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GPS EME Experiment |
 the pdf file is also located here 
Transmitter setup used for EME experiment May 27 2009 In this experiment I transmitted powerful 3 Hz light pulses at the moon which were synchronized with the GPS system 1 Hz sync signal. A remote receiver (in France) was listening for any reflected signals by using the program Spectrum Lab set up in the time domain mode and also synchronized with the GPS 1 Hz sync signal. 3 Hz was chosen for the transmitter so at least two pips would be visible on the screen at any time. The Spectrum Lab software allows the pulse signals to build up over time like a photographic time exposure. With the time domain setup, pulses are added together as long as they remain synchronized with, or maintain a fixed phase relative to the highly accurate GPS time standard. In other words, the pulses need to remain temporally coherent over a period of time. (at least an hour in this experiment) Here is how Spectrum Lab's time domain scope settings were configured: 
At the transmitter, the one second GPS sync signal was used to trigger a pulse generator to produce a 3 Hz pulse rate which was in turn used to trigger the strobe tube. The strobe transmitter generated flashes with 10 joules energy and 1 millisecond pulse width. This is about 10 kilowatts peak power per pulse at a rate of 3 pulses per second. A 22 x 28cm fresnel lens focused this into a beam width of 0.5 x 1.5 degrees. At the receiver, F1AVY (Yves) had a 24 cm telescope with an ultra low noise PGP detector aimed at the dark side of the moon. The GPS sync signal was used to trigger the time domain scope's horizontal channel. The receiver's output was fed to the vertical channel. The time domain scope then added the signals together many times until random noise was reduced to almost zero and hopefully leaving the weak lunar echo pulses visible. But despite good conditions on both ends of the path and an hour of recording time, no signals were detected in France. Here is a printout provided by G3RUH (James) from his lunar tracking program that shows what the round-trip and one-way delay times were during the experiment. I think this changing delay time could be one reason the experiment didn't work as planned. This is due to the changing time-of-flight of the pulses during the one hour "exposure". Since the transmitter used 1 millisecond pulses, any drift in the time of arrival of more than 1 millisecond would cause the pulses to no longer line up properly when summed by the software. As can be seen in the printout, the delay time changed by about 3.427 milliseconds during the hour. Here is one possible solution for this problem. Another factor that could have affected the time delays would be the accuracy of tracking. In this experiment, both transmitter and receiver were guided manually. The transmitter was illuminating the whole moon, while the receiver had a smaller FOV. The time delay of echo pulses received from near the edge of the moon will LAG about 11 milliseconds behind echo's from near the center of the moon's disk. So, with a small RX FOV, the delay time of the narrow pulses could drift rapidly - by several milliseconds in less than a minute as the telescope's FOV drifts toward the edge of the moon's disk before being re-centered. The best time to try these type experiments is not known for certain. One thing that can affect this is the earth-shine noise, which is maximum at new moon and gradually decreases to a minimum at full moon phase. Other things that can affect signals and noise: Transmitter power and pulse width, Receiver sensitivity and selectivity, The illuminated area vs. receiver field of view, Albedo in localized areas (such as Tycho), Lunar distance, Rapid changes in time delay due to tracking errors or Earth's rotation, The moon's elevation above your local horizon, Optics etc... A total lunar eclipse is an ideal situation possibly but there is still some light refracted onto the moon by the Earth's atmosphere. |
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