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
[edit]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. Since the start of several satellite gravity missions, more data is being gathered about the Earth's gravity field.[8][9]
| Event | Recorded by | Reference(s) |
|---|---|---|
| 2016 earthquake in Papa New Guinea | GRACE | [10] |
| 2017 earthquake in Papa New Guinea | ||
| 2019 earthquake in Peru |
Earth's gravitational field
[edit]Earth's gravitational field reflects its surface mass redistribution and its inner structure.[11]
Satellite orbit
[edit]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.[12] 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.[13]
Research
[edit]Researchers typically use data from satellites that:[14]
- orbit as low as possible (usually 200 &endash; 500 km from the ground)
- uninterrupted orbit
- separation of gravitational and non-gravitational forces
Observations
[edit]The main observation principles are satellite-to-satellite tracking in two modes:
High-low mode
[edit]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.[15] 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
[edit]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.[16] 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.
Atmospheric impacts
[edit]A certain portion of the time-variable gravity signal received by a satellite is caused by atmospheric mass variability.[17] To minimize temporal aliasing, the trihourly non-tidal atmosphere gravity field is subtracted from the gravitational value reported by the satellite.
Accuracy
[edit]While satellite gravity gradient data is very precise in determining the short-wavelength part of the static gravity field model, it is highly inaccurate in measuring the long-wavelength part.[18] Data collected from ground-level also tends to be more accurate than from satellites in geophysical explorations due to their proximity to the Earth.[19]
Geophysics
[edit]The gravitational models from satellite gravity are important in many geoscience applications such as geophysics, hydrology, and glaciology.[20]
Earthquakes
[edit]GRACE satellite gravity missions reported volumetric disturbance of rocks for most shallow earthquakes with a moment magnitude greater than 8.0.[10]
References
[edit]- ^ "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 launches of several satellite gravity missions have revolutionized the estimation and representation of Earth’s gravitational field." - [8] (ScienceDirect)
- ^ "Since the emergence of satellite gravity field missions, especially the Gravity Recovery And Climate Experiment (GRACE, Tapley et al. 2004) and the Gravity field and steady-state Ocean Circulation Explorer (GOCE, Drinkwater et al. 2003), the quality of global gravity field models has significantly improved." - [9] (Springer)
- ^ a b "Earthquakes involve mass redistribution within the solid Earth and the ocean, and as a result, perturb the Earth's gravitational field. For most of the shallow (<60 km) earthquakes with Mw > 8.0, the GRACE satellite gravity measurements suggest considerable volumetric disturbance of rocks." - [10] (Wiley Online Library)
- ^ "The gravitational field of the Earth reflects Earth's surface mass redistribution and its inner structure and dynamics." - [11] (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." - [12] (IAG website)
- ^ "The satellite motion and the parameters describing the orbit are observed to draw conclusions about the forces acting on the satellite." - [13] (IAG website)
- ^ "The main characteristics of these satellite missions are: ..." - [14] (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." - [15] (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 ..." - [16] (IAG website)
- ^ "It has to be noted that a certain fraction of the time-variable gravity signal picked up by a satellite gravity mission is caused by atmospheric mass variability. To avoid temporal aliasing, the 3-hourly non-tidal atmosphere and ocean de-aliasing model AOD1B (Dobslaw et al., 2017) is subtracted already during the Level-2 processing of monthly GRACE/-FO gravity fields. To provide users with the flexibility to restore atmospheric signals, the monthly mean estimate of the atmospheric background model is provided along with the GravIS TWS products." - [17] (ESSD)
- ^ "Although satellite gravity gradient data plays a great role in determining short-wavelength part of static gravity field model, accuracy of the long-wavelength part of gravity field model recovered by them are poor, which leads to only a few applications in time-variable gravity investigation." - [18] (SciOpen)
- ^ "With the advent of satellite technology, it became possible to collect gravity data on a global scale but the resolution, and accuracy of such data are often inferior to that obtained from ground-based measurements (Kamto et al., 2023; Anderson et al., 2020).This is due to their distance from the Earth's surface (Beránek and Mrlina, 2023; KHATRI et al., 2013; Ahmadi et al., 2020). This presents a significant challenge in geophysical exploration, as the full potential of satellite gravity data is not realized due to these limitations (Alhassan and Aliyu, 2022)." - [19] (ScienceDirect)
- ^ "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." - [20] (ScienceDirect)