Post by ka9q on Mar 1, 2010 2:34:14 GMT -4
Recently I learned about this story:
legacy.jefferson.kctcs.edu/observatory/apollo11/
I've run a preliminary radio link budget using the best numbers for Baysinger's receiving station and found that although it's quite marginal, it's entirely consistent with his very noisy recording.
A link budget is simply an accounting of the signal and noise powers reaching the detector in a radio receiver so that the performance of the link can be calculated.
His recording would have been of the VHF transmitter in Neil Armstrong's PLSS, which transmitted both his voice and that of Buzz Aldrin to a receiver in the lunar module, which relayed it to earth over S-band.
Although NASA did not use VHF beyond earth orbit, Baysinger was wise to try it as Eagle's S-band transmitter would have been out of the question for him. During the EVA, Eagle transmitted wideband FM for video, with voice riding on a subcarrier. This was an "all or nothing" mode; anything less than a signal strong enough to produce video would produce nothing at all, not even audio. At all other times the downlink was PM, with voice on a narrowband FM subcarrier that could be (and was) received with an antenna of a practical size for radio hams. The PM mode was required to provide Doppler and ranging, so it was always used during flight. (The LM had a single transmitter switchable between FM and PM, the CSM had one transmitter of each type.)
The bottom line is that for the following parameters:
PLSS transmitter power of 500 mW on 259.7 MHz
cable and diplexer losses of -1.2 dB
PLSS antenna gain of -2 dB
distance 386,000 km
receiving antenna size 96 sq ft
receiving antenna efficiency 100%
receiving system noise temperature 300K
then the signal-to-noise ratio in a 3 kHz bandwidth would have been -10.8 dB. This is certainly not broadcast quality or even good quality, but neither is his recording. I am much more certain of some of these numbers than others. The PLSS figures came from a Bellcomm analysis of A15 surface propagation (using PLSS figures from A14) so they're probably good, although the -2 dB antenna gain seems pessimistic to me.
The distance came from JPL Horizons for the center of the moon as seen from Louisville KY on 0400 UTC July 21, 1969, minus the 1700 km radius of the moon.
The receiving antenna area came from the above article. Every antenna has an efficiency factor, but it is not given here so I assumed unity, which is perhaps a little optimistic.
I am least confident with my figure for system noise. An antenna pointed at the horizon will see noise from the earth filling half of its beamwidth. The sky will fill the other half. The noise temperature of the sky depends on direction and frequency; at 300 MHz it is already quite low, in the tens of kelvins in most directions.
To this "antenna noise" would have to be added the noise temperature of Baysinger's preamplifier. When I got involved in amateur satellite communications in about 1980 it has been fairly easy to get VHF amplifier noise temps well below 100 kelvins. But I don't know what he was actually using.
Radio hams have been involved in EME ("moonbounce") since the 1950s. It's exactly as the name suggests. They usually run full legal power into the biggest antennas they can build, and even then the links are often highly marginal. But it turns out that when the moon is rising or setting (it was just setting at the time of Baysinger's recording), the antenna picks up multipath reflections from the ground that can either enhance or subtract from the direct signal. Many hams doing EME with marginal setups have made quick contacts during a peak in the multipath fading.
Careful listening to Baysinger's tape shows that the signal did indeed fade quite slowly. At times it's pretty much gone. If I knew his antenna height above ground I could calculate the expected fade period for that VHF frequency.
While it's never possible to rule out a sufficiently elaborate hoax, so far I haven't found anything that says it was impossible for Baysinger to do what he did.
Soon I expect the hoax crowd to get wind of this one. That should be fun.
legacy.jefferson.kctcs.edu/observatory/apollo11/
I've run a preliminary radio link budget using the best numbers for Baysinger's receiving station and found that although it's quite marginal, it's entirely consistent with his very noisy recording.
A link budget is simply an accounting of the signal and noise powers reaching the detector in a radio receiver so that the performance of the link can be calculated.
His recording would have been of the VHF transmitter in Neil Armstrong's PLSS, which transmitted both his voice and that of Buzz Aldrin to a receiver in the lunar module, which relayed it to earth over S-band.
Although NASA did not use VHF beyond earth orbit, Baysinger was wise to try it as Eagle's S-band transmitter would have been out of the question for him. During the EVA, Eagle transmitted wideband FM for video, with voice riding on a subcarrier. This was an "all or nothing" mode; anything less than a signal strong enough to produce video would produce nothing at all, not even audio. At all other times the downlink was PM, with voice on a narrowband FM subcarrier that could be (and was) received with an antenna of a practical size for radio hams. The PM mode was required to provide Doppler and ranging, so it was always used during flight. (The LM had a single transmitter switchable between FM and PM, the CSM had one transmitter of each type.)
The bottom line is that for the following parameters:
PLSS transmitter power of 500 mW on 259.7 MHz
cable and diplexer losses of -1.2 dB
PLSS antenna gain of -2 dB
distance 386,000 km
receiving antenna size 96 sq ft
receiving antenna efficiency 100%
receiving system noise temperature 300K
then the signal-to-noise ratio in a 3 kHz bandwidth would have been -10.8 dB. This is certainly not broadcast quality or even good quality, but neither is his recording. I am much more certain of some of these numbers than others. The PLSS figures came from a Bellcomm analysis of A15 surface propagation (using PLSS figures from A14) so they're probably good, although the -2 dB antenna gain seems pessimistic to me.
The distance came from JPL Horizons for the center of the moon as seen from Louisville KY on 0400 UTC July 21, 1969, minus the 1700 km radius of the moon.
The receiving antenna area came from the above article. Every antenna has an efficiency factor, but it is not given here so I assumed unity, which is perhaps a little optimistic.
I am least confident with my figure for system noise. An antenna pointed at the horizon will see noise from the earth filling half of its beamwidth. The sky will fill the other half. The noise temperature of the sky depends on direction and frequency; at 300 MHz it is already quite low, in the tens of kelvins in most directions.
To this "antenna noise" would have to be added the noise temperature of Baysinger's preamplifier. When I got involved in amateur satellite communications in about 1980 it has been fairly easy to get VHF amplifier noise temps well below 100 kelvins. But I don't know what he was actually using.
Radio hams have been involved in EME ("moonbounce") since the 1950s. It's exactly as the name suggests. They usually run full legal power into the biggest antennas they can build, and even then the links are often highly marginal. But it turns out that when the moon is rising or setting (it was just setting at the time of Baysinger's recording), the antenna picks up multipath reflections from the ground that can either enhance or subtract from the direct signal. Many hams doing EME with marginal setups have made quick contacts during a peak in the multipath fading.
Careful listening to Baysinger's tape shows that the signal did indeed fade quite slowly. At times it's pretty much gone. If I knew his antenna height above ground I could calculate the expected fade period for that VHF frequency.
While it's never possible to rule out a sufficiently elaborate hoax, so far I haven't found anything that says it was impossible for Baysinger to do what he did.
Soon I expect the hoax crowd to get wind of this one. That should be fun.
