European Remote Sensing Satellite

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ERS satellite over Antarctica (photomontage)
Model: The ERS satellite in the payload fairing of Ariane 4

The European Remote Sensing Satellite s ERS-1 and ERS-2 are two satellites of the European Space Agency (ESA) that were used for remote sensing of the earth's surface. Both satellites are no longer in operation. ERS-1 was ESA's first Earth observation satellite and one of its most important satellite developments in the 1980s.

Development and construction

Initial project studies began in 1978. DASA was the prime contractor for both missions and supplied both the platform and some key instruments. The satellites were equipped with several (“multidisciplinary”) measurement techniques for different spectral ranges ( UV / VIS range, infrared range, microwaves ). ERS-1 and ERS-2 were built by a consortium of companies under the system management of Dornier- System GmbH in Friedrichshafen . This company also developed the SAR sensor. The ERS ground station was also built near Dornier in the Antarctic.

Satellite orbits and image tracks

The two satellites were launched into orbit with Ariane 4 rockets on July 17, 1991 and April 21, 1995 . The ERS satellites orbited the earth in about 100 minutes and run in almost polar orbits , which means that they can be made synchronous with the sun . This means that the planes of the orbit are always at the same angle to the sun and that the recorded image strips also have roughly the same conditions in terms of lighting and contrast at different times .

Stripe-like scanning of the earth

The satellite orbits were designed in such a way that their sensors covered almost every point on earth at least once in 35 days .

This strip-like overflying is due to two effects: the earth's rotation and the precession of the orbital planes. The orbits of satellites represent ellipses or circles and run approximately according to Kepler's laws . Their levels remain largely fixed in space in the surrounding space ( reference system of the stars) , so that our home planet rotates under these orbits. This means that ERS and similar satellites can gradually scan the earth's surface in contiguous image strips .

If a near-polar satellite (flying north-south) were to circle the earth exactly 14 times a day, it would come back to almost the same strip after each day. So would have z. B. ERS-2 this orbital period of 102.57 minutes, he could be the earth's surface , although daily observed along certain meridians, but not the intervening areas. You change and stabilize the orbits so that it runs at a certain distance from the previous meridian every day.

Satellite orbit

The ERS satellites orbited the earth in a sun-synchronous orbit in an initial 800 km with an inclination of 98.5 °. The railway track passes the poles for about 900 km. The satellites scanned the earth in strips and reached the starting point after 35 days. The orbit was designed as a frozen orbit .

Instruments of the ERS satellites

Model of the ERS (spotlight in the foreground as a size reference)

The most important measuring device was a C-band Synthetic Aperture Radar with a ground resolution of 30 m × 30 m. It could be swiveled 12 ° to the left and right and covered a 100 km wide strip on the earth. Due to the sun-synchronous orbit, it always saw the surface at the same local time.

In addition to the instruments of the ERS-1, the ERS-2 carries the GOME spectrometer . Other measuring devices are:

  1. Radar altimeter for measuring heights above sea or ice: a Ku-band transmitter with 13.8 GHz, which emitted vertical microwaves and measured the transit time of the echo. From this data on wave heights, wind, sea level, tides, ice surfaces and geoid shape could be derived.
  2. ATSR (Along-Track Scanning Radiometer ): an imaging infrared radiometer (IRR) combined with a passive microwave probe (MWS). The IRR measured cloud, ground and sea temperature in four channels with an accuracy of 0.2 to 0.5 °. In addition, the visible area for vegetation analysis was recorded. The MWS had two channels for determining the total water content in the atmosphere over a soil track 20 km wide.
  3. GOME (Global Ozone Monitoring Experiment) was a high resolution spectrometer for UV and visible radiation. From 1996 onwards, ESA supplied 3-day data sets on cloud cover and the atmospheric ozone and nitrogen dioxide distribution via CD-ROM or the Internet . GOME also detected some other trace gases (bromine monoxide, sulfur dioxide , formaldehyde , chlorine dioxide , O2-O2 dimer) and aerosols in the air. This instrument is only on board ERS 2
  4. MWS / MWR ( Microwave Sounder & Radiometer ): passive radiometer (23.8 and 36.5 GHz) for the analysis of water vapor in the atmosphere. This improves the altitude determination (altimetry), since water vapor and water droplets extend the apparent path of the echo signal.
  5. SAR ( Synthetic Aperture Radar ) including AMI mode ( active microwave instrumentation ):
    1. Image mode for images of the earth's surface with a resolution of 8–20 m within a 100 km strip,
    2. Wave mode for analyzing ocean waves and determining wind direction and speed. The windscatterometer measured the changed reflection of the sea, which depends on the small ripple waves and their wind energy.
  6. PRARE (Precise Range and Range Rate Equipment): All-weather distance measurement for high-precision orbit determination and for satellite geodesy - e.g. B. to analyze the earth's gravity or plate tectonics.
  7. LRR ( Laser -Retroreflector): Infrared reflector for pulsed laser beams from special ground stations, which had the associated measuring devices for measuring the railway.

