11:670:451 / 16:712:552 Remote Sensing of the Ocean and Atmosphere
Homework 4
Due Wednesday April 16, 2008
1. To achieve 3 cm precision in measuring sea surface height using satellite altimetry,
what must be the effective time duration, in nanoseconds, of the radar pulse?
To distinguish 3 cm of sea level variability with pulse-limited altimetry, the radar pulse
itself must be of order 3 cm length. If the pulse duration is T, its length is L = T/c where
c is the speed of light. Therefore we need an effective pulse duration of T = L/c =
0.03/3x108 = 10-10 seconds = 0.1 nanoseconds.
In practice, the pulse duration is longer than this and signal compression techniques
are applied in the signal processing to deduce the travel time of an “effective pulse” of
order this duration.
2. Explain why the Significant Wave Height of wind waves on the sea surface alters the
shape of the altimeter radar reflections shown in the figure below.
At high SWF the radar return begins early because the wave crests are closer to the
satellite, and the return lasts longer because the wave troughs are further from the
satellite. Hence, the returned power is stretched out over a greater time interval when
wave height is higher.
3. What properties of the ionosphere and troposphere are observed or analyzed in order to
make corrections to altimeter radar travel times? How is this information obtained?
The number of free electrons in the ionosphere affects the radar pulse transmission
time. Electron content varies from day to night (fewer free electrons at night), from
summer to winter (fewer during summer), and as a function of the solar cycle (fewer
during the solar minimum). This information is obtained by an analysis of travel time
variability for transmissions of GPS satellite signals, and in the case of the Poseidon
altimeter the transmission of dual-frequency pulses gives a differential response related
