[Laser] more EME stuff
Tim Toast
toasty256 at yahoo.com
Wed Jul 8 06:02:41 EDT 2009
Hi all,
I was searching the net for similar time delay effects on optical pulses. In the astronomical "zoo" out there in space are pulsars, which normally have a very steady rate of sending out radio bursts. Some of these are in orbit around another star. These "binary" pulsars, when the plane of their orbits are roughly aligned to the direction of earth, have a period which seems to vary at a regular rate - related to the time it takes for the pulsar to orbit its companion.
When receiving these pulses on earth with a radio telescope, instead of a nice steady ticking, they alternately run fast or slow depending on which part of the orbit the star is in. When it is moving away from us, the frequency slows down a bit, and when it is moving towards us, the frequency increases. As much as this sounds like the doppler effect, it is NOT. Most of the frequency drift in the rate of the pulses is caused by just the changing time of flight for the pulses. In the case of the pulsar, the distance can change by thousands or millions of miles over a period of time.
If i had searched radar or doppler radar i would probably find a similar case. About 300 kilometers change in distance over an hour, would shift the phase of a short pulse about 1 millisecond relative to GPS.
James G3RUH, sent me this table showing the exact time delays during the experiment last May:
i hope it doesn't get scrambled by the word wrap...or non-wordwrap.
-----------------------------------
Dear Tim,
I reworked the round-trip-light-time for the path AL-Moon-FR and the range
change is about +/-1.7 ms. Not so large as Self-Moon-Self, mainly because
one station (AL) is approaching the Moon, and the other (FR) is receding,
which compensates somewhat.
The light times given below are to the centre of gravity of the Moon, so
there is the curve of the Moon to be accounted for one-way, but not relevant
when considering the change in time two-way.
Hope useful.
73 de James G3RUH
______________________________________________________________________________
2009 May 27 [Wed]
Moon - Alabama | Moon - France
| RTLT -
OWLT | OWLT 2.396136s
UTC Az El s | Az El s ms
----------------------- -----------|------------------- ----------------------
2000 126.224 70.292 1.191799 | 276.141 24.952 1.202734 -1.603
2001 126.701 70.451 1.191785 | 276.301 24.782 1.202797 -1.554
2002 127.185 70.609 1.191772 | 276.462 24.612 1.202860 -1.504
2003 127.675 70.766 1.191758 | 276.622 24.442 1.202922 -1.456
2004 128.173 70.922 1.191745 | 276.782 24.273 1.202985 -1.406
2005 128.678 71.077 1.191732 | 276.942 24.103 1.203048 -1.356
2006 129.190 71.231 1.191720 | 277.102 23.933 1.203110 -1.305
2007 129.710 71.383 1.191707 | 277.261 23.764 1.203173 -1.256
2008 130.237 71.535 1.191695 | 277.421 23.595 1.203236 -1.204
2009 130.772 71.685 1.191684 | 277.580 23.425 1.203299 -1.153
2010 131.314 71.834 1.191672 | 277.740 23.256 1.203362 -1.102
2011 131.865 71.982 1.191661 | 277.899 23.087 1.203425 -1.050
2012 132.423 72.128 1.191650 | 278.058 22.917 1.203489 -0.996
2013 132.990 72.273 1.191639 | 278.217 22.748 1.203552 -0.945
2014 133.564 72.417 1.191629 | 278.375 22.579 1.203615 -0.892
2015 134.148 72.559 1.191619 | 278.534 22.410 1.203678 -0.839
2016 134.739 72.700 1.191610 | 278.693 22.241 1.203742 -0.784
2017 135.340 72.840 1.191600 | 278.851 22.073 1.203805 -0.731
2018 135.949 72.978 1.191591 | 279.009 21.904 1.203868 -0.677
2019 136.567 73.114 1.191582 | 279.168 21.735 1.203932 -0.622
2020 137.194 73.249 1.191573 | 279.326 21.567 1.203996 -0.567
