Satellite geodesy

In satellite geodesy geodesy refers to the means of observation of artificial satellites.

As an independent branch of geodesy she established herself in 1960, when the first geodetic satellites were launched. From fixed ground stations or mobile radio receivers directions, distances and velocities of the satellites to be measured, from which one can calculate the coordinates of the stations and / or the precise satellite orbit. Specific probes can measure the height above sea level or characteristics of the Earth's gravity field, which allows the determination of the mathematical figure of the earth and of the geoid.

Characteristics and measurement principles

One characteristic of satellite geodesy is the high speed of the missile and its movement in a complex field of forces ( earth gravity, various perturbations by the Moon, upper atmosphere, solar radiation, magnetic fields, etc.). In low Earth orbits the satellites run at 7-8 miles per second, so a time error of millionths of a second means already several decimetres location error. High standards also provide the wireless technology, data transmission and the constant worldwide availability of the spatial reference system in which the orbit determination must be made. To the high altitude and difficult optical visibility, however, was a problem only in the early years.

For use geodetic satellites or for the geodetic use of other Earth satellites, there are basically four methodological procedures:

• Some special procedures for the three groups are led under the measurement methods.

By optimizing the methods 1-3, the Geodesy, the point determination and the definition of reference systems could since 1970 of a few meters accuracy in the centimeter range and are sometimes even increased in the sub-millimeter range. This means that, for example, Today, continental drift and plate tectonics, earthquakes or detectable by the finest variations in the Earth's rotation.

Classification according to measurement methods

In satellite geodesy a number of very different measurement methods will be applied. They can be broken down as follows:

Direction measurements

• Visual: at the beginning of space (1957 to about 1970). Measurement in the telescope or binoculars against the background of the starry sky or with special theodolites; achievable accuracy of 10-50 " Special measurement methods in the Moonwatch projects and for balloon satellites.

• Minimize the time error by flash satellites tracked cameras and highly precise Uhrsysteme.

See also: ballistic camera, startrail, star passage

Distance measurements

• Electronic distance measurement with microwaves ( eg SECOR until about 1970, GPS, see below) and Radar: today between satellite (SST s, u ) and with velocity measurement ( PRARE ) to a few mm.
• Laser Ranging by runtime measurement of ultra -short laser pulses. Since about 1965 (± 5 m exactly ), now also a few mm.
• Doppler effect, see also hyperbolic and radio navigation. The best known method 1964-1995 was "Transit" ( NNSS, ± 20 m to 30 cm), today global DORIS system about ± 10 cm.
• Pseudoranging: runtime measurement encoded microwaves, clock error is calculated from over-determination. Measurement Method of GPS NAVSTAR, GLONASS and the future Galileo, accuracy mm - cm depending on the method.
• All of these measurements should be corrected for the Earth's atmosphere to increase their accuracy by longer measurement series and special railway and evaluation methods. " Zweiwegmessungen " ( there and back) are more accurate than single measurements.

Altimetry

Or satellite altimetry over the sea, the future via ice: Run- time measurement of a radar pulse that is reflected from the sea surface. Accuracy 1978 ( Seasat ) 20 cm, now in the cm range. Important method of geoid determination and for Oceanography (wind, waves, ocean currents ), use in the domains of ERS satellites of ESA.

TerraSAR -X was launched in June 2007; Since 2010, he has a "twin" ( called tandem ) who accompanies him in under a kilometer away in space. TS -X is provided with a unique SAR sensor; it provides very high resolution images ( wavelength only 31 mm). The oceanographic applications of TS -X data are: Calculation of the sea state, wind fields, coastlines, ice, oil film and ship detection. In the tandem constellation, it is also possible to detect movements and so as to determine ocean currents, ice drift, and ship speeds.

SST and speed

• Satellite -to- Satellite Tracking (SST ): microwave distance measurement between satellites. First attempts in 1975, highly successful in twin satellite GRACE (2004) for local details in the gravitational field.
• Speed: from the differences of radar measurements, but especially with Doppler effect (transit, DORIS system) and with " Precise Range and range rate Exp " ( PRARE, for various probes from 1990).

Gradiometry

• Measurement of gravity gradients ( difference in the gravitational field at different places of the satellite). For the first time in GOCE 2008.
• Measuring accelerations in the satellite by accelerometer and gyro systems. Development issues for 10 years, first at GOCE.

Remote Sensing and Cartography

(see special article ): photos or digital images of the Earth's surface, or multispectral scanner, Side Looking Radar etc. Geodetic used primarily as interferometry over local processes of Geodynamics.