197 
Parameter 
Approximate Value 
Estimated Value 
Standard Deviation 
Ro [m] 
825185.4208 
825185.4208 
10' 8 (fixed) 
AR [m] 
7.89809 
7.89769 m 
0.00083 
To [s] 
97.7800 
97.7800 
10' 8 (fixed) 
AT [s] 
0.0023828 
0.0023828 
10' 8 (fixed) 
foo [Hz] 
398.573 
398.573 
10' 8 (fixed) 
foi [Hz/col] 
0.0 
- 0.0057 
0.00102 
ÎD2 [Hz/lin] 
0.0 
- 0.0087 
0.00759 
f D3 [Hz/ col lin] 
0.0 
0.0000028 
0.0000046 
do [m] 
33.7 
33.8218 
0.00434 
di [m/col] 
0.0 
-0.0001480 
0.000009 
d2 [m/lin] 
0.0 
0.0000289 
0.000015 
d3 [m/coMin] 
0.0 
0.0000000093 
0.0000000031 
d 4 [m/col 2 ] 
0.0 
- 0.0000000273 
0.0000000092 
ds [m/lin 2 ] 
0.0 
0.00000004445 
0.0000000035 
Table 1: Estimated parameters and their standard deviations (first set of parameters). 
(ascending and descending ERS-1 SAR images, SPOT 
images, orthophotos, reference DTM, land-use map, etc.), 
has been made available to ORFEAS participants. The 
results analysed in the following were obtained through 
the ORFEAS data set. 
Two ascending ERS-1 images of the ORFEAS data set 
were chosen for the processing. They were acquired at 
September 12 and 15, 1991. The baseline length is 161.5 
m, which is about optimal for DEM generation. From the 
original images two sub-images of 1500 pixels in range by 
5000 pixels in azimuth were extracted and processed with 
the ISAR software. The mean coherence of the filtered 
images equals 0.57. The considered area has an 
extension of approximately 25 by 35 km. The maximum 
height difference within the area is about 1150 m. This 
area includes many portions affected by foreshortening, 
layover and even shadow that make difficult the phase 
unwrapping. 
The images were processed in batch mode with the ISAR 
software. Before phase unwrapping, the interferogram 
was compressed 4 times in azimuth (complex average). 
The unwrapping generated four major zones of 
integration. The zones were manually "welded” and the 
unwrapped phases were checked and corrected for 
aliasing errors. 
5.1 InSAR Calibration 
Input data for the InSAR calibration are the precise master 
and slave orbits and the GCPs. In order to derive accurate 
orbits, the precise state vectors calculated by the 
GeoForschungZentrum (Germany) were used. The orbits 
are described by polynomials of the 5 th order; the 
polynomial coefficients were estimated by LS adjustment 
using the 7 state vectors closest to the image acquisition 
interval. 
The GCP identification was carried out using the 
orthophotos (scale 1:25000) of the ORFEAS data set and 
the SAR amplitude and coherence images. The 
identification in image space was realised using both the 
SAR amplitude and coherence images. In fact, depending 
on the land cover type and topography, such images bring 
quite complementary information. 
An example of GCP measured on the amplitude image is 
shown in Figure 2. In this case the river and the bridge 
can be easily recognised in both images and the point 
was chosen nearby the bridgehead. The example of 
Figure 3 regards homologous points measured on the 
coherence image. In this case a little stream is the linear 
feature which can be recognised in both images. In the 
coherence image it appears as decorrelated (low 
coherence) with respect to the background. 
Being very difficult to recover a sufficient set of full GCPs, 
we decided to measure height GCPs, i.e. points not well 
identified in planimetry whose height can be determined 
accurately. Such points were chosen in the centre of very 
flat fields. The identification of a suitable set of GCPs was 
very demanding. For the entire scene 20 GCPs were 
collected: 13 of them are full GCPs and 7 are only height 
GCPs. 
The InSAR calibration was performed with two different 
sets of parameters. They differ for the parameterization 
employed for the interferometric constant Die. In the first 
set a second order polynomial was used (with do, di, d2, 
d3, d4 and ds as parameters, see equation (6)), while in 
the second one, a bilinear polynomial (with do, di, d2 and 
d3 as parameters) was adopted. 
5.2 Analysis of the First Set of Parameters 
The calibration requires the selection of the suited set of 
parameters. In order to avoid a very ill-conditioned normal 
matrix, from the original parameter set (Ro, AR, T 0 , AT, foo, 
f D i, 1d2, fD3, do, d-i, d2, d3, d 4 and ds), the parameters Ro 
and foo were excluded from the adjustment.