Laser altimetry What corrections need to be made to estimate

Laser altimetry What corrections need to be made to estimate the elevation of the surface from the travel time of the laser pulse from the satellite to the ground? How is the elevation determined from a laser altimeter waveform (e.g. ICESat)? For (iii) and (iv) refer to Figure below. Why does the bottom figure show two peaks, when the top figure only shows one? Why is the peak in the top figure stronger than either of the peaks in the bottom figure?

Solution

i) There can be several corrections that needs to be made in this case. Corrections may need to be made for atmospheric refraction of the signal. There can be obstruction due to clouds due to which the return waveform energy may be reduced. Also, there can be fluctuations in the readings due to unevenness of the ground surface. So, a correction needs to be done by averaging out the fluctuations to account for this.

ii) We observe the return time of the strongest (energy wise) wave-form received, say t. If c be the speed of light, then ct is the distance travelled by the wave to reach the ground surface and return back. So, the elevation is equal to ct/2.

iii) The top figure shows a uniform ground surface A. When a waveform hits such a surface, all the waveform is reflected in a similar fashion and reach the satellite at the same time.

However, in the bottom figure the ground surface has two seperate level B and C. When the waveform reaches the ground half of the waveform is reflected by B and half by C. However, since their levels are different, there is a path difference between the two waveforms. So the waveform reflected by B reaches earlier and that from C reaches later. So, two peaks are observed.

iv)However, since the same waveform is being divided into two other waveforms, their energies are halved as compared to the previous waveform. So, two peaks observed are of half the energy as compared to the previous waveform.