NASA NTRS · 20210004634
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Explore connections, maps & timelines
Vigue, Y., Thomas, J.B., Lichten, S.M., Jefferson, D.C., C.E. Dunn. 1993-11-30. Time and Position Accuracy Using Codeless GPS. https://ntrs.nasa.gov/citations/20210004634
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This presentation provides a review of Position Navigation and Timing activities at the Glenn Research Center. Topics include 1) contributions to simulation studies for the Space Service Volume of the Global Navigation Satellite System, 2) development and integration efforts for a Software Defined Radio (SDR) waveform for the Space Communications and Navigation (SCaN) testbed, currently onboard the International Space Station and 3) a GPS L5 testbed intended to explore terrain mapping capabilities with communications signals. Future directions are included and a brief discussion of NASA, GRC and the SCAN office.
We have become impatient waiting for a web page to load, but the first member of our species evolved about 150,000 years ago - a geological instant as brief and as transitory as a text message. The shortest generation time of a bacterium is a sprint at under ten minutes, whereas a 200-year old whale, turtle or tree is not unknown. Life is a phenomenon that integrates processes ranging from the near instantaneous reactions of photosynthesis to the more stately pace of evolution. Here I will elucidate these processes with radically different time scales that go to creating and maintaining the diversity of life on earth, the clocks that nature uses to time them, and how modern biology is being used to alter the natural time scales.
We propose an index of climate change based on practical climate indicators such as heating degree days and the frequency of intense precipitation. We find that in most regions the index is positive, the sense predicted to accompany global warming. In a few regions, especially in Asia and western North America, the index indicates that climate change should be apparent already, but in most places climate trends are too small to stand out above year-to-year variability. The climate index is strongly correlated with global surface temperature, which has increased as rapidly as projected by climate models in the 1980s. We argue that the global area with obvious climate change will increase notably in the next few years. But we show that the growth rate of greenhouse gas climate forcing has declined in recent years, and thus there is an opportunity to keep climate change in the 21st century less than "business-as-usual" scenarios.