A method and system are provided for determining gas concentration at a region of a planetary surface. A gas filter correlation radiometry (GFCR) system, provided at a location above a planetary surface, includes a processor and a GFCR sensor having a first gas cell containing a target gas and a second gas cell that does not contain the target gas. Using the GFCR sensor, first images of a region of the planetary surface are generated via capture through the first gas cell and second images of the region are generated via capture through the second gas cell. Using the processor, ratio images are generated using the first images and the second images. Concentration of the target gas over known altitude variations of the region is determined where the concentration of the target gas is a function of the ratio images and the altitude variations of the region.