Subject: Space-tech Digest #55 Contents: George Ott Lunar Surface Comm. Lou Adornato Re: Lunar Surface Comm. Jordin Kare Re: Lunar Surface Comm. Gord Deinstadt Re: AIAA solicits your Moon/Mars ideas Joe Beckenbach Re: AIAA solicits your Moon/Mars ideas George Ott Fairly Stupid Robots Henry Spencer Re: Fairly Stupid Robots Edward Wright Re: Fairly Stupid Robots ------------------------------------------------------------ Date: Fri, 16 Mar 90 09:48 CST From: GOTT@wishep.physics.wisc.edu Subject: Lunar Surface Comm. To: space-tech@CS.CMU.EDU X-VMS-To: IN%"space-tech@cs.cmu.edu" Several weeks ago there was a question posted regarding communications on the moon between two bases 1000km apart. Several ideas were proposed including fiber optic cable, plain old ordinary cable, relaying everything via the Earth, using relay satellites in a variety of lunar orbits and microwave and laser links on towers. I vote for lasers on towers. The formula used to calculate the surface distance from an observer to the horizon is: X = R * ATAN [SQRT {h/R*((h/r)+2)}] where R is the objects radius h is the observers height X is the surface distance (note: this equation was donated by yaron@astro.as.utexas.edu) According to the CRC 65th edition the radius of the moon is 1738.3 km plus or minus 1.1 km. For the purposes of the following calculations I chose R to be 1738.5km. NOTE: the distance from the tower to the horizon is X, the total distance the tower can "see" is 2*X h (in meters) X (km) 2*X (km) Number of towers needed to cover 1000km ------------- ------- -------- ------------------------------ 100 18.6 37.2 27 200 26.4 52.8 19 400 37.3 74.6 14 500 41.7 83.4 12 1000 60.0 120.0 9 2000 83.4 166.8 6 A note on tower height: I stopped at 2000m tall towers because that's where it started to look like the payback in increased range gets eaten by the extra tower height. Tower 2km tall in 1/6 gee with no wind loads (i.e. built on the moon) would be easy to build. I assume that the primary purpose of this link is to talk between the two bases. Some other communication needs are 1. To teleoperate things 2. To communicate with vehicals and crew out wandering around the surface 3. Navagation beacons. The need for 1, 2 and 3 will probably be concentrated around the two bases. I propose a total of 12 towers: Two 1000m tall ones, one at each base, and ten 500m tall relays between the two bases. The 1000m towers will serve as the platforms for the interbase comm. link, for teleoperation and for "local" communications i.e. to crew and vehicals. The 500m towers will act as relays between the bases. The comm. link itself will be based on lasers both for the higher bandwidth than radio and to avoid mucking up the radio astronomers. (After all, radio astronomers are people too :) Power will be provided by the appropriate radioisotope generator. Although the electronics will need hardening, it should be a simpler task than shielding kilometers of fiber optic cable. When the growth of the bases requires more bandwidth either a parallel set of towers could be installed or more/upgraded hardware could be stuck on each tower. As other bases are built they can be added to the system. It should be possible to build the towers out of available materials, the lasers and detectors may need to be imported from earth. Practice building towers out of lunar materials will be useful when we start building power satellites, radio astronomy antenna farms, geosynch. orbit communications platforms etc. Towers are simple. (as compared to anything in any orbit) Towers are resistant to radiation. (as compared to fiber optics) Towers are expandable. (use all the bandwidth you want. we'll add more) Towers will lead to a 3.3ms lightspeed delay. (rather than the seconds for moon-earth-moon relays) Towers can be built with local materials. (this means cheap) I apologize for the length, I wanted to make sure I covered all of my reasoning ... well, most of it anyway. George K. Ott University of WI - Madison High Energy Physics Department gott@wishep.physics.wisc.edu ------------------------------ Date: Mon, 19 Mar 90 13:39:25 CST From: Lou Adornato To: space-tech@CS.CMU.EDU Subject: Lunar Surface Comm. Can anyone out there answer the following: 1) Using present technology, how tighly can a communications laser be focused? Is it reasonable to expect that tighter focusing in the next 20 years, or are we at a fundamental limit? 2) What is the threshold of detection given the present technology? In other words, at 10Mbps, how much power needs to come out of the small end of my attenuation function? Again, is this fundamental or subject to incremental improvements? 