Identification of the Location Phase Screen of Ers-envisat Permanent Scatterers

The phase time series of a point-wise Permanent Scatterer shows an additional phase shift passing from ERS to ENVISAT. This shift depends both on the existing 30MHz frequency shift between ERS and ENVISAT and on the location of the PS within the slant range resolution cell. The phase shift changes from one PS to another, but it is constant for all the ENVISAT images and is called "location phase screen" . In this presentation we show the experimental evidence of its existence. A data set of 90 ERS images and 7 ENVISAT images (ERS-like mode) on Milano has been used. Moreover, the location phase screen can be exploited to combine coherently ERS/ENVISAT SAR images and to identify the precise location of the Permanent Scatterers. 1 ERS AND ENVISAT INTERFEROGRAMS ON PS’s A key difference between ERS and ENVISAT SAR images is the 30 MHz central frequency change from 5.3 to 5.33 GHz. This change entails advantages as well as disadvantages: classical interferometry on distributed scatterers is made impossible, unless the normal baseline ranges from –1000 m to – 3000 m [1], [2], [3] so that the wavenumber shift ensures sufficient range common band. In the case of point scatterers (or at least scatterers with a reduced slant range extension) the baseline range allowing for a prediction of the phase signature from the ERS to the ENVISAT operating frequency is extended and crossinterferograms can be created without strict constraints on the normal baseline. In fact point scatterers are imaged coherently with both systems by definition. Their phase histories can be combined moving from one to the other frequency. Of course this requires the correction of the deterministic phase term depending on the scatterer position coupled with the normal baseline and the frequency shift. In fact, the phase shift due to the change of frequency from f0 to f0+∆f, for a given PS with slant range position and elevation respectively ∆r and ∆q (both relative to the center of the sampling cell taken as origin of the coordinates) is:       ∆ + ∆ + ∆ ∆ = ∆ θ θ π φ sin tan 4 0 0