Satellite laser ranging

Satellite laser ranging

The Earth moves around the ecliptic around the sun at an average speed of 107 218.8 km/h (between 29.291 km/s and 30.287 km/s). At the same time the Earth rotates on itself around an axis at the speed (at the equator) of 1674.364 km/h (465 m/s). Our natural satellite (moon) and artificial turn at different speeds, 1.17 km/s for the Moon, 3.88 km/s for the GPS satellites or 7.68 km/s for ISS. Despite these relative velocities it is possible to accurately measure the distance between our satellite and Earth. For this we use the satellite laser ranging (SLR).
The satellite laser ranging test measures the time for a round trip of a laser pulse emitted from a ground station and returned from reflectors located on the satellite. The distance between the satellite and the observation site is approximately equal to half the round trip time, multiplied by the speed of light. Many satellites are equipped with laser reflectors reflect light in the exact direction of the transmitter. The angular accuracy of retro reflectors is a few seconds of a degree. The accuracy of a measurement laser telemetry is 1 to 2 cm. SLR technique is sensitive to the position of center of mass of the Earth. This simple and highly accurate (1 cm) provides measurements from the center of mass of the Earth. It defines an absolute reference ground for scientific applications (geodesy, earth physics, oceanography, plate tectonics, seismology, planetary lunar orbit control of satellites,...). This technique provides, for example, daily estimates of polar motion with an accuracy of about 0.3 millisecond degree.

Another technique as the technique of Laser Moon (LLR) is used to determine the obliquity of the ecliptic, and the orientation of the solar system in the extragalactic reference.

NB: a laser rangefinder is a device for measuring distances. A laser beam is projected onto a target that in turn refers to the beam. The electronic control unit calculates the phase difference between the transmission and reception.

Image: Satellites LAGEOS (Laser Geodynamics Satellite) are artificial satellites in Earth orbit. Contituted by a brass ball covered with aluminum on which were placed 426 retroreflectors rhombohedric (cube corners), they are entirely passive. Placed on a circular orbit mean (5 900 km altitude) and very massive (406 kg for a 60-cm diameter), they are very stable and remain in orbit for 8.4 million years. Also, are they filled a time capsule giving the position, the current and projected continents. 422 are cube corner blocks made ​​of quartz glass and 4 germanium in order to reflect the infrared and allow the study of variation of the altitude of the satellite. 35 ground stations participating in distance measurements by calculating the time taken by a laser beam to return to its point of emission. The measurement accuracy is centimetric and can measure the movement of tectonic plates with comparable accuracy. There are other satellites equipped with retro-reflectors (ERS, Jason, Starlet, Etalon, two GPS satellites (-35, -36,...) and five retro-reflectors were deposited on the Moon.

Image: Laser ranging satellite.
The satellite LAGEOS-1 is a ball of brass covered with aluminum on which are placed 426 rhomboedric retroreflectors (cube corner), entirely passive.