Geodetic Data Collection Techniques - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 53

1. The round-trip time of the radar pulse going from the satellite to the ocean's
surface and back to the satellite: the measured value of satellite altitude.
2. The waveform of an echo signal, which contains an ascending front, a flat top,
and a decay area.
3. The amplitude of an echo signal, that is, the automatic gain control value of the
signal. Based on the analysis of the waveform, structure, and round-trip time of
the echo signal, we can obtain information like sea level altitude, sea level tilt,
ocean currents, effective wave height, sea surface backscattering coefficients,
and wind field (see Leuliette et al. 2004).

In satellite altimetry, the satellite is seen as a mobile platform on which a radar
altimeter transmits a microwave pulse to the Earth's surface and receives the signal
reflected back. Suppose that the altitude of the satellite is and the propagation
velocity of the signal is c; we can then use ʔ t, the round-trip time of the radar signal

observed, to calculate:

c ʔ

t
2 :

a ¼ ð 2 : 22 Þ

Because of water's good reflective properties, this method is particularly suitable
for marine areas. The radiation of radar signals can instantaneously cover an
annular region with a radius of thousands of kilometers on the sea surface (often
referred to as the signal footprint). The size of the annular region is related to the
spatial resolution of the incident microwave beam. So, the observed value is an
el ev ation to the average instantaneous sea level. Its difference from the geoid height
is H. Assume that the satellite altitude with respect to the reference ellipsoid is h,
which can be derived from the satellite orbit in the geocentric reference system
through calculation. If neglecting additional corrections, we can simplify the basic
satellite altimetry equation to:

h ¼ N þ H þ a : ð2:23Þ

Figure 2.16 shows that the radar altimeter can be adopted to scan the sea surface
directly so as to scan the marine geoid approximately. Therefore, satellite altimetry
is an effective method for directly drawing a geoid map. It is important that it can
detect a very large marine area in a rather short time and make o ut a detailed sea
level expression with a very high spatial-temporal resolution. H means a distur-
bance (noise) in establishing the geoid, but is an ob se rved signal for the research of
ocean dynamics. From the extensive analysis of H, we can obtain an important
understanding of the structure of the ocean floor and seabed features.