Geodetic Data Collection Techniques - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 54

Fig. 2.16 Rationale of
satellite altimetry
Satellite Altimeter and Its Operational Principle
A satellite altimeter is a satellite-borne microwave radar that usually consists of a
transmitter, a receiver, a timing system, and a data collection system. It is generally
13.9 GHz in the emission frequency with 2 kW in transmitting power and works at
an altitude of 800 km. Its radar antenna is parabolic with a diameter of 0.6-1 m. To
guarantee at the same time the accuracy of measurement, the resolution, altitude,
and other indices, the radar pulses transmitted must have a comparatively large
time-frequency bandwidth. Thus, the satellite altimeter employs the pulse com-
pression technique to transmit and receive pulses. The compressed pulses can be of
nanoseconds (10 9 s) in width, which means that the pulse compression technique
has solved a big problem in radio theory in that the width of pulses in the time
domain and frequency domain cannot be enlarged simultaneously. The product of
the width of pulses in the time and frequency domains is referred to as the
compression ratio.
The operational principle of a satellite altimeter is as follows: The transmitter
sends modulated compressed pulses to the Earth's surface with a certain pulse
repetition frequency (PRF) through its antenna; after the reflection by the ocean's
surface, the pulses return and are received by the receiver, which measures the time
difference between the transmitted pulse and the received pulse. According to this
time difference and the reflected waveform, we can determine the distance from the
satellite to the ocean's surface. The width of the radar beam transmitted by the
satellite is approximately 1 ; as a result, when the radar beam reaches the ocean's
surface, the radius of the signal track is about 3-5 km. Therefore, the distance
measured by the altimeter is equal to the average distance from the satellite to the
circular area with the radius of 3-5 km. On this basis, instrument adjustment, sea
state correction, tropospheric refraction correction, ionospheric effect correction,
and correction for periodic effects of the sea surface, etc. have to be taken into
account.