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Explore the many instruments the European Space Agency use to observe the Earth. ESA offer data from a wide range of optical, radar, atmospheric, altimetric and gravimetric instrumentation.

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  • Instrument - Altimeters

    Instrument - Altimeters

    RA (ERS)

    The Radar Altimeter (RA) was a Ku-band (13.8 GHz) nadir-pointing active microwave sensor, on board ERS-1 and ERS-2 missions, designed to measure echoes from ocean and ice surfaces.

  • Instrument - Spectrometers/Radiometers

    Instrument - Spectrometers/Radiometers


    The MIRAS instrument is a passive microwave 2-D interferometric radiometer onboard the SMOS satellite.

  • Instrument - Altimeters

    Instrument - Altimeters

    RA (ERS) Overview

    RA Applications Significant Wave Height Measured by the ERS Radar Altimeter The ERS Radar Altimeter (RA) operated in two modes: ocean mode and ice mode. The RA operated by timing the two-way delay for a short duration radio frequency pulse, transmitted vertically downwards. The required level of range measurement accuracy (better than 10 cm) calls for a pulse compression technique (chirp). The instrument employed frequency modulation and spectrum analysis of the pulse shape. In ocean mode a chirped pulse of 20 micro-s duration was generated with a band width of 330 MHz. For tracking in ice mode an increased dynamic range is used, obtained by reducing the chirp bandwidth by a factor of four to 82.5 MHz, though resulting in a coarser resolution. The Radar Altimeter for ERS-1 and ERS-2 was designed to meet very demanding constraints and had the following major objectives: Precise altitude (ocean surface elevation for the study of ocean currents, the tides and the global geoid) - global measurements of the height of the ocean waves (as significant wave height) - such measurements are extremely important to marine users and scientists wishing to understand the ocean's dynamic behaviour. The Radar Altimeter provided measurements to latitude 82°, north and south, extending to regions which previously had no regular observations - this included the major wave-generation regions in the Southern oceans. Significant wave height (SWH) - measurements of the satellite's height above the local mean sea surface, with an unprecedented precision (equivalent to 1 cm in 100 km) - the applications of this dataset are numerous, for example the operational monitoring of the boundaries of major ocean currents, likely to have significant economic benefits. Ocean surface wind speed - global measurements of wind speed - these can be used to complement the SAR and Scatterometer wind field measurements and also combined with the Radar Altimeter measurements of SWH to distinguish swell from wind-driven waves. Various ice parameters (surface topography, ice types, sea/ice boundaries) - the ability to make measurements over ice with the long term monitoring of the topography of the ice sheets providing a vital warning capability for any substantial shift in the world's climate. Design ERS-2 platform and payload The Radar Altimeter antenna consisted of a reflector, waveguide feed, tripod plus supporting structure, horn feed and the waveguide. In ocean mode a chirped pulse of 20 micro-s duration was generated with a band width of 330 MHz. For tracking in ice mode an increased dynamic range is used, obtained by reducing the chirp bandwidth by a factor of four to 82.5 MHz, though resulting in a coarser resolution. The Frequency Generator units provided the transmit signal at a frequency of 450 MHz to the chirp generator. This generated a chirped output with a bandwidth of 165 MHz (ocean) and 41.25 MHz (ice), gated within a pulse of 20 micro-s. This signal was up-converted and multiplied (using C- and L-band LO signals) to 13.8 GHz, with 330 MHz (ocean) and 82.5 MHz (ice) bandwidths. The required power output level (42 dBm) was generated by the High Power Amplifier (HPA), which was realised as a Travelling Wave Tube and Electronic Power Conditioner (TWT/EPC) combination. A harmonic filter at the TWT output attenuated the harmonics of the RF signal. The transmitter signal was fed to the antenna. The returned signal was routed to the receiver via the Front End Electronics, with an insertion loss of approximately 1.6 dB. The received chirp signal was deramped by mixing it with the LO chirp at a frequency of 15.025 GHz. The deramped output (first IF) was at 1.225 GHz. The signal was then amplified to recover the conversion loss, filtered and mixed with a second LO chirp (1.3 GHz) to provide a second IF of 75 MHz. The second IF signal was filtered, using a surface acoustic wave (SAW) device with a bandwidth of 3.2 MHz and passed via a step attenuator. This provided an overall gain adjustment over a 62 dB range, implemented as two 31 dB step attentuators with a step size of 1 dB. The output was then coherently detected by a quadrature IF mixer to obtain the I- and Q-components of the received signal. The Processor and Data Handling Sub-system (PDHSS) performed tracking and the