Geodetic satellite
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A geodetic satellite is an artificial earth satellite that is used for special geometric or physical measurements of the earth's surface or the earth's gravity field .
Areas of application
There are essentially four areas of application for geodetic satellites:
- Use as a high target for measurements from terrestrial satellite stations - see Geometric Satellite Geodesy
- as a test probe in the earth's gravitational field - see orbit determination and dynamic satellite geodesy
- than the combination of these methods that is common today, whereby the precise orbit data - e.g. B. GPS satellites - allow rapid location fixes on the ground and vehicles. For the measurement methods used, see below
- as a sensor or as an active measuring platform for the purpose of remote sensing of the earth's surface - see earth observation satellite .
Track height and inclination
The chosen orbital height of the satellites depends on the range of applications and is usually at least 800 km. From this height, his field of vision above the earth's surface is large enough to enable long-range measurements, and there are hardly any more orbital disruptions due to the high atmosphere . However, if a satellite is to probe the gravity field directly , an orbit height of less than 500 km is required (see GRACE and GOCE project).
For remote sensing purposes, the orbit height is usually 800 km above the earth's surface, for LASER satellites 1000–4000 km, for satellite navigation systems, however, far higher, in order to enable the largest possible viewing areas. With the Navstar GPS satellites and GLONASS , the orbit height is even around 20,000 km.
The selected orbit inclination (symbol i for inclination ) depends on the geographical area of the application spectrum and can assume all values between 0 ° ( synchronous satellite ) and 90 ° (low polar orbit ). Often there are values around 30 ° (energetically favorable research satellites ) and around 60 ° (global navigation ), as well as close to 90 ° ( sun-synchronous orbits). For special purposes of satellite geodesy, 57 ° is sometimes chosen ( critical inclination ) and slightly declining orbits with i ~ 98 °. The lower the value, the more you can benefit from the earth's rotation when the satellite is launched , which is 460 m / s near the equator, but the satellites can then hardly be observed at higher geographical latitudes . Orbital inclinations over 90 ° (at which the satellite moves from east to west) must be bought with a few percent additional starting energy.
Measurement methods
The most important measuring methods of the first two areas of application are:
- Distance measurement with LASER (see SLR ), microwaves or radar
- Speed measurement according to the Doppler principle or using satellite-to-satellite tracking (SST)
- Direction measurement (visual), photographically , with CCD sensors or with radio interferometry, formerly also minitrack
- Remote sensing (photographic or with optical sensors) in visible light, infrared or UV
- Gradiometry (measurement of gravity and earth orbit gradients )
- Dynamic satellite geodesy (physically conditioned changes in the orbit elements ) in order to derive harmonic coefficients of the earth's gravity field.
Occasionally satellites are also used geodetically with completely different areas of application, e.g. B. some news and weather satellites . The Envisat , which started on March 1, 2002, received several corresponding measuring devices. Many special satellites can also be temporarily reprogrammed for experimental ideas, such as ATS-6 , which was used in the 1970s for the first tests of SST techniques.
See also
- Passive: Echo 1 and 2 , PAGEOS , Explorer 19 , laser satellites
- Active: Topex
- Remote sensing: Landsat , Seasat , ERS 1 and ERS-2