Groot, Jos
AIRBORNE REPEAT PASS INTERFEROMETRY FOR DEFORMATION
MEASUREMENTS
Jos GROOT, Matern OTTEN, Erik van HALSEMA
TNO Physics and Electronics Laboratory, The Netherlands
groot@fel.tno.nl, otten@fel.tno.nl, vanhalsema@fel.tno.nl
KEY WORDS: SAR, Airborne Interferometry, RPI, Deformation, Dike
ABSTRACT
In ground engineering the need for deformation measurements is urgent. SAR interferometry can be used to
measure small (sub-wavelength) deformations. An experiment to investigate this for dike deformations was set
up, using the C-band SAR system PHARUS (PHased ARray Universal SAR). This paper describes the progress
made in the research.
1 MOTIVATION FOR THE EXPERIMENT
During extremely high floods a river dike can lose its stability, which can lead to catastrophic inundation of the
polder to be protected. Loss of stability is warned for by increasing deformations. For an early warning system,
which should support decisions to evacuate, input of actual information on deformations is essential. Only
remote sensing techniques can supply such information in an efficient way. Repeat pass airborne SAR
interferometry, which is capable of monitoring mm-level deformations, is the technique investigated in this
experiment. The spaceborne equivalent was not investigated because of the unpredictability of floodings, and the
high rate at which dikes deform during floodings.
2 EXPERIMENTAL CONSIDERATIONS
Accurate deformations can only be derived from high quality (highly coherent) interferograms. This puts
stringent requirements on the orbit and beam directions. These are discussed in this chapter.
2.1 Orbit and beam direction conditions
If the incidence angle of the two passes (for a single resolution element) differs too much, little or no coherence
remains. If the absolute value of the perpendicular baseline B , (directly related to the incidence angle difference)
exceeds the critical (zero coherence) perpendicular baseline (Zebker and Villasenor, 1992), no coherence can be
obtained. In our case the critical baseline is about 173 m.
The difference of the (horizontal) azimuthal beam directions of the two passes, Aq [deg], should fulfil a similar
inequality. The coherence drops to zero if Aq exceeds 1.9? in our case (Geudtner, 1995).
Assuming a linear, multiplicative model we can write
ala Y. nl
173 1.0
y (for O<|B, |<173, 0°<|Ag|<1.9°; O otherwise) (1)
for the measured coherence of an object with real coherence p,. If we allow for a coherence loss of at most 10 %
due to baseline and 30 % due to azimuthal difference effects (totalling 37 % loss), we obtain the approximate
conditions B ,«20 m and Aq«0.6?. For a single pass the aircraft position should therefore be within about 10 m of
a planned flight track, while the beam azimuth should vary by less than 0.3°. The next section describes how this
could be achieved.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B5. Amsterdam 2000. 305