necessary processing of the radar echoes in order to maintain the echo within the radar range-window. Measurements over Ocean The return pulse shape as a function of time is the convolution of three functions: the average flat surface impulse response, which is a function incorporating the antenna beam weighting and the geometric spreading of the radar pulse along the original surface the probability distribution of surface heights over the sea surface, expressed in terms of delay times the altimeter system point-target response, which is a function of pulse width Over ocean surfaces, the distribution of the heights of reflecting facets is gaussian or near-gaussian, and the echo waveform has a characteristic shape that can be described analytically, as a function of the standard deviation of the distribution which is closely related to the ocean wave height. The resulting return pulse shape is shown in the figure. In general terms the ocean mode encompassed the following echo characteristics time delay with respect to the transmitted pulse - this provides the measure of altitude slope of the echo leading edge, which is related to the width of the height distribution of reflecting facets, and thus to wave height parameters such as SWH the power level of the echo signal, which depends on small scale surface roughness, and thus on surface wind-field parameters over the ocean Real echoes are composed of the sums of signals from many point scatterers, each with individual phase and amplitude. Therefore, the individual echoes have statistical characteristics superimposed on the pulse shape. In order to reduce uncertainties in the determination of pulse characteristics, the altimeter averages pulses together to reduce this statistical effect. When in ocean tracking mode, the mean sea-level point (mid point of the leading edge) on the time axis is maintained in the centre of the range window. The time interval between the transmitted pulse and this point is effectively the classical radar measurement of range. Measurements over Ice From other surfaces the waveform shape does not always conform to the simple Brown Model. The return echo from sea ice appears more specular than that from the ocean and has a peaked trace. The variability of the range measurement is of the same order as that from the ocean and this surface can therefore be tracked using the altimeter ocean tracking mode. The situation is different for continental ice, as the typical return echo has unpredictable shape and more importantly can have a larger variability in surface elevation An altimeter waveform over continental ice where the typical return echo has unpredictable shape and can have a larger variability in surface elevation. In order to maintain track of the surface, the Radar Altimeter, in ice mode, benefited from a wider observation window. The required increase in the size of the observation window was obtained by reducing the pulse bandwidth by a factor of four. This solution did not change the intermediate frequency (IF) bandwidth and was equivalent to enlarging the filter bandwidth without changing the filter bank; therefore it did not introduce major hardware changes into the system. In ice mode, tracking the echo of unpredictable shape was achieved by tracking the centre of gravity of the return pulse rather than the leading edge. This technique was used as the location of the centre of gravity is always unique, whereas there may be more than one leading edge, so avoiding any ambiguities. The main instrument parameters and technical characteristics of the Radar Altimeter are listed below: Mass: <= 96 kg Antenna diameter: 1.2 m DC power: <=134.5 W Data rate: <= 15 kbit/sec Bandwidth: ocean mode: 330 MHz ice mode: 82.5 MHz Pulse repetition frequency: 1020 Hz RF transmit power: 50 W Pulse length: 20 micro-s chirp Altitude measurement: 10 cm (1s, SWH = 16 m) Significant wave height: 0.5 m or 10% (1s) whichever is smaller Backscatter coefficient: 0.7 dB (1s) Echo waveform samples: 64 x 16 bits at 20 Hz Beam width: 1.3° Sea surface elevation: better than 10 cm Spatial Resolution: Footprint is 16 km - 20 km, depending on sea state Waveband: Microwave: Ku-band: 13.8GHz Sensor Modes ERS Radar Altimeter sea level map The ERS Radar Altimeter operated in 2 modes: ocean mode and ice mode. Beam width = 1.3° foot print = 16 - 20 m (depending on sea state). RA-1 operated by timing the two-way delay for a short duration radio frequency pulse, transmitted vertically downwards. The required level of range measurement accuracy (better than 10 cm) calls for a pulse compression technique (chirp). The instrument employs frequency modulation and spectrum analysis of the pulse shape. RA-1 provided measurements leading to the determination of: Precise altitude (ocean surface elevation for the study of ocean currents, the tides and the global geoid) Significant wave height Ocean surface wind speed Various ice parameters (surface topography, ice types, sea/ice boundaries) More details about RA (ERS-2) Sensor Modes are available in an ESA Bulletin Instrument Operations Find out about RA (ERS-2) instrument operations

