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Instrument Error

(A) Tracker Bias
This correction arises from the calibration bias in the discrete samples of the return waveform used in the onboard tracking algorithm which is designed to accomodate linear changes in the height (constant velocity) of the altimeter. When there is a rapid acceleration in height, for example when the altimeter passes over a narrow ocean trench, there is a corresponding induced height error which must be compensated for.

(B) Waveform Sampler Gain Calibration Bias
This correction occurs due to the fact that the amplitude of the received signal varies with the cross section of the monitored surface. An automatic gain controller is used for this signal attenuation adjustment, but rapid changes in echo strength mislead the circuit that tracks the position of the leading edge of the pulse, thereby producing a calibration error.

(C) Pointing Errors
Actually a combined effect of the satellite antenna gain pattern in relation to the antenna off-nadir pointing error on the shape of the return waveform, this correction occurs when the sub-satellite point is near the edge of the area illuminated by the altimeter. The resulting radar echo distortion produces an unwanted error bias.

(D) Antenna Gain Pattern Bias

(E) Average Pulse Shape Uncertainty and Time Tag Bias
The error in return pulse shape stems from the uncertainty due to random variability of the pulses used to calculate the mean echo. The residuals associated with averaging, say, 1,000 pulses, therefore contribute noise to the measurement. Also, the aging of microwave parts and long term clock drifts can induce height errors. Clock drifts can be accounted for by comparing the altimeter clock with some reference. Drifts in the height measurement induced by aging can be partially compensated for with the altimeter's internal calibration mode.

Propagation Medium Corrections


Last Modified: Sat Jul 17, 1998