Satellite geodesy

Under Satellitengeodäsie means the surveying the earth by means of artificial satellites .

It established itself as an independent branch of geodesy around 1960 when the first geodetic satellites were launched. The directions, distances and speeds of the satellites are measured by fixed ground stations or with mobile radio receivers, from which the coordinates of the stations and / or the exact satellite orbit can be calculated. Special probes can measure the height above sea level or properties of the earth's gravity field , which enables the mathematical figure of the earth and the geoid to be determined.

Characteristics and measurement principles

A characteristic of satellite geodesy is the high speed of the missiles and their movement in a complicated field of force (earth's gravitational field, various orbital disturbances by the moon, high atmosphere, solar radiation, magnetic fields, etc.). In orbits close to the earth, the satellites run at less than 8 kilometers per second, which is why a time error of millionths of a second already means several decimeters of location error. Radio technology, data transmission and the constant worldwide availability of the spatial reference system in which the orbit must be determined also make high demands . The high altitude and difficult visual visibility, however, were only a problem in the early years.

For the use of geodetic satellites and for the geodetic use of other earth satellites, there are basically four methodological approaches:

1. Geometric satellite geodesy : Direction and distance measurements to set up networks for determining the position of the measuring points, calculating their coordinates and the exact shape of the earth
2. Dynamic satellite geodesy : speed measurement and orbit determination of satellites as well as analysis of orbital disturbances to determine the earth's gravitational field
3. Combined processes that are most important today: from precise path data - e.g. B. GPS satellites - they allow quick and precise location on the ground, the navigation of vehicles and the location of other satellites and probes.
4. Earth observation satellites as sensors or active measuring platforms for remote sensing of the earth's surface. They are not discussed here.

For groups 1 to 3, some methods are listed in the section on measurement methods. By optimizing these methods, earth measurements, point determination and the definition of reference systems have been increased since 1970 from a few meters accuracy to the centimeter range and sometimes even to the sub-millimeter range. So today z. B. the continental shifts caused by plate tectonics and earthquakes or the finest fluctuations in the earth's rotation can be detected.

Classification according to measurement methods

A number of very different measurement methods are used in satellite geodesy. They can be broken down as follows:

Direction measurements

• Visually: at the beginning of space travel (1957 to around 1970). Measurement with a telescope or binoculars against the background of the starry sky or with special theodolites ; achievable accuracy 10–50 arc seconds
  • Special measurement methods in the Moonwatch projects and for balloon satellites .
• Photographic : 1957 to around 1980, ballistic cameras with photo plates , special satellite cameras with films; Focal length 20–100 cm, accuracy 1–5 arcseconds, which means e.g. B. across Europe in the network WEST showed a few meters over 2,000 kilometers. Later displaced by radio processes (GPS), laser satellites and CCD sensors .
  • Minimization of time errors through flashlight satellites , tracking cameras and high-precision clock systems.
• CCD cameras: since around 1995, increased from 2005 (automatic control, digital evaluation methods). Accuracies up to 0.5 arc seconds.
• Initially also with radio waves (automatic, but relatively imprecise) and interferometry (high effort).
• Astrometry by scanning the starry sky and time measurement (see Hipparcos ): can be used indirectly geodetically as an extraterrestrial reference system .

Distance measurements

• Electronic distance measurement with microwaves (e.g. SECOR until around 1970; GPS see below) and with radar : today also between satellites (SST, see below) and with speed measurement ( PRARE ) to a few mm.
• Laser ranging by measuring the transit time of extremely short laser pulses. Since around 1965 (± 5 m accurate), today also a few mm.
• Doppler effect , see also hyperbolic and radio navigation . The best-known method from 1964 to 1995 was transit (NNSS, ± 20 m to 30 cm), today the global DORIS system around ± 10 cm.
• Pseudoranging : time of flight measurement of coded microwaves, clock errors are calculated from overdetermination. Measurement method from GPS -NAVSTAR, GLONASS and the future Galileo, accuracy mm – cm depending on the method.
• All of the measurements mentioned have to be corrected because of the earth's atmosphere and their accuracy increased through longer series of measurements and special orbit and evaluation methods. "Two-way measurements" (there and back) are more accurate than one-way measurements.

Height measurement

Height measurement or satellite altimetry over the sea, in future also over ice surfaces: measurement of the transit time of a radar pulse that is reflected from the sea surface. Accuracy 1978 ( Seasat ) around 20 cm, today in the centimeter range. Important method of geoid determination and for oceanography (wind, waves, ocean currents ), use, etc. a. at ESA's ERS satellites.

TerraSAR-X was launched in June 2007; since 2010 he has had a “twin” (named TanDEM-X ) who accompanies him in space less than a kilometer away. TS-X is equipped with a unique SAR sensor; it delivers particularly high-resolution images (wavelength only 31 mm). The oceanographic applications of TS-X data are: calculation of sea state parameters, wind fields, coastlines, ice, oil film and ship detection. In the tandem constellation, it is also possible to detect movements and thus determine ocean currents, sea ice drift and ship speeds.

SST and speed

• Satellite-to-Satellite Tracking (SST): Microwave distance measurement between satellites. First attempts in 1975, extremely successful with the twin satellite GRACE (2004) for local details in the gravitational field. From probably April 2018, distance measurement using laser interferometry in GRACE-Follow On .
• Speed : from differences in radar measurements, but above all 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 gravity field at different points on the satellite). For the first time at GOCE 2008.
• Measurement of accelerations in satellites using accelerometer and gyro systems. Development problems for 10 years, for the first time at GOCE.

Remote sensing and cartography

(See special article): Photos or digital recordings of the earth's surface, multispectral scanners, side looking radar, etc. Can be used geodetically, especially as interferometry in local processes of geodynamics .

Literature and web links

• Günter Seeber, satellite geodesy . De Gruyter Publishing House, Berlin 1989
• Satellite Geodesy Research Group: Program 2011–2015 , Bonn 2010 (PDF; 6 MB)

Footnotes

1. ↑ Susanne Lehner: Pirates and Monster Waves - Satellite radar observations of the sea surface. Deutsches Museum, February 16, 2011, archived from the original on July 17, 2013 ; Retrieved July 17, 2013 .