81–88 of 88 posts
https://www.jpl.nasa.gov/news/radar-astronomy-used-to-resear... (1991)
Earlier quoted context omitted.
I didn't see any discussion on your page of the retroreflectors the Apollo astronauts left on the moon. These were put there for distance measuring but they might be useful for laser-based communication too. Caveat: Retroreflectors only reflect in the same direction as the incoming beam. But I'd guess that imperfections in their construction together with the roughly 1 degree of arc spanned by 2 stations on opposite…
1 degree and 54 minutes, it turns out. Amazing when you think about that, isn't it?
Earlier quoted context omitted.
1 degree and 54 minutes, it turns out. Amazing when you think about that, isn't it?
I was thinking earth was about twice the diameter of the moon, hence 1 degree. Turns out it's closer to a factor of 4. Should have looked it up.
: yeso; units
Currency exchange rates from FloatRates (USD base) on 2025-10-06
3749 units, 113 prefixes, 120 nonlinear units
You have: 2 arcsin(earthradius/moondist)
Unknown unit 'arcsin'
You have: 2 asin(earthradius/moondist)
You want: dms
1 deg + 53 arcmin + 57.540656 arcsecEarlier quoted context omitted.
I have no understanding of the physics involved, but could the broadcast location be reverse engineered? (With triangulation and clever math?)
Your location is very very visible to any plane or satellite passing overhead.
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The entire HF band, including the parts already used for something, is only 27 MHz of bandwidth, it's full of noise, and at any given time only a fraction of it can propagate to the other side of the world, dependent on time of day and, literally, sunspots. This antenna has 1100 MHz of bandwidth, the analog front end has 40 MHz for any given conversation, and noise levels are much lower. It could conceivably deliver…
I didn't see any discussion on your page of the retroreflectors the Apollo astronauts left on the moon. These were put there for distance measuring but they might be useful for laser-based communication too. Caveat: Retroreflectors only reflect in the same direction as the incoming beam. But I'd guess that imperfections in their construction together with the roughly 1 degree of arc spanned by 2 stations on opposite…
Earlier quoted context omitted.
I didn't see any discussion on your page of the retroreflectors the Apollo astronauts left on the moon. These were put there for distance measuring but they might be useful for laser-based communication too. Caveat: Retroreflectors only reflect in the same direction as the incoming beam. But I'd guess that imperfections in their construction together with the roughly 1 degree of arc spanned by 2 stations on opposite…
The APOLLO lunar laser ranging experiment uses a 3.5 meter telescope as a laser turret and manages to get about 2,400 photons back from those retroreflectors every half an hour, and it's a challenge just to find the things as the spot's a few km wide by the time it gets to the moon. Good luck.
I don't know what wavelength they're using, but at 555nm, 1.22λ/d would be 0.193 microradians, which, unless I'm doing the math wrong, works out to a 74-meter Airy-spot radius at the distance to the moon. At that sort of size, you'd think the majority of the photons in the desired wavelength band would be from their laser rather than stray Earthshine.
I was doing calculations based on λ = 350nm and a 500-mm reflector, and no retroreflector, and getting rather sad estimates of 3 joules of light transmitted per returned photon (per receiver). While that's clearly a feasible commnications system, it's going to be pretty limited in bandwidth. I'm not sure if C-band radio is better?
Earlier quoted context omitted.
I have no understanding of the physics involved, but could the broadcast location be reverse engineered? (With triangulation and clever math?)
Your location is very very visible to any plane or satellite passing overhead.
Expected array gain: ~39.3 dBi / EIRP: ~63.1 dBW Tx power: 1 W per antenna Yeah... so free space path loss at legal frequencies for hams this thing can transmit on is ~283dB. Neat idea but consider me skeptical. Having said that I can see some interesting applications for this kind of gear, EME seems overly optimistic though.
At those power levels they would have to use some kind of highly error-corrected modulation and coding scheme to provide enough coding gain to overcome the path loss. I agree they are pretty optimistic, but until they detail their modulation scheme, it's hard to tell. A few years ago I was experimenting with 900 MHz LoRa for a work project -- we had need to communicate a very small data payload from inside elevator c…