TECHNOLOGY
From its vantage point
1336 kilometers (830 miles) above the Earth, TOPEX/Poseidon
can measure the height of the ocean surface directly underneath the
satellite with an accuracy of 4-5 centimeters (better than 2 inches).
Traveling in excess of 7 kilometers (4 miles) every second as it traces
out its orbit, TOPEX/Poseidon covers the global oceans every 10 days
(the "repeat period" of
the satellite orbit).
The accurate determination of the ocean height is made by first
characterizing the precise height of the spacecraft above the center
of the Earth. This is achieved through a technique called "precise
orbit determination" (POD), of which satellite-tracking
information is the most important ingredient. The baseline tracking
system for TOPEX/Poseidon is the onboard NASA retroreflector
array, which serves as a target for 10-20 satellite laser
ranging (SLR) stations that dot the Earth's surface. The CNES
DORIS (Doppler Orbitography
and Radiopositioning Integrated by Satellite) system provides an
important additional set of tracking data. Anchored by approximately
50 ground-based beacons, the DORIS receiver measures the Doppler
shift of microwave signals
to support POD. NASA's experimental GPS receiver onboard TOPEX/Poseidon
provides precise, continuous tracking of the spacecraft by monitoring
range and timing signals from up to 6 GPS spacecraft at the same
time. In order to produce accurate estimates of the satellite orbital
height, POD combines the satellite tracking information with accurate
models of the forces (e.g.,
gravity, aerodynamic
drag) that govern the satellite motion. For TOPEX/Poseidon,
this process supports the determination of the satellite orbital
height with an accuracy of 2-3 centimeters (1 inch).
The second component of the ocean height measurement is the range
from TOPEX/Poseidon to the ocean surface. The satellite carries
two radar altimeters
for providing this information: the NASA TOPEX instrument beams
microwaves at 13.6 and 5.3 GHz; the CNES POSEIDON instrument, at
13.65 GHz. To take a measurement, an onboard altimeter bounces microwave
pulses off the ocean surface and measures the time it takes the
pulses to return to the spacecraft. This measurement, multiplied
by the speed of light, gives the range from the satellite to the
ocean surface. After correction for atmospheric and instrumental
effects, the TOPEX/Poseidon range measurements are accurate to 3-4
centimeters. The range measurements are subtracted from POD-derived
estimates of the satellite orbital height, resulting in ocean height
measurements that are good to 4-5 centimeters (better than 2 inches)
relative to the center of the Earth.
This accuracy figure pertains to a few-kilometer spot on the ocean surface directly
beneath the TOPEX/Poseidon satellite. By averaging the few-hundred thousand measurements
collected by the satellite in the time it takes to cover the global oceans (10 days),
global mean sea level can be determined with a precision of several millimeters.
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