Earth Probes is a NASA program that complements EOS (Earth Observing System). Earth Probes are specific, highly-focused missions that investigate processes requiring special orbits or short development cycles of one to three years. Within this program are the ESSP (Earth System Science Pathfinder) missions, the UnESS (University Earth Systems Science) program, and various specialized Earth-observing projects such as TOMS (Total Ozone Mapping Spectrometer), Triana, and SRTM (Shuttle Radar Topography Mission).