> between a LEO and a GEO satellite. We were trying to work out how strong the > received beam would have to be at GEO, in order to be seen against the earth If the optics on the satellite are so bad that the earth can only be resolved to one pixel then you asked the correct question and the laser has to be very bright although we already have lasers that can outshine the total light from the sun heading to a nearby star for a picosecond or so. It would be expensive to send much information on such a laser (I think they take hours to recharge). But if the optics of this GEO can resolve down to 1 meter and can track your ground laser then the ground laser only has to be brighter than the amount of light put out by one meter of earth's surface. If the laser is shining on the ground instead of the sky then you can test this yourself. Distance should be irrelevant - just resolution. You could just use a lightbulb sitting above 1 square meter of black paper. If the ground optics sending the laser to the satellite keep the beam width and position down to 1 meter, then you don't need much power at all to outshine the whole earth. For example the light output from a 20W flourescent light is plenty to outshine the earth if it is focused on one square meter of geosat. Having intermediate optics on both ends (say accuracy and resolution of 1 mile) should make laser power very low. If we assume 20W per meter outshines earth at Satellite with no resolution, then resolution of 1 mile of the earth will be about 200 million times better and if we assume ground scope spreads laser light over 1 square degree of sky, that is about 40,000 times worse then spreading all over the sky. So power_needed/40,000*200,000,000=20W and power_needed=4 millwatts! I'm not confident in this answer, but I do think it should be easy with modest optics and good tracking. - George Roberts http://www.pobox.com/~gr ----------------------------------------------------------------- To unsubscribe from SeeSat-L, send a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@satobs.org List archived at http://www.satobs.org/seesat/seesatindex.html
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