observations

The main observation principles are satellite-to-satellite tracking in two modes:

High-low mode

In satellite-to-satellite tracking in the high-low mode (SST-HL), a satellite that orbits Earth from a lower altitude is tracked by other satellites from above.^[12] The lower satellite is a probe within Earth's gravity field. Observed tridimensional accelerations are associated with gravitational accelerations. Non-gravitational forces acting on the satellite are measured using accelerometers.

Low-low mode

In satellite-to-satellite tracking in the low-low mode (SST-LL), two satellites are placed in the same orbit with a large distance between them.^[13] The difference in acceleration of both satellites is precisely measured. Changes in the gravity field of either satellite affecting the distance between both satellites is also considered.

Geophysics

The gravitational models from satellite gravity are important in many geoscience applications such as geophysics, hydrology, and glaciology.^[14]

References

^ "The Earth is a dynamic system—it has a fluid, mobile atmosphere and oceans, a continually changing distribution of ice, snow, and groundwater, a fluid core undergoing hydromagnetic motion, a mantle undergoing both thermal convection and rebound from glacial loading of the last ice age, and mobile tectonic plates. These processes affect the distribution of mass in the Earth and produce variations in the Earth's gravitational field on a variety of spatial and temporal scales (Figure 1.1)." - [1] (The National Academies Press)
^ "Gravity keeps satellites in orbit" - [2] (Science Learning Hub)
^ "According to Newton’s law of gravitation, the attraction between two bodies is proportional to the product of their mases and inversely proportional to the square of the distance between them." - [3] (IAG website)
^ "Gravimetry, an experimental method providing data about this potential field, utilises terrestrial, airborne, shipborne, and satellite sensors." - [4] (ScienceDirect)
^ "With the beginning of the space age in 1957, a new branch of geodesy was created, satellite geodesy." - [5] (PMC)
^ "Over the first three decades of space techniques, the emphasis of gravimetry was to obtain a more detailed determination of spatial variations in the Earth's static gravity field (Table 1.1). All of these variations also affect the elevation of the Earth's surface, which is usually easier to measure than the gravitational field. However, there are differences of effect reflecting the depths of the causative density variations, and hence their nature. Consequently, since the inference of isostasy well over a century ago, gravity has always been analyzed in conjunction with topographic elevation." - [6] (The National Academies Press)
^ "With the entry into the space age, Sputnik-1 was put into orbit by the Soviet Union on 4 October 1957—satellite gravimetry also began..." - [7] (PMC)
^ "The gravitational field of the Earth reflects Earth's surface mass redistribution and its inner structure and dynamics." - [8] (ScienceDirect)
^ "If a satellite passes above an Earth’s mass inhomogeneity (or anomaly), its trajectory (orbit) has a perturbation, i.e., the satellite position gets closer or further away from the Earth." - [9] (IAG website)
^ "The satellite motion and the parameters describing the orbit are observed to draw conclusions about the forces acting on the satellite." - [10] (IAG website)
^ "The main characteristics of these satellite missions are: ..." - [11] (IAG website)
^ "Satellite-to-satellite tracking in the high-low mode (SST-HL): a low Earth orbiting (LEO) satellite is tracked by high orbiting GNSS satellites, relative to a network of ground stations. The satellite is a probe within the Earth’s gravity field, which can be precisely tracked without interruption. The observed 3-D accelerations correspond to the gravity accelerations. The non-gravitational forces acting on the satellite are measured by accelerometers." - [12] (IAG website)
^ "Two LEO satellites are placed in the same orbit, separated by several hundred kilometres, and the range (distance) between them is measured with the highest possible accuracy. Basically, the acceleration difference between the two satellites is measured. The two satellites can be considered as one instrument in which ..." - [13] (IAG website)
^ "The global gravitational models from satellite gravimetry, typically in terms of spherical harmonic coefficients, are crucial in geodetic, geodynamic, geophysical, hydrological, glaciological, oceanographic, and many other geoscience applications." - [14] (ScienceDirect)

[1] "The Earth is a dynamic system—it has a fluid, mobile atmosphere and oceans, a continually changing distribution of ice, snow, and groundwater, a fluid core undergoing hydromagnetic motion, a mantle undergoing both thermal convection and rebound from glacial loading of the last ice age, and mobile tectonic plates. These processes affect the distribution of mass in the Earth and produce variations in the Earth's gravitational field on a variety of spatial and temporal scales (Figure 1.1)." - [1] (The National Academies Press)

[2] "Gravity keeps satellites in orbit" - [2] (Science Learning Hub)

[3] "According to Newton’s law of gravitation, the attraction between two bodies is proportional to the product of their mases and inversely proportional to the square of the distance between them." - [3] (IAG website)

[4] "Gravimetry, an experimental method providing data about this potential field, utilises terrestrial, airborne, shipborne, and satellite sensors." - [4] (ScienceDirect)

[5] "With the beginning of the space age in 1957, a new branch of geodesy was created, satellite geodesy." - [5] (PMC)

[6] "Over the first three decades of space techniques, the emphasis of gravimetry was to obtain a more detailed determination of spatial variations in the Earth's static gravity field (Table 1.1). All of these variations also affect the elevation of the Earth's surface, which is usually easier to measure than the gravitational field. However, there are differences of effect reflecting the depths of the causative density variations, and hence their nature. Consequently, since the inference of isostasy well over a century ago, gravity has always been analyzed in conjunction with topographic elevation." - [6] (The National Academies Press)

[7] "With the entry into the space age, Sputnik-1 was put into orbit by the Soviet Union on 4 October 1957—satellite gravimetry also began..." - [7] (PMC)

[8] "The gravitational field of the Earth reflects Earth's surface mass redistribution and its inner structure and dynamics." - [8] (ScienceDirect)

[9] "If a satellite passes above an Earth’s mass inhomogeneity (or anomaly), its trajectory (orbit) has a perturbation, i.e., the satellite position gets closer or further away from the Earth." - [9] (IAG website)

[10] "The satellite motion and the parameters describing the orbit are observed to draw conclusions about the forces acting on the satellite." - [10] (IAG website)

[11] "The main characteristics of these satellite missions are: ..." - [11] (IAG website)

[12] "Satellite-to-satellite tracking in the high-low mode (SST-HL): a low Earth orbiting (LEO) satellite is tracked by high orbiting GNSS satellites, relative to a network of ground stations. The satellite is a probe within the Earth’s gravity field, which can be precisely tracked without interruption. The observed 3-D accelerations correspond to the gravity accelerations. The non-gravitational forces acting on the satellite are measured by accelerometers." - [12] (IAG website)

[13] "Two LEO satellites are placed in the same orbit, separated by several hundred kilometres, and the range (distance) between them is measured with the highest possible accuracy. Basically, the acceleration difference between the two satellites is measured. The two satellites can be considered as one instrument in which ..." - [13] (IAG website)

[14] "The global gravitational models from satellite gravimetry, typically in terms of spherical harmonic coefficients, are crucial in geodetic, geodynamic, geophysical, hydrological, glaciological, oceanographic, and many other geoscience applications." - [14] (ScienceDirect)

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