Spaceborne differential interferometric Synthetic Aperture Radar technology（D-InSAR） in long temporal series is an important technology in the land subsidence measurement

extraction of stable pointwise target is a critical procedure in long temporal D-InSAR processing chain.As we know

now the main two kinds of method have been proposed.One is amplitude method that can be performed to model the similar behaviors with phase dispersion with no resolution loss by means of a time series analysis of amplitude data.The other is based on coherence stability that can be computed in the stack of interferograms.The two methods have their own defects

the former requires a minimum of 30 images to detect the stable pointwise target precisely

the latter will directly lead to lower resolution than the original images.In this paper we introduce an optimized extraction method of stable pointwise target combining amplitude and coherence analysis.In this paper

amplitude detection method is first implemented and the number of stable pointwise targets is extracted.In the step

the data need to be radiometrically calibrated in order to ensure the images to be comparable.And the experiential threshold of amplitude dispersion is selected to extract stable pointwise targets.Aiming at solving problems of low density and ununiformity distribution of pointwise targets extracted by amplitude detection

a multi-step detection method is proposed.The multi-step detection method first uses amplitude detection

the proper threshold is selected to extract candidates of stable pointwise targets

which can ensure most of stable pointwise targets are included and greatly improve computation efficiency by the threshold.Further temporal coherence is utilized to extract the precise pointwise targets.In the step

the model of temporal coherence is established

and the partial atmosphere disturb and orbit error are removed by lowpass filters

sequentially the phase caused by the DEM error is estimated by a rough search of parameter space

the average effective phase are separated in interferogram stack.As the temporal coherence coefficient is proportional to the number of interferograms

the proper threshold is chosen in terms of the proportional relationship.Lastly

pointwise targets of their pixel value larger than the referred threshold are extracted.With the multi-step detection method

more reliable stable pointwise targets are extracted.The evaluation of the method is finally carried out with 26 ERS-1/2 C-band SAR images acquired over Shanghai urban covering 94 km2 from 1992 to 2000.Amplitude detection method and multi-step detection method are implemented at the same region respectively.The effectiveness of two methods is evaluated by the number and density of extracted pointwise targets.By comparison

pointwise targets of extracted by amplitude detection have the lower density than multi-step detection method.And reliability is evaluated by testing the target corresponding to the land type.It is known that stable pointwise targets points corresponding to the stable man-made objects in the urban.After the ground investigation in shanghai urban

we find the most of extracted points are usually corresponding to stable man-made objects

e.g.building and bridge.It fully validates the reliability of multi-step detection method.In summary

the testing results show that the presented method can extract stable pointwise targets effectively and reliably.