Wavelets improve resolution of digital elevation maps

Synthetic aperture radar (SAR) takes advantage of the Doppler-shifted radar echoes generated by a satellite`s motion to synthesize large radar antennas and generate high-resolution images. To create digital elevation models of the Earth, interferometric SAR (IFSAR) is used, in which the phases of two SAR images taken of the same surface area are used to calculate terrain elevation.

May 1st, 1997

Wavelets improve resolution of digital elevation maps

-Andrew Wilson

Synthetic aperture radar (SAR) takes advantage of the Doppler-shifted radar echoes generated by a satellite`s motion to synthesize large radar antennas and generate high-resolution images. To create digital elevation models of the Earth, interferometric SAR (IFSAR) is used, in which the phases of two SAR images taken of the same surface area are used to calculate terrain elevation.

While IFSAR is accurate in determining the fine structure of terrain, it is prone to large-scale errors due to uncertainties in estimating the satellite`s position and errors that may result from scene changes that occur between observations. Stereo SAR uses ground control points to derive topographical elevation models and has good accuracy at coarser scales. However, it does not have the high-resolution accuracy of IFSAR.

Engineers at Vexcel (Boulder, CO) are exploring ways to combine the two techniques to obtain more accurate digital elevation maps. They have turned to orthogonal discrete wavelet transforms (DWTs). A DWT is used to decompose two registered IFSAR images taken from different viewing angles into different spatial frequency bands.

Final images are then generated by combining the averaged high-pass portions of the processed IFSAR data with low-pass portions of stereo SAR imagery to produce a higher-quality map. According to Carolyn Johnston at Vexcel, this process corrects local errors due to thermal and high-frequency noise and large-scale errors due to systematic problems. For information, contact Carolyn Johnston at cpj@vexcel.com.

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