Precise measurements of CH4 in a column of near surface air, and in partial columns above this, would be very valuable in identifying sources/sinks of atmospheric CH4, and its transport. For this purpose we have proposed a grating mapping spectrometer (GMS) for deployment as an Instrument of Opportunity (IOO) on the NPOESS that acquires data in the 2990 to 3050 cm-1 spectral region. It will provide measurements of CH4 absorption of sunlight in the weaker CH4 features in the region, and of thermal emission in the stronger CH4 features in the region. It is the combination of the two that provides the vertical information. The IOO will acquire spectra on a crosstrack swath centered on nadir, and with 1/2 width of 55 degrees on each side of nadir (about 2800 km full width swath on the ground for a nominal 828 km satellite altitude). This with footprints that are about 3.1 km on a side at nadir. The small footprint facilitates cloud screening, and identification of CH4 source hotspots. A capability to project the slit to nadir along the direction from satellite to sun will be utilized for over the ocean viewing in order to facilitate measurements in solar glitter. It will obtain spectra with resolution n < 0.58 cm-1 and sample spacing < 0.17 cm-1. Based on the spectral characteristics and currently achievable very low-noise we do a linear error analysis (Rodgers, ) for the simultaneous retrieval of multi-column CH4, humidity, and surface parameters and 13CH4 total column. We show that useful multi-column CH4 retrievals can be obtained, with good near surface sensitivity in sunlit conditions. We also show the 13CH4 column can be retrieved with precision better than 3%. Retrieval of 13CH4 column in the earth's atmosphere is analogous in difficulty to retrieval of the major CH4 isotope column in the Martian atmosphere by a similar GMS deployed on a Mars orbiter. We show that H2O vertical information can be retrieved from these measurements and discuss the potential for ethane column retrieval.