Direct light propagation measurement The noise in measuring the time will be proportional the duration of the pulse For mm-level measurements, need a pulse of the duration equivalent of a few millimeters Pulse strength also enters you need to be able to detect the return pulse) In general, direct time measurement needs expensive equipment a laser system capable of mm-level ranging to satellites costs - $1M 02/25/02 12.540Lec06
02/25/02 12.540 Lec 06 6 Direct light propagation measurement • The noise in measuring the time will be proportional the duration of the pulse • For mm-level measurements, need a pulse of the duration equivalent of a few millimeters. • Pulse strength also enters (you need to be able to detect the return pulse). • In general, direct time measurement needs expensive equipment. • A laser system capable of mm-level ranging to satellites costs ~$1M
Reflecting the signal back With optical (laser) systems you want to reflect signal back: a plain mirror wont do this unless perfectly normal to ray ·Usea" corner cube” reflector.n2 D shown on next page For satellites, need to"spoil the cube (i.e corner not exactly 90 degrees because station not where it was when signal transmitted) 02/25/02 12.540Lec06
02/25/02 12.540 Lec 06 7 Reflecting the signal back • With optical (laser) systems you want to reflect signal back: a plain mirror won’t do this unless perfectly normal to ray. • Use a “corner cube” reflector. In 2-D shown on next page • For satellites, need to “spoil” the cube (i.e., corner not exactly 90 degrees because station not where it was when signal transmitted)
Corner cube reflector Corner Cube reflector The return angle is twice the coming and outgoing rays are parallel corner angle For go degree 90- corner. return Is 180 degrees 02/25/02 12.540Lec06
02/25/02 12.540 Lec 06 8 Corner cube reflector Corner Cube Reflector Incoming and outgoing rays are parallel i i 90-i 90-i The return angle is twice the corner angle For 90 degree corner, return is 180 degrees
Alternative way to measure distance Instead of generating a short pulse and measuring round trip propagation time(also requires return pulse be detected), you can measure phase ditterence between outgoing and incoming continuous wave Schematic shown on next page Basic method used by interferometer 02/25/02 12.540Lec06
02/25/02 12.540 Lec 06 9 Alternative way to measure distance • Instead of generating a short pulse and measuring round trip propagation time (also requires return pulse be detected), you can measure phase difference between outgoing and incoming continuous wave • Schematic shown on next page • Basic method used by interferometer
Difference measurement(stays constant with time and depends on distance) 1.00 Outgoing Incoming 0.00 Outgoing + coming+△t 0.4 .8 4 Distance 02/25/02 12.540Lec06
02/25/02 12.540 Lec 06 10 Difference measurement (stays constant with time and depends on distance) -1.00 -0.50 0.00 0.50 1.00 0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 Outgoing Incoming Outgoing + ∆t Incoming + ∆t Signal voltage Distance