Tropospheric Correction Techniques in repeat-pass SAR Interferometry Zhenhong Li, Jan-Peter Muller, and Paul Cross Department of Geomatic Engineering, University College London, Gower Street, London, WC1E 6BT, UK, Email: zhli@ge.ucl.ac.uk ; jpmuller@ge.ucl.ac.uk ; paul.cross@ge.ucl.ac.uk [Abstract] For the first time, the application of a spaceborne near infrared (IR) water vapour product from the NASA Moderate Resolution Imaging Spectroradiometer (MODIS) for reducing water vapour effects of ERS-2 measurements are presented. The MODIS near IR water vapour product has been available since the end of February 2000 with a spatial resolution of 1km (at nadir) and a moderate accuracy of 5-10%. Coincident data with ERS-2 used in this study were located over Mt. Etna and Southern California. The results showed that the MODIS near IR water vapour product could be useful in reducing atmospheric effects in ERS-2 SAR interferometric data. Taking these results into account and the fact that, the MEdium Resolution Imaging Spectrometer (MERIS) has a higher accuracy and is on board the same platform, ENVISAT, as the Advanced Synthetic Aperture Radar (ASAR), future prospects look optimistic. The major limitation for either MODIS or MERIS is the reliance on cloud-free water vapour retrievals with the percentage of cloud free observations as low as 25%. In this paper, also for the first time, 3D GPS water vapour (or wet refractivity) models were applied to correct ERS-2 SAR interferometric data. Firstly, the WAter Vapour Extraction Software (WAVEs), developed at UCL, was used to construct a 3D water vapour (or wet refractivity) model with an appropriate resolution making full use of the spatial structure of zenith wet delays; then the difference in LOS (Line-of- Sight) path delays were calculated and applied to correct InSAR measurements. The results showed that the volumetric fields derived using the 3D GPS water vapour model can reduce water vapour effects significantly. It also showed that such 3D models are sensitive to the distribution of GPS receivers. Presently, a new way to fuse MODIS or MERIS with ground-based GPS networks within the WAVEs package is under investigation at UCL. 1. Introduction Tropospheric delay (especially that part due to water vapour) in radio signal propagation is known to be one of the major limitations of repeat-pass Interferometric Synthetic Aperture Radar (InSAR) [1, 2, 3, 4]. A 20% spatial or temporal change in relative humidity is estimated to cause between 80 and 290 m of topographic error for baselines between 400 and 100 m, and the corresponding error is of the order of 10 to 14 cm in the case of deformation estimates [4]. So far, several methods have been proposed to reduce the tropospheric effects on interferograms; temporal stacking, calibration and the use of permanent scatterers are the three most common methods. Temporal stacking reduces the spatially uncorrelated variance by averaging N independent interferograms in time with a factor of N. Reference [5] demonstrated an analytical method to determine the length of observation time necessary to measure a given deformation rate using the temporal stacking method. In [6], permanent scatterers and stacking were combined to reduce tropospheric effects and other short wavelength noise to recover a fault slip signal. Calibration utilises independent sources such as GPS and microwave radiometer (MWR) measurements to reduce tropospheric effects [3]. It has been suggested that numerical modelling could also be applied to the calibration [7, 8]. In this paper, two distinct correction methods are discussed and tested: the use of contemporaneous Near Infrared (IR) water vapour measurements from MODIS and the application of a 3D water vapour model derived from a dense network of contemporaneous GPS measurements. 2. MERIS, MODIS and 3D GPS water vapour model (WAVEs) The MEdium Resolution Imaging Spectrometer (MERIS) is a key payload on ESA’s ENVISAT, an advanced polar- orbiting Earth observation satellite launched on 1 March 2002. MERIS allows for the global retrieval of total columnar atmospheric water vapour of the Earth every 3 days, with two water vapour channels in the near infrared. MERIS near infrared water vapour products are available at two spatial resolutions. In full resolution (FR) mode each pixel has a nominal resolution of 300m with an instantaneous field of view (IFOV) of 0.019°, with a nadir spatial sampling of 260 m across track by 290 m along track. In reduced resolution (RR), the nominal resolution is 1.2km where each pixel ____________________________________________________________ Proc. of FRINGE 2003 Workshop, Frascati, Italy, 1 – 5 December 2003 (ESA SP-550, June 2004) 61_li