http://clasp-research.engin.umich.edu/missions/cygnss/docs/CYGNSS_FactSheet_October2014.pdf I wonder what the return signal strength of the reflected signals is?

On 12/16/16 9:54 PM, Mark Sims wrote: > http://clasp-research.engin.umich.edu/missions/cygnss/docs/CYGNSS_FactSheet_October2014.pdf > > I wonder what the return signal strength of the reflected signals is? There's quite a few of these bistatic radar using GPS as illuminator things over the years, airborne and space borne. You get a fairly big signal: if you're in LEO, the signal has already gone 20,000 km from GPS satellite to surface of earth,reflects and goes 400-1000 km more. The entire earth is illuminated, and the antenna on the spacecraft sees pretty much everything within a thousand km (depending on the height. The reflectivity of ocean water is quite high, and even soil is pretty good. The number is usually worked as sigma0 (pronounced sigma naught), which is the normalized radar cross section - RCS in square meters per square meter of surface. Typical numbers for L-band range from -20 to +10 dB - depending on the material and whether there are features (waves, furrows) that result in bragg scattering in a preferred direction (this is how you measure the wind speed with a radar from space) The trick on this kind of measurement is not detecting the signal in the first place, it's getting some kind of spatial resolution - the signal from thousands of square km is big, the signal from any little square is small. GPS is useful because you can build a fairly simple receiver, record the raw bits, and then, on the ground, post process to extract the direct signals (which gives you the satellite position and time very accurately) and get the reflections.. By combining the data from multiple satellites (made much easier because you know the time and position of each recording), you can get measurements for discrete areas on the surface of the Earth. It's a sort of multilateration process, and solving a big set of linear equations - much like any form of tomography. Each GNSS satellite/observer pair gives you a "reflected power vs delay" curve, a given delay maps into a sort of egg shaped ellipse on the surface of the earth. You can form a linear equation for each egg/slice/ellipse, and then iteratively solve the system (since the measurements are noisy, etc.) > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. >

Hi Mark, Here are a couple more novel and inspiring uses of GPS... 1) One can determine snow depth by monitoring the SNR of each SV: GPS Snow Depth http://xenon.colorado.edu/spotlight/index.php?action=kb&page=10 GPS Snow Sensing http://xenon.colorado.edu/presentations/Chapman/SnowChapman.pdf 2) One can determine ocean or tide levels by using both a RHCP antenna looking up and a LHCP antenna looking down: GPS & Sea Level Case Study http://xenon.colorado.edu/spotlight/index.php?action=kb&page=67 How does a GPS Tide Gauge work? http://xenon.colorado.edu/reflections/GPS_reflections/TideGauge.html The Accidental Tide Gauge: A GPS Reflection Case Study From Kachemak Bay, Alaska http://www.kristinelarson.net/wp-content/uploads/2015/10/LarsonIEEE_2013.pdf 3) One can use high-rate GPS as a seismometer: GPS & Earthquakes http://xenon.colorado.edu/spotlight/index.php?action=kb&page=46 Observing Seismic Waves Using High-Rate GPS: The 2002 Denali Fault Earthquake https://www.unavco.org/community/publications_and_reports/proposals/2007/facility2007/section3/UNV-GRID-SPREAD-EC.pdf /tvb