  • Instrument - Altimeters

    Instrument - Altimeters

    RA (ERS) Products Information

    Reprocessed ESA ERS Altimetry (REAPER) dataset The full-mission reprocessing of the ESA ERS Radar Altimetry (RA) Level 2 data was completed and the dataset is available online to the ESA Earth Observation user community. The REAPER (REprocessing of Altimeter Products for ERS) project covers both the ERS-1 and the ERS-2 Altimetry missions. The reprocessed dataset spans from the start of the ERS-1 mission in July 1991 to June 2003, when the loss of the ERS-2 on-board data storage capability occurred causing the end of the ERS-2 global mission coverage. The reprocessing campaign was performed with REAPER processor version 1.08 and use of the latest reprocessed ESA precise orbit products (see ESA news "Homogeneous reprocessed ERS-1 and ERS-2 Orbit dataset"). The output products were delivered in netCDF format (version 3.6) and were based on dump orbits (i.e. not split in pole-to-pole passes). The REAPER Product Handbook provides exhaustive information on the ERS RA product content, format and improvements. It provides also, for user's convenience, a list of passes with known data quality issues (i.e. orbital manoeuvres, telemetry problems, etc). Data Access The ERS-1/2 REAPER Altimeter dataset is composed of the following three product types and is freely accessible online upon Fast Registration: Radar Altimeter REAPER Geophysical Data Record - GDR (ERS_ALT_2) Radar Altimeter REAPER Meteo Product - METEO (ERS_ALT_2M) Radar Altimeter REAPER Sensor Geophysical Data Record - SGDR (ERS_ALT_2S) Data Processing and Quality Improvement Major improvements with respect to the previous ESA RA products format (OPR - Ocean Product - and WAP - Waveform product) were implemented (e.g. the four Envisat RA-2 retrackers, RA calibration improvement, new reprocessed Precise Orbit Solution, ECMWF ERA-interim model, NICO09 ionospheric correction until 1998, GIM ionospheric correction up to 2003, new SSB, etc.). The assessment of the REAPER data quality versus the ERS OPR and WAP data showed a clear improvement in terms of accuracy over the tandem periods between ERS-1, ERS-2 and Envisat missions (currently assessed periods). However, the REAPER dataset presented some limitations (such as the use of poor MWR Wet tropospheric correction, out of range PTR corrections, etc.) that were fully described in the Product Handbook. Intercalibration between ERS-1, ERS-2 and Envisat, between all three radar altimeters and all three microwave radiometers, led to several improvements, including the incorporation of inter-mission bias corrections. Envisat style retrackers were used in the REAPER processing, allowing easier comparison. REAPER ERS-2 data was cross-calibrated to the released Envisat v2.1 product and the REAPER ERS-1 data to REAPER ERS-2. Validation reports for all three product types (GDR/SGDR/METEO) are available to users: REAPER Calibration Monitoring Cal/Val evaluation of REAPER ERS altimeter data RA L2 Validation Report ERS REAPER Performance and QA Website Contact Information If you have any questions about this project, contact: Dr. Philippe Goryl Email: Organisation: ESA-ESRIN Via Galileo Galilei Frascati (RM) 00044 Italy