2021 137.830 73.382 1.191565 | 279.484 21.398 1.204059 -0.512
2022 138.476 73.513 1.191557 | 279.642 21.230 1.204123 -0.456
2023 139.130 73.643 1.191549 | 279.800 21.062 1.204187 -0.399
2024 139.795 73.771 1.191542 | 279.958 20.893 1.204250 -0.344
2025 140.468 73.897 1.191535 | 280.115 20.725 1.204314 -0.287
2026 141.151 74.021 1.191528 | 280.273 20.557 1.204378 -0.229
2027 141.844 74.144 1.191521 | 280.430 20.389 1.204442 -0.173
2028 142.547 74.264 1.191515 | 280.588 20.221 1.204505 -0.116
2029 143.259 74.383 1.191509 | 280.745 20.053 1.204569 -0.058
2030 143.982 74.499 1.191503 | 280.903 19.886 1.204633 0.000
2031 144.714 74.614 1.191498 | 281.060 19.718 1.204697 0.059
2032 145.456 74.726 1.191492 | 281.217 19.551 1.204761 0.117
2033 146.208 74.836 1.191488 | 281.374 19.383 1.204825 0.177
2034 146.970 74.944 1.191483 | 281.531 19.216 1.204890 0.237
2035 147.742 75.050 1.191479 | 281.689 19.049 1.204954 0.297
2036 148.524 75.153 1.191475 | 281.846 18.882 1.205018 0.357
2037 149.317 75.254 1.191471 | 282.003 18.715 1.205082 0.417
2038 150.119 75.353 1.191467 | 282.159 18.548 1.205146 0.477
2039 150.931 75.449 1.191464 | 282.316 18.381 1.205210 0.538
2040 151.753 75.543 1.191461 | 282.473 18.214 1.205275 0.600
2041 152.584 75.634 1.191459 | 282.630 18.048 1.205339 0.662
2042 153.426 75.722 1.191456 | 282.787 17.881 1.205403 0.723
2043 154.277 75.808 1.191454 | 282.944 17.715 1.205467 0.785
2044 155.137 75.891 1.191453 | 283.100 17.549 1.205532 0.849
2045 156.007 75.972 1.191451 | 283.257 17.382 1.205596 0.911
2046 156.886 76.049 1.191450 | 283.414 17.216 1.205660 0.974
2047 157.773 76.124 1.191449 | 283.570 17.050 1.205725 1.038
2048 158.670 76.196 1.191449 | 283.727 16.885 1.205789 1.102
2049 159.575 76.264 1.191448 | 283.884 16.719 1.205854 1.166
2050 160.489 76.330 1.191448 | 284.040 16.553 1.205918 1.230
2051 161.410 76.393 1.191449 | 284.197 16.388 1.205982 1.295
2052 162.340 76.453 1.191449 | 284.353 16.223 1.206047 1.360
2053 163.277 76.509 1.191450 | 284.510 16.057 1.206111 1.425
2054 164.221 76.563 1.191451 | 284.667 15.892 1.206176 1.491
2055 165.172 76.613 1.191453 | 284.823 15.727 1.206240 1.557
2056 166.129 76.660 1.191454 | 284.980 15.562 1.206305 1.623
2057 167.093 76.704 1.191457 | 285.136 15.398 1.206369 1.690
2058 168.062 76.744 1.191459 | 285.293 15.233 1.206434 1.757
2059 169.037 76.782 1.191462 | 285.450 15.068 1.206498 1.824
---------------------------------------------------------------------------
So, during the hour we were sending and receiving, the round trip delay time drifted by a total of about +3.427 milliseconds.
What i am wondering is: When using the time domain setup and 1 millisecond pulses, would the time standard used to analyze the data have to drift by the same amount to properly track the signal? And if so, would this mean GPS is "too" steady as a standard for this or can it still be used? I know the GPS satellites do this all the time to compensate for distance changes and doppler effects when sending out their time data signals.
Anyway, i thought maybe using pulses several times longer than any phase delay effects might be a way to avoid this problem when using the time domain receiving setup. This, along with a narrow bandpass filter on the receiver and lasers as transmitters, low frequency modulation, good tracking and small similar FOV's for both transmitter and receiver, plus a few more unknown things tossed in probably... then that one photon in a million might get detected??
happy light beam bouncing!
tim
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