3) What references exist for someone with limited knowledge of laser communications technology (but with an engineering background and enough physics to not understand eigenfunctions?). I'm talking about transmission _and_ detection technoology, preferably line of sight. Lou Adornato | Statements herein do not represent the opinions or attitudes Cray Research | of Cray Research, Inc. or its subsidiaries. lfa@cray.com | (...yet) ------------------------------ Date: Mon, 19 Mar 90 15:49:56 PST From: Jordan Kare To: lfa@vielle.cray.com, space-tech@CS.CMU.EDU Subject: Re: Lunar Surface Comm. >From mnr@daisy.learning.cs.cmu.edu Mon Mar 19 12:46:26 1990 >Date: Mon, 19 Mar 90 13:39:25 CST >From: Lou Adornato >To: space-tech@cs.cmu.edu > >Can anyone out there answer the following: > 1) Using present technology, how tighly can a communications laser > be focused? Is it reasonable to expect that tighter focusing in > the next 20 years, or are we at a fundamental limit? Laser range is limited (in vacuum) by diffraction, and good optics are at the diffraction limit. The only way to focus more tightly is to build a bigger mirror. There is considerable excitement in the physics world about ``super-Gaussian'' transmission and ``Photon torpedos'', both referring to ways to do better than the diffraction limit. From what I understand of the physics, these either a) do not do fundamentally better than conventional optics (they control the wavefront differently, but put no more power into the central peak of the beam) or b) require impossible initial conditions, like infinite source size or infinite energy, or c) require impractical conditions, like multi-octave swept-frequency sources, that are unlikely to be available in laser wavelengths any time soon. Proposals in category c) are also not clearly workable even if the sources _are_ available. So I wouldn't rule out the possibility of doing better, but I'd bet heavily on the diffraction limit holding in the near term. To a rough approximation, R=dD/lambda, where R is the useful range, the transmitting and receiving mirrors are diameter d and D, and lambda is the wavelength. You can do better with shorter lambda; hence, for instance, a visible laser will have a longer range than an infrared laser. On the other hand, you may be able to get a more efficient, more powerful infrared laser -- and diffraction-limited mirrors are easier to make for long wavelengths, so you may be able to get bigger mirrors.... > 2) What is the threshold of detection given the present technology? > In other words, at 10Mbps, how much power needs to come out of the > small end of my attenuation function? Again, is this fundamental or > subject to incremental improvements? Laboratory systems get to within a small factor (~3) of the statistical limit, which is effectively a few photons per bit for decent signal/noise ratio. I seem to recall a recent test that got to 20 photons/bit. Assuming ~1 eV photons, 10 Mbps would require ~few x 10^8 eV/sec, or ~10^-10 watts received. You should have a comfortable margin at ~10^-9 watts received (-60 dBm). If you start with ~100 mW (easy with current diode lasers) you have ~80 db available for link losses. Of course, I'm not including problems like scattered sunlight adding to the noise floor. > 3) What references exist for someone with limited knowledge of laser > communications technology (but with an engineering background and > enough physics to not understand eigenfunctions?). I'm talking > about transmission _and_ detection technoology, preferably line of > sight. Any number of handbooks, etc. are available about fiber optics; free-space paths are pretty rare now, so I don't know what to recommend offhand. > >Lou Adornato | Statements herein do not represent the opinions or attitudes >Cray Research | of Cray Research, Inc. or its subsidiaries. >lfa@cray.com | (...yet) > Jordin Kare jtk@mordor.uucp jtk@mordor.s1.gov ------------------------------ Date: Mon, 19 Mar 90 10:55:16 EST From: Gord Deinstadt To: uunet!cs.cmu.edu!space-tech@uunet.UU.NET Subject: AIAA solicits your Moon/Mars ideas Marc, I have been thinking along similar lines for some time. My ultimate goal was a system of tiny robots that could assemble the next generation of larger robots, and so on until they are big enough to do "real" work. But, even if they were less capable they could still do a lot. The biggest problem I see is power. A 20-pound robot can't store much energy relative to its size, at least not with batteries. I'm not sure how constraining this is, since a lot of the work can