• to encourage development of techniques for satellite data assimilation in numerical mo 
dels and model tuning, 
• to stimulate and develop scientific research investigations using all-weather high or me 
dium resolution data from different microwave sensors over land, coastal zones, or ice 
at regional scales. 
To serve these scientific objectives a payload of instruments has been defined and developed 
which include ([ESA 1988]) 
• the Precise Range and Range Rate Equipment PRARE which is an active micro- 
wave ranging system to be used for orbit parameter determination at decimeter level of 
accuracy as well as for various geodetic applications; 
• the Radar Altimeter which will provide high precision measurements of the distance 
from the satellite to the underneath surface. Over oceans and smooth ice sheets, ac 
curacies significantly better than 10cm are expected, whereas over rough ice a special 
mode is available with coarser resolution and more agility. With precise knowledge of 
the orbit, the absolute elevation of ocean and ice surfaces can be calculated for use 
in geodesy, ocean and ice topography, etc. The instrument will also operate over land 
surfaces where the terrain is not too mountainous; 
• the Along-Track Scanning Radiometer and Microwave Sounder ATSR-M which 
is a passive instrument consisting of a 4-channel infrared radiometer providing measu 
rements of sea-surface and cloud-top temperatures, and a 2-channel microwave sounder 
providing information on the total water-vapour content of the atmosphere; 
• the Active Microwave Instrument AMI, which is a multipurpose radar sensor with 
3 basic modes of operation: 
1. the wind mode: 
the purpose of the wind mode is to obtain measurments on wind speed and direction 
at the sea surface. The scatterometer will measure the echo power of the signal 
transmitted from the satellite; the echo power is affected by the surface wind 
conditions. Wind mode specifications include wind direction range of 0 — 360°, 
wind speed range of 4 - 24 m/s, spatial resolution of 50 km, swath width and 
location of 200 km to 700 km off subsatellite track; 
2. the wave mode: 
the SAR system can be operated in a special wave mode: regularly spaced sample 
areas within the 100 km swath will be imaged; these images will be transformed into 
directional spectra providing measurements of ocean wave length and direction; 
wave mode specifications include wave direction range of 0 — 180°, wave length 
range of 100 - 1000 m, sample area size of 5 km x 5 km every 200 - 300 km within 
the radar swath; 
3. the imaging mode: 
the normal SAR imaging mode will deliver strips of high resolution radar imagery; 
SAR system specifications include spatial resolution of 30m x 30m, radiometric 
resolution of 2.5 dll at rr u = - IN (///, swat h width of 100 km, incidence angle of 23° 
at mid-swath, radar frequency of5.3(lhz ( C - Hand ), VV polarization, peak power 
of 4.8 KW allowing only 10 minutes of imaging per orbit, data rate of lOOMHps 
preventing image recording on board and allowing operation only in direct line of 
sight of a ground receiving station.