Broadband focal plane arrays, operating in UV-to-SWIR wavelength range, are required for atmospheric monitoring of greenhouse gases. Currently, separate image sensors are used for different spectral sub-bands: GaN for UV, Si for visible, and InGaAs for SWIR, requiring expensive component-level integration for hyper-spectral imaging. Also, the size of the InGaAs focal plane arrays is currently limited by the InP substrate area. We propose a GaAs/InGaP/InGaAs based photodiode on standard GaAs substrates for large-format (4096 x 4096) focal plane arrays with the following characteristics: (1) Wavelength = 0.25 to 2.5 micron; (2) Quantum Eficiency > 30% in UV (0.25 to 0.4 micron), > 80% in Visible (0.4 to 0.9 micron), and > 70% in IR (0.9 to 2.5 micron) subbands; (3) Photodiode Area (single element) = 15 x 15, 25 x 25, and 50 x 50 micron square; (4) RoA > 35 Ohm-cm^2 at 300K; and (5) Bandwidth > 1 GHz. Additionally, feasibility of UV-to-SWIR graded optical filters will be investigated. Based on P.I.'s experience on SCIAMACHY, this project will enable one image sensor for 8 spectroscopic channels currently orbiting on European Space Agency's ENVISAT. Also, feasibility of large-format image sensors on GaAs substrates will be demonstrated.