ERS-1 SATELLITE - SCATTEROMETER OCEAN SURFACE WIND VECTORS

T. Liu (NASA, USA)

Announced availability: 1 January 1994

Data Access

ERS-1 scatterometer ocean surface wind vectors are available upon request from Tim Liu at NASA/JPL. The data are contained on eight Exabyte tapes.

Background

European Remote Sensing Satellite (ERS-1)
In July 1991 the European Space Agency (ESA) launched the first European Remote Sensing Satellite (ERS-1) as a forerunner to a new generation of satellites for environmental monitoring. ERS-1 uses advanced microwave techniques to acquire measurements and images regardless of cloud and sunlight conditions. ERS-1 parameters include those of sea state, sea surface winds, ocean circulation, sea surface temperature, sea and ice levels, as well as all-weather imaging of ocean, ice and land.

ERS-1 has been placed in a near-polar orbit at a mean altitude of about 780 km with an instrument payload comprising active and passive microwave sensors and a thermal infrared radiometer.

Instrumentation
ERS-1 carries a core set of active microwave sensors supported by additional, complementary instruments:

  • Active Microwave Instrument (AMI) combining the functions of a Synthetic Aperture Radar (SAR) and a Wind Scatterometer. The SAR operates in image mode for the acquisition of wide-swath, all-weather images over the oceans, polar regions, coastal zones and land. In wave mode the SAR produces images (about 5 km x 5 km) at regular intervals for the derivation of the length and direction of ocean waves.

  • The Wind Scatterometer uses three antennae for the generation of sea surface wind speed and direction.

  • Radar Altimeter (RA) provides accurate measurements of sea surface elevation, significant wave heights, various ice parameters and an estimate of sea surface wind speed.

  • Along Track Scanning Radiometer (ATSR) combining an infrared radiometer and a microwave sounder for the measurement of sea surface temperature, cloud top temperature, cloud cover and atmospheric water vapor content.

  • Precise Range and Range-rate Equipment (PRARE) is included for the accurate determination of the satellite's position and orbit characteristics, and for precise position determination (geodetic fixing).

  • Laser Retro-reflectors (LRR) allow measurement of the satellite's position and orbit via the use of ground-based laser ranging stations.

    Wind Scatterometer
    The purpose of the Wind Scatterometer is to obtain information on wind speed and direction at the sea surface for incorporation into models, global statistics and climatological datasets. It operates by recording the change in radar reflectivity of the sea due to the perturbation of small ripples by the wind close to the surface. This is possible because the radar backscatter returned to the satellite is modified by wind-driven ripples on the ocean surface and, since the energy in these ripples increases with wind velocity, backscatter increases with wind velocity.

    The three antennae generate adar beams looking 45 deg forward, sideways, and 45 deg backwards with respect to the satellite's flight direction. These beams continuously illuminate a 500-km wide swath as the satellite moves along its orbit. Thus three backscatter measurements of each grid point are obtained at different viewing angles and separated by a short time delay. These `triplets' are input into a mathematical model to calculate surface wind speed and direction.

    The main technical characteristics of the Wind Scatterometer are as follows:

  • Spatial resolution: >=45 km (along and across track)
  • Radiometric resolution
    (4 m/sec):
    <=8.5% (mid beam)
    <=9.7% (fore/aft beam)
    (24 m/sec):
    <=6.5% (mid beam)
    <=7.0% (fore/aft beam)
  • Radiometric stability
    CMIS: <=0.57 dB
    IIS: <=0.46 dB
  • Cross polarization: >=15 dB
  • Swath width: >=500 km
  • Swath stand-off: 200 km to right of sub-satellite track
  • Localization accuracy: +-5 km (along and across track)
  • Wind direction range/accuracy: 0 - 360deg / +-20deg
  • Wind speed range/accuracy: 4 m/s - 24 m/s / 2 m/s or 10 % <o> A transmit pulse is produced by the Scatterometer electronics and is amplified by the IF Radar unit, converted to an RF signal in the transmitter/converter unit and amplified by the High-Power Amplifier. The transmit signal is routed to the correct antenna by the Circulator Assembly which in this mode is under the control of the Scatterometer electronics.

    The received signal is down-converted, amplified by the IF Radar and routed to the Scatterometer electronics. A measurement sequence of 3.763 seconds corresponds to 25 km along the sub-satellite track at a satellite altitude of 785 km and is continuously repeated in the wind mode without any gap. This sequence involves four sets of measurements, regularly spaced, for each antenna beam (fore, mid and aft). Each series corresponds to 32 measurement pulses on each beam. Noise measurements and internal calibration are regularly performed in the interval between the transmitted pulse and the reception of the return echo.

    For the mid-beam, the return echos are filtered and sampled in complex form I and Q, while for the fore and aft-beams, as the doppler variation is significant over the swath width (20 KHz near swath to 140 KHz far swath), a programmable doppler compensation law is applied to the received signal before filtering and complex sampling.

    Data File Information

    The ocean surface wind vectors observations, collected between March 1992 and June 1993, were calculated using an algorithm developed at IFREMER, France.

    For more information, please contact:

    Tim Liu
    Jet Propulsion Laboratory
    California Institute of Technology
    Mail Stop 300-323
    4800 Oak Grove Drive
    Pasadena,CA 91109
    USA

    email: liu@pacific.jpl.nasa.gov
    Phone: (818) 354-2394
    FAX: (818) 393-6720

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