be done while standing still, which makes a power cord acceptable. You still have to have a lunar power station, though. Hmm- microwave transmitters a few thousand miles apart, locked in phase, should be able to beam power from Earth to a smallish rectenna on the Moon. Anyhow, I'm convinced you can do a whole lot with fairly stupid robots on the Moon, provided you're willing to do it slowly. But slowly is relative; we could put up a bunch of robots next year, so they'd have a big head start over a manned base. In fact I think this is a great way to build the infrastructure for a manned base, not to mention useful chores such as assaying minerals. (Note to Henry S.: I said *we* could put up robots next year, not that *NASA* could. :-) :-( -- Gord Deinstadt gdeinstadt@geovision.UUCP ------------------------------ To: space-tech@CS.CMU.EDU Reply-To: jerbil@csvax.caltech.edu Date: Mon, 19 Mar 90 09:09:41 -0800 From: Joe Beckenbach > The biggest problem I see is power. A 20-pound robot can't store much > energy relative to its size, at least not with batteries. I'm not > sure how constraining this is, since a lot of the work can be done > while standing still, which makes a power cord acceptable. You still > have to have a lunar power station, though. Hmm- microwave transmitters > a few thousand miles apart, locked in phase, should be able to beam > power from Earth to a smallish rectenna on the Moon. Why not simply let each be solar-powered? True, a little additional stuff will be necessary to allow the 'bots to turn off at nearly the same time, but that could be done in a small plain area by having each one turn off when the sunshine level drops below a critical value. So think of it as a 50% duty cycle through each month. 1/3 :-) > (Note to Henry S.: I said *we* could put up robots next year, not that > *NASA* could. :-) :-( And should there be numerous robots and small, then multiple launches are needed anyway. A good reason to choose small or medium new-commercial launch slots? Yeah, that's it. Put up instruments, a couple of small propulsion experiments, a commsat or two, and a batch of robots all on one bus. Put this in orbit around Luna, and then send each item on its way, like blowing dandelion seeds. Joe Beckenbach --- Joe Beckenbach jerbil@cs.caltech.edu (818) 356-6767 ------------------------------ Date: Mon, 19 Mar 90 14:00 CST From: GOTT@wishep.physics.wisc.edu Subject: Fairly Stupid Robots To: space-tech@CS.CMU.EDU X-VMS-To: IN%"space-tech@cs.cmu.edu" How large and intelligent does a robot have to be to be useful for setting up a lunar base? Would a semi-intelligent bulldozer do the trick or do we need something with some sort of arms? What are the early tasks that need to be accomplished? Or that could accomplished using "off the shelf robots" or at least robots built from off the shelf parts.... One bobcat One Macintosh Si One satellite dish (for comm.) One SNAP generator for power Three grey scale CCD cameras for seeing :) George K. Ott UW Madison HEP Dept. gott@wishep.physics.wisc.edu ------------------------------ From: henry@zoo.toronto.edu Date: Mon, 19 Mar 90 17:04:03 EST To: space-tech@CS.CMU.EDU Subject: Fairly Stupid Robots > Or that could accomplished using "off the shelf robots" or at least robots > built from off the shelf parts.... Remember that you need either (a) plenty of power to run heaters, (b) a heated garage, or (c) robots built to soak at cryogenic temperatures during the lunar night. The Surveyor landers tried to survive using small battery-powered heaters in crucial places plus durable hardware; not all of them made it through one lunar night, and I don't think any made it through two. The Lunokhods and the Apollo instrument networks did better, but they had isotope power. If I were trying to use off-the-shelf hardware, I think I'd opt for the heated garage. Henry Spencer at U of Toronto Zoology uunet!attcan!utzoo!henry henry@zoo.toronto.edu ------------------------------ Date: Mon, 19 Mar 90 10:45:30 CST From: "Edward V. Wright" To: uiucuxc!uunet.UU.NET!gd%geovision@uunet.UU.NET, uiucuxc!CS.CMU.EDU!space-tech@uunet.UU.NET Subject: "fairly stupid robots" > we could put up a bunch of robots next year Are you talking about off-the-shelf robots? Are there really robots available that can reproduce themselves? If they can, what materials will they require that can't be obtained on the Moon? I doubt you'll be taking along a complete semiconductor factory at first, so I assume that the "brains," at least, will have to be imported from Earth. ------------------------------ End of Space-tech Digest #55 *******************