<p>NASA and the Indian Space Research Organization (ISRO) are collaborating on the Synthetic Aperture Radar (SAR) mission known as NISAR, which was launched on 30 July 2025. It will undergo a 90-day initial phase operation. The NISAR mission features two SAR sensors on a single satellite platform: one operating in the L-band from NASA and the other in the S-band from ISRO. NISAR will provide simultaneous data from two Interferometric SAR (InSAR) pairs—one in the S-band and one in the L-band—on a 12-day repeat pass orbit cycle. In this paper, we explore the potential for multi-frequency complementary interferometric phase fusion using simulated data from both the S-band and L-band. We propose a methodology aimed at generating a high-quality, improved fused S-band interferogram by combining the complementary phase information from the L-band interferogram. The methodology consists of three main processes. The first process for SAR Raw Data Simulation. This process generates S-band and L-band SAR interferometric pairs utilizing the NISAR sensor parameters. During this simulation, two datasets were created: the first simulated with short vegetation, and the second simulated with shrub vegetation surface backscattering coefficients.The second process is the InSAR Process.This step generates interferograms for both the S-band and L-band using the simulated datasets.The third process is the InSAR Phase Fusion Process.The final step involves fusing the InSAR phases to create a new, improved fused S-band interferogram. A comparative analysis is presented for the wrapped and unwrapped interferograms from the L-band, S-band, and fused S-band data. The fused S-band interferogram demonstrates high-resolution, unwrapped phase values while minimizing phase errors. Results indicate significant improvements over single-frequency InSAR pairs, highlighting the promise of multi-frequency phase fusion for future Earth observation and surface deformation studies.</p>