t B3. Istanbul 2004 
  
latest global geoid 
meter) : 
lar pulses towards 
f 1000 pulses per 
signal, called the 
ring the reception 
rd before switching 
detail step by step 
round surface. For 
ce, the reflection is 
gure 2. 
  
  
n flat surface 
ed by the on-board 
signal is received 
the pulse in the 
hits the ground, the 
face being a disc 
s the corresponding 
orresponding to the 
wough" the ground 
hrough” the ground 
ing with increasing 
dio wave the signal 
tance, the returned 
' till vanishing down 
"ate. 
| reflecting surfaces 
s the exploitation of 
of strong variations 
nding of the ground 
specular waveforms 
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B3. Istanbul 2004 
22.1 Specular waveforms : 
the return signal from very reflective surfaces like water bodies. 
In this case, the reflected energy is concentrated in a narrow 
cone of reflection, which gives a very strong return signal 
received by the altimeter in a very short period of time; this 
gives a very sharp waveform as presented on figure 3 (left). 
22.2 Non specular waveforms : Non specular waveforms 
result from the interaction of the altimeter's transmitted pulse 
with scattering surface found in rough terrain. In this case, the 
return signal power is much lower than for specular waveform 
and reception of return scattered signal is spread over a larger 
time than for specular echo (the cone of reflection extends 
much wider from the vertical axis) 
  
  
Mt 300 
120 2500 
10000 Flanc 
: e 291 Frontde 
989) Pmav? 2 montée 
$ 3 150 
4610 : 
3 Q 
Sm 1000 
a0 50 
rrr rn " ps : : : 
in a 94 F9 6 1^ 4 2 39 43 8 
  
     
  
  
  
  
  
  
  
Figure 3 : Specular (left) and non specular (right) waveforms 
(note that Y power scale is not the same for both cases) 
23 Retracking 
Because of highly complex waveforms, particularly in non 
specular case, altimeter data over land must be post-processed 
to produce accurate surface elevation. This post-processing, 
called “retracking”, is required because the leading edge (also 
called the *ramp") of the terrain return waveform deviates from 
the on-board altimeter tracking gate (predicted location of 
waveform ramp mid-point), causing a significant error in the 
telemetered range measurement. Retracking altimetry data is 
done by computing the starting point of waveform's leading 
edge from the altimeter tracking gate and correcting the satellite 
range measurement (and surface elevation) accordingly. Figure 
4. illustrates this concept . 
  
  
à 
signal power tracking gate : ; 
actual location of 
ramp mid-point 
  
Y 
Altimeter 
waveform 
NU g 
correction 
  
range measu rement 
  
Y 
  
  
  
  
Figure 4. Retracking correction 
Specular waveforms result of 
3. EXPLOITATION OF SPECULAR DATA ON WATER 
BODIES 
3.1 Threshold values for selection of specular echoes 
Water bodies situated up to 18 km off-nadir can return strong 
specular signal . From our experience, a water body echo 
should be within the following threshold values : 
0,5 gate « ramp duration « 1 gate 
(1 gate = 12,12 ns equivalent to about 2m range for ERS) 
rear edge slope « -0,11 Neper/gate 
(Neper is the logarithmic value of the return signal) 
coefficient of reflection > 22dB 
(coefficient of reflection = total return energy / emitted energy) 
3.2 Matching of specular data with water bodies 
Though the range measured between the satellite and a water 
body is very accurate (thanks to the sharp return signal), the 
main problem is that the altimeter tracking system keeps locked 
to the water body even when it is well off-nadir (more than 10 
km is commonly observed) causing a slope error which has to 
be corrected to get the water body elevation with enough 
accuracy. 
  
  
  
HA 
A 
D 
Z 
water 
body 
ground surface 
H 
. . I L | 
ellipsoid « Pa 
  
  
  
  
  
Figure 5. Off-nadir signal geometry 
Simple geometric consideration as shown on figure 5 brings the 
corrected value : 
H = Z — SORT (D? —L’) 
with 
H : ellipsoid altitude of water body 
D : Altimeter range measurement 
Z : ellipsoid altitude of satellite 
L : horizontal distance between satellite nadir and water body 
The major cause of inaccuracy in the determination of altitude 
H comes from the inaccurate horizontal position of the water 
body itself (small scale available topographical maps give that 
position with about 250 m absolute accuracy, which makes a 
vertical error of about 4 meters for a water body situated 10 km