ESA, 3-axis-stabilised, Earth-pointing satellite in yaw steering mode (YSM). Elliptical orbit in Sun-synchronous, near polar, with a mean altitude of 785 km, an inclination of 98.5deg. and a local solar time at the descending node of 10.30 a.m.
European Remote Sensing (ERS) was the first ESA program in Earth observation with the overall objectives to provide environmental monitoring, in particular in the microwave spectrum. ERS-1 was launched by Ariane-4 on 17 July 1991. In March 2000, a computer and gyro control failures led to the end of ERS-1’s mission after far exceeding its planned lifetime. The ERS-1 was mainly operated in a 35-day repeat cycle with, in addition, a 3-day cycle (ice phase) in 1992 and 1994 and by ERS-2 in 2011. ERS-1 had also a 168-day repeat cycle (geodetic phase) in 1994 and 1995. It included 6 sensors:
- Active Microwave Instrument (AMI): Synthetic Aperture Radar and wind scatterometer (both in the C-band)
- Along-Track Scanning Radiometer (ATSR): Infrared and visible ranges, sea surface temperatures and the vegetation cover of land surfaces
- Laser Retro-Reflector (LRR): Satellite position using ground-based laser stations
- Microwave Sounder Supplied (MSS): atmospheric humidity
- Precise Range And Range-Rate Equipment (PRARE): ERS orbit and trajectory determination
Radar Altimeter (RA): Measurements of the distance from the ocean surface and of wave heights
ERS-2 was launched by ESA in 1995 to follow-up the first ERS mission on the same orbit. Both ERS-1 and ERS-2 were operated in a 1-day difference (known as tandem mode) from 1995 to 2010 to develop SAR interferometry applications. The two satellites acquired a combined data set extending over two decades. In 2001, after the failure of several on-board gyro systems, an innovative new scheme for flying and controlling the ERS-2 mission without gyros was invented by a group of engineers across ESA and industry- the “gyro-less” yaw steering mode or “Zero-Gyro Mode”. In 2003, a failure in the on-board data storage system led to directly relay science data to ground at the time of acquisition. The mission ended on 5 September 2011, after the satellite’s average altitude had already been lowered from 785 km to about 573 km. At this height, the risk of collision with other satellites or space debris is greatly reduced. Over a series of burns in July, August and September, ERS-2 was finally depleted of all fuel and the batteries were switched off, leaving the spacecraft in an orbit where it will reenter the Earth’s atmosphere and safely disintegrate within 25 years, in accordance with international standards.
ERS-2 included 1 additional instrument compared to ERS-1:
- Global Ozone Monitoring Experiment (GOME): Absorption spectrometer to the presence of O3 – Ozone, trace gases and aerosols in the stratosphere and troposphere
Envisat was was launched in 2002 by ESA. Its goal was to succeed to ERS, and thus extend related data sets. With 10 instruments aboard and eight tons, Envisat was the largest civilian Earth observation mission. The overall objectives were: studying and monitoring the Earth’s environment on various scales, from local through regional to global, monitoring the Earth’s resources, both renewable and non-renewable. Major disciplines covered: meteorology, climatology, environment, atmospheric chemistry, vegetation, hydrology, land use, ocean and ice processes.
The Envisat mission ended on 08 April 2012, following the unexpected loss of contact with the satellite. Just weeks after celebrating its tenth year in orbit, this loss of communication was very sudden. Several rigorous attempts followed in order to re-establish contact, but without success. Despite continuous commands sent from a widespread network of ground stations and a team of engineers, there was no reaction from the satellite. As there were no signs of degradation before the loss of contact, the team has been collecting other information to help understand the satellite’s condition. Possible failure scenarios are: a) the loss of the power regulator, blocking telemetry and telecommands, or b) A short circuit, triggering a ‘safe mode’ – a special mode ensuring Envisat’s survival. A second anomaly may have occurred during the transition to safe mode, leaving the satellite in an intermediate and unknown condition.
Envisat had a low Earth Sun-Synchroneous orbit, and included more advanced imaging radar, radar altimeter, temperature-measuring radiometer instruments, and 2 atmospheric sensors monitoring trace gases:
- Advanced Along Track Scanning Radiometer (AATSR): provided by the UK and Australia.
- Advanced SAR (ASAR):
- DORIS: provided by France.
- Global Ozone Monitoring by Occultation of Stars (GOMOS):
- Medium Resolution Imaging Spectrometer (MERIS): passive optical push-broom wide-field instrument (CCD technology) intended to measure reflected radiation from the Earth’s surface, ocean, and clouds with high spectral resolution (high radiometric resolution in the VIS range). Objectives: ocean monitoring marine biophysical and biochemical parameters (chlorophyll, suspended particles); Atmosphere monitoring cloud distribution, cloud altitude, water vapor column content, aerosols; Land: monitoring, vegetation/biomass, inland water, agriculture/forestry.
- Michelson Interferometer for Passive Atmospheric Sounding (MIPAS): high-resolution Fourier-Transform Spectrometer (FTS) for atmospheric constituents on a global scale; Limb emission sounding instrument operating between 4.15 and 14.6 µm (685-2410 cm-1), during day and night times of the orbit, over 20 trace gases, including the complete family of nitrogen-oxygen compounds and several CFCs, especially in the stratosphere and in cloud-free regions in the upper troposphere. Developped by EADS Astrium GmbH.
- MicroWave Radiometer (MWR):
- Radar Altimeter-2 (RA-2): including a Laser Retro-Reflector (LRR)
- Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY): passive sensor, air quality & climate purposes, trace gases (pollutants + green-house gases CO2 – Carbon dioxide & CH4 – Methane) and aerosols, provided by Germany and the Netherlands.