Doppler satellite
As Doppler satellites are artificial earth satellites denotes that for measuring the Doppler effect by their serve web speed and the rotation of the earth is caused. This can be done with various instruments:
- Devices for measuring these frequency shifts,
- a very stable oscillator to broadcast constant radio frequencies, or
- a transponder to answer the incoming radio signals from a ground station (see also secondary radar ).
At the beginning of space travel in 1957, the Doppler measuring principle was used to determine the orbit of Sputnik and Explorer satellites . A few years later they started their own navigation satellites , which were perfected in the 1970s and 1980s for the purposes of astrometry and earth measurement .
The method of using the Doppler effect on a large scale for navigation and soon also for geodesy goes back to the NNSS (Navy Navigation Satellite System) or transit system of the USA: the US Navy launched several Earth trips from 1960 onwards satellites orbiting on polar orbits and made the system available for civil use as early as 1964 - after the Cuban Missile Crisis was finally resolved . The at least 5 satellites formed an approximately 1100 km high spherical shell of 5 circular satellite orbits in which the earth rotates like in a precisely measured cage .
The Doppler location determination of military and civilian ships is related to the methods of hyperbola navigation , but no direct route differences are determined, but integrated speeds for minutes . With this newly developed satellite positioning based on multiple Doppler satellite (Transit satellites of NASA that could and its Russian equivalent) ship - and air navigation from the hitherto standard 1-mile - accuracy increased to 50 m to ± 20, and a few years later it was possible for the first time to carry out international, extensive railway and national surveys with meter accuracy.
Further developments: NOVA and DORIS
From about 1965 American satellite Doppler type came NOVA ( N on-gravitational O ? Rbit (corrected) V ehicle Acceleration ) is used, with special acceleration - and shock sensors and the non-gravitational could consider forces on the satellite orbits. This enabled navigation to be increased to an accuracy of 20 m worldwide, and longer national survey campaigns even to 20–30 cm.
For the "Doppler networks" - z. For example, the transnational DÖDOC and WEDOC campaigns, in which extensive control and surveying networks of the zeroth order were set up with point spacings of 100–500 km - in addition to the classic 400 MHz frequency, one with 150 MHz was set up, so that the frequency-dependent influence of the high atmosphere could be eliminated (it ranges from a few meters to a maximum of 100 m).
Around 1985 France developed a measuring system called DORIS , in which the ground stations do not measure the Doppler effect of the signals coming from orbit , but in which this takes place in the satellites themselves. This enables savings in effort and in the organization of measurement campaigns. Since around 1990, DORIS has been contributing to the monitoring of the earth's rotation and the earth's geodetic reference system with around 50 ground stations , but also to the analysis of the charge density in the ionosphere .