Later developments allowed artificial satellites transmitting navigation signals more accurate and opened a new era in navigation technology. The satellites were first used in determining the position in a two – dimensional system used by the US Navy and called TRANSIT. This system was the precursor to the current GPS system. Geoid surface is determined by the mean sea level. This is so the surface is obtained if the free surface of seas and oceans should be placed in communication and if it came under the continents surrounding the planet.
Geoid shape is very close to that of a rotating ellipsoid flattened at the poles but the area is not a regular geometric surface. Undulations more or less steep geoid is due to density differences that exist from place to place on Earth.
Reference ellipsoid surface is instead a regular geometric surface defined so as to better approximate the geoid for the territory considered. Because the shape of the geoid is not uniform, to achieve national cartography, different states or groups of states use different ellipsoids reference chosen so as to better approximate the geoid within the territory of interest. Usually you can check the coordinates using a mobile phone tracker apps.
Resorting to appropriate methods of processing and using a certain number of points noted in the two systems, it is possible to move without particular problems, the X and Y coordinates of the GPS coordinates corresponding national geodetic systems. The problem is actually in relation to allowances. Time is the basic form of existence of matter in motion. Generally, to establish a uniform time scales against which to relate observations, it is necessary to define two sizes: the unit of time (or second day) and the age or origin of time chosen.
Time scale consists of the origin of the time axis, defined and recognized internationally, a unit of measurement (second) and make sense. To this end, to measure a period of time, it can use regular and continuous vibration period of a measuring instrument.
The last unit of measure, second, relied on Earth’s rotation around its axis, today, it relying on the natural frequency of a chemical. The time has particular importance in satellite geodesy, because a satellite position and coordinates of the Earth’s surface, are functions of time because of Earth’s rotation. With technological development, precession and nutation phenomena that are most diurnal rotation of the Earth, they began to know as precisely, and these remarkable repercussions in defining time and coordinate systems.
During dynamic is uniform time scale which describe the motion of bodies in a specified reference system and moving one theory of gravity general theory of relativity and Newtonian mechanics. Dynamic Theory of Relativity time depends on the coordinate system used as a reference system.
For example, a fixed clock on Earth will spread periodic variations under 1.6 ms to TDB caused by the Earth’s gravitational field of the sun. Sometimes, in describing the orbital motion of the satellites in near-Earth (with low orbit), there is no need to use TDB as how both satellite and Earth are influenced by the same disturbance.
The time used in the calculation of the satellite is called dynamic time terrestrial TDT and is abbreviated uniform scale of time for movement in the terrestrial gravitational field, scale that relate to Earth’s center of mass.