International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B3. Istanbul 2004 
So, we will report the results of a quantitative evaluation of 
ERS altimeter accuracy relatively to terrain height and slope 
variations within the whole impact area of the altimeter pulse 
contributing to the return signal used for the height 
measurement; the purpose of this evaluation is to empirically 
predict the accuracy of ERS altimetric data from terrain and 
ERS signal fluctuations, not only in completely flat areas but 
also in moderately rough terrain after correcting some 
systematic errors ; such predicted accuracy will determine 
whether ERS altimeter data can contribute to the ground 
control strategy according to the required accuracy 
specifications of the mapping project (orthoimage, densified 
DIM...) 
2. PRINCIPLES OF RADAR ALTIMETRY 
2.1 General principles 
  
satellite 
  
Pulse emitted 
-- ud 
nn ~~ 
- ——— 
Returned pulse 
A 
Ground surface 
AN 
geoid 
  
  
  
  
  
  
  
ellipsoid 
TOUR 
  
  
  
Figure 1. General principle of radar altimetry 
The ERS radar altimeter measurement consists in 
measuring the distance H between the satellite and the 
near nadir reflecting ground surface (see figure 1.). This 
distance is derived from the travel time of a radar pulse 
emitted by the satellite and returned back after reflection 
on the ground surface. If T is the time between emission 
and reception of the pulse and C the propagation speed of 
the pulse, we get H by : 
H=(C*T)/2 
The satellite height Hs above WGS84 ellipsoid is known 
with sub-decimeter accuracy through DORIS and GPS 
positioning systems. The ground altitude Ze referred to 
WGS84 ellipsoid is then computed from : 
Ze=Hs-H 
The ground altitude Zg refered to local geoid (equivalent 
to mean sea level) is finally computed, taking in account 
the height shift N between WGS84 ellipsoid and local 
geoid (the value of N is known from the latest global geoid 
model with an accuracy better than one meter) : 
Zg=Ze-N 
2.2 Waveform 
The satellite altimeter emits spherical radar pulses towards 
nadir within a narrow cone at the rate of 1000 pulses per 
second. The varying power of the return signal, called the 
“waveform” is sampled and memorised during the reception 
gate adjusted by the tracking system on board before switching 
again to emission mode. 
To explain the waveform shape we have to detail step by step 
the reflection sequencing of the wave on ground surface. For 
an ideally flat and equally reflecting surface, the reflection is 
going through the main steps presented on figure 2. 
  
  
  
  
  
Figure 2. Waveform with reflection on flat surface 
First, when the reception mode is activated by the on-board 
tracking system, a low power noise signal is received 
corresponding to parasite reflection of the pulse in the 
ionosphere and atmosphere. 
When the leading edge of the radar pulse hits the ground, the 
returned signal rises up, the reflection surface being a disc 
linearly spreading with time, which makes the corresponding 
return signal increase up to a maximum corresponding to the 
passage of the rear edge of the pulse "through" the ground 
surface. 
After the rear edge of the pulse passed "through" the ground 
level, the reflecting surface turns to a ring with increasing 
radius and area but like in a spherical radio wave the signal 
intensity decreases with the travelled distance, the returned 
signal to the altimeter decreases accordingly till vanishing down 
to the noise level or being cut by reception gate. 
Significant return signal is available from reflecting surfaces 
situated up to 18 km off nadir, which makes the exploitation of 
altimetric data particularly delicate in case of strong variations 
of the surface reflectivity . 
We face two main types of waveform depending of the ground 
surface reflectivity : specular and non specular waveforms 
described in following subsections . 
puissance 
BH 8 8 BS 
  
   
International Archiv 
uel dilly 
221 Specular w 
the return signal froi 
In this case, the rel 
cone of reflection, 
received by the alti 
gives a very sharp w 
22.2 Non specul 
result from the inte 
with scattering surf: 
return signal power 
and reception of ret 
time than for speci 
much wider from the 
EE 5 
Pmax/2 
  
  
  
  
> 
TI TTI TA TTT TEI TITY 
6" X dy íi 
Figure 3 : Specular 
(note that Y pov 
23 Retracking 
Because of highly 
specular case, altinx 
to produce accurate 
called “retracking”, 
called the *ramp") o 
the on-board altim. 
waveform ramp mic 
telemetered range n 
done by computing 
edge from the altime 
range measurement i 
4. illustrates this con 
  
  
& 
signal power — tr: 
  
  
Figu