Multi-sensor Climatology onboard GRACE

The thermosphere lies between the exosphere and the mesosphere. The temperature in this layer can reach up to 4,500 degrees Fahrenheit. The thickness of this layer is about 513 km [NASA, 2018]. The thermosphere is the top level of the Earth atmosphere, located from 100 to 1000 km altitude. At 100 km already, the air density is twelve orders of magnitude lower than at the Earth’s surface. However, the remaining air is enough to exert a significant force on satellites orbiting the Earth at low heights. This perturbation is mainly due to high orbital velocity of 7.5 km/s, and the proportional relation between the air drag and the square of the speed. Since the space-borne accelerometer could measure the total non-conservative accelerations acting on the satellites directly, the air drag component could be isolated with the help of solar and earth albedo radiation pressure models, then the atmospheric density can be estimated, which provides necessary data for making evaluation and improvement of the existing atmospheric models.

The GRACE spacecraft trajectories contain the influence of the total exterior geopotential, and other forces, on each satellite. The strongest gravitational force acting on the satellite is the generated by the Earth gravity field; however, there are also other perturbing forces that should be considered even though their perturbations are relative small compared to the acceleration due to the spherical-symmetric component of the Earth’s gravity field. The gravitational and non-gravitational forces are mainly conformed by the forces described in Fig 1.

The non-gravitational accelerations are computed by subtracting the modelled gravitational accelerations from the total acceleration derived after polynomial fitting to the GNV1B positions. The next step consists of subtracting solar radiation pressure, albedo effect and thermal infrared from the non-gravitational accelerations, in order to arrive at the atmospheric acceleration. Finally, the thermosphere density is estimated by using observed and modelled all forces acting on GRACE satellite.