Measurement Process
9
H|0|alt|altimeter|93|101|
H|0|urad|radiometer|502|511|
H|0|locate|precise orbit determination|547|573|
H|0|ocean|ocean circulation|669|685|
H|0|tides|tides|691|695|
G|0|ot|ocean topography|55|70|
G|0|alt|altimeter|78|86|
G|0|ms|microwave signal|360|375|
G|0|mr|radiometer|407|416|
    TOPEX/Poseidon's primary instrument for measuring ocean topography is an altimeter. The altimeter bounces a microwave pulse off the surface of the ocean. By measuring how long the signal takes to return, the altimeter determines the distance between the satellite and the ocean surface. 
    Air, water vapor, clouds, and rain slow down the return of the microwave signal. A second instrument called a radiometer is used to correct for the influence of water in the atmosphere. 
    Altimeter and radiometer measurements, in conjunction with precise orbit determination, are used to obtain a detailed map of the ocean's topography. From this, scientists calculate ocean circulation and tides.