Innovation through combination

After the start of ERS-2, the SAR sensors of ERS-1 and ERS-2 were able to record the same earth surface in very short time intervals (usually one day) and this data could be used for interferometry . The slightly different orbits of the two satellites (usually a few 100 meters) lead to slightly different "angles" of the same area of ​​the earth's surface. By computationally combining the two recordings, either digital elevation models of the earth's surface could be created or small movements of the earth's surface between the two recordings could be recorded and made visible with an accuracy of about one centimeter (differential radar interferometry, DInSAR).

The satellites provided data on changes in the earth's surface before or after a volcanic eruption or on changes in the earth's surface caused by earthquakes. The expansion of a lava chamber on Mount Etna or the forecast of a mudslide of a volcano in Iceland were other examples.

A similar combination of two SAR sensors is carried out with the TerraSAR-X satellite . From the end of 2010, it initiated a multi-year interferometric mission together with the almost identical TanDEM-X satellite .

Additional benefit

In addition to achieving research goals, the satellite is also used for the International Charter for Space and Natural Disasters .

State of the satellites

ERS-1 has not been active since March 10, 2000, but has doubled its planned service life.

In June 2003, the ERS-2 tape storage failed. The satellite could no longer buffer the signals that it registered in 100 minutes while orbiting the earth. It only sent the data it was recording when it was in contact with a ground station for 10 minutes. However, this disadvantage was offset as much as possible by an extensive international network of ground stations.

Since February 2001 problems with the gyro sensors led to certain restrictions in the usability of some sensors. In 2003, these problems were partially offset by a new software control system. Otherwise ERS-2 works perfectly until 2011.

At the end of 2007 and the beginning of 2008, a tandem mission was carried out with the ESA satellite Envisat, during which important new data was obtained through the staggered overflight (approx. 30 minutes difference). B. About rapidly changing glaciers in the Arctic.

On July 5, 2011, ESA announced the end of the ERS-2 mission. From July 6th, the orbit height of the satellite was lowered by several brake ignitions from 800 km to 550 km, where the risk of collision is lower. In addition, all tanks were emptied and the batteries discharged to prevent explosions on board from generating further space debris. The last command to ERS-2 was sent on September 5, 2011 at 13:16 UTC. At 5981 days (over 16 years), ERS-2 was the ESA's longest active satellite. Template: future / in 5 yearsERS-2 will then burn up in the earth's atmosphere within the next 25 years (until 2036 ).

literature

  • D. Zhao, C. Kuenzer, C. Fu, W. Wagner: Evaluation of the ERS Scatterometer derived Soil Water Index to monitor water availability and precipitation distribution at three different scales in China . In: Journal of Hydrometeorology . 2008, doi : 10.1175 / 2007JHM965.1 .
  • Dieter Gottschalk: ERS-1 Mission and System Overview . In: The Geosciences . 1991, doi : 10.2312 / geosciences . 1991.9.100 . - and further articles in issue 9 (4-5) of the journal Die Geoswissenschaften .

Web links

Individual evidence

  1. ↑ ERS satellite missions complete after 20 years. ESA, September 12, 2011, accessed September 12, 2011 .
  2. ↑ ESA: Proba-1 Sets New Record. Retrieved on January 8, 2019 (English): "Proba-1 will surpass ERS-2, making it ESA's longest operated Earth observation mission of all time."
  3. ^ Pioneering ERS environment satellite retires. ESA, July 5, 2011, accessed July 6, 